The LUX-ZEPLIN (LZ) experiment

Journal Pre-proof The LUX-ZEPLIN (LZ) experiment

D.S. Akerib, C.W. Akerlof, D.Yu. Akimov, A. Alquahtani, S.K. Alsum, T.J. Anderson, N. Angelides, H.M. Araújo, A. Arbuckle, J.E. Armstrong, M. Arthurs, H. Auyeung, X. Bai, A.J. Bailey, J. Balajthy, S. Balashov, J. Bang, M.J. Barry, D. Bauer, P. Bauer, A. Baxter, J. Belle, P. Beltrame, J. Bensinger, T. Benson, E.P. Bernard, A. Bernstein, A. Bhatti, A. Biekert, T.P. Biesiadzinski, B. Birrittella, K.E. Boast, A.I. Bolozdynya, E.M. Boulton, B. Boxer, R. Bramante, S. Branson, P. Brás, M. Breidenbach, J.H. Buckley, V.V. Bugaev, R. Bunker, S. Burdin, J.K. Busenitz, J.S. Campbell, C. Carels, D.L. Carlsmith, B. Carlson, M.C. Carmona-Benitez, M. Cascella, C. Chan, J.J. Cherwinka, A.A. Chiller, C. Chiller, N.I. Chott, A. Cole, J. Coleman, D. Colling, R.A. Conley, A. Cottle, R. Coughlen, W.W. Craddock, D. Curran, A. Currie, J.E. Cutter, J.P. da Cunha, C.E. Dahl, S. Dardin, S. Dasu, J. Davis, T.J.R. Davison, L. de Viveiros, N. Decheine, A. Dobi, J.E.Y. Dobson, E. Druszkiewicz, A. Dushkin, T.K. Edberg, W.R. Edwards, B.N. Edwards, J. Edwards, M.M. Elnimr, W.T. Emmet, S.R. Eriksen, C.H. Faham, A. Fan, S. Fayer, S. Fiorucci, H. Flaecher, I.M. Fogarty Florang, P. Ford, V.B. Francis, F. Froborg, T. Fruth, R.J. Gaitskell, N.J. Gantos, D. Garcia, V.M. Gehman, R. Gelfand, J. Genovesi, R.M. Gerhard, C. Ghag, E. Gibson, M.G.D. Gilchriese, S. Gokhale, B. Gomber, T.G. Gonda, A. Greenall, S. Greenwood, G. Gregerson, M.G.D. van der Grinten, C.B. Gwilliam, C.R. Hall, D. Hamilton, S. Hans, K. Hanzel, T. Harrington, J. Harrison, C. Hasselkus, S.J. Haselschwardt, D. Hemer, S.A. Hertel, J. Heise, S. Hillbrand, O. Hitchcock, C. Hjemfelt, M.D. Hoff, B. Holbrook, E. Holtom, J.Y-K. Hor, M. Horn, D.Q. Huang, T.W. Hurteau, C.M. Ignarra, M.N. Irving, R.G. Jacobsen, O. Jahangir, S.N. Jeffery, W. Ji, M. Johnson, J. Johnson, P. Johnson, W.G. Jones, A.C. Kaboth, A. Kamaha, K. Kamdin, V. Kasey, K. Kazkaz, J. Keefner, D. Khaitan, M. Khaleeq, A. Khazov, A.V. Khromov, I. Khurana, Y.D. Kim, W.T. Kim, C.D. Kocher, A.M. Konovalov, L. Korley, E.V. Korolkova, M. Koyuncu, J. Kras, H. Kraus, S.W. Kravitz, H.J. Krebs, L. Kreczko, B. Krikler, V.A. Kudryavtsev, A.V. Kumpan, S. Kyre, A.R. Lambert, B. Landerud, N.A. Larsen, A. Laundrie, E.A. Leason, H.S. Lee, J. Lee, C. Lee, B.G. Lenardo, D.S. Leonard, R. Leonard, K.T. Lesko, C. Levy, J. Li, Y. Liu, J. Liao, F.-T. Liao, J. Lin, A. Lindote, R. Linehan, W.H. Lippincott, R. Liu, X. Liu, C. Loniewski, M.I. Lopes, B. López Paredes, W. Lorenzon, D. Lucero, S. Luitz, J.M. Lyle, C. Lynch, P.A. Majewski, J. Makkinje, D.C. Malling, A. Manalaysay, L. Manenti, R.L. Mannino, N. Marangou, D.J. Markley, P. MarrLaundrie, T.J. Martin, M.F. Marzioni, C. Maupin, C.T. McConnell, D.N. McKinsey, J. McLaughlin, D.-M. Mei, Y. Meng, E.H. Miller, Z.J. Minaker, E. Mizrachi, J. Mock, D. Molash, A. Monte,

C.W. Akerlof and D.Yu. Akimov

Nuclear Inst. and Methods in Physics Research, A () xxx

M.E. Monzani, J.A. Morad, E. Morrison, B.J. Mount, A.St.J. Murphy, D. Naim, A. Naylor, C. Nedlik, C. Nehrkorn, H.N. Nelson, J. Nesbit, F. Neves, J.A. Nikkel, J.A. Nikoleyczik, A. Nilima, J. O’Dell, H. Oh, F.G. O’Neill, K. O’Sullivan, I. Olcina, M.A. Olevitch, K.C. Oliver-Mallory, L. Oxborough, A. Pagac, D. Pagenkopf, S. Pal, K.J. Palladino, V.M. Palmaccio, J. Palmer, M. Pangilinan, S.J. Patton, E.K. Pease, B.P. Penning, G. Pereira, C. Pereira, I.B. Peterson, A. Piepke, S. Pierson, S. Powell, R.M. Preece, K. Pushkin, Y. Qie, M. Racine, B.N. Ratcliff, J. Reichenbacher, L. Reichhart, C.A. Rhyne, A. Richards, Q. Riffard, G.R.C. Rischbieter, J.P. Rodrigues, H.J. Rose, R. Rosero, P. Rossiter, R. Rucinski, G. Rutherford, J.S. Saba, L. Sabarots, D. Santone, M. Sarychev, A.B.M.R. Sazzad, R.W. Schnee, M. Schubnell, P.R. Scovell, M. Severson, D. Seymour, S. Shaw, G.W. Shutt, T.A. Shutt, J.J. Silk, C. Silva, K. Skarpaas, W. Skulski, A.R. Smith, R.J. Smith, R.E. Smith, J. So, M. Solmaz, V.N. Solovov, P. Sorensen, V.V. Sosnovtsev, I. Stancu, M.R. Stark, S. Stephenson, N. Stern, A. Stevens, T.M. Stiegler, K. Stifter, R. Studley, T.J. Sumner, K. Sundarnath, P. Sutcliffe, N. Swanson, M. Szydagis, M. Tan, W.C. Taylor, R. Taylor, D.J. Taylor, D. Temples, B.P. Tennyson, P.A. Terman, K.J. Thomas, J.A. Thomson, D.R. Tiedt, M. Timalsina, W.H. To, A. Tomás, T.E. Tope, M. Tripathi, D.R. Tronstad, C.E. Tull, W. Turner, L. Tvrznikova, M. Utes, U. Utku, S. Uvarov, J. Va’vra, A. Vacheret, A. Vaitkus, J.R. Verbus, T. Vietanen, E. Voirin, C.O. Vuosalo, S. Walcott, W.L. Waldron, K. Walker, J.J. Wang, R. Wang, L. Wang, Y. Wang, J.R. Watson, J. Migneault, S. Weatherly, R.C. Webb, W.-Z. Wei, M. While, R.G. White, J.T. White, D.T. White, T.J. Whitis, W.J. Wisniewski, K. Wilson, M.S. Witherell, F.L.H. Wolfs, J.D. Wolfs, D. Woodward, S.D. Worm, X. Xiang, Q. Xiao, J. Xu, M. Yeh, J. Yin, I. Young, C. Zhang

PII: DOI: Reference:

S0168-9002(19)31403-2 https://doi.org/10.1016/j.nima.2019.163047 NIMA 163047

To appear in:

Nuclear Inst. and Methods in Physics Research, A

Received date : 21 October 2019 Accepted date : 24 October 2019 Please cite this article as: D.S. Akerib, C.W. Akerlof, D.Yu. Akimov et al., The LUX-ZEPLIN (LZ) experiment, Nuclear Inst. and Methods in Physics Research, A (2019), doi: https://doi.org/10.1016/j.nima.2019.163047. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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C.W. Akerlof and D.Yu. Akimov

Nuclear Inst. and Methods in Physics Research, A () xxx

© 2019 Published by Elsevier B.V.

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The LUX-ZEPLIN (LZ) Experiment

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D.S. Akeriba,b , C.W. Akerlofc , D. Yu. Akimovd , A. Alquahtanie , S.K. Alsumf , T.J. Andersona,b , N. Angelidesg , H.M. Ara´ ujoh , A. Arbucklef , J.E. Armstrongi , M. Arthursc , H. Auyeunga , X. Baij , A.J. Baileyh , J. Balajthyk , S. Balashovl , J. Bange , M.J. Barrym , D. Bauerh , P. Bauern , A. Baxtero , J. Bellep , P. Beltrameq , J. Bensingerr , T. Bensonf , E.P. Bernards,m , A. Bernsteint , A. Bhattii , A. Biekerts,m , T.P. Biesiadzinskia,b , B. Birrittellaf , K.E. Boastu , A.I. Bolozdynyad , E.M. Boultonv,s , B. Boxero , R. Bramantea,b , S. Bransonf , P. Br´ asw , M. Breidenbacha , x x j o y f J.H. Buckley , V.V. Bugaev , R. Bunker , S. Burdin , J.K. Busenitz , J.S. Campbell , C. Carelsu , D.L. Carlsmithf , B. Carlsonn , M.C. Carmona-Benitezz , M. Cascellag , C. Chane , J.J. Cherwinkaf , A.A. Chilleraa , C. Chilleraa , N.I. Chottj , A. Colem , J. Colemanm , D. Collingh , R.A. Conleya , A. Cottleu , R. Coughlenj , W.W. Craddocka , D. Currann , A. Currieh , J.E. Cutterk , J.P. da Cunhaw , C.E. Dahlab,p , S. Dardinm , S. Dasuf , J. Davisn , T.J.R. Davisonq , L. de Viveirosz , N. Decheinef , A. Dobim , J.E.Y. Dobsong , E. Druszkiewiczac , A. Dushkinr , T.K. Edbergi , W.R. Edwardsm , B.N. Edwardsv , J. Edwardsf , M.M. Elnimry , W.T. Emmetv , S.R. Eriksenad , C.H. Fahamm , A. Fana,b , S. Fayerh , S. Fioruccim , H. Flaecherad , I.M. Fogarty Florangi , P. Fordl , V.B. Francisl , F. Froborgh , T. Fruthu , R.J. Gaitskelle , N.J. Gantosm , D. Garciae , V.M. Gehmanm , R. Gelfandac , J. Genovesij , R.M. Gerhardk , C. Ghagg , E. Gibsonu , M.G.D. Gilchriesem , S. Gokhaleae , B. Gomberf , T.G. Gondaa , A. Greenallo , S. Greenwoodh , G. Gregersonf , M.G.D. van der Grintenl , C.B. Gwilliamo , C.R. Halli , D. Hamiltonf , S. Hansae , K. Hanzelm , T. Harringtonf , J. Harrisonj , C. Hasselkusf , S.J. Haselschwardtaf , D. Hemerk , S.A. Hertelag , J. Heisef , S. Hillbrandk , O. Hitchcockf , C. Hjemfeltj , M.D. Hoffm , B. Holbrookk , E. Holtoml , J.Y-K. Hory , M. Hornn , D.Q. Huange , T.W. Hurteauv , C.M. Ignarraa,b , M.N. Irvingk , R.G. Jacobsens,m , O. Jahangirg , S.N. Jefferyl , W. Jia,b , M. Johnsonn , J. Johnsonk , P. Johnsonf , W.G. Jonesh , A.C. Kabothai,l , A. Kamahaah , K. Kamdinm,s , V. Kaseyh , K. Kazkazt , J. Keefnern , D. Khaitanac , M. Khaleeqh , A. Khazovl , A.V. Khromovd , I. Khuranag , Y.D. Kimaj , W.T. Kimaj , C.D. Kochere , A.M. Konovalovd , L. Korleyr , E.V. Korolkovaak , M. Koyuncuac , J. Krasf , H. Krausu , S.W. Kravitzm , H.J. Krebsa , L. Kreczkoad , B. Kriklerad , V.A. Kudryavtsevak , A.V. Kumpand , S. Kyreaf , A.R. Lambertm , B. Landerudf , N.A. Larsenv , A. Laundrief , E.A. Leasonq , H.S. Leeaj , J. Leeaj , C. Leea,b , B.G. Lenardok , D.S. Leonardaj , R. Leonardj , K.T. Leskom , C. Levyah , J. Liaj , Y. Liuf , J. Liaoe , F.-T. Liaou , J. Lins,m , A. Lindotew , R. Linehana,b , W.H. Lippincottp , R. Liue , X. Liuq , C. Loniewskiac , M.I. Lopesw , B. L´ opez Paredesh , c n a e e l e W. Lorenzon , D. Lucero , S. Luitz , J.M. Lyle , C. Lynch , P.A. Majewski , J. Makkinje , D.C. Mallinge , A. Manalaysayk , L. Manentig , R.L. Manninof , N. Marangouh , D.J. Markleyp , P. MarrLaundrief , T.J. Martinp , M.F. Marzioniq , C. Maupinn , C.T. McConnellm , D.N. McKinseys,m , J. McLaughlinab , D.-M. Meiaa , Y. Mengy , E.H. Millera,b , Z.J. Minakerk , E. Mizrachii , J. Mockah,m , D. Molashj , A. Montep , M.E. Monzania,b , J.A. Moradk , E. Morrisonj , B.J. Mountal , A.St.J. Murphyq , D. Naimk , A. Naylorak , C. Nedlikag , C. Nehrkornaf , H.N. Nelsonaf , J. Nesbitf , F. Nevesw , J.A. Nikkell , J.A. Nikoleyczikf , A. Nilimaq , J. O’Delll , H. Ohac , F.G. O’Neilla , K. O’Sullivanm,s , I. Olcinah , M.A. Olevitchx , K.C. Oliver-Mallorym,s , L. Oxboroughf , A. Pagacf , D. Pagenkopfaf , S. Palw , K.J. Palladinof , V.M. Palmaccioi , J. Palmerai , M. Pangilinane , S.J. Pattonm , E.K. Peasem , B.P. Penningr , G. Pereiraw , C. Pereiraw , I.B. Petersonm , A. Piepkey , S. Piersona , S. Powello , R.M. Preecel , K. Pushkinc , Y. Qieac , M. Racinea , B.N. Ratcliffa , J. Reichenbacherj , L. Reichhartg , C.A. Rhynee , A. Richardsh , Q. Riffards,m , G.R.C. Rischbieterah , J.P. Rodriguesw , H.J. Roseo , R. Roseroae , P. Rossiterak , R. Rucinskip , G. Rutherforde , J.S. Sabam , L. Sabarotsf , D. Santoneai , M. Sarychevp , A.B.M.R. Sazzady , R.W. Schneej , M. Schubnellc , P.R. Scovelll , M. Seversonf , D. Seymoure , S. Shawaf , G.W. Shutta , T.A. Shutta,b , J.J. Silki , C. Silvaw , K. Skarpaasa , W. Skulskiac , A.R. Smithm , R.J. Smiths,m , R.E. Smithf , J. Soj , M. Solmazaf , V.N. Solovovw , P. Sorensenm , V.V. Sosnovtsevd , I. Stancuy , M.R. Starkj , S. Stephensonk , N. Sterne , A. Stevensu , T.M. Stiegleram , K. Stiftera,b , R. Studleyr , T.J. Sumnerh , K. Sundarnathj , P. Sutcliffeo , N. Swansone , M. Szydagisah , M. Tanu , W.C. Taylore , R. Taylorh , D.J. Taylorn , D. Templesab , B.P. Tennysonv , P.A. Termanam , K.J. Thomasm , J.A. Thomsonk , D.R. Tiedti , M. Timalsinaj , W.H. Toa,b , A. Tom´ash , T.E. Topep , M. Tripathik , D.R. Tronstadj , C.E. Tullm , W. Turnero , L. Tvrznikovav,s , M. Utesp , U. Utkug , S. Uvarovk , J. Va’vraa , A. Vachereth , A. Vaitkuse , J.R. Verbuse , T. Vietanenf , E. Voirinp , C.O. Vuosalof , S. Walcottf , W.L. Waldronm , K. Walkerf , J.J. Wangr , R. Wangp , L. Wangaa , Y. Wangac , J.R. Watsons,m , J. Migneaulte , S. Weatherlyi , R.C. Webbam , W.-Z. Weiaa , M. Whileaa , R.G. Whitea,b , J.T. Whiteam , D.T. Whiteaf , T.J. Whitisa,an , W.J. Wisniewskia , K. Wilsonm , M.S. Witherellm,s , F.L.H. Wolfsac , J.D. Wolfsac , D. Woodwardz , S.D. Worml , X. Xiange , Q. Xiaof , J. Xut , M. Yehae , J. Yinac , I. Youngp , C. Zhangaa a

b

SLAC National Accelerator Laboratory, Menlo Park, CA 94025-7015, USA Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, Stanford, CA 94305-4085 USA c University of Michigan, Randall Laboratory of Physics, Ann Arbor, MI 48109-1040, USA

Preprint submitted to Elsevier

October 26, 2019

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National Research Nuclear University MEPhI (NRNU MEPhI), Moscow, 115409, RUS e Brown University, Department of Physics, Providence, RI 02912-9037, USA f University of Wisconsin-Madison, Department of Physics, Madison, WI 53706-1390, USA g University College London (UCL), Department of Physics and Astronomy, London WC1E 6BT, UK h Imperial College London, Physics Department, Blackett Laboratory, London SW7 2AZ, UK i University of Maryland, Department of Physics, College Park, MD 20742-4111, USA j South Dakota School of Mines and Technology, Rapid City, SD 57701-3901, USA k University of California, Davis, Department of Physics, Davis, CA 95616-5270, USA l STFC Rutherford Appleton Laboratory (RAL), Didcot, OX11 0QX, UK m Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA 94720-8099, USA n South Dakota Science and Technology Authority (SDSTA), Sanford Underground Research Facility, Lead, SD 57754-1700, USA o University of Liverpool, Department of Physics, Liverpool L69 7ZE, UK p Fermi National Accelerator Laboratory (FNAL), Batavia, IL 60510-5011, USA q University of Edinburgh, SUPA, School of Physics and Astronomy, Edinburgh EH9 3FD, UK r Brandeis University, Department of Physics, Waltham, MA 02453, USA s University of California, Berkeley, Department of Physics, Berkeley, CA 94720-7300, USA t Lawrence Livermore National Laboratory (LLNL), Livermore, CA 94550-9698, USA u University of Oxford, Department of Physics, Oxford OX1 3RH, UK v Yale University, Department of Physics, New Haven, CT 06511-8499, USA w Laborat´ orio de Instrumenta¸ca ˜o e F´ısica Experimental de Part´ıculas (LIP), University of Coimbra, P-3004 516 Coimbra, Portugal x Washington University in St. Louis, Department of Physics, St. Louis, MO 63130-4862, USA y University of Alabama, Department of Physics & Astronomy, Tuscaloosa, AL 34587-0324, USA z Pennsylvania State University, Department of Physics, University Park, PA 16802-6300, USA aa University of South Dakota, Department of Physics & Earth Sciences, Vermillion, SD 57069-2307, UK ab Northwestern University, Department of Physics & Astronomy, Evanston, IL 60208-3112, USA ac University of Rochester, Department of Physics and Astronomy, Rochester, NY 14627-0171, USA ad University of Bristol, H.H. Wills Physics Laboratory, Bristol, BS8 1TL, UK ae Brookhaven National Laboratory (BNL), Upton, NY 11973-5000, USA af University of California, Santa Barbara, Department of Physics, Santa Barbara, CA 93106-9530, USA ag University of Massachusetts, Department of Physics, Amherst, MA 01003-9337, USA ah University at Albany (SUNY), Department of Physics, Albany, NY 12222-1000, USA ai Royal Holloway, University of London, Department of Physics, Egham, TW20 0EX, UK aj IBS Center for Underground Physics (CUP), Yuseong-gu, Daejeon, KOR ak University of Sheffield, Department of Physics and Astronomy, Sheffield S3 7RH, UK al Black Hills State University, School of Natural Sciences, Spearfish, SD 57799-0002, USA am Texas A&M University, Department of Physics and Astronomy, College Station, TX 77843-4242, USA an Case Western Reserve University, Department of Physics, Cleveland, OH 44106, USA

We describe the design and assembly of the LUX-ZEPLIN experiment, a direct detection search for cosmic WIMP dark matter particles. The centerpiece of the experiment is a large liquid xenon time projection chamber sensitive to low energy nuclear recoils. Rejection of backgrounds is enhanced by a Xe skin veto detector and by a liquid scintillator Outer Detector loaded with gadolinium for efficient neutron capture and tagging. LZ is located in the Davis Cavern at the 4850’ level of the Sanford Underground Research Facility in Lead, South Dakota, USA. We describe the major

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subsystems of the experiment and its key design features and requirements.

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1. Overview

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tanium [7]. The cryostat stands inside the Davis Campus water tank, which provides shielding from laboratory gam-

the LUX-ZEPLIN (LZ) experiment, a search for dark mat-

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mas and neutrons. An additional set of PMTs immersed

ter particles at the Sanford Underground Research Facil-

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in the water observe an Outer Detector (OD) comprised of

ity (SURF) in Lead, South Dakota, USA. LZ is capable

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acrylic vessels (AVs) surrounding the cryostat. The AVs

of observing low energy nuclear recoils, the characteristic

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contain organic liquid scintillator loaded with Gadolinium

signature of the scattering of WIMPs (Weakly Interact-

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(GdLS) for efficient neutron and gamma tagging. The wa-

ing Massive Particles). It is hosted in the Davis Cam-

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ter tank and OD are penetrated by various TPC services,

pus water tank at SURF, formerly the home of the LUX

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including vacuum insulated conduits for LXe circulation,

experiment[1]. LZ features a large liquid xenon (LXe)

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instrumentation cabling, neutron calibration guide tubes,

time projection chamber (TPC), a well-established tech-

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and the cathode high voltage (HV) connection.

nology for the direct detection of WIMP dark matter for

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masses greater than a few GeV. The detector design and

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mize the amount of underground fabrication and assem-

experimental strategy derive strongly from the LUX and

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bly of the various detector sub-systems. The LZ TPC is

ZEPLIN–III experiments [2, 3]. A Conceptual Design Re-

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assembled and integrated into the ICV in a surface lab-

port and a Technical Design Report were completed in

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oratory cleanroom at SURF, with the ICV outer diame-

2015 and 2017, respectively [4, 5]. The projected cross-

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ter taking maximal advantage of the available space in the

section sensitivity of the experiment is 1.5 × 10−48 cm2 for

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Yates shaft. The OCV and OD, being larger than the ICV,

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cannot be transported underground in monolithic form.

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a 40 GeV/c2 WIMP (90% C.L.) [6].

One goal of the experimental architecture is to mini-

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In this article we describe the design and assembly of

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Therefore the OCV is segmented into three flanged com-

The LZ TPC monitors 7 active tonnes (5.6 tonnes fidu-

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ponents and integrated and sealed in the Davis Campus

cial) of LXe above its cathode. Ionizing interactions in

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water tank, while the OD is subdivided into ten hermetic

the active region create prompt and secondary scintillation

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AVs. This architecture does not require any underground

signals (‘S1’ and ‘S2’), and these are observed as photo-

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titanium welding or acrylic bonding.

electrons (PEs) by two arrays of photomultiplier tubes

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Besides the instrumented skin and OD, several other

(PMTs). The nature of the interaction, whether electronic

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design choices distinguish LZ from its LUX predecessor.

recoil (‘ER’) or nuclear recoil (‘NR’), is inferred from the

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The cathode HV connection, for example, is made at a

energy partition between S1 and S2. The location of the

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side port on the cryostat, while the PMT cables from the

event is measured from the drift time delay between S1

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lower half of the TPC are pulled from the bottom. The

and S2 (z coordinate) and from the S2 spatial distribution

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heat exchanger for LXe condensation, evaporation, and

(x and y coordinates). The TPC is housed in an inner

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circulation is located in a separate and dedicated cryostat

cryostat vessel (ICV), with a layer of ‘skin’ LXe acting as a

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outside the water tank, with LXe being circulated to and

high voltage stand-off. The skin is separately instrumented

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from the bottom of the detector through vacuum insu-

with PMTs to veto gamma and neutron interactions in

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lated transfer lines. To continuously reject heat from the

this region. The ICV is suspended inside the outer cryo-

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LN thermosyphon systems, a cryocooler is installed above

stat vessel (OCV), cooled by a set of LN thermosyphons,

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the water tank in the Davis Campus. This eliminates the

and thermally isolated by an insulating vacuum. Both the

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need to transport LN to the underground, except during

ICV and OCV are fabricated from low radioactivity ti-

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cryocooler maintenance and repair.

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A cutaway drawing of the experiment is shown in Fig. 1.

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Figure 1: Rendering of the LZ experiment, showing the major detector subsystems. At the center is the liquid xenon TPC (1), monitored by two arrays of PMTs and serviced by various cable and fluid conduits (upper and lower). The TPC is contained in a double-walled vacuum insulated titanium cryostat and surrounded on all sides by a GdLS Outer Detector (2). The cathode high voltage connection is made

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horizontally at the lower left (5). The GdLS is observed by a suite of 8” PMTs (3) standing in the water (4) which provides shielding for the detector. The pitched conduit on the right (6) allows for neutron calibration sources to illuminate the detector.

The experimental strategy is driven by the need to con- 93 This volume measures approximately 1.5 m in diameter

78

trol radon, krypton, and neutron backgrounds. Control of 94 and height, and is viewed by two arrays of PMTs. The

79

dust on all xenon-wetted parts is essential, since it can be 95 liquid phase produces prompt S1 pulses. This is topped by

80

an important source of radon. Kr is removed from the ven- 96 a thin layer of vapor (8 mm thick) where delayed S2 elec-

81

dor supplied xenon using an off-site charcoal chromatogra- 97 troluminescence light is produced from ionization emitted

82

phy facility. This purification step takes place prior to the 98 across the surface. Around and underneath the TPC, the

83

start of underground science operations. Gamma back- 99 Xe Skin detector contains an additional ∼2 tonnes of liq-

84

grounds are highly suppressed by the self-shielding of the100 uid, also instrumented with PMTs. This space is required

85

TPC, and by careful control and selection of detector ma-101 for dielectric insulation of the TPC but it constitutes an

86

terials. Neutrons from spontaneous fission and alpha cap-102 anti-coincidence scintillation detector in its own right. An

87

ture on light nuclei are efficiently tagged and vetoed by103 overview of the Xenon Detector is shown in Fig. 2.

88

the OD and skin.

89

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2. The Xenon Detector: TPC and Skin

90

The Xenon Detector is composed of the TPC and its

91

Xe Skin Veto companion. The central TPC contains 7

92

tonnes of active LXe which constitutes the WIMP target.

104

The design of the Xenon Detector optimizes: i) the

105

detection of VUV photons generated by both S1 and S2,

106

through carefully chosen optical materials and sensors both

107

in the TPC and the Xe Skin; and ii) the detection of ioniza-

108

tion electrons leading to the S2 response, through carefully

109

designed electric fields in the various regions of the TPC.

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110

The hardware components involved in the transport and

111

detection of photons and of electrons in the detector are

112

described in Sections 2.1 and 2.3. Section 2.4 describes the

113

flow and the monitoring of the LXe fluid itself.

114

2.1. Optical Performance of the TPC Both the S1 and the S2 signals produced by particle

116

interactions consist of vacuum ultraviolet (VUV) photons

117

produced in the liquid and gas phases, respectively. It is

118

imperative to optimize the detection of these optical sig-

119

nals. For the S1 response, the goal is to collect as many

120

VUV photons as possible, as this determines the thresh-

121

old of the detector. This is achieved primarily by the

122

use of high quantum efficiency (QE) PMTs optimized for

123

this wavelength region, viewing a high-reflectance cham-

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115

124

ber covered in PTFE, and by minimizing sources of photon

125

extinction in all materials. Good photocathode coverage

126

is also essential. For the S2 response, the gain of the elec-

127

troluminescence process makes it easier to collect enough photons even at the lowest energies, and the main design

129

driver is instead to optimize the spatial resolution, espe-

130

cially for peripheral interactions.

urn al P 128

131

The TPC PMTs are 3–inch diameter Hamamatsu R11410–

22, developed for operation in the cold liquid xenon and

133

detection of the VUV luminescence. The “-22” variant was

134

tuned for LZ in particular: both for ultra-low radioactiv-

135

ity and for resilience against spurious light emission ob-

136

served at low temperature in previous variants [8]. The

137

average cold QE is 30.9% after accounting for the dual

138

photoelectron emission effect measured for xenon scintil-

139

lation [9]. Key parameters were tested at low temperature

140

for all tubes, including pressure and bias voltage resilience,

Figure 2: The assembled Xenon Detector. Upper panel labels: 1-141

gain and single photoelectron response quality, afterpuls-

Top PMT array; 2-Gate-anode and weir region (liquid level); 3-Side

142

ing and dark counts. These are critical parameters that

143

directly influence the overall performance of the detector.

inch). Lower panel photo by Matthew Kapust, Sanford Underground144

The procurement, radioassay and performance test cam-

Research Facility.

145

paign lasted for nearly three years. The PMTs are pow-

146

ered by resistive voltage divider circuits attached to the

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skin PMTs (1-inch); 4-Field cage; 5-Cathode ring; 6-Reverse field region; 7-Lower side skin PMTs (2-inch); 8-Dome skin PMTs (2-

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tubes inside the detector. The voltage ladder is that rec-156 241 units arranged in close-packed hexagonal pattern. A

148

ommended by Hamamatsu, using negative bias to extract157 downward-looking “top” array located in the gas phase

149

the signal near ground potential (two independent cables158 features 253 units arranged in a hybrid pattern that tran-

150

are used for signal and bias). The nominal operating gain159 sitions from hexagonal near the center to circular at the

151

of the PMTs is 3.5 × 106 measured at the end of the signal160 perimeter. This design was chosen to optimize the posi-

152

cables.

tion reconstruction of the S2 signal for interactions near

162

the TPC walls, a leading source of background in these

163

detectors. The structural elements of the arrays are made

164

from low-background titanium. These include a thin plate

165

reinforced by truss structures with circular cut-outs to

166

which the individual PMTs are attached. In the bottom

167

array this plate sits at the level of the PMT windows.

168

The exposed titanium is covered with interlocking PTFE

169

pieces to maximize VUV reflectance. The PMTs are held

170

by Kovar belts near their mid-point and attached to this

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of

161

plate by thin PEEK rods. In the top array the struc-

172

tural plate is located at the back of the tubes, and the

173

gaps between PMT windows are covered with more com-

174

plex interlocking PTFE pieces secured to the back plate.

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171

175

The array designs ensure that mechanical stresses induced

176

by the thermal contraction of PTFE and other materials

177

does not propagate significantly to the PMT envelopes. A

178

number of blue LEDs (Everlight 264-7SUBC/C470/S400)

179

are installed behind plastic diffusers between PMTs at the

180

face of both arrays. These are used to help optimize and

Figure 3: Arrays of R11410–22 PMTs viewing the TPC. Upper panel:181

calibrate the PMT gains and the timing response of the

front view of the top PMT array within its assembly and transportation enclosure. Note the circular PMT arrangement at the periph-

182

detector. The assembly and transport of the PMT arrays

ery transitioning to compact hexagonal towards the center, and the183

required a robust QA process to prevent mechanical dam-

coverage of non-sensitive surfaces by interlocking pieces of highly184

age, dust contamination, and radon-daughter plate-out.

reflective PTFE. Photo by Matthew Kapust, Sanford Underground

185

At the center of this program were specially-designed her-

186

metic enclosures that protected the arrays during assem-

as well as 18 2-inch dome PMTs, which are part of the skin veto187

bly, checkout, transport and storage until assembly into

system. Also visible are the titanium support trusses and the LXe188

the TPC at SURF.

Research Facility. Lower panel: Back view of bottom array PMTs

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in hexagonal arrangement, showing cable connections and routing

distribution lines. Most surfaces are covered in PTFE to aid light collection.

189

A key element of the optical systems is the ∼20 km

190

of cabling used for PMT and sensor readout. A 50-Ohm

154

Two PMT arrays detect the xenon luminescence gener-191 coaxial cable from Axon Cable S.A.S. (part no. P568914Aˆ) ated in the TPC. These are shown in Fig. 3. An upward-192 was selected for electrical and radioactivity performance.

155

looking “bottom” array immersed in the liquid contains193 This cable has a copper-made inner conductor and braid,

153

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194

and extruded FEP insulator and outer sleeve (1.3-mm232 (PDE) for S1 and S2 light in the TPC. These include pho-

195

nominal diameter).

196

to the external feed-throughs means that signal attenu-234 ers) and absorption by impurities in the liquid bulk. With

197

ation, dispersion and cross-talk are important considera-235 realistic values for these parameters our optical model pre-

198

tions. The individual cables were pre-assembled into bun-236 dicts a photon detection efficiency of around 12% for S1

199

dles which are routed together through two conduits that237 light.

200

carry the cables from the top and bottom of the detector.238

201

An additional consideration is the potential for radon em-239 robust reconstruction of low energy events at the edge of

202

anation. This is especially important for the fraction of240 the TPC, in particular from the decay of Rn daughters de-

203

the cabling located near room temperature. Low intrinsic241 posited on the field cage wall, termed “wall events”. These

204

radioactivity of the cable materials can be easily achieved,242 events may suffer charge loss, thus mimicking nuclear re-

205

but dust and other types of contamination trapped within243 coils [12]. If mis-reconstructed further into the active re-

206

the fine braid during manufacture can be problematic. We244 gion, they can be a significant background. Our aim is

207

developed additional cleanliness measures with Axon to245 to achieve ∼106 :1 rejection for this event topology in the

208

mitigate this and have opted for a jacketed version which246 fiducial volume. A detailed study of this issue led to the

209

acts as a further radon barrier. In addition, the xenon247 adoption of a circular PMT layout near the detector edge,

210

flow purging the cable conduits is directed to the inline248 with the final PMT row overhanging the field cage inner

211

radon-removal system described in Sec. 3.2.

The 12 m span from the detector233 ton absorption by the electrode grids (wires and ring hold-

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The optical design of the S2 signal is optimized for

249

walls, an optimized distance between the top PMT ar-

After the PMTs, the next main optical component of250 ray and the liquid surface, and an absorbing optical layer

213

the detector is the PTFE that defines the field cage and251 (Kapton foil) covering the lateral conical wall in the gas

214

covers the non-sensitive detector components. The optical252 phase.

215

performance of the detector depends strongly on its VUV

216

reflectivity – VUV photons reflect several times off PTFE

217

surfaces before detection – and the radiopurity of this ma-254

218

terial is also critical due to its proximity to the active vol-255 Xe Skin, the region containing around 2 tonnes of LXe

219

ume. We identified both the PTFE powder and process256 between the field cage and the inner cryostat vessel. A

220

that optimized radiological purity and VUV reflectivity in257 primary motivation for this liquid is to provide dielec-

221

liquid xenon during a long R&D campaign [10, 11]. The258 tric insulation between these two elements. In addition

222

PTFE selected was Technetics 8764 (Daiken M17 powder),259 to its electrical standoff function, it is natural to instru-

223

whose reflectivity when immersed in LXe we measured as260 ment this region for optical readout so that it can act as

224

0.973 (>0.971 at 95% C.L.). Our data are best fitted by261 a scintillation-only veto detector, especially effective for

225

a diffuse-plus-specular reflection model for this particular262 gamma-rays.

226

material, which was tested using the procedures described263 light from particle interactions or electrical breakdown in

227

in Ref. [10]. Most PTFE elements were machined from264 this region could leak in to the TPC unnoticed and create

228

moulds of sintered material, while thinner elements are265 difficult background topologies. To further suppress this

229

‘skived’ from cast cylinders manufactured from the same266 pathology, the LZ field cage is designed to optically isolate

230

powder.

231

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253

267

Other factors influence the photon detection efficiency268 7

2.2. The Xe Skin Detector An important component of the Xenon Detector is the

Also, if the skin were not instrumented,

the skin from the TPC. The side region of the skin contains 4 cm of LXe at

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the top, widening to 8 cm at cathode level for increased285 minimize field effects were epoxied to the ICV wall with

270

standoff distance. This is viewed from above by 93 1–inch286 MasterBond EP29LPSP cryogenic epoxy. Holes were ma-

271

Hamamatsu R8520-406 PMTs. These are retained within287 chined into the PTFE tiles to fit around the buttons, and

272

PTFE structures attached to the external side of the field288 PTFE washers were attached to the buttons with PTFE

273

cage, located below the liquid surface. At the bottom of289 screws to secure the tiles in place. These are visible in

274

the detector a ring structure attached to the vessel con-290 Fig. 4.

275

tains a further 20 2–inch Hamamatsu R8778 PMTs view-

276

ing upward into this lateral region, as shown in Fig. 4.

2.3. TPC Electrostatic Design

of

291

The S2 signature detected from particle interactions

293

in the liquid xenon comes from the transport of ioniza-

294

tion electrons liberated by the recoiling nucleus or elec-

295

tron, and their subsequent emission into the gas phase

296

above the liquid, where the signal is transduced into a

297

second VUV pulse via electroluminescence. Great care is

298

required to ensure that the various electric field regions

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292

299

in the detector achieve this with high efficiency and with

300

low probability for spurious responses. The LZ detector

301

is instrumented as a traditional three-electrode two-phase

302

detector, with cathode and gate wire-grid electrodes establishing a drift field in the bulk of the liquid, and a separate

304

extraction and electroluminescence region established be-

305

tween the gate and an anode grid. The former sits 5 mm

306

below the surface and the latter is 8 mm into the gas. The

307

nominal operating pressure of the detector is 1.8 bara. At

308

nominal fields, each electron emitted into the gas generates

309

∼820 electroluminescence photons.

urn al P 303

Figure 4: Upper panel: CAD section of the TPC below the cathode

310

The nominal 300 V/cm drift field established in the

311

active region of the detector requires application of an

attached to the ICV that ensures high reflectance in the skin region312

operating voltage of −50 kV to the cathode grid, which

showing the location of the 2” bottom side skin (1) and lower dome (2) PMTs. Lower panel: Photograph showing the PTFE panelling

and the lower side skin PMT ring at the bottom of the vessel.

The dome region of the skin at the bottom of the de-

278

tector is instrumented with an additional 18 2–inch R8778

279

PMTs. These are mounted horizontally below the bottom

280

array, with 12 looking radially outward and 6 radially in-

281

ward. To enhance light collection, all PMTs in that region

282

and array truss structures are dressed in PTFE. Moreover,

283

PTFE tiles line the ICV sides and bottom dome. To attach

284

the PTFE lining, low profile titanium buttons designed to

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313

allows LZ to meet its baseline performance for particle

314

discrimination. The design goal is −100 kV, the maxi-

315

mum operating voltage for the system. The system to

316

deliver the HV to the cathode grid contains some of the

317

highest fields in the detector. The HV is delivered from

318

the power supply (Spellman SL120N10, 120 kV) via a

319

room-temperature feed-through and into a long vacuum-

320

insulated conduit entering the detector at the level of the

321

cathode grid, as shown in Fig. 5. Most of the system was

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322

tested to −120 kV in liquid argon, except for the flexible346 the potential along the dielectric, preventing very high field

323

component connecting the grading structure to the cath-347 regions inside the detector. The maximum field in the liq-

324

ode, for which a similarly-shaped part was tested in liquid348 uid xenon is 35 kV/cm. This voltage grading system ends

325

argon to surface fields 30% higher than those needed to349 in a bayonet connector that allows rapid engagement to

326

meet the design goal.

350

the cathode ring during installation, minimizing exposure

351

of the detector to radon. Inside the ICV there is a significant insulation stand-

353

off distance of 4–8 cm between the field cage and the inner

354

vessel, and there is no instrumentation or cabling installed

355

along the length of the skin region, where the field is high

356

and the possibility of discharges and stray light production

357

would be concerning. This region is instead optimized for

358

optical readout, becoming an integral part of the LZ veto

359

strategy.

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The drift region is 145.6 cm long between cathode and

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360

361

gate electrodes, and 145.6 cm in diameter, enclosed by a

362

cylindrical field cage which defines the optical environment

363

for scintillation light and shapes the electric field for elec-

364

tron transport. The field cage is constructed of 58 layers

Figure 5: The interface of the high voltage system with the cathode. 1-Polyethylene high voltage cable; 2-LXe displacer; 3-LXe space; 4Stress cone; 5-grading rings.

329

330

331

332

333

334

335

336

337

338

339

340

341

342

343

344

345

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328

The cable enters the xenon space through two o-rings365 of PTFE, which provides insulation and high reflectivity, at the core of the feed-through system mounted on top366 with a set of embedded titanium electrode rings. The layof the water tank. The space between them is continu-367 ers are 25 mm tall, the Ti rings 21 mm tall, and each layer ously pumped to provide a vacuum guard, monitored by368 of PTFE is azimuthally segmented 24 times. Due to their a Residual Gas Analyzer. Located at room temperature369 proximity to the active volume, these are critical materials and far from the detector, a leak-tight seal to the xenon370 for LZ. The metal rings are made from the same batch of space can be reliably achieved and the feed-through mate-371 titanium used for the cryostat [7] (the PTFE is described rials are not a radioactivity concern. Another key feature372 above). A key design driver was to achieve a segmented of the cathode HV system is the reliance on a single span373 field cage design: to prevent the excessive charge accumuof polyethylene cable (Dielectric Sciences SK160318), con-374 lation observed in continuous PTFE panels, and to better necting the power supply all the way to the cathode in the375 cope with the significant thermal contraction of PTFE beliquid xenon many meters away. This 150 kV-rated cable376 tween ambient and low temperature. The field cage emfeatures a conductive polyethylene sheath and center core377 beds two resistive ladders connecting the metal rings, each

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and contains no metal components, avoiding differential378 with two parallel 1 GΩ resistors per section (the first step contraction and thermal stress issues, and precluding the379 has 1 GΩ in parallel with 2 GΩ to tune the field close to appearance of insulation gaps between the dielectric and380 the cathode ring). This ladder ensures a vertical field with the sheath, which contract equally. The HV line ends in381 minimal ripple near the field cage. a complex voltage-grading structure near the cathode grid382

The lower PMT array cannot operate near the high ring where the conductive sheath splays away. This grades383 potential of the cathode and so a second, more compact 9

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Figure 6: The electron extraction region assembly on the loom. 1-Anode grid; 2-Gate grid. During TPC operations the anode is above the LXe surface and the gate is below. The liquid level is registered to this assembly by three weir spillovers.

ladder is required below that electrode. This reverse field409 glued region, and the tensioning weights are released after

385

region (RFR) contains only 8 layers with two parallel 5 GΩ410 curing. This woven mesh has several advantages over wires

386

resistors per section, and terminates 13.7 cm away at a411 stretched in a single direction. The load on the ring set is

387

bottom electrode grid which shields the input optics of412 azimuthally uniform and purely radial, allowing the mass

388

the PMTs from the external field.

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384

413

of the rings to be minimized. The region of non-uniform

The electrode grids are some of the most challenging414 field near the wires is smaller for a mesh, which improves

390

components of the experiment, both to design and to fab-415 the uniformity and hence energy resolution obtained in

391

ricate. Mechanically, these are very fragile elements that416 the S2 channel. Finally, a mesh grid has lower field trans-

392

nonetheless involve significant stresses and require very417 parency than stretched wires, resulting in a more uniform

393

fine tolerances for wire positioning. Besides optimizing418 overall drift field. To preserve high S2 uniformity, it is im-

394

the conflicting requirements of high optical transparency419 portant that the woven mesh have uniform wire spacing.

395

and mechanical strength, electrical resilience was an addi-420 The loom design included several features to achieve high

396

tional major driver – spurious electron and/or light emis-421 uniformity during fabrication, and great care was taken in

397

sion from such electrodes is a common problem in noble422 subsequent grid handling to avoid displacing wires.

398

liquid detectors [13, 14].

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The anode and gate grids are depicted in Fig. 6. All

400

grids are made from 304 stainless steel ultra-finish wire [15]

401

woven into meshes with a few mm pitch using a custom

402

loom. Key parameters of the four LZ grids are listed in

403

Table 1. Each wire is tensioned with 250 g weights on both

404

ends, and the mesh is glued onto a holder ring. The glue,

405

MasterBond EP29LPSP cryogenic epoxy, is dispensed by

406

a computer-controlled robotic system. It includes acrylic

407

beads that prevent external stresses from being transferred

408

to the wire crossings. A second metal ring captures the

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◦

Table 1: TPC electrode grid parameters (all 90 woven meshes).

Electrode

Voltage

Diam.

Pitch

Num.

(kV)

(µm)

(mm)

Anode

+5.75

100

2.5

1169

Gate

−5.75

75

5.0

583

Cathode

−50.0

100

5.0

579

Bottom

−1.5

75

5.0

565

423

At the top of the detector, the electron extraction and

424

electroluminescence region, which contains the gate-anode

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tain the strongest surface fields of any cathodic element

445

in the detector ('52 kV/cm, with no grid deflection, and

446

'58 kV/cm with 1.6 mm combined gate/anode deflection).

447

A major QA program was implemented to ensure the

448

high quality of the grids throughout manufacture, clean-

449

ing, storage and transport, and to prevent damage and

450

dust contamination. A key feature of this program was

451

a series of measurements of the high voltage behavior in

452

Xe gas of 1/10th scale and full scale prototype grids, and

453

the final cathode, gate and anode grids. Emission of sin-

454

gle electrons to a rate as low as ∼Hz was measured via an

455

S2-like electroluminescent signal with PMTs. These mea-

456

surements confirmed earlier work [14] showing that elec-

457

tron emission is strongly reduced by passivation, thus the

458

production gate grid was passivated in citric acid for 2

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459

Figure 7: The electron extraction and electroluminescence region.460

hours at ∼125◦ F at AstroPak [16]. 2.4. Fluid Systems and Sensors

1-TPC PMT; 2-Anode grid; 3-Gate grid; 4-Weir; 5-Xe Skin PMT.

Purified and sub-cooled LXe is prepared by the LXe

461

462

tower and delivered to the Xenon Detector through two

463

vacuum-insulated supply lines that connect at the ICV

464

bottom center flange (see Sec. 3.2). One line flushes the

465

lower dome and side skin, the other fans out through a

466

manifold into seven PTFE tubes that penetrate the lower

467

PMT array and supply LXe to the TPC. The fluid returns

468

to the external purification system by spilling over three

469

weirs that establish the liquid surface height. The weirs

470

have 23.3 cm circumference, are uniformly spaced in az-

471

imuth around the top of the TPC, and are mounted to the

472

gate grid ring so that the liquid level is well registered to

473

the location of the gate and anode grids. The weirs drain

474

through three tubes that penetrate the ICV in the side

475

skin region and descend in the insulating vacuum space.

476

The three lines are ganged together near the bottom of the

477

ICV and return liquid to the purification circuit through

478

a common vacuum-insulated transfer line.

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425

system, is one of its most challenging aspects (see Fig. 7.)

426

It establishes the high fields that extract electrons from the

427

liquid and then produce the S2 light. The quality of the S2

428

signal is strongly dependent on both the small- and large-

429

scale uniformity achieved in this region. In particular, the

430

anode contains the finest mesh of any LZ grid (2.5 mm

431

pitch) as this drives the S2 resolution.

An important consideration was the electrostatic de-

433

flection of the gate-anode system. We have directly mea-

434

sured the deflection of the final grids as a function of

435

field using a non-contact optical inspection method. This

436

matches expectations from electrostatic and mechanical

437

modeling, and predicts a ∼1.6 mm decrease in the 13 mm

438

gap at the 11.5 kV nominal operating voltage. As a conse-

439

quence the field in the gas phase varies from 11.5 kV/cm

440

at the center to 10.1 kV/cm at the edge. The combined

441

effect of field increase and gas gap reduction increases

442

the S2 photon yield by 5% at the center.

443

can be corrected in data analysis. The gate wires sus-

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432

This effect

479

A variety of sensors monitor the behavior and perfor-

480

mance of the TPC. Six Weir Precision Sensors (WPS) mea-

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481

sure the liquid level to within ≈20 µm in the gate-anode517 vessels (see Sec. 5). Both vessels were designed in com-

482

region. An additional WPS is installed in the lower dome518 pliance with the ASME Boiler and Pressure Vessel Code

483

to monitor filling and draining of the detector. Long level519 Section VIII Div. 1.

484

sensors (LLS) are installed in the LXe tower for providing520

485

information during detector filling and for monitoring dur-521 connected by a large flange near the top. The maximum

486

ing normal operation. RF loop antennae (LA) and acous-522 outer diameter of the ICV is constrained by the cross-

487

tic sensors (AS) monitor the electrostatic environment of523 section of the Yates shaft. Its tapered shape is to reduce

488

the detector during electrode biasing and thereafter dur-524 the electric field near the cathode. The TPC structure is

489

ing operation. The combination of WPS and AS sensors525 anchored to the bottom of the ICV through six dedicated

490

will also be used to detect disturbances of the fluid system526 ports in the dished end. Three angled ports below the

491

and especially the liquid surface, such as bubbling or drip-527 main flange are provided for the LXe weir drain return

492

ping. These will be aided by dedicated resistors installed528 lines. Two ports at the top head and the central port at

493

in the bottom array that will be used to create bubbles in529 the bottom are for the PMT and instrumentation cables.

494

the LXe so that their signature can be characterized. At530 The high voltage port has been machined on the inside to

495

the top of the detector, a hexapod structure connects the531 form a curvature minimizing the electric field around the

496

top PMT array to the ICV lid through six displacement532 cathode HV feed-through. Five plastic blocks are attached

497

sensors, allowing the displacement and tilt between these533 to the tapered part of the ICV wall to prevent the ICV

498

two elements to be measured to within 0.1 degrees. This is534 from swinging during a seismic event.

499

especially important to prevent major stresses arising dur-535

500

ing cool-down of the TPC. Finally, PT100 thermometers536 largest into the conveyance of the Yates shaft. A port in

501

are distributed at both ends of the detector. By design, all537 the center of the top head hosts the low energy neutron

502

sensors and their cabling are excluded from the side skin538 source deployed in a large tungsten “pig” (see Sec. 5). A

503

region and other high electric field regions. All sensors539 reinforcing ring allows the top AVs to rest on the OCV

504

are read out by dedicated electronics attached to flanges540 head.

505

enclosed in the signal breakout boxes.

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The ICV consists of a top head and a bottom vessel

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The OCV consists of three segments in order to fit the

506

3. Cryogenics and Xe Handling

507

3.1. Cryostat and cryogenics

The Xenon Detector and its LXe payload are contained

509

in the Inner Cryostat Vessel. The Outer Cryostat Vessel

510

provides its vacuum jacket. As shown in Fig. 8, the OCV

511

is supported at the bottom by three legs. The same as-

512

sembly provides shelves for the GdLS AVs located under-

513

neath the OCV. The ICV is suspended from the top head

514

of the OCV with a mechanism enabling its levelling from

515

above. Three long tubes run vertically to deploy calibra-

516

tion sources into the insulating vacuum space between the

Jo

508

541

The entire cryostat assembly is made out of a carefully

542

selected ultra-radiopure Grade-1 titanium sourced from a

543

single titanium supplier [7]. After a comprehensive mate-

544

rial search campaign, a 5 metric ton Titanium Gr-1 slab

545

was procured from Timet and used to fabricate all the

546

stock material required for the cryostat. Initially it was

547

cut into three pieces in order to roll the plates with mul-

548

tiple thicknesses, forge all the flanges, and the ports and

549

to draw the welding wires. The ICV and OCV were fab-

550

ricated from this material at Loterios in Milan, Italy (see

551

Fig. 8). The cleaning and etching of the ICV and OCV is

552

described in Sec. 8.

553

The ICV is maintained at its operating temperature

554

by a set of closed-loop thermosyphon heat pipes utilizing

12

Journal Pre-proof

1

fins on the ICV exterior and are serviced by three ther-

568

mosyphon lines. The coldheads are placed at a height

569

just below the LXe level. The cooling power of each ther-

570

mosyphon is set by adjusting the amount of process nitro-

571

gen in each circuit. Fine adjustment is provided by PID-

572

controlled trim heaters located on each coldhead. Two ad-

573

ditional thermosyphon circuits remove heat from the LXe

574

tower (see Sec. 3.2).

575

The total heat budget of the experiment is estimated

576

to be 700 W. The largest contributing item, at 349 W, is

577

due to the inefficiency of the primary two-phase xenon cir-

578

culation heat exchanger. The thermosyphon trim heaters

579

and the heat leak into the ICV each account for about

580

115 W.

re-

2

567

581

Figure 8: The ICV and OCV during a test assembly at Loterios in

582

The online Xe purification system continuously removes

583

electronegative impurities from the Xe while also provid-

584

ing some measure of Rn removal and control. Rejection

585

of electronegatives begins during the final assembly of the

586

detector with a TPC outgassing campaign described in

587

Sec. 7. The electron lifetime goal is 800 µs, sufficient to

588

drift charge from the cathode to the anode while suffering

Italy, prior to cleaning and etching. The ICV is suspended from the

top dome of the OCV. 1-ICV; 2-middle section OCV; 3-top dome

section OCV; 4-ICV weir drain port; 5-OCV cathode high voltage589 port.

555

nitrogen as the process fluid. The thermosyphons deliver

556

heat from the ICV to the Cryogen On Wheels (COW), a

557

bath of LN located above the water tank in the Davis Cav-

558

ern. A cryocooler, model SPC-1 from DH industries, recondenses the boil-off nitrogen from the COW and trans-

560

fers the heat to the chilled water system. During cry-

561

ocooler maintenance and repair, the COW can be filled

562

by transporting LN to the Davis Cavern from the surface.

563

Four 450 liter storage dewars located underground, act as

564

an intermediate LN repository to enable this mode of op-

Jo

559

565

eration.

3.2. Online Xe handling and purification

urn al P

5

of

4

Six copper coldheads are bolted to welded titanium

pro

3

566

an acceptable signal reduction factor of 1/e.

590

An overview of the system is shown in Fig. 9. Xe

591

gas is pumped through a hot zirconium getter at a de-

592

sign flow rate of 500 standard liters per minute (SLPM),

593

taking 2.4 days to purify the full 10 tonne Xe inventory

594

in a single pass. The getter, model PS5-MGT50-R from

595

SAES [17], operates at 400 ◦ C. For thermal efficiency, the

596

getter features a heat exchanger to couple the inlet and

597

outlet Xe gas streams, substantially reducing the 3 kW

598

heat burden at 500 SLPM. A pre-heater ensures that the

599

gas strikes the getter bed at the operating temperature.

600

Besides electronegative removal, the getter bed also serves

601

as a permanent repository for the tritium and

602

labeled methane species that calibrate the beta decay re-

13

14

C radio-

Journal Pre-proof

Storage and Recovery Getter

Radon Removal

Compressors Cable Breakout

Cable Breakout

HX/ Gas Gap Cryovalves

LXe-TPC

pro

Weir Drain Pipe

Reservoir

Sub Cooler

Cable Stand Pipe

Vacuum

Weir

LXe Skin

of

Xe Vapor 2 Phase HX

Liquid Xenon Tower

Cathode High Voltage

TPCPTFE Walls

LXe Skin

re-

Flow Restrictor

Water Tank

Figure 9: Overview of the online Xe purification system. LXe in the Xenon Detector (right) spills over a weir drain and flows horizontally to the Liquid Xenon tower, which stands outside the water tank. It is vaporized in a two phase heat exchanger, pumped through a hot zirconium

urn al P

getter, and returned to the detector after condensing. Cryovalves control the flow of LXe between the LXe tower and the Xenon Detector. A radon removal system treats Xe gas in the cable conduits and breakout feed-throughs before sending it to the compressor inlet.

603

sponse of the TPC (see Sec. 5).

619

the online purification circuit and to efficiently exchange

Circulation flow is established by two all-metal diaphragm 620 heat between them. There are four vessels in the tower:

605

gas compressors, model A2-5/15 from Fluitron [18]. The621 the reservoir vessel, the two-phase heat exchanger (HEX),

606

two compressors operate in parallel, each capable of 300622 the subcooler vessel, and the subcooler HEX.

607

SLPM at 16 PSIA inlet pressure. The system operates623

608

with one compressor at reduced flow rate during peri-624 Detector via the weir system. It features a standpipe con-

609

odic maintenance. The total achieved gas flow is trimmed625 struction to decouple its liquid level from that in the weir

610

by a bellows-sealed bypass proportional valve, model 808626 drain line. LXe flows from the bottom of the reservoir

611

from RCV. Both circulation compressors have two stages,627 into the two-phase HEX, where it vaporizes after exchang-

612

each featuring copper seals plated onto stainless steel di-628 ing heat with purified Xe gas returning from the getter.

613

aphragms.

614

limit radon ingress from air.

Jo

604

The reservoir vessel collects LXe departing the Xenon

All-metal sealing technology was chosen to629 The two-phase HEX is an ASME-rated brazed plate de630

vice made by Standard Xchange consisting of corrugated

615

The LXe tower is a cryogenic device standing on the631 stainless steel. On its other side, condensing LXe flows into

616

floor of the Davis Cavern outside the water tank and at a632 the subcooler vessel and subcooler HEX. The vessel sepa-

617

height somewhat below the Xenon Detector. Its primary633 rates any remaining Xe gas from the LXe, while the HEX

618

purpose is to interface the liquid and gaseous portions of634 cools the LXe to below its saturation temperature. The

14

Journal Pre-proof

HEX consists of three isolated elements: the LXe volume,673 or 12.7 days, allowing 90% of these atoms to decay. The

636

an LN thermosyphon coldhead cooled to 77 K, and a thin674 sequestration is accomplished by a gas chromatographic

637

thermal coupling gap. The power delivered to the LXe can675 process that employs 10 kilograms of synthetic charcoal

638

ucher GmbH) cooled to be varied from 90 W and 480 W by adjusting the compo-676 (Saratech Spherical Adsorbent, Bl¨

639

sition of the He/N2 gas mixture in the gap. An additional677 190 K [19]. The technique was previously demonstrated in

640

thermosyphon coldhead integrated with the reservoir re-678 Ref.[20]. The charcoal was etched in nitric acid and rinsed

641

moves excess heat during cooldown and operations. Both679 with distilled water to reduce its radon emanation rate.

642

the reservoir and the subcooler vessels are equipped with680

643

LXe purity monitors (LPMs) to monitor electronegatives681 of the Xe is also provided by two coldtrap mass spectrom-

644

entering and exiting the Xenon Detector. Each LPM is682 etry systems [21, 22]. These devices monitor for the pres-

645

a small, single-phase TPC which drifts free electrons over683 ence of stable noble gas species such as 84 Kr and 40 Ar and

of

635

pro

Besides the LPMs, surveillance of the impurity content

646

a distance, and measures the attenuation of the electrons684 also for electronegatives such as O2 . Ten standard liter

647

during that transit.

685

samples of Xe gas are collected and passed through a cold-

LXe flows between the LXe tower and the Xenon De-686 trap cooled to 77 K, a temperature at which Xe is retained

649

tector through three vacuum insulated transfer lines that687 while many impurities species pass through. The outlet of

650

run across the bottom of the Davis Cavern water tank.688 the coldtrap is monitored by a Residual Gas Analyzer (an

651

Two lines connect to the bottom of the ICV and supply689 RGA200 from SRS). The sensitivity for detecting

652

sub-cooled LXe to the TPC and skin regions of the Xenon690 Xe is better than 10 parts-per-quadrillion (g/g). The cold-

653

Detector. The third line returns LXe from the ICV weir691 trap is cooled either with a pulse tube refrigerator (model

654

drain system to the reservoir. The lines are constructed692 PT60 from Cryomech Inc.) or with an open flask dewar

655

by Technifab with an integrated vacuum insulation jacket.693 of liquid nitrogen. One of these systems is permanently

656

They are further insulated from the water by an additional694 plumbed to fixed locations in the Xe handling system; the

657

vacuum shell. Cryogenic control valves from WEKA reg-695 other acts as a mobile utility system to be deployed as

658

ulate the LXe flow in each of the three lines.

urn al P

659

re-

648

696

84

Kr in

needed. Both are highly automated and allow for multiple

Conduits connect to the ICV at its lower flange and697 measurements per day. To recover the ten tonne Xe inventory to long term

upper dome to service PMT and instrumentation cables to698

661

the TPC. The lower conduit, which is vacuum insulated699 storage, two high pressure gas compressors (Fluitron model

662

and filled with LXe, travels across the water tank floor,700 D1-20/120) pump Xe gas into 12 Xe storage packs. The

663

penetrates its side wall, and mates with a vertical LXe701 recovery compressors use the same all-metal diaphragm

664

standpipe. Its cables emerge into gaseous Xe and then702 technology as the circulation compressors. Each Xe stor-

665

connect to breakout feed-throughs at the standpipe top.703 age pack consist of 12 DOT-3AA-2400 49.1 liter cylinders

666

Two upper conduits filled with gaseous Xe connect the704 sealed with Ceoduex D304 UHP tied diaphragm valves

667

ICV top head to breakout feed-throughs and service cables705 and ganged together in a steel frame. Each pack weighs

668

to the upper part of the Xenon Detector.

Jo

660

706

1,800 kg when full. During Xe recovery heat is added to

669

The Xe gas in the cable conduits and breakout feed-707 the LXe by electrical heaters and by softening the insulat-

670

throughs are treated for radon removal by a cold synthetic708 ing vacuum of the lower cable conduit. Two backup diesel

671

charcoal column drawing 0.5 SLPM of Xe gas flow. The709 generators are provided in case of a power outage. The

672

system is designed to sequester

222

Rn for three half-lives,710 emergency recovery logic is described in Sec. 6.3. 15

Journal Pre-proof

711

All elements of the online Xe handling system were748 in a single pass through the system; two passes are envi-

712

cleaned for ultra high vacuum with solvents and rinsed in749 sioned to achieve the required concentration. The process-

713

de-ionized water. Orbital welds conform to ASME B31.3.750 ing is monitored by a coldtrap Xe mass spectrometry sys-

714

Where possible, stainless steel components have been etched751 tem for quality assurance. After processing, the Xe storage

715

in citric acid to reduce radon emanation.

716

3.3. Removal of Kr from Xe 85

753

condensing into the Xenon Detector.

of

Beta decay of

packs are shipped from SLAC to SURF in preparation for

Kr in the LXe is a challenging ER754 4. Outer Detector

718

background source. The acceptable Kr concentration, de-

719

rived by assuming an isotopic abundance of

720

2 × 10−11 , is Kr/Xe < 0.3 parts-per-trillion (g/g). This

721

concentration is achieved prior to the start of LZ opera-

722

tions by separating trace Kr from the Xe inventory with a

723

gas charcoal chromatography process.

85

Kr/Kr ∼

A total of 800 kg of Calgon PCB activated charcoal is

725

employed, divided evenly into two columns. The charcoal

726

was washed with water to remove dust and baked under

727

an N2 purge for 10 days, ultimately achieving a charcoal

The principal purpose of the Outer Detector is to tag

756

neutrons scattering events in the TPC. Most neutrons orig-

757

inate from radioactive impurities in material immediately

758

adjacent to the TPC, such as those from (α,n) processes

759

on PTFE. The OD is a near-hermetic liquid scintillator

760

detector designed to capture and tag neutrons within a

761

time window that allows the signals to be correlated with

762

the NR in the TPC.

re-

724

755

pro

717

752

temperature of 150 ◦ C. During processing the Xe inven-

729

tory is mixed with He carrier gas circulated by a compres-

The detection medium for the OD is gadolinium-doped

764

liquid scintillator contained within segmented acrylic ves-

765

sels that surround the OCV. Neutrons are detected pre-

urn al P

728

763

sor (model 4VX2BG-131 from RIX). The Xe/He mixture

731

is passed through one of the two charcoal columns at a

732

pressure of 1.7 bara. Trace Kr reaches the column outlet

733

first and is directed to an LN-cooled charcoal trap where

734

it is retained. A Leybold-Oerlikon Roots blower and screw

735

pump located at the charcoal column outlet then activates,

736

dropping the column pressure to 10 mbar. This purges the

737

purified Xe from the column. The Xe is separated from the

738

He carrier gas by freezing it at 77 K in an LN-cooled heat

739

exchange vessel. This freezer is periodically warmed to va-

740

porize the Xe ice, and the recovered Xe gas is transferred

741

at room temperature by a Fluitron Xe recovery compressor

742

to one of the twelve Xe storage packs described above.

Jo

730

743

The entire 10 tonne Xe inventory is processed in 16 kg

744

batches. The chromatographic and Xe purge cycles each

745

take about 2 hours. Two charcoal columns are employed to

746

allow processing and column purging to proceed in paral-

747

lel. A Kr rejection factor greater than 1000 can be achieved

766

dominantly through capture on 155 Gd and 157 Gd; a total of

767

7.9 MeV (155 Gd) or 8.5 MeV (157 Gd) is released in a post-

768

capture cascade of, on average, 4.7 gammas. About 10% of

769

neutrons capture on hydrogen, emitting a single 2.2 MeV

770

gamma. Gammas induce scintillation within the LS which

771

is subsequently collected by the 120 8–inch PMTs that

772

view the OD from a support system inside the water tank.

773

To maximize light collection efficiency, there is both a

774

Tyvek curtain behind, above and below the PMTs, and

775

a layer of Tyvek surrounding the cryostat.

776

The OD has been designed to operate with a neutron

777

detection efficiency of greater than 95%. To optimize this

778

efficiency, the concentration of Gd was chosen such that

779

capture on H is sub-dominant. Furthermore, the time be-

780

tween a signal in the TPC and a neutron capture in the OD

781

impacts the efficiency (see Fig. 10). The level of Gd chosen

782

for LZ reduces the average capture time of thermal neu-

783

trons in liquid scintillator from 200 µs to 30 µs. However,

784

there is a significant population of neutrons that survive

16

several times longer than 30 µs. Simulations demonstrate

786

that neutrons can spend significant time scattering and

787

thermalizing within the acrylic walls of the OD vessels. To

788

minimize this effect, the acrylic walls are designed to be as

789

thin as is structurally possible. Using less acrylic also re-

790

duces the number of H-captures. The use of a 500 µs time

791

window allows for an efficiency of 96.5% for a 200 keV

792

threshold, while achieving a deadtime of less than 5%.

0 keV 100 keV 200 keV

10 8

pro

Inefficiency (%)

785

of

Journal Pre-proof

6 4

0

200

400

600

re-

2 800 1000 OD Veto Window ( s)

Figure 10: Monte Carlo derived OD inefficiency as a function of veto

urn al P

window (time between S1 in the TPC and signal in the OD). The energy thresholds referenced in the legend are for electron recoils.

Figure 11: The outer detector system in an exploded view. The four large side vessels are shown in green and the 5 smaller top and

793

794

bottom vessels are shown in blue. Also shown are water displacers

4.1. Outer Detector systems

in red, and the stainless steel base in grey.

A total of ten ultra-violet transmitting (UVT) Acrylic

795

Vessels have been fabricated by Reynolds Polymer Tech-810 ICP-MS (see Sec. 8.1)and found to be sufficiently low in

796

nology in Grand Junction, Colorado. These consist of four811 radioactive contaminants.

797

side vessels, three bottom vessels, two top vessels and a812

798

‘plug‘ which can be removed for photoneutron source de-813 ear alkyl benzene (LAB) solvent doped at 0.1% by mass

799

ployment, see Fig. 11. The AVs were designed as seg-814 with gadolinium. The full chemical make-up of the GdLS

800

mented to allow transport underground and into the water815 cocktail is shown in Table 2. Gd is introduced into LAB us-

801

tank with no acrylic bonding necessary on site. All acrylic816 ing trans-3-Methyl-2-hexenoic acid (TMHA) as a chelation

802

walls for the side AVs are nominally 1–inch thick. For the817 agent. Other components are the fluor, 2,5-Diphenyloxazole

803

top and bottom AVs, the side and domed acrylic walls are818 (PPO), and a wavelength shifter, 1,4-Bis(2-methylstyryl)-

804

0.5–inch thick, whereas the flat tops and bottoms are 1–819 benzene (bis-MSB). The emission spectrum of this mix

805

inch thick for structural reasons. The AVs contain various820 spans 350 to 540 nm, with peaks at 400 and 420 nm, and

806

penetrations for conduits and cabling. All sheets of acrylic821 the absorption length in this range is of order 10 m. LAB,

807

used for fabrication were tested for optical transmission822 TMHA and PPO are purified in order to remove metallic

808

and were found to exceed 92% between 400 and 700 nm,823 and coloured impurities to improve optical transmission;

809

meeting requirements. Acrylic samples were screened with824 LAB and TMHA by thin-film distillation and PPO by

Jo

The liquid scintillator used in the OD consists of a lin-

17

Journal Pre-proof

825

water-extraction and recrystallization. For the chelated862 to the OD, i.e. inside the GdLS; while external sources

826

Gd product, a self-scavenging method is used to induce863 can be subdivided again into radioactivity from LZ com-

827

precipitation of uranium and thorium isotopes to improve864 ponents and radioactivity from the Davis cavern itself (see

828

radiopurity. Twenty-two tonnes of GdLS contained in 150865 Table 3).

829

55-gallon drums are shipped to SURF and transferred into866

830

the AVs through a reservoir. Exposure to air is minimized,867 been measured through a campaign with an LS Screener, a

831

as oxygen, radon and krypton negatively impact the GdLS868 small detector containing 23 kg of liquid scintillator viewed

832

performance. The GdLS is bubbled with nitrogen while869 by three low background LZ PMTs, fully described in

833

in the reservoir in order to remove dissolved oxygen and870 Ref. [23]. The LS Screener took data with both loaded

834

maximize the light yield. Furthermore, a light yield test is871 (with Gd) and unloaded (no Gd) samples of the liquid

835

performed on each drum of GdLS before transfer into the872 scintillator that is used in the Outer Detector. The un-

836

AVs. The test apparatus consists of a dark box containing873 loaded sample allowed a clear determination of what con-

837

a radioactive source, one PMT, and a small sample of the874 taminants were introduced during the Gd-doping process,

838

GdLS.

of

pro

875

as well as a clearer low energy region to allow a measure-

A total of 120 Hamamatsu R5912 8–inch PMTs view876 ment of the

14

re-

839

Radioactive contaminants internal to the GdLS have

C concentration, particularly important as

the GdLS and AVs from a Tyvek curtain situated 115 cm877 its rate influences the choice of energy threshold. Use of

841

radially from the outer wall of the acrylic (see Fig. 1). The878 pulse shape discrimination allowed for efficient separation

842

interaction rate in the OD from radioistopes in the PMT879 of alpha decays from betas and gammas, and constraints

843

system is predicted to be only 2.5 Hz due to the shielding880 were placed on activity from the

844

provided by the water gap. The PMTs are arranged in 20881 decay-chains,

845

ladders spaced equally around the water tank in φ with 6882 of carbon in the LS),

846

PMTs per ladder. Each PMT is held by a ‘spider’ support883 and significant findings of the LS Screener were the domi-

847

and covered by a Tyvek cone.

848

urn al P

840

884

40

K,

14

238

U,

235

U and

232

Th

C, 7 Be (from cosmogenic activation 85

Kr and

176

Lu. Some surprising

nance of the rate from nuclides within the 235 U chain, and

A dedicated Optical Calibration System (OCS) has885 the presence of

176

Lu, now known to be introduced when

been designed for PMT monitoring and measurement of886 doping with gadolinium, since neither were observed in

850

the optical properties of the GdLS and acrylic. Thirty887 the unloaded LS sample. A more aggressive purification

851

LED duplex optical fibres are mounted with the PMT888 of the GdLS resulted in a decrease in activity of almost

852

supports, with an additional five beneath the bottom AVs889 all contaminants. The new, lower activities were used in

853

placed specifically to check transmission through the acrylic. 890 combination with the LS Screener results to predict a rate

854

Short, daily calibrations with the OCS will be performed891 of 5.9 Hz above the nominal 200 keV veto threshold for

855

in order to check the PE/keV yield at the veto threshold,892 the OD.

856

and weekly calibrations will be used to check PMT gains893

857

and optical properties.

858

4.2. Performance

Jo

849

The biggest contribution to the rate in the OD is from

894

the radioactivity within the Davis cavern. Contamination

895

of the cavern walls with on the order of tens of Bq/kg for

896

40

K,

238

U and

232

Th has been established using dedicated

859

The performance of the OD strongly depends on its897 measurements of the γ-ray flux with a NaI detector [24],

860

event rate. The sources can be divided into internal and898 and simulation studies suggest a rate above 200 keV of

861

external; internal backgrounds are contamination intrinsic899 27 ± 7 Hz, concentrated in the top and bottom AVs. 18

Journal Pre-proof

Table 2: Chemical components in 1 L of GdLS.

Molecular Formula

Molecular Weight (g/mol)

Mass (g)

LAB

C17.14 H28.28

234.4

853.55

PPO

C15 H11 NO

221.3

3.00

bis-MSB

C24 H22

310.4

0.015

TMHA

C9 H17 O2

157.2

2.58

Gd

Gd

157.3

0.86

GdLS

C17.072 H28.128 O0.0126 N0.0037 Gd0.0015

233.9

860.0

Rates are given in the case of the nominal 200 keV threshold.

External

Internal

Component

Many attributes of the LZ detector response require

920

OD Rate (Hz) 921

in situ calibration. Calibration goals range from low-level

0.5

922

quantities such as PMT gain and relative timing to high-

Cryostat

2.5

923

level quantities such as models of electron recoil and nu-

OD

8.0

Davis Cavern

31

GdLS

5.9

PMTs & Bases

0.9

TPC

Total

51

924

clear recoil response. To these ends, the LZ detector in-

925

cludes significant accommodations for a suite of calibra-

926

tions. Large-scale accommodations (some visible in Fig. 1)

927

include three small-diameter conduits to transport exter-

With an expected overall rate of ∼50 Hz, the OD can928 nal sources to the cryostat side vaccum region, one large-

urn al P

900

5. Calibrations

re-

Type

919

pro

Table 3: Predictions for the event rate in the Outer Detector.

of

Acronym

901

be expected to operate with an efficiency of 96.5% for a929 diameter conduit to transport large (γ,n) sources to the

902

200 keV threshold. The energy threshold of the OD is930 cryostat top, and two evacuated conduits to enable neu-

903

nominally a number of photoelectrons corresponding to931 tron propagation from an external neutron generator to

904

an energy deposit of 200 keV, predicted to be 10 PE by932 the cryostat side.

905

photon transport simulations. The threshold is chosen to

906

eliminate the rate from internal

907

is a low energy β-decay with an endpoint of 156 keV. The

908

OD may be operated instead with a 100 keV threshold,

909

depending on the observed rate, which would decrease the

910

inefficiency at a window of 500 µs from 3.5% to 2.8%.

14

933

C contamination, as it

The impact of the OD on NR backgrounds is charac-

912

terized through neutron Monte Carlo simulations. The

913

total NR background in 1000 livedays is predicted to be

914

reduced from 12.31 to 1.24 NR counts when the OD and

915

skin vetoes are applied, with the OD providing most of

916

the vetoing power. Due to the spatial distribution of these

917

NRs in the LXe TPC, the OD is necessary to utilize the

918

full 5.6 tonne fiducial volume.

Jo

911

5.1. Internal sources

934

Gaseous sources can mix with the LXe in order to reach

935

the central active volume, where self-shielding limits cali-

936

bration via external gamma sources. The baseline suite of

937

such ‘internal’ sources is listed in Group A of Table 4.

938

Long-lived gaseous sources (3 H,

14

C) can be stored as

939

a pressurized gas, with purified Xe serving as the carrier.

940

Because the nuclide is long-lived, it must be in a chemi-

941

cal form that can be efficiently removed by the getter (see

942

Sec. 3.2). The LZ implementation builds on the successful

943

example of LUX, in which isotopically-labeled CH4 served

944

as the calibration gas. CH4 was seen to be efficiently re-

945

moved, as long as it did not contain trace amounts of other

946

labeled hydrocarbons [25, 26].

19

Journal Pre-proof

947

The short-lived gaseous sources (83m Kr, 131m Xe, 220 Rn)

948

are stored in the form of their parent nuclide, which can

949

be handled and stored in a compact solid form and placed

950

within ‘generator’ plumbing in which it emanates the cal-

951

ibration daughter.

954

Kr, and is deposited in aqueous solution on high pu-

rity, high surface area charcoal before baking (as in [27]). 131

I serves as the parent nuclide of

131m

Xe, and is com-

955

mercially available in a pill form of high Xe emanation ef-

956

ficiency.

957

is available commercially from Eckert & Ziegler as a thin

958

electroplated film for optimal Rn emanation. In the LZ

959

implementation, these generator materials are housed in

960

transportable and interchangeable plumbing sections (see

961

Fig. 12). These assemblies contain both a port for material

962

access and a pair of sintered nickel filter elements (3 nm

963

pore size, Entegris WG3NSMJJ2) to prevent contamina-

964

tion by the parent nuclide of the active Xe.

Th serves as the parent nuclide of

220

Rn, and

re-

228

of

953

Rb serves as the parent nuclide of

pro

952

83m

83

Figure 12: TOP: Three example solid materials containing parent

965

Both long-lived and short-lived gaseous sources require

966

precise dose control on the injected activity, accomplished

dosed with

967

via a gas handling system dedicated to injection control.

permeable pill (2) containing

urn al P

968

nuclides that emit daughter calibration gaseous sources. Charcoal

plated

A specific GXe cylinder supplies the carrier gas to trans-

228

83

Rb is fixed to the a 1/2-inch VCR plug (1). A gas131

I and a disk source (3) of electro-

Th can also be fixed in place. BOTTOM: Photograph of

a typical gaseous source generator. Carrier Xe gas flows from left

969

port small quantities of calibration gas through a series of

970

high-precision Mass Flow Controllers (Teledyne-Hastings

(4), accessed via a 1/2-inch VCR port. This region is bounded by a

971

HFC-D-302B) and volumes of precise pressure measure-

pair of filter elements (5) of 3 nm pore size sintered nickel and then

972

ment (MKS 872). Once a dose of calibration gas has been

a pair of lockable manual valves for isolation during shipping and

973

isolated, the volume containing the dose is flushed into the

975

976

977

installation.

main GXe circulation flow path, either before the getter984 laser ranger (visible at the top of Fig. 13), supplying live for noble-element calibration species or after the getter for985 data for an active feedback protocol to a SH2141-5511 long-lived CH4 -based species. 986 (SANYO DENKO Co) stepper motor. A ∼100 µm nylon

Jo

974

to right. The active parent material is stored in the central region

987

composite filament suspends the sources, rated to a maxi-

988

mum load of 12 kg. The external sources themselves are in

989

most cases commercial sources (Eckert & Ziegler type R).

990

A special case is the AmLi source, custom fabricated but

991

of the same form factor. To enable smooth transport up

992

and down the conduit, each source is epoxied and encap-

993

sulated at the lower end of a 5” long by 0.625” diameter

994

acrylic cylinder. The top end contains the capsule holder

5.2. External sources

978

External sources are lowered through three 23.5 mm

979

ID 6 m long conduits to the vacuum region between the

980

ICV and OCV. Each conduit is capped by a deployment

981

system (Fig. 13) which raises and lowers the sources with

982

final position accuracy of ±5 mm. The position measure-

983

ment is accomplished via an ILR1181-30 Micro-Epsilon

20

Journal Pre-proof A ∼140 kg tungsten shield block is designed to be de-

995

allowing connection to the filament and includes a ferro-1013

996

magnetic connection rod for recovery in case of filament1014 ployed at the top of LZ via a crane. In the unlikely event

997

breakage.

the shield block were to become lodged inside the LZ water

1016

tank, it would be possible to separately remove the conical

1017

structure which contains the gamma source and the Be.

1018

5.4. Deuterium-deuterium neutron sources

of

1015

An Adelphi DD-108 deuterium-deuterium (DD) neu-

1020

tron generator produces up to 108 neutrons per second. A

1021

custom upgrade will allow up to 109 n/s. The neutrons

1022

are delivered through the Davis Cavern water tank and

1023

Outer Detector via dedicated neutron conduits. There are

1024

two sets of conduits, one level and one inclined at 20 de-

1025

grees from the horizontal. Each conduit assembly includes

1026

a 2–inch diameter and a 6–inch diameter path, and all are

re-

pro

1019

1027

Figure 13: LEFT: One of three external source deployment systems,

1028

Fig. 14, the generator is permanently mounted on an Ekko

stepper motor and gear/winding assembly (enclosed in a KF50 Tee1029

Lift (model EA15A), and surrounded by custom neutron

at the back). A transparent plate makes visible the region in which1030

shielding material. A kinematic mounting plate, located

including the laser ranging system (top black component) and the

1031

assembly, showing the acrylic body, the source region (at bottom),

1032

in the concrete floor. This is designed to provide precise,

1033

repeatable positioning.

and the filament connection, black skids, and laser reflector (at top).

5.3. Photoneutron sources

999

1001

1002

1003

1004

1005

1006

1007

1008

1009

1010

1011

1012

1034

The DD-108 produces 2450 keV mono-energetic neu-

1035

trons. This source has already been used by LUX to ob-

A selection of photoneutron (γ,n) sources, including1036 tain a precise, in-situ calibration of the low-energy nuclear YBe, 124 SbBe, 204 BiBe and 205 BiBe, are planned to cal-1037 recoil response [28]. In addition to this mode of operation,

ibrate the nuclear recoil energy range from below 1 keV1038 LZ obtains 272 keV quasi mono-energetic neutrons by reup to about 4.6 keV. This range corresponds to the ex-1039 flecting the 2450 keV beam from a deuterium oxide (D O) 2 pected energy depositions from 8 B solar neutrino coherent1040 target. This allows the lowest nuclear recoil energies to be scattering. Only about one neutron is produced for every1041 calibrated with decreased uncertainty. 104 gammas emitted, so a significant quantity of gamma shielding is required (see Fig. 14). The neutrons are quasi1042 6. Electronics and Controls mono-energetic at production (within a few percent) but 1043 The signal processing electronics, described in detail in undergo additional scatterings before they reach the liq1044 Sec. 6.1, processes the signals from 494 TPC PMTs, 131 uid xenon. The utility of this calibration source is derived 1045 skin PMTs, and 120 outer-detector PMTs. The electronics from the endpoint energy the neutron deposits, which sim1046 is designed to ensure a detection efficiency for single phoulations indicate will be clearly distinguishable after a few 1047 toelectrons (PEs) of at least 90%. The PMT signals are days of calibration. 1048 digitized with a sampling frequency of 100 MHz and 14-bit

Jo

1000

88

between the forks of the lift, will bolt to threaded inserts

urn al P

the sources are installed and removed. RIGHT: An external source

998

filled with water during dark matter search. As shown in

21

Journal Pre-proof

1073

The Data Acquisition system (DAQ) is designed to al-

1075

low LED calibrations of the TPC PMTs in about 10 min-

1076

utes. This requires an event rate of 4-kHz, resulting in a

1077

∼340 Mb/s total waveform data rate. Monte Carlo sim-

1078

ulations predict a total background rate is about 40 Hz.

1079

The background rate between zero and 40 keV is about

1080

0.4 Hz. Due to the maximum drift time of 800 µs in the

1081

TPC, the rate for TPC source calibrations is limited to 150 Hz. A 150 Hz calibration rate results in a 10% prob-

is at top. Depicted below is a conical insert that houses radioactive1083

ability of detecting a second calibration event within the

source. RIGHT: Rendering of the DD generator in its boron-doped1084

drift time of the previous calibration event.

shielding assembly, all mounted on a movable positioning system.

1086

and monitoring all LZ systems. It is described in detail in

1087

Sec. 6.3.

1050

fiers are adjusted to optimize the dynamic range for the

1057

1058

1059

1060

1061

1062

1063

1064

1065

1066

1067

1068

1069

1070

1071

1072

urn al P

1056

PMTs. The dynamic range for the TPC PMTs is defined1088 6.1. Signal Flow by the requirement that the S2 signals associated with a 1089 The processing of the signals generated by the TPC full-energy deposition of the 164 keV 131m Xe activation 1090 PMTs is schematically shown in Fig. 15. The TPC and line do not saturate the digitizers. Larger energy deposi1091 skin PMTs operate at a negative HV, supplied by the HV tions will saturate a number of channels of the top PMT system, using MPOD EDS 20130n 504 and MPOD EDS 1092 array, but the size of the S2 signal can be reconstructed 1093 20130p 504 HV distribution modules from from WIENER using the S2 signals detected with the bottom PMT array. 1094 Power Electronics [29]. HV filters are installed at the HV The saturation of a few top PMTs for S2 signal will not 1095 flange on the breakout box. The PMT signals leave the impact the accuracy of the position reconstruction. For 1096 breakout box via a different flange and are processed by the skin PMTs, the dynamic range is defined by the re1097 the analog front-end electronics. The amplified and shaped quirement that the skin PMT signals associated with the 1098 signals are connected to the DAQ. The digitized data are interaction of a 511-keV γ-ray in the skin do not saturate 1099 sent to Data Collectors and stored on local disks. The the digitizers. Simulations show that such an interaction 1100 PMTs of the outer-detector system operate at positive HV. can generate up to 200 PEs in a single PMT, depending 1101 The same type of amplifier used for the TPC and skin on the location of the interaction. The dynamic range 1102 PMTs is also used for the outer-detector PMTs. for the outer-detector PMTs is defined by the requirement 1103 The PMT signals are processed with dual-gain amthat the size of the outer-detector PMT signals associated 1104 plifiers. The low-gain channel has a pulse-area gain of with neutron capture on Gd, which generates a γ-ray cas1105 four and a 30-ns full width at tenth maximum (FWTM) cade with a total energy between 7.9 and 8.5 MeV, do not 1106 shaping-time constant. The high-gain channel has a pulsesaturate the analog and digital electronics. Such events 1107 area gain of 40 and a shaping time of 60-ns (FWTM). The generate at most 100 PEs in a single PMT. For muon in1108 high-gain channel is optimized for an excellent single PE teractions in the outer detector, a few PMTs may saturate, 1109 response. The shaping times and gains are derived from

Jo

1055

re-

accuracy. The gain and shaping parameters of the ampli-

1054

The slow control system is responsible for controlling

1085

1049

1053

pro

1082

ing assembly, which will be made of tungsten alloy. The main body

1052

of

1074

Figure 14: LEFT: Brass mockup of the photoneutron source shield-

1051

depending on the location of the muon track.

22

Journal Pre-proof

DS Master

Level 1 DS

DAQ Master

Data Extractors

DDC-32s

Low/High Gain Amp

External Triggers

1133

trol/Monitoring system, not shown in Fig. 15, for slow

1134

operations such as running setup/control and operator di-

1135

agnostics. The entire system runs synchronously with one

1136

global clock. The performance of the entire signal processing chain

1138

has been evaluated in an electronics chain test. Pre-production

1139

prototypes of the analog and digital as well as signal ca-

1140

bles of the same type and length as those to be installed

1141

at SURF were used. The measured response of a four-

Figure 15: Schematic of the signal processing of the TPC PMTs.

1142

sample wide S1 filter is shown in Fig. 16. The measured

The TPC and outer-detector PMTs use dual-gain signal processing.1143

single photoelectron (PE) efficiency is 99.8%, much better

The skin PMTs only utilize the high-gain section of the amplifiers.1144

than the requirement of 90%. The threshold at which the

The signals are digitized using the DDC-32 digitizer, developed for 1145

false-trigger rate is 1 Hz is 43 ADC Counts (ADCC) or

1146

16% of a single PE.

re-

Counts (#)

LZ in collaboration with SkuTek [30]. Event selection is made by the Data Sparsification (DS) system [31].

pro

Xenon Space

Event Building

HV Supply

HV Filter

Data Collectors

of

1137

PMTs

300

one assumption: the DAQ has a usable dynamic range of

1111

1.8 V at the input. A 0.2 V offset is applied to the digitizer

1112

channels in order to measure signal undershoots of up to

200

1113

0.2 V. Measurements with prototype electronics and the

150

1114

LZ PMTs have shown that the same amplifier parameters

100

1115

can be used for all of them. For the TPC and the Outer

50

1116

Detector PMTs, both high-gain and low-gain channels are

1117

digitized; for the skin PMTs, only the high-gain channel

1118

is digitized.

urn al P

1110

The top-level architecture of the DAQ system is shown

1120

schematically in Fig. 15. The DDC-32s digitizers, evel-

1121

oped for LZ in collaboration with SkuTek [30], continu-

1124

1125

1126

1127

1128

1129

1130

1131

1132

200 300 400 500 Four sample wide S1 Filter peak output (ADCC)

The output of the four-sample wide S1 filter, in units of ADC Counts (ADCC), is shown.

ously digitize the incoming PMT signals and store them1147 6.2. Data Flow and Online Data Quality in circular buffers. When an interesting event is detected, 1148 The data flow is schematically shown in Fig. 17. Five the Data Extractors (DE) collect the information of inter1149 event builders (EB) assemble the events by extracting the est from the DDC-32s. The DEs compress and stack the 1150 relevant information from the Data Collector disks, DAQ1extracted data using their FPGAs and send the data to 1151 DAQ15. A 10 Gigabit per second (Gbs) line connects the Data Collectors for temporary storage. The Event Builder 1152 Data Collectors to the Event Builders. The event files are takes the data organized by channels and assembles the 1153 stored on the 16 TB local disk array of each EB, before buffers into full event structures for online and offline anal1154 being transferred to the 192 TB RAID array installed on ysis. The DAQ operation is controlled by the DAQ Master 1155 the surface. From there, the event files are distributed to for high-speed operations such as system synchronization 1156 the data-processing centers for offline data processing and and waveform selection, and by the DAQ Expert Con1157 analysis.

Jo

1123

100

Figure 16: Measurements of the TPC PMT response to a single PE.

1119

1122

250

23

Journal Pre-proof

DAQ 1

CPU DAQM + DQM

Tape Archive

1186

egories: 1) protection against xenon loss and contamina-

1187

tion, 2) experiment parameter monitoring and logging, 3)

1188

control over LZ subsystems, and 4) providing the interface

x% of Data

DAQ 15

CPU Event Building

Event Files

RAID Surface

RAID US DS

Data (Re)processing 1189

to operators.

Simulations

In order to minimize risks associated with possible xenon

1190

SURF Underground

SURF Surface

RAID UK DS

1191

loss or contamination, instruments and subsystems are di-

1192

vided in two major groups with respect to the perceived

1193

impact of their possible malfunction on the integrity of the

1194

xenon supply. Those where equipment failure or operator

Simulations

Figure 17: A schematic of the LZ data flow.

of

Data (Re)processing

Redundant connections exist between the surface and1195 error can lead to xenon loss or contamination are desig-

1159

the Davis Cavern. A pair of managed 10 Gbs switches is1196 nated “critical” and the rest are “non-critical”. During a

1160

installed on the surface and underground. Each switch on1197 power failure, a combination of UPS and generator power

1161

the surface is connected via two fibers, travelling through1198 will provide continuity to the critical components of the

1162

the Yates and the Ross shafts, to the two underground1199 slow control system.

1163

switches. Since each link supports 10 Gbs, this configura-1200

1164

tion can support data transfer rates of up to 40 Gbs.

pro

1158

re-

The PLC system provides automatic protection in the

1201

case of an emergency. If the Xe pressure inside the ICV

A fraction of the data is analyzed online by the Detec-1202 reaches a threshold value, or if there is an extended power

1166

tor Quality Monitor (DQM), running on a dedicated on-1203 outage in the Davis Cavern, the Xe compressors will acti-

1167

line server, installed in the Davis Laboratory. The DQM1204 vate and the vaporized xenon will be safely transferred to

1168

applies elements of the offline analysis to the data being1205 the Xe storage packs. In these scenarios Xe transfer occurs

1169

collected, in order to monitor the performance of the de-1206 automatically without assistance from a human operator.

1170

tector. For example, during

1171

will monitor the electron life time and the energy reso-1208 locks to protect the experiment from erroneous operator

1172

lution. Various detector parameters, such as PMT mul-1209 commands or equipment failure during routine operations.

1173

tiplicity, hit distributions across both PMT arrays, and1210

1174

trigger rates, will be monitored. If significant deviations1211 its interaction with the experiment subsystems and infras-

1175

from prior observed patterns are seen, experts will be au-1212 tructure is shown in Fig. 18. The core of the slow control

1176

tomatically notified via the slow control system.

1177

6.3. Controls

1179

1180

1181

1182

1183

1184

1185

83m

Kr calibrations, the DQM1207 Additionally, the PLC is programmed with a set of inter-

The functional diagram of the slow control system and

1213

system is composed of three components: (1) the inte-

1214

grated supervisory control and data acquisition (SCADA)

1215 software platform Ignition from Inductive Automation [32], The Controls system performs supervisory control and1216 (2) a Siemens SIMATIC S7-410H dual-CPU PLC with Remonitoring of all the major subsystems of the experiment,1217 dundant Hot Backup, allowing bumpless transfer from one

Jo

1178

urn al P

1165

including cryogenics, fluid handling, detector diagnostic1218 active CPU to the backup, and (3) associated I/O modsensors, high voltage, and electronics monitoring. Not1219 ules [33]. Non-critical instruments connect directly to the included in this system are the SURF-managed controls1220 Ignition server, typically using MODBUS-TCP over the ensuring personnel safety (for example, oxygen deficiency1221 slow control local network. Critical instruments are manalarms and sensors). The functionality provided by the1222 aged by the PLCs, which in turn communicates with the Controls system can be classified in the following four cat-1223 Ignition server. 24

Journal Pre-proof

1224

The Ignition server provides a single operator interface,1262 function as PROFIBUS slaves to the Master PLC. These

1225

alarm system, and historical record for all slow control in-1263 smaller PLCs are either provided and programmed by the

1226

strumentation. It provides authorized users with access to1264 instrument vendor or, in the case of the xenon compres-

1227

specific controls. In addition, the Ignition server provides1265 sors, built and programmed by LZ.

1228

the scripting engine for experiment automation. Ignition1266

1229

also provides a GUI for accessing historical data in the1267 and two recovery) is run by a dedicated Beckhoff CX8031

1230

form of plots by local and remote clients. The config-1268 PLC, which governs compressor startup and shutdown se-

1231

uration data (properties of sensors and controls, alarms,1269 quences. The PLCs running the two xenon recovery com-

1232

and user preferences) are stored in the local configuration1270 pressors have the extra feature that they are capable of ini-

1233

database.

of

tiating xenon recovery in response to an over-pressure sce-

pro

1271

Each of the four xenon compressors (two circulation

1234

For critical systems, which include xenon handling,1272 nario, triggered by xenon pressure transducers connected

1235

cryogenics, and grid high voltage, PLCs add an additional1273 directly to each PLC. The intent is that emergency xenon

1236

layer between the slow control software and the physical1274 recovery will be initiated and coordinated by the Master

1237

instruments. This ensures uninterrupted control of critical1275 PLC, with this slave-initiated recovery as a backup.

1238

instruments and separates the low level logic governed by1276

1239

PLCs from the higher level operations run by the Ignition1277 Controls is based on the wide range of highly customizable,

1240

scripting engine.

re-

The choice of Ignition as the software platform for LZ

1278

easy to use core functions and tools for development of ef-

The PLC system is responsible for the automated re-1279 ficient and robust SCADA systems. Ignition also comes

1242

covery of xenon in emergency scenarios and must be ro-1280 with a versatile toolbox for GUI design and a comprehen-

1243

bust against single-point failures. This begins with the1281 sive library of device drivers supporting most of the hard-

1244

the Siemens dual-CPU system. The system is powered by1282 ware used in LZ. One of these drivers supports Siemens

1245

redundant 24 VDC supplies fed by two separate electrical1283 PLCs allowing to export the PLC tags (internal variables).

1246

panels, one of which is supported by an 8 kVA APC Sym-1284 Devices connected to the PLC can be exposed to Ignition

1247

metra UPS. Both panels are also supported by backup1285 as sensors and controls. For the rest of the devices the

1248

diesel powered electrical generator, capable of powering1286 preferred protocol is MODBUS which is also supported by

1249

the PLC system and xenon recovery compressors for the1287 the Ignition server. For those devices that do not support

1250

time required to complete xenon recovery.

urn al P

1241

1288

MODBUS (e.g. RGA units), two interfacing methods are

Each Siemens CPU has its own connection to a com-1289 envisaged: 1) custom Java drivers written in the Ignition

1252

mon set of S7-300 I/O Modules, which connect directly1290 software development kit, and 2) Python MODBUS server

1253

to individual instruments/sensors, as well as to a set of1291 with plugin system or custom Python drivers.

1254

PROFIBUS-DP Y-links, allowing for redundant control of1292

1255

PROFIBUS instruments. In either case, redundancy does1293 els, organized into a tree view. At a lower GUI level, the

1256

not in general extend to the I/O modules and instruments.1294 panels represent the experiment subsystem via functional

1257

However, those instruments critical for xenon recovery are1295 diagrams with relevant information on the state of sensors

1258

duplicated to eliminate single-point failures.

Jo

1251

1296

The operator interface is comprised of a set of pan-

and controls displayed in real time. The authorized system

1259

Several integrated instruments, including the xenon1297 experts can also use these GUI panels to alter the controls

1260

compressors, vacuum pump systems, and the liquid ni-1298 not protected by the PLC interlocks. At a higher level,

1261

trogen generator, have their own dedicated PLCs which1299 several panels show the real-time status of the system and 25

Journal Pre-proof

LN Distribution Thermosiphons

SURF surface (direct fiber link)

Local Direct Local HMI Dev. (Ignition Client) control

Remote HMI (Ignition client)

Gids HV Local switch

Xe gas system Local Network

Master PLC

Circulation Compressor 1

PLC

Compressor 2

PLC

Ignition Server HW Interface Tag manager DB interface

PLC

Compressor 2

PLC

pro

Compressor 1

Detector

Alarm system Automation Historian

ICE module

Config. DB

Recovery

DAQ & Electronics

of

Storage

SURF gateway VPN

Online DB

UPS/Generator Powered

Calibration

Environment PMT HV

Run Control Offline DB

re-

Figure 18: Slow control functional diagram.

subsystems from which a trained operator is able to tell1321 increase from LUX to LZ. This included creating more

1301

the overall health of the system. These panels also allow1322 floor space with a platform for cable breakouts, conversion

1302

to assess high level information, for example alarm sta-1323 of the compressor room roof to a space for purification and

1303

tus, current operation mode, status of automation scripts,1324 Xe sampling equipment, and converting two previously un-

1304

etc. At a very high level, a single summary plot of the1325 used excavations to spaces for Xe storage and Rn removal

1305

entire system can be prepared by slow control and sent to1326 equipment.

1306

run control for display in a single status panel on the run1327

1307

control GUI.

1308

7. Detector Assembly

urn al P

1300

1328

port of the 12-foot tall AVs. Each AV was contained in

1329

a heavy steel frame. At the top of the Yates shaft the

1330

frame was mounted in a rotatable assembly, slung under

1331

the cage, and lowered to the 4850’ level. The rotatable

1309

LZ is being installed inside the existing 25’ diameter,

1310

20’ tall Davis Cavern water tank used for the LUX ex-

1311

periment. Access to the water tank is down the 4850’

1312

vertical Yates shaft and through horizontal drifts. Some

1313

large items of equipment are segmented and transported

1314

underground in sections, including the OCV (three sec-

1315

tions), and the tall AVs (four sections). Other items, like

1316

the ICV and its TPC payload, and the LXe tower, are

1317

transported and installed after being fully assembled on

1318

the surface.

1332

1333

1334

1335

Jo

Underground assembly started in 2018 with the trans-

1336

1337

1338

1339

assembly was used to move each 5000 pound unit around duct work and other obstacles between the cage and the Davis Cavern upper deck. Each AV was wrapped in two plastic bags to keep dust from contaminating the acrylic and the water tank. The AVs are sensitive to large temperature changes so timing and speed were also important considerations during transport. After the four tall AVs were placed inside the water

1340

tank, the three sections of the OCV were transported un-

1341

derground and installed on support legs inside. After a

1319

Substantial changes to the infrastructure of the Davis

1320

Campus were required to accommodate the detector size

1342

26

leak cheak, and a final washing of the water tank and its

Journal Pre-proof

1363

subsequent assembly steps are designed to keep the TPC

1364

under nitrogen purge until it can be evacuated and filled

1365

with Xe gas. The TPC is assembled and installed into the ICV ver-

1367

tically. To move underground, the assembly is rotated to

1368

horizontal, set in a transport frame, and moved to the

1369

Yates headframe. Slings attached to the underside of the

1370

cage are used to lift the assembly into a vertical position

1371

underneath the cage. This process is reversed at the 4850’

1372

level, returning it to horizontal, for transport down the

1373

drift. It is set vertical again on a shielding platform over

1374

the water tank. A temporary clean room is constructed

1375

around the ICV, and the last protective bag is removed.

1376

The ICV is connected to the OCV top by three tie bars

Figure 19: Intermediate assembly. A section through the Davis cam-1377

(instrumented threaded rods) that allow for precision lev-

pro

re-

pus showing the OCV ready to receive the ICV, and the ICV sus-1378 pended above with cable conduits. 1-Deck; 2-Water tank.

of

1366

eling of the TPC relative to the liquid Xe surface while

1379

minimizing thermal losses. During final lowering into the

1380

OCV the ICV is supported by the tie bars. Once sealed,

1381

the ICV is evacuated, and the AVs and OD PMTs are as-

equipment, the OCV top was removed and stored to pre-

1344

pare for ICV installation, and cleanroom protocols were

1345

instituted for entering and exiting the area. An under-

1346

ground radon reduction system treats the air supplied to

1347

the water tank during the remainder of the assembly.

urn al P

1343

1382

sembled around the OCV in the water tank. A rendering

1383

of the ICV just prior to nesting within the OCV is shown

1384

in Fig. 19.

1385

1348

Much of the TPC assembly work was done in the Sur-

1349

face Assembly Lab (SAL) at SURF because of its superior

1386

A second radon reduction system is available in the

Davis Cavern to supply air depleted of radon.

It has

3

1387

demonstrated an output of 100 mBq/m given input air

1388

of 70 Bq/m3 .

logistics compared to the underground. A well sealed class

1351

1000 clean room with reduced radon air supply [34] and

1352

a clean monorail crane was used for cryostat acceptance,

1353

TPC assembly, and insertion of the TPC into the ICV.

1354

The Reduced Radon System (RRS) has been measured

1355

to produce air laden with radon at a specific activity of

1356

4 mBq/m3 given input air at about 8.5 Bq/m3 . The PMT

1357

and instrumentation cables are routed through three rein-

1358

forced bellows attached to the ICV, and the entire assem-

1359

bly is transported underground together. During trans-

1360

port the bellows are sealed and the ICV is purged with

1361

boil off LN. This begins the process of removing H2 O, O2 ,1397

1362

and Kr from the large mass of PTFE in the detector. All1398 the ER and NR backgrounds resulting from radioactiv-

Jo

1350

1389

The water and GdLS are co-filled to minimize stress in

1390

the AV walls. The GdLS will be transported underground

1391

in 150 sealed barrels. Each is placed on a scale in a clean

1392

tent before connection to the liquid transfer system. The

1393

liquid is pumped into the GdLS reservoir and distributed

1394

to the AVs. Once the liquids are filled detector commis-

1395

sioning can begin.

1396

1399

27

8. Materials Screening & Backgrounds Material screening is the primary route to controlling

ity in the experiment. Measurements of radioactive nu-

Journal Pre-proof 212

Pb, resulting in 212 Bi-212 Po sequential events which can

1400

clides in and on all detector components are required. The1438

1401

ubiquitous and naturally occurring radioactive materials1439 be tagged. Radon daughters are readily identified through

1402

(NORM) of particular concern are the γ-ray emitting nu-1440 their alpha decay signatures and can be used to character-

1403

clides

1404

and their progeny. The U and Th chains are also respon-1442 tions in the active region, providing a useful complement to

1405

sible for neutron production following spontaneous fission1443 estimating radon concentration from the beta decay con-

1406

and (α,n) reactions. Kr and Rn outgassing from materials1444 tribution to the ER background.

1407

into the Xe also results in ER backgrounds, and α-emitting1445

1408

Rn daughters can contribute to neutron backgrounds when1446 less than 2 µBq/kg of LXe, equivalent to 20 mBq. All

1409

deposited on certain materials.

137

Cs; and

60

Co, as well as

238

U,

235

U,

232

Th,1441 ize the

1447

222

Rn and

220

Rn decay chain rates and distribu-

of

K;

The specific activity of

222

Rn in LZ is required to be

xenon-wetted components in LZ have been constructed

pro

40

1410

Fixed contamination, referring to non-mobile NORM1448 from low-radon materials selected through dedicated mea-

1411

1449 surement facilities. Due to the large number of expected nuclides embedded within materials, is the dominant source

1412

of neutron and γ-ray emission in LZ. To ensure a sub-1450 emitters, these facilities were required to achieve sensi-

1413

dominant contribution from fixed contaminants, relative1451 tivity of 0.2 mBq to

1414

to irreducible backgrounds, all materials considered for1452 room temperature, however, the expected emanation can

1415

use in the construction of the experiment are screened for1453 depend strongly on temperature depending on the source

1416

NORM nuclides to ≈0.2 mBq/kg sensitivities, equivalent1454 material. A conservative approach is adopted in estimat-

1417

to tens of parts per trillion (ppt) g/g for

1418

This ensures a maximum contribution from fixed contam-1456 duction at LXe temperatures wherever this is supported

1419

ination of less than 0.4 NR and 1 × 10−6 events/(kev ·1457 in the literature. Significant background from 220 Rn is not

1420

kg · year) ER in the LZ exposure. For materials such as1458 expected given its very short half-life; we conservatively in-

1421

PTFE, which are produced in granular form before being1459 clude in our background model a contribution from

232

Rn. Measurements are made at

re-

U and

Th.1455 ing radon emanation in our model, taking credit for a re-

urn al P

238

222

222

220

Rn

1422

sintered in molds, plate-out constitutes additional risk be-1460 of 20% of the ER counts from

1423

cause surface contamination of the granular form becomes1461

1424

contamination in bulk when the granules are poured into1462 ing the manufacture and assembly of components as well

1425

molds. A limit of 10 mBq/kg of 210 Po and 210 Pb in PTFE1463 as dust and particulates contribute to the LZ background.

1426

maintains an NR contribution of <0.1 count in the LZ1464 The α-particle emitting Rn-daughters can induce neutrons

1427

exposure.

1465

The accumulation of

222

Rn.

Rn-daughters plated-out dur-

via (α,n) processes, particularly problematic for materials

Radon emanated from within detector components is1466 with large cross-sections for this process such as flourine,

1429

the dominant contributor to background in LZ, primar-1467 present in the TPC walls (PTFE). Plate-out on the inside

1430

ily through the “naked” beta emission from

1431

222

Jo

1428

Rn sub-chain as it decays to

214

214

Bi. The

Pb in the1468 of the TPC walls causes α-particles and recoiling ions to

214

Bi beta1469 enter the active volume. The risk of mis-reconstructing 214

Po1470 these recoils into the fiducial volume in particular sets

1432

decay itself is readily identified by the subsequent

1433

alpha decay that would be observed within an LZ event1471 stringent constraints on plate-out on the PTFE. LZ in-

1434

timeline (T1/2 =160 µs). Similar coincidence rejection also1472 stituted a target for plate-out of

210

Pb and

210

Po of less

2

1435

occurs where beta decay is accompanied by a high-energy1473 than 0.5 mBq/m on the TPC walls and below 10 mBq/m2

1436

γ-ray, which may still be tagged by the Xe Skin or OD ve-1474 everywhere else.

1437

toes even if it leaves the active Xe volume.

220

Rn generates1475 28

Generic dust containing NORM also releases γ-rays

Journal Pre-proof

1476

and induce neutron emission. Dust is further expected1513 allows calculation of isotopic concentrations. A typical as-

1477

to be the single largest contributor to radon emanation.1514 say lasts 1–2 weeks per sample to accrue statistics at the

1478

Dust contamination is limited to less than 500 ng/cm2 on1515 sensitivities required for the LZ assays. These direct γ-ray

1479

all wetted surfaces in the detector and xenon circulation1516 assays probe radioactivity from the bulk of materials and

1480

system. Under the conservative assumption that 25% of1517 may identify equilibrium states.

1483

1484

Rn is released from the dust, via either emanation or1518

Sixteen HPGe detectors located in facilities both above-

recoils out of small grain-size particulates, this limits the1519 and underground have been used for LZ, with differences in 222

Rn activity from dust to less than 10 mBq.

8.1. HPGe + MS Techniques and Instruments

of

1482

222

1520

detector types and shielding configuration providing use-

1521

ful dynamic range both in terms of sensitivity to partic-

1522

ular nuclides and to varying sample geometries. The de-

pro

1481

1485

The LZ screening campaign deploys several mature tech1523 tectors are typically several hundreds of grams to several

1486

niques for the identification and characterization of ra-1524 kilograms in mass, with a mixture of n-type, p-type, and

1487

dioactive species within these bulk detector materials, pri-1525 broad energy Ge (BEGe) crystals, providing relative effi-

1488

marily γ-ray spectroscopy with High Purity Germanium1526 ciency at the tens of percent through to in excess of 100% (HPGe) detectors and Inductively-Coupled Plasma Mass1527 (as compared to the detection efficiency of a (3 × 3)-inch

re-

1489

1490

Spectrometry (ICP-MS), supported by Neutron Activation1528 NaI crystal for 1.33 MeV γ-rays from a 60 Co source placed

1491

Analysis (NAA). These complementary techniques collec-1529 25 cm from the detector face). While p-type crystals can

1492

tively produce a complete picture of the fixed radiological1530 be grown to larger sizes and hence require less counting

1493

contaminants.

1531

time due to their high efficiency, the low energy perfor-

Sensitivity to U and Th decay chain species down to1532 mance of the n-type and broad energy crystals is superior

1495

≈10 ppt has been demonstrated using ultralow-background1533 due to less intervening material between source and ac-

1496

HPGe detectors. HPGe can also assay

1497

other radioactive species emitting γ-rays. This technique1535 and assayed for several days to weeks in order to accrue

1498

is nondestructive and, in addition to screening of candi-1536 sufficient statistics, depending on the minimum detectable

1499

date materials, finished components can be assayed prior1537 activity (MDA). The detectors are generally shielded with

1500

to installation. Under the assumption of secular equilib-1538 low-activity Pb and Cu, flushed with dry nitrogen to dis-

1501

rium, the U and Th content, assuming natural terres-1539 place the Rn-carrying air, and sometimes are surrounded

1502

trial abundance ratios, may be inferred from the mea-1540 by veto detectors to suppress background from Compton

1503

surement of isotopic decay emissions lower in their respec-1541 scattering that dominates the MDA for low-energy γ-rays.

1504

tive chains. However, secular equilibrium can be broken1542 To reduce backgrounds further, most of the detectors are

1505

through removal of reactive nuclides during chemical pro-1543 operated in underground sites [35, 36], lowering the muon

1506

cessing or through emanation. HPGe readily identifies the1544 flux by several orders of magnitude. We also utilize a num-

1507

concentrations of nuclides from mid- to late-chain

1508

and

1509

several hundred keV. Background-subtracted γ-ray count-1547 pre-screenings before more sensitive underground assays.

1510

ing is performed around specific energy ranges to identify1548

1511

radioactive nuclides. Taking into account the detector ef-1549 tors, a cross-calibration program was performed using all

1512

ficiency at that energy for the specific sample geometry1550 detectors active in 2014. This involved the blind assay of

232

Jo

urn al P

1494

60

Co,

40

K, and1534 tive Ge. Clean samples are placed close to the Ge crystal

238

U1545 ber of surface counters, some of them employing active

Th, particularly those with energies in excess of1546 cosmic rate veto systems, that are particularly useful for

29

To ensure uniform analysis outputs for all HPGe detec-

Journal Pre-proof

1551

a Marinelli beaker containing ≈ 2 kg of Rhyolite sourced1589 simply monitored and reported as an equivalent concen-

1552

from the Davis cavern at SURF. This sample had previ-1590 tration, and lower concentration samples allow for external

1553

ously been characterized using the MAEVE p-type HPGe1591 calibrations and a significantly relaxed cleaning schedule,

1554

detector at LBNL. Across the eight detectors online at1592 allowing to measure less demanding samples at the rate

1555

the time, assays for both

1556

of each-other. As additional detectors have been brought1594 order of a few hundred ppt. Finally, some initial work has

1557

online, consistency has been assured by cross-calibration1595 been done to develop measurements of potassium using

1558

intra-facility.

U and

232

Th were within 1σ1593 of several per day with sensitivities to U and Th on the

1596

of

238

the cold plasma configuration, leading to measurements of

ICP-MS offers precise determination of elemental con-1597 a few hundred ppb of K.

1559

NAA has been used by LZ to assay PTFE to sub-ppt

tamination with potentially up to 100× better sensitivity1598

1561

for the progenitor U and Th concentrations compared to γ-1599 g/g levels of

1562

ray spectroscopy. Since ICP-MS directly assays the

1563

235

U, and

232

238

pro

1560

238

U and

232

Th that is not well suited to

U,1600 HPGe, due to the sensitivity, nor ICP-MS, due to diffi-

Th progenitor activity it informs the contri-1601 culty in digesting PTFE. However, as with ICP-MS, this

bution to neutron flux from (α,n) in low-Z materials, but1602 technique requires small sample masses, does not assay fin-

1565

also the contribution from spontaneous fission, which in1603 ished components, and assumptions of secular equilibrium

1566

specific materials can dominate. However, it cannot iden-1604 need to be made since this technique measures the top

1567

tify daughter nuclides in the U and Th decay chains that1605 of the U and Th chains. Samples are irradiated with neu-

1568

are better probed by HPGe. The ICP-MS technique as-1606 trons from a reactor to activate some of the stable nuclides,

1569

says very small samples that are atomized and measured1607 which subsequently emit γ-rays of well-known energy and

1570

with a mass spectrometer. As a destructive technique,1608 half-life that are detected through γ-ray spectroscopy. El-

1571

it is not used on finished components. The limitation of1609 emental concentrations are then inferred, using tabulated

1572

ICP-MS is that the sample must be acid soluble and that1610 neutron-capture cross sections convolved with the reac-

1573

several samples from materials must be screened to probe1611 tor neutron spectra. Depending on the surface treatment,

1574

contamination distribution and homogeneity. Assays take1612 NAA can probe both bulk and surface contamination. Its

1575

1–2 days per material, dominated by the sample prepa-1613 application and sensitivity are limited by the composition

1576

ration time, where extreme care must be taken to avoid1614 of the material.

1577

contamination of solvents and reactants.

urn al P

re-

1564

1615

1578

1579

ICP-MS assays for LZ materials have been performed

using several facilities, the majority of which operate Ag-1616

8.2. Radon Emanation Techniques and Instruments Radon emanation measurements of all xenon-wetted

1582

ity to U and Th in materials at the level of several ppt.1619 mental operation have been performed using four facilities

1583

Protocols and methodologies for sample preparation are1620 available to LZ, listed in Table 5. In all of the facilities

1584

largely based on well established procedures [38–40].

Jo

1581

ilent 7900 ICP-MS systems within minimum ISO Class 61617 components within the inner cryostat and those in the cleanrooms [37]. These are capable of achieving sensitiv-1618 gas system that come into contact with Xe during experi-

1580

1621

radon is accumulated in emanation chambers, where sam-

1585

These measurements take significant time owing to the1622 ples typically remain for two weeks or more. In many cases

1586

need for high statistics, standard addition calibration of1623 multiple measurements are performed per sample. The

1587

high-concentration samples, and frequent machine clean-1624 radon is then transferred using a carrier gas to a detector

1588

ing. For more routine measurements, the backgrounds are1625 to be counted. 30

Journal Pre-proof

In one of the stations, the radon atoms are collected1663 powered microscopy and x-ray fluorescence techniques.

1627

and counted by passing the radon-bearing gas through liq-

1628

uid, dissolving most of the radon. The scintillator is then

1629

used to count Bi-Po coincidences, detected through gated

1630

coincidence logic, using one PMT viewing the scintillator.

1631

In the other three stations, radon atoms and daughters are

1632

collected electrostatically onto silicon PIN diode detectors

1633

to detect 218 Po and 214 Po alpha decays. The radon screen-

1634

ing systems have been developed such that anything from

1635

individual components through to sections of LZ pipework

1636

may be assayed.

1664

8.4. Cleaning procedures and protocols (ASTM standards)

1665

A rigorous program of cleanliness management is im-

1666

plemented to ensure that the accumulated surface and dust

1667

contamination are monitored, tracked and do not exceed

1668

requirements. All detector components that contact xenon must be cleaned and assembled according to validated

1670

cleanliness protocols to achieve the dust deposition levels

1671

below 500 ng/cm2 and plate-out levels below 0.5 mBq/m2

1672

for the TPC inner-walls, and 10 mBq/m2 everywhere else.

1673

Witness plates accompany the production and assembly

1674

of all detector components to ensure QC and demonstrate

1675

QA through the plate-out and dust assays. The titanium

1676

cryostat was cleaned by AstroPak [16] to ASTM standard

1637

All stations were initially evaluated using calibrated

1638

sources of radon, with a cross-calibration program per-

1639

formed to ensure the accuracy of each system’s overall ef-

1640

ficiency and ability to estimate and subtract backgrounds.

1641

The radon-emanation screening campaign extends be-

1642

yond the material selection and construction phase and

1643

into detector integration and commissioning phases. A

1644

system is available underground at SURF in order to screen

1645

large-scale assembled detector elements and plumbing lines.

1646

As pieces or sections are completed during installation of

1647

gas pipework for the LZ experiment, they are isolated and

1648

assessed for Rn emanation and outgassing for early iden-

1649

tification of problematic seals or components that require

1650

replacement, cleaning, or correction.

1651

8.3. Surface Assays (XIA, Si, dust microscopy)

IEST-STD-CC1246 (rev E) level 50R1 (ICV) and level

1678

VC-0.5-1000-500UV (OCV). The ICV cleaning standard

1679

is equivalent to the requirement that mass density of dust

1680

be less than 100 ng/cm2 . The vessels were etched accord-

1681

ing to ASTM B-600.

urn al P

re1677

1682

As described in Sec. 7, detector integration is done in a

1683

reduced-radon cleanroom at the SAL at SURF. Dust and

1684

plate-out monitoring on-site was continuously performed

1685

to measure and maintain compliance with tolerable dust

1686

and plate-out levels.

1687

8.5. Backgrounds summary

1688

1652

Two sensitive detectors have been used to carry out

1653

assays of Rn plate-out to ensure the requirements are met

1654

and inform the experiment background model. The first

1655

is the commercial XIA Ultralo-1800 surface alpha detec-

1656

tor system, suitable for routine screening of small sam-

1657

ples including witness plates and coupons deployed dur-

1658

ing component assembly and transport to track exposure.

1659

The second detector employs a panel of large-area Si de-

1660

tectors installed in a large vacuum chamber. These sys-

1661

tems exceed the requisite sensitivity to

1689

1690

1691

Jo

1692

1693

1694

1695

1696

1662

of

1669

pro

1626

2

210

1697

Po at the level 1698

of 0.5 mBq/m . Dust assays are performed using high1699

31

Measured material radioactivity and anticipated levels

of dispersed and surface radioactivity are combined with the Monte Carlo simulations and analysis cuts to determine background rates in the detector. Table 8.3 presents integrated background ER and NR counts in the 5.6 tonne fiducial mass for a 1000 live day run using a reference cutand-count analysis, both before and after ER discrimination cuts are applied. For the purposes of tracking material radioactivity throughout the design and construction of LZ, Table 8.3 is based on a restricted region of interest relevant to a 40 GeV/c2 WIMP spectrum, equivalent to approximately 1.5–6.5 keV for ERs and 6–30 keV for NRs.

Journal Pre-proof

1700

1737 new calibrations, updated reconstruction and identificaThe expected total from all ER(NR) background sources

1701

is 1195(1.24) counts in the full 1000 live day exposure.1738 tion algorithms, etc.) is expected to take place at one or

1702

Applying discrimination against ER at 99.5% for an NR1739 both locations, with the newly generated files mirrored at

1703

acceptance of 50% (met for all WIMP masses given the1740 both sites and made available to the collaboration. Sim-

1704

nominal drift field and light collection efficiency in LZ [4])1741 ulation and data processing at the U.K. data center are

1705

suppresses the ER(NR) background to 5.97(0.62) counts.1742 performed using distributed GridPP resources [44, 45].

1706

Radon presents the largest contribution to the total num-

1707

ber of events. Atmospheric neutrinos are the largest con-

1708

tributor to NR counts, showing that LZ is approaching1744

1709

the irreducible background from coherent neutrino scat-1745 HEP frameworks, specifically Geant4 for simulations [46]

1710

tering [41].

1746

of

9.1. The Offline Software Stack

The LZ Offline software stack is based on standard

pro

1743

and Gaudi for reconstruction [47]. The simulation package is called BACCARAT [48]. This

1747

1711

9. Offline Computing

1748

software provides object-oriented coding capability specif-

1749

ically tuned for noble liquid detectors, working on top of

1712

The LZ data is stored, processed and distributed using

1713

two data centers, one in the U.S. and one in the U.K. Both

1714

data centers are capable of storing, processing, simulating

1715

and analyzing the LZ data in near real-time. Resource

1716

optimization, redundancy, and ease of data access for all

1717

LZ collaborators are the guiding principles of this dual

1718

data-center design.

the Geant4 engine. BACCARAT can produce detailed, ac-

re-

1750

1751

1752

1753

grounds, which are crucial both for detector design and during data analysis. The physics accuracy of the LZ simulations package was validated during the science run of LUX, as described in [49].

urn al P

1754

curate simulations of the LZ detector response and back-

1755

1756

1719

LZ raw data are initially written to the surface stag-

1720

ing computer at SURF, which was designed with sufficient

1721

capacity to store approximately two months of LZ data,

1722

running in WIMP-search mode. This guarantees conti-

1723

nuity of operations in case of a major network outage.

1724

The surface staging computer at SURF transfers the raw

1725

data files to the U.S. data center, where initial process-

1726

ing is performed. The reconstructed data files are made

1727

available to all groups in the collaboration and represent

1728

the primary input for the physics analyses. The U.S. data

1729

center is hosted at the National Energy Research Scientific

1730

Computing (NERSC) center [42]. Data simulation and re-

1731

construction at NERSC is performed using a number of

1732

Cray supercomputers [43].

1757

1758

1759

1760

1761

1762

1763

1764

1765

Jo

1766

1767

1768

BACCARAT is integrated into the broader LZ analy-

sis framework, from production to validation and analysis. Two output formats are supported, a raw simulation output at the interaction level, and a reduced tree format at the event level. Both output files are written in the ROOT data format [50]. A Detector Electronics Response package can be used to emulate the signal processing done by the front-end electronics of LZ. It reads raw photon hits from BACCARAT to create mock digitized waveforms, organized and written in an identical format to the output of the LZ data acquisition system. These can be read in by the analysis software, providing practice data for framework development and analysis.

1769

The data processing and reconstruction software (LZ

1770

analysis package, or LZap for short) extracts the PMT

1733

The raw and reconstructed files are mirrored to the

1734

U.K. data center (hosted at Imperial College London) both

1735

as a backup, and to share the load of file access and pro-

1736

cessing. Subsequent reprocessing of the data (following

1771

1772

1773

32

charge and time information from the digitized signals, applies the calibrations, looks for S1 and S2 candidate events, performs the event reconstruction, and produces

Journal Pre-proof

1774

the so-called reduced quantity (RQ) files, which represent1812 liver experiment software in a fast, scalable, and reliable

1775

the primary input for the physics analyses.

1813

way. Files and file metadata are cached and downloaded on

LZap is based on the Hive version of the Gaudi code,1814 demand. CVMFS features robust error handling and se-

1777

which is specially designed to provide multi-threading at1815 cure access over untrusted networks [53]. The LZ CVMFS

1778

the sub-event level. The framework supports the develop-1816 server is visible to all the machines in the U.S. and U.K.

1779

ment of different physics modules from LZ collaborators1817 data centers. All the LZ software releases and external

1780

and automatically takes care of the basic data handling1818 packages are delivered via CVMFS: this ensures a unified

1781

(I/O, event/run selection, DB interfaces, etc.). Gaudi fea-1819 data production and analysis stream, because every user

1782

tures a well established mechanism for extending its input1820 can access identical builds of the same executables, remov-

1783

and output modules to handle custom formats. This func-1821 ing possible dependencies on platform and configuration.

1784

tionality has has been exploited in the design of the LZ1822

1785

raw data and RQ formats.

1823

pro

of

1776

A rigorous program of Mock Data Challenges has been

enacted, in order to validate the entire software stack and

All non-DAQ data, i.e. any data that is not read out by1824 to prepare the collaboration for science analysis. The first

1787

the DAQ with each event, is stored in a database known1825 data challenge simulated the initial LZ commissioning and

1788

as the “conditions database”. This database can auto-1826 tested the functionality of the reconstruction framework.

1789

matically understand the interval of validity for each piece1827 The second data challenge covered the first science run of

1790

of data (based on timestamps), and supports data ver-1828 LZ and tested the entire data analysis chain, including cal-

1791

sioning for instances such as when better calibrations be-1829 ibrations, detailed backgrounds and potential signals. The

1792

come available. This design implements a hierarchy of data1830 third and final data challenge is currently underway (2019)

1793

sources, which means that during development of code or1831 and will test the complete analysis strategy, validating the

1794

calibrations it is possible to specify alternate sources, al-1832 readiness of the offline system just before the underground

1795

lowing for the validation of updated entries.

urn al P

1796

re-

1786

1833

installation of LZ.

All LZ software is centrally maintained through a soft-

1797

ware repository based on GitLab [51]. GitLab provides a1834 10. Conclusion

1798

continuous integration tool, allowing for automatic testing

1799

and installation of the offline codebase on the U.S. and

1800

U.K. data center servers. Build automation is inherited

1801

from the Gaudi infrastructure and supported via CMake.

1802

Release Management and Version Control standards were

1803

strictly enforced from a very early stage of the experiment

1804

to ensure sharing, verifiability and reproducibility of the

1835

1836

1837

1838

1839

Jo

1840

1841

1805

results. Each code release undergoes a battery of tests be-

1806

fore being deployed to production. Release management

1807

ensures that all the changes are properly communicated

1808

and documented, to achieve full reproducibility.

1842

1843

menting the LZ conceptual and technical designs described in Refs.

[4, 5]. The start of science operations is ex-

pected 2020. The projected background rate enables a 1000 day exposure of the 5.6 tonne fiducial mass, with a spin-independent cross-section sensitivity of 1.5×10−48 cm2 (90% C.L.) at 40 GeV/c2 . This will probe a significant portion of the viable WIMP dark matter parameter space. LZ is also be sensitive to spin-dependent interactions, through 129

the odd neutron number isotopes

1845

and 21.2% respectively by mass). For spin-dependent WIMP-

Software distribution is achieved via CernVM File Sys-

1810

tem (CVMFS) [52]. CVMFS is a CERN-developed net-

1846

1847

work file system based on HTTP and optimized to de33

Xe and

131

1844

1809

1811

Considerable progress has been made towards imple-

Xe (26.4%

neutron(-proton) scattering a sensitivity of 2.7×10−43 cm2 (8.1×10−42 cm2 ) is expected at 40 GeV/c2 .

Journal Pre-proof

1848

1849

11. Acknowledgements

1886

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Nuclide

Type

Energy [keV]

τ1/2

83m

γ

32.1 , 9.4

1.83 h

γ

164

11.8 d

α, β, γ

various

10.6 h

β

18.6 endpoint

12.5 y

β

156 endpoint

5730 y

Kr

131m 220

Xe

Rn

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A

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sources released into GXe circulation, B: sealed sources lowered down

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calibration, grouped according to deployment method. A: gaseous

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AmLi

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Cf

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2.65 y

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AmBe

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11,000 endpoint

432 y

γ

122

0.74 y

γ

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1.91 y

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228

Jo

2056

57

B

C

Th

432 y

22

Na

γ

511,1275

2.61 y

60

Co

γ

1173 , 1333

5.27 y

Ba

γ

356

10.5 y

54

Mn

γ

835

312 d

88

YBe

(γ,n)

152

107 d

133

D

36

Co

(a)

124

SbBe

(γ,n)

22.5

60.2 d

205

BiBe

(γ,n)

88.5

15.3 d

206

BiBe

(γ,n)

47

6.24 d

DD

n

2450

−

D Ref.

n

272 → 400

−

Journal Pre-proof

Table 5: Primary material radio-assay techniques, indicating isotopic sensitivity and detection limits, as well as typical throughput or singlesample measurement duration.

Typical

Sample

Sampling

Sensitivity

Sensitivity

Mass

Duration

238

HPGe

U,

235

232

Th

chains, 60

40

137

Co,

U,

K,

Cs

−11

5 × 10

−10

10

g/g U,

g/g Th

kg

Up to 2 weeks

U, 10

g/g

mg to g

232

of chain), K

238

GD-MS

235

U,

232

of chain)

Radon

222

Surface α

Rn

10

210

210

Pb,

Bi,

210

g/g to

−14

10

−10

10

g/g

g/g

0.1 mBq

Jo

Emanation

−12

sensitive as other techniques, large

sample digestion,

g

day)

kg

UCL, IBS, BHUC, U. Alabama

for non-metals,

MITR-II, HPGe

weeks

minimal sample

assay at

preparation

U. Alabama

Days

matrix effects, cannot analyze ceramics and other

National Research Council Canada

157

3

2

insulators Non-destructive,

UCL ×2,

large samples,

U. Maryland,

weeks

limited by size of

SDSM&T ×2,

emanation chamber

U. Alabama ×2

samples, large surface area required

37

926

Destructive, minimal

1 to 3

<1 week

Assayed

Boulby ×7

Days to

Non-destructive, thin g to kg

U. Alabama ×2,

Irradiated at

2

120 α/(m ·

Po

mg to g

Samples

SURF ×6, LBNL ×1,

Destructive, useful

U,

Th (top

very versatile, not as

preparation critical

U,

Th (top

Days

urn al P

NAA

235

U,

Systems if > 1)

Destructive, requires

−12

Th (top

of chain)

238

Notes

re-

ICP-MS

235

U,

Number of

samples

emitter

232

Locations (and

destructive and

Non-destructive,

any γ-ray

238

Destructive/Non-

of

Isotopic

pro

Technique

175

SDSM&T (Si), Brown (XIA), Boulby (XIA), U. Alabama (Si)

306

Journal Pre-proof

Table 6: The estimated backgrounds from all significant sources in the LZ 1000 day WIMP search exposure. Mass-weighted average activities 8

are shown for composite materials. Solar B and hep neutrinos are only expected to contribute at very low energies (i.e. WIMP masses) and are excluded from the table. ER

NR

(cts)

(cts)

9

0.07

40

0.39

0.2

0.05

-

0.05

40.0

-

-

0.16

-

0.12

5

0.06

4.6

0.00

-

0.06

Cosmogenic Activation

0.2

-

Xenon Contaminants

819

0

222

Rn (1.81 µBq/kg)

681

-

220

Rn (0.09 µBq/kg)

111

-

of

Background Source

Detector Components Surface Contamination 2

pro

Dust (intrinsic activity, 500 ng/cm ) 2

Plate-out (PTFE panels, 50 nBq/cm ) 210

Bi mobility (0.1 µBq/kg) 2

Ion misreconstruction (50 nBq/cm ) Pb (in bulk PTFE, 10 mBq/kg)

Laboratory and Cosmogenics Laboratory Rock Walls

urn al P

Muon Induced Neutrons

re-

210

nat

Kr (0.015 ppt)

24.5

-

nat

Ar (0.45 ppb)

2.5

-

322

0.51

67

-

255

-

Diffuse Supernova Neutrinos

-

0.05

Atmospheric Neutrinos

-

0.46

1,195

1.03

5.97

0.51

Physics 136

Xe 2νββ

7

13

Total

Jo

Solar Neutrinos: pp+ Be+ N

Total (with 99.5 % ER discrimination, 50 % NR efficiency) Sum of ER and NR in LZ for 1000 days, 5.6 tonne FV, with all analysis cuts

38

6.49

Journal Pre-proof *Declaration of Interest Statement

of

Declaration of interests ☐ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

re-

pro



Jo

urn al P