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The Bern Cryogenic Detector System for dark matter search
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Nuclear Instruments and Methods in Physics Research A 547 (2005) 359–367 www.elsevier.com/locate/nima
The Bern Cryogenic Detector System for dark matter search S. Janos, M. Hauser, K. Pretzl, F. Nydegger, H.U. Schu¨tz, S. Lehmann, M. Hess, J.C. Roulin Laboratory for High Energy Physics, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland Received 17 January 2005; received in revised form 15 March 2005; accepted 16 March 2005 Available online 16 May 2005
Abstract In this paper we report on the design, construction, and operation of a cryogenic system designed to cool a Superheated Superconducting Granule (SSG) detector for dark matter search. The volume accommodating SSG detector is 20 cm in diameter and 67 cm long. The system provides a lowest operating temperature of 115 mK for longlasting (several months) experiments in the Bern Underground Laboratory. r 2005 Elsevier B.V. All rights reserved. PACS: 95.35.+d; 7.20.Mc Keywords: Cold dark matter; Cryogenic detectors
1. Introduction The quest for dark matter is one of the most pressing challenges to cosmology and astro-particle physics. Its direct detection with earthbound detectors will provide decisive explanations of its true nature. Since its discovery by the Swiss astronomer Fritz Zwicky in 1933 [1] a lot more has been learned, mostly in recent years, about this ubiquitous matter, which gravitationally binds stars in galaxies [2], galaxies in clusters and in Corresponding author. Tel.: +41 31 6314065;
fax: +41 31 6314487 E-mail address: [email protected] (S. Janos).
large-scale structures [3]. From a number of astrophysical observations and from the cosmic microwave background radiation experiment WMAP [4], we have learned that the universe contains about 30% matter and 70% dark energy. Most of this matter is invisible and of exotic nature. In case the dark matter consists of exotic particles which interact weakly with ordinary matter, so-called weakly interacting massive particles (WIMPs), they should be detectable in principle. A recent review on direct dark matter searches can be found in Ref. [5] and references therein. WIMPs can be detected by measuring the nuclear recoil energy when one of these particles
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interacts with a nucleus of the detector material. The energy of the recoiling nucleus can be measured in several ways, e.g., as a small rise of temperature, electric charge liberation, or photon emission. To observe a WIMP, the detector must be capable of registering very small nuclear recoil energies below 1 keV [6–9]. In terms of sensitivity to very small nuclear recoil energies, cryogenic detectors promise significant advantages [10–12]. At temperatures near the absolute zero, the heat capacity is very small and the nuclear recoil energy can be transformed into a measurable signal. Several groups in the USA, Europe, and in Japan are searching for WIMPs employing different detection techniques [13]. One of these innovative detection systems has been developed also at the University of Bern [14,15]. The detector is called ORPHEUS in analogy to Greek mythology and is situated in the Bern Underground Laboratory 30 m below the Institute of Physics.
2. General consideration For a dark matter search it is of extreme importance to reduce the external and internal radioactive backgrounds. The experiment must be
carried out in an underground laboratory to suppress the radioactive background coming from the cosmic rays. In addition, it must be also locally shielded against the natural radioactivity of the rock and the radioactivity of the materials surrounding the detector. Therefore, it was necessary, to position the cold box containing the detector chamber (Fig. 1) as far as possible from the dilution cryostat, which is made from standard cryogenic materials (e.g., stainless steel, Al, Alfoils) with usually too high residual radioactivity for our purpose. The detector chamber and the cold box thermal shields (1.7, 4.5, 77 K) have been constructed using low activity materials (mainly OFHC Cu). Lot of effort has gone into the background measurements and into the construction of an adequate radioactive shielding respecting the dimensions of the laboratory furnished with vacuum, cryogenic, and electronic equipment. The measured fluxes of muons and neutrons in the Bern Underground Laboratory are 7.6 104 muons/cm2 s and 2 104 neutrons/ cm2 s, respectively. The neutrons are moderated and captured by the boron-doped polyethylene inside the lead shield. Thus, the neutron flux reaching the detector is estimated as 2 106 neutron/cm2 s. The passive shielding for the
Fig. 1. Vertical cross-section of the ORPHEUS cryogenic system.
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Fig. 2. Cold box and the passive radioactive shield.
ORPHEUS detector is shown on Fig. 2. It is divided into two closely fitting halves and each half is supported by a platform on rails. Looking from outside to inside it consists of the following materials: 2 cm scintillator veto counter (not shown), 15 cm lead, 4 cm of OFHC copper and 18 cm of 5% boron-doped polyethylene. All materials used in the shielding and also in the vicinity of the SSG detector (e.g., electrical connectors, wires, screws) were tested for their radioactive purity. Because of the limiting height of the Bern Underground Laboratory (3.2 m), the thermal extension contacts from the dilution cryostat to the cold box with detector chamber cannot be vertical, but horizontal. However, such horizontal extension requires the cooling power of the dilution refrigerator to be transferred through a very long (about 1.7 m) copper rod—cold finger— with a flexible mechanical connection to the mixing chamber. Similarly, the thermal shields connecting the dilution refrigerator cryostat and the side access, must be also flexibly connected to each other in order to compensate for the
vertical thermal contractions of about 5 mm relative to the horizontal side access during cooling to 77 or 4 K, respectively. In addition, the 4 K connections must also be leak-tight to separate the outer vacuum space (OVC) from the inner vacuum space (IVC) as IVC contains the helium exchange gas during the cooldown to 4 K. This was achieved using 28 membrane pairs of 316 L stainless steel bellows with dimensions +65/90 for the 4 K connection and +120/140 for the 77 K connection (Comvat AG, 9469 Haag, Switzerland). The bellows provide the flexibility required but are insufficient for heat transfer. The thermal connection between the horizontal side access and the vertical 4 K cryostat thermal shield is provided by a bundle of 98 Cu wires (RRR ¼ 180, +1.2 mm, l ¼ 10 cm). Such a thermal bypass was then fastened with stainless screws to the bellows copper flanges (Fig. 3) using a thin layer of Apiezon-N grease (Apiezon Products Ltd., London, UK) to improve essentially the contact conductance [16]. Before applying the Apiezon-N grease, both contact surfaces were polished with 600-grit emery paper, then
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Fig. 3. Thermal bypass mounted on the stainless steel bellows.
ultrasonically cleaned in acetone followed by alcohol and dry-blown in clean nitrogen gas. A similar thermal bypass was used also for 77 K thermal shield. From a practical point of view, Apiezon–N grease provides a very simple method to increase thermal contact conductance at helium temperatures. Previous work with gold coating [17] showed that the improvement in the thermal contact conductance was much below the magnitude realized by Apiezon–N grease. However, new results for the copper to copper joints with gold-plated surfaces below 400 mK [18], when linearly extrapolated to 1 K, are in disagreement with data from Ref. [17]. As far as we know, the copper to copper joints coated with a thin layer of Apiezon–N grease have not been investigated yet below 1.6 K [19,20]. The thermal connection for the 1.7 K thermal shield was made of 63 Cu wires (+1.8 mm, l ¼ 6 cm). The flexible thermal connection between the vertical cold finger (OFHC copper, +30 mm, l ¼ 32 cm) and the horizontal cold finger (OFHC copper, +20 mm, l ¼ 133 cm) was made of 93 Cu wires (+1.4 mm, l ¼ 10 cm). The ends of the
copper wires were electron beam welded to the bulk copper cylinders. One end of such connection was then welded to the horizontal cold finger and the other one was fastened to the vertical cold finger with four M6 screws. Our plan to replace the conventional readout system by a SQUID system installed in the same cold box and in the same detector chamber was a decisive one for choosing a rigid support between thermal shields and for the detector chamber instead of a swinging support system used in the Cryogenic Dark Matter Search (CDMS) collaboration [21,22]. However, the problem with a rigid support is rather high heat loads in comparison with Kevlar hanger loops used in the CDMS. Our experience from the nitrogen and helium cooling runs in 1996–1997 showed, that the heat conduction via copper sideaccess tubes with thermal bypasses alone were insufficient to reach the low temperatures required (about 300 mK). To solve this problem, we developed additional liquid nitrogen and liquid helium cooling systems for the cold box. This additional cooling allowed us to achieve an operating temperature of 115 mK on the ORPHEUS detector.
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3. Detector principle The idea to use superheated superconding granules as a particle detector goes back to 1967 [23]. The detection principle is based on the phase transition of the type I superconductor from the metastable superheated state to the normal conducting state. Interaction of an incoming particle (e.g., WIMP) within a superconducting granule produces quasiparticles, which spread over the volume of the granule. The quasiparticles lose their energy via electron–phonon interaction. The resulting temperature increase can induce such a phase transition into the normal state. Disappearance of the Meissner–Ochsenfeld effect allows the magnetic field to enter the normal conducting granule, and therefore the phase transition can be detected with a pickup loop around the detector. The detected signal is proportional to the volume of the granule and to the strength of the magnetic field. The detector threshold is given by the minimum deposited energy needed to raise the granule temperature from the operating temperature to the phase transition temperature. The sudden character of phase transitions of the granules (100 ns) allows for fast time correlation between SSG signals and those of other detectors (e.g., scintillation veto counters surrounding the detector). We operated the detector at as low as 100 eV threshold. Our detector consists of many billions of small grains (e.g., 30 mm diameter) embedded in a dielectric medium (teflon, paraffin, plasticine) with a volume filling factor of 10%. The detector is operated in the superheated superconducting state at a constant temperature of 115 mK and in an external magnetic field of 285 G. We studied the response of the SSG detector to nuclear recoil energies in the keV range by irradiating Sn, Al, and Zn SSG detectors with a 70 MeV neutron beam in experiments performed at the Paul Scherrer Institute in Villigen (Switzerland) [24–26]. Recoil energies down to 1 keV were measured. These experiments confirmed that SSG detectors can also be used to detect very small recoil energies in elastic WIMP-nucleus scatterings and encouraged us to develop a large SSG detector
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for dark matter search. A review of SSG investigations can be found in Ref. [27].
4. Detector The detector chamber (Fig. 1) is made of electroformed copper (produced by Steiger S.A., Vevey, Switzerland) to guarantee a low level of radioactivity. Inside the detector chamber (+12 cm, l ¼ 54 cm) there are supports made of Delrin (Polyoxymethylene) for 56 pickup coils. A signal coil has a length of 6.8 cm, a diameter of 1.8 cm, 1500 turns of 60 mm diameter high purity Cu wire, and contains 8 g of tin grains immersed in teflon powder (Goodfellow Cambridge Ltd., Cambridge, UK) with a 10% volume filling factor. The size of the pickup coil is limited by the required signal-to-noise ratio between 10 and 20. Each pickup coil is connected to a J-FET preamplifier working at room temperature [28]. In order to investigate the sensitivity on the grain size we filled one group of signal coils with smaller tin granules (+27.774.1 mm), and the other one with larger tin granules (+3672.2 mm). The total of target material in the detector chamber amounts to 448 g. The granules were produced by gas atomization at the Technical University Clausthal, Germany, and mechanically sieved in our laboratory using a sieving machine Type TS18-IG (SWECO Europe, Nivelles, Belgium). The granules are located in a an external magnetic field with a homogeneity better than 1.6% produced by a superconducting solenoid made from multifilament NbTi wire in copper cladding (SUPERCON, USA Type MR 24, Cu:NbTi ¼ 7:1, +0.4 mm, l ¼ 9.556 km, 19109 turns). The solenoid is wound on the high purity copper body which is directly thermally contacted to the detector chamber (Fig. 1) and is operated in high vacuum. In order to provide adequate thermal contact between tin granules and the detector chamber, the free space in the detector chamber (1 l) is filled with superfluid helium. Liquid 4He as a contact agent for granules below 1 K has great advantage because of its high thermal conductivity [29–31]. The main difficulty is that the detector chamber with 114 electrical feedthroughs and two
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condensation lines for 4He (each 14 m long) must be leak tight against superfluid helium.
5. Cooling systems To cool the detector chamber below 1 K we use a Kelvinox 300 dilution refrigerator (Fig. 1). The cryostat has two side-access ports near the bottom. One of these ports connects the side-access thermal shields (77, 4.5, and 1.7 K). The second port at the opposite side is used for IVC pumping. The liquid nitrogen bath has a volume of 36 l and is refilled automatically every 12 h using a Model 180 Liquid Level Controller with solenoid-operated fill valve (American Magnetics Inc., Oak Ridge, Tennessee, USA). The liquid helium bath has a useful volume of 22 l and is refilled automatically every 8 h using a Model 136 Helium Level Controller (American Magnetics Inc., Oak Ridge, Tennessee, USA). A Kelvinox IGH gas handling system and ITC 503 temperature controller (Oxford Instruments, UK) were used for monitoring the dilution refrigerator. The main operation parameters were displayed in real time on the Internet. 3He evaporating in the still is pumped through a 10 cm diameter tube by an Alcatel Roots pump RSV 601B backed by a Alcatel 3He rotary pump 2063 H. When the backing pressure of the 3He rotary pump rises above 800 mbar, both 3He pumps are switched off. The 3He gas is purified in two successive charcoal traps at 77 K followed by the liquid helium-cooled trap installed immediately in front of a dilution refrigerator in the cryostat [32] to absorb traces of air. It was necessary to regenerate this trap only after 1 month of continuous operation. The 3He flow rate is measured by a Teledyne–Hastings flow meter HFM-200 (Hampton, VA, USA). Typically 3 He circulation rate was 450 mmol/s when applying 10 mW into the still heater. The gas charge of the dilution refrigerator is 64 l of 4He and 16 l of 3He at NTP. Additional liquid nitrogen and helium cooling systems using a continuous flow cryostat mode and a liquid bath were developed for the cold box. These cooling systems for 77 and 4.5 K thermal shields used vacuum isolated transfer lines with needle valves (Janis Transfer lines Type SF-100,
Janis Research Company Inc., Wilmington, MA 01887) and a membrane GF3 gas flow pump (Oxford Instruments) to regulate flow through heat exchangers soldered directly onto the thermal shields. The heat loads to the 4.5 K (77 K) chambers were 0.4 and 28.5 W, respectively. This arrangement allowed us to maintain the 77 K shield continuously at 77 K for 11 days without changing the 200 l liquid nitrogen storage dewar. Likewise, the 100 l liquid helium storage dewar was changed after 30 h. Changing the liquid helium and liquid nitrogen storage dewars for additional cold box cooling has practically no influence on the temperature of the detector chamber.
6. Cryogenic seals The choice of superfluid helium leaktight cryogenic seals on demountable joints was primarily governed by requirements for low radioactive background due to their proximity to the detector chamber. Commercially available aluminium Helicoflex gaskets, which are leaktight against superfluid helium [33], require stainless steel nuts and bolts and have high radioactivity due to 60Co, whose beta decay yields two high energy g-rays. The aluminium contains 40K, which decays by electron capture yielding a high-energy g. Polycarbonate-based cryogenic seals have been examined at CERN [34] down to 4.2 K using flanges made of aluminium alloys (AlMg, AlMgSi) and the bolts made of stainless steel 304 L. No experiments have been performed with polycarbonates using the OFHC copper flanges and brass bolts to achieve very low residual radioactivity and tightness against superfluid helium. An all copper seal, e.g., CF-type, with a nose profile on the copper flanges and with a copper gasket [22], is mechanically rather complicated and not very appropriate for repeated assembling and disassembling. Another candidate, the conventional cryogenic vacuum seal gasket made from indium wire, is too radioactive for our purpose because of indium beta decay. We decided to use high-purity lead wire of 1 mm diameter (Puratronic grade, 99.998%, Alfa Aesar,
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Karlsruhe, Germany) as the gasket material. The joint is tightened with brass screws, because copper screws would break easily. Since the lead wire is considerably harder than indium one, it requires more pressure to get a reliable seal. However, it is difficult to apply large pressure on the seal using the brass bolts only. Therefore, for the start, we tightened the lead wire seal using stainless steel bolts. Then the seal was tested using a leak detector. When the test was satisfactory, the stainless steel screws were replaced by the brass screws. Throughout all this procedure the leak detector signal was monitored. This procedure allowed us to make very reliable seals leaktight even against superfluid helium.
7. Electrical wiring, electrical feedthrough, and thermal anchoring of wires (1) The wiring of the signal coils and the test pulse consists of 114 Cu wires (RRR ¼ 120, +100 mm) in twisted pairs running from room temperature to 4.5 K, 114 superconducting NbTi in CuNi matrix wires (Supercon, USA, Type SWM, +80 mm) in twisted pairs running from 4.5 K to the detector chamber, 114 Cu wires (+100 mm) in twisted pairs inside the detector chamber connecting the signal coils with the electrical feedthrough into the detector chamber. The wires enter the cryostat at room temperature flange through two 61-pin feedthroughs (Framatome, France, Type 851-02H24-61P50). These serial production high-vacuum connectors are designed for room temperature applications and are not suitable for low temperature because their flange is sealed with a rubber O-ring. We have used these connectors also at 77 and 1.7 K thermal shields, because at these positions vacuum tightness is not required (77 K thermal shield is in OVC and 1.7 K shield is in the IVC). There were, however, additional requirements for leak tightness on electrical feedthroughs mounted on the 4.5 K thermal shield, which separates the OVC and IVC and also for detector chamber, which contains superfluid helium. The urgent need for high-pin density electrical feedthrough for superfluid helium applications stimulated our effort to
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test different types of many pin connectors. We have found a simple modification of the Framatome connectors which enabled us to obtain very reliable high-pin density feedthroughs leak tight against superfluid helium [35]. (2) The wiring of the solenoid consists of one twisted pair of Cu wires (+0.4 mm) from room temperature to 4.5 K followed by one pair of single filament superconducting NbTi (+0.15 mm) wires without copper matrix (Supercon, USA, Type T48B-M) from 4.5 K to the solenoid. (3) For thermometry we used eight bundles of 36 AWG phosphor-bronze quad leads (Lake Shore, USA, Type QL-36) from room temperature down to the sensors. The resistances of the thermometers were measured by the 4-wire method (see below). (4) Thermal anchoring of wires: Twisted pairs of copper and NbTi (CuNi) signal wires were glued, using General Electric 7031 varnish (General Electric Company, Schenectady, USA), onto the OFHC copper cylinders with a diameter of 26 mm.The glued tempered length was 74 cm [36]. The cylinders were greased with Apiezon N grease and fastened with M5 screws to the flanges at 77, 4.5, and 1.7 K, respectively. In the same way, the solenoid current leads were glued to the OFHC copper cylinders with a diameter of 49 mm achieving the glued tempered length of 246 cm.
8. Thermometry The basic thermometers which we use for diagnostic purposes are 12 calibrated Cernox resistors model CX-1050-CU (Lake Shore Cryotronics Inc, Westerwille, OH43082), two calibrated Ruthenium oxide resistors (Oxford Instruments) and three calibrated Ruthenium oxide resistors from Rivac Technology bv, Waalre, Netherlands. The detector chamber is fitted with three thermometers: calibrated Cernox resistor used from 1.4 to 300 K, calibrated Ruthenium oxide resistor used below 2 K and Germanium resistor thermometer Model 5He3A (Scientific Instruments Inc., Mangonia Park, FL33407) used below 3 K. The resistance values are measured using AVS-46 and
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AVS-47 AC resistance bridges (RV.Elektroniikka Oy, Vantaa, Finland) and two Lake Shore DRC91C temperature controllers.
9. Operation Filling the cryostat’s nitrogen reservoir and the cold box nitrogen thermal shield with liquid nitrogen starts precooling to 77 K. At the same time, there is 1 bar of helium gas in the detector chamber and the 3He exchange gas in the IVC. It takes 13 days for the detector chamber to reach 124 K (Fig. 4). Then we start cooling the helium thermal shield in the cold box with liquid helium using a GF3 membrane pump and gas flow controller. It takes another 2.5 days for the detector chamber to reach 14 K with a helium gas flow of 12 l/min. After checking the temperatures in the cryostat to see whether they have reached thermal equilibrium with the detector chamber, we transfer liquid helium in the cryostat’s helium reservoir. The reservoir is filled in 40 min using 35 l of LHe to cool the cryostat and fill the 22 l reservoir. During this procedure the detector chamber reaches helium temperature. Then we pump IVC for 8 h to remove the 3He exchange gas. When the 3He-signal on leak detector falls below 106 mbarl/s, we start to pump the 1 K pot and slowly condense and circulate the helium mixture with the rotary pump. After all the helium mixture condensed, we start the Roots pump and apply 10 mW into the still heater to increase the circulation. It takes 4 h for the detector chamber to reach 115 mK.
10. Conclusion We have constructed and operated a cryogenic system, which can keep a SSG detector for dark matter search below 200 mK with temperature stability better than 75 mK for more than 2 months. Our experiments have demonstrated the cryogenic feasibility of a large SSG detector for the next-generation dark matter search.
Acknowledgements This work was supported in part by the Schweizerischer Nationalfonds zur Fo¨rderung der wissenschaftlichen Forschung. We are thankful to Dr. Jan Nye´ki for the critical comments to the manuscript and to Dr. Hajime´ Yoshiki for very useful technical information. References [1] [2] [3] [4] [5]
[6] [7] [8] [9] [10] [11]
Temperature (K)
350
150
[12] [13] [14] [15] [16]
100
[17]
300 250 200
50 [18]
0 0
2
4
6
8 10 12 Time (days)
14
16
Fig. 4. Cool-down curve of the detector chamber.
18 [19] [20]
F. Zwicky, Helv. Phys. Act. 6 (1933) 110. V. Rubin, Comm. Astrophys. 8 (1979) 79. Y. Mellier, Space Sci. Rev. 100 (2002) 73. D.N. Spergel, et al., Astrophys. J. Suppl. Ser. 148 (2003) 175. K. Pretzl, in: W. Greiner, et al. (Eds.), Proceedings of Structure and Dynamics of Elementary Matter, Nato Science Series, vol. 166, Kluwer Academic Publishers, Dordrecht, 2004, p. 289. P.F. Smith, J.D. Lewin, Phys. Rep. 187A (1990) 203. R. Bernabei, Riv Nuovo Cimento 5 (1995) 1. G. Jungman, M. Kamionkowski, K. Griest, Phys. Rep. 267 (1996) 195. L. Bergstro¨m, Rep. Prog. Phys 63 (2000) 793. D. Twerenbold, Rep. Prog. Phys 59 (1996) 349. N.E. Booth, B. Cabrera, E. Fiorini, Annu. Rev. Nucl. Part. Sci. 46 (1996) 471. K. Pretzl, Nucl. Instr. and Meth. A 454 (2000) 127. J. Gascon, Nucl. Instr. and Meth. A 520 (2004) 96. K. Borer, et al., Nucl. Instr. and Meth. A 520 (2004) 112. K. Borer, et al., Astropart. Phys. 22 (2004) 199. L.J. Salerno, P. Kittel, A.L. Spivak, Cryogenics 34 (1994) 649. P. Kittel, A.L. Spivak, L.J. Salerno, Adv. Cryogen. Eng. 37A (1992) 241. I. Didschuns, A.L. Woodcraft, D. Bintley, P.C. Hargrave, Cryogenics 44 (2004) 293. E. Gmelin, M. Asen-Palmer, M. Reuther, R. Villar, J. Phys. D: Appl. Phys 32 (1999) R19. A.L. Woodcraft, J. Phys. D: Appl. Phys 34 (2001) 2932.
ARTICLE IN PRESS S. Janos et al. / Nuclear Instruments and Methods in Physics Research A 547 (2005) 359–367 [21] J.D. Taylor, et al., Adv. Cryogen. Eng. 41 (1996) 1971. [22] P.D.Barnes, The cryogenic dark matter search, Ph.D. Thesis, The University of California, Berkeley, 1996. [23] H. Bernas, et al., Phys. Lett 24 A (1967) 721. [24] C. Berger, et al., Nucl. Instr. and Meth A 330 (1993) 285. [25] M. Abplanalp, et al., Nucl. Instr. and Meth. A 344 (1994) 239. [26] M. Abplanalp, et al., Nucl. Instr. and Meth. A 360 (1995) 616. [27] K. Pretzl, J. Low Temp. Phys. 93 (1993) 439. [28] K. Borer, M. Furlan, Nucl. Instr. and Meth. A A365 (1995) 491.
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[29] O.V. Lounasmaa, Experimental Principles and Methods Below 1 K, Academic Press, New York, 1974, pp. 251–252. [30] F. Pobell, Matter and Methods at Low Temperatures, Springer, Berlin, 1992, pp. 21–22. [31] Y. Fujii, K. Sakai K, M. Ogura, Cryogenics 33 (1993) 1010. [32] A. Savada, S. Inoue, Y. Masuda, Cryogenics 26 (1986) 486. [33] H. Ishimaru, H. Yoshiki, Cryogenics 31 (1991) 456. [34] P. Wertelaers, CERN Report 98-07, 1, September 1998. [35] S. Janos, F. Nydegger, Cryogenics 43 (2003) 463. [36] I.V. Velichkov, Cryogenics 30 (1990) 527.
Nuclear Instruments and Methods in Physics Research A 547 (2005) 359–367 www.elsevier.com/locate/nima
The Bern Cryogenic Detector System for dark matter search S. Janos, M. Hauser, K. Pretzl, F. Nydegger, H.U. Schu¨tz, S. Lehmann, M. Hess, J.C. Roulin Laboratory for High Energy Physics, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland Received 17 January 2005; received in revised form 15 March 2005; accepted 16 March 2005 Available online 16 May 2005
Abstract In this paper we report on the design, construction, and operation of a cryogenic system designed to cool a Superheated Superconducting Granule (SSG) detector for dark matter search. The volume accommodating SSG detector is 20 cm in diameter and 67 cm long. The system provides a lowest operating temperature of 115 mK for longlasting (several months) experiments in the Bern Underground Laboratory. r 2005 Elsevier B.V. All rights reserved. PACS: 95.35.+d; 7.20.Mc Keywords: Cold dark matter; Cryogenic detectors
1. Introduction The quest for dark matter is one of the most pressing challenges to cosmology and astro-particle physics. Its direct detection with earthbound detectors will provide decisive explanations of its true nature. Since its discovery by the Swiss astronomer Fritz Zwicky in 1933 [1] a lot more has been learned, mostly in recent years, about this ubiquitous matter, which gravitationally binds stars in galaxies [2], galaxies in clusters and in Corresponding author. Tel.: +41 31 6314065;
fax: +41 31 6314487 E-mail address: [email protected] (S. Janos).
large-scale structures [3]. From a number of astrophysical observations and from the cosmic microwave background radiation experiment WMAP [4], we have learned that the universe contains about 30% matter and 70% dark energy. Most of this matter is invisible and of exotic nature. In case the dark matter consists of exotic particles which interact weakly with ordinary matter, so-called weakly interacting massive particles (WIMPs), they should be detectable in principle. A recent review on direct dark matter searches can be found in Ref. [5] and references therein. WIMPs can be detected by measuring the nuclear recoil energy when one of these particles
0168-9002/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2005.03.155
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interacts with a nucleus of the detector material. The energy of the recoiling nucleus can be measured in several ways, e.g., as a small rise of temperature, electric charge liberation, or photon emission. To observe a WIMP, the detector must be capable of registering very small nuclear recoil energies below 1 keV [6–9]. In terms of sensitivity to very small nuclear recoil energies, cryogenic detectors promise significant advantages [10–12]. At temperatures near the absolute zero, the heat capacity is very small and the nuclear recoil energy can be transformed into a measurable signal. Several groups in the USA, Europe, and in Japan are searching for WIMPs employing different detection techniques [13]. One of these innovative detection systems has been developed also at the University of Bern [14,15]. The detector is called ORPHEUS in analogy to Greek mythology and is situated in the Bern Underground Laboratory 30 m below the Institute of Physics.
2. General consideration For a dark matter search it is of extreme importance to reduce the external and internal radioactive backgrounds. The experiment must be
carried out in an underground laboratory to suppress the radioactive background coming from the cosmic rays. In addition, it must be also locally shielded against the natural radioactivity of the rock and the radioactivity of the materials surrounding the detector. Therefore, it was necessary, to position the cold box containing the detector chamber (Fig. 1) as far as possible from the dilution cryostat, which is made from standard cryogenic materials (e.g., stainless steel, Al, Alfoils) with usually too high residual radioactivity for our purpose. The detector chamber and the cold box thermal shields (1.7, 4.5, 77 K) have been constructed using low activity materials (mainly OFHC Cu). Lot of effort has gone into the background measurements and into the construction of an adequate radioactive shielding respecting the dimensions of the laboratory furnished with vacuum, cryogenic, and electronic equipment. The measured fluxes of muons and neutrons in the Bern Underground Laboratory are 7.6 104 muons/cm2 s and 2 104 neutrons/ cm2 s, respectively. The neutrons are moderated and captured by the boron-doped polyethylene inside the lead shield. Thus, the neutron flux reaching the detector is estimated as 2 106 neutron/cm2 s. The passive shielding for the
Fig. 1. Vertical cross-section of the ORPHEUS cryogenic system.
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Fig. 2. Cold box and the passive radioactive shield.
ORPHEUS detector is shown on Fig. 2. It is divided into two closely fitting halves and each half is supported by a platform on rails. Looking from outside to inside it consists of the following materials: 2 cm scintillator veto counter (not shown), 15 cm lead, 4 cm of OFHC copper and 18 cm of 5% boron-doped polyethylene. All materials used in the shielding and also in the vicinity of the SSG detector (e.g., electrical connectors, wires, screws) were tested for their radioactive purity. Because of the limiting height of the Bern Underground Laboratory (3.2 m), the thermal extension contacts from the dilution cryostat to the cold box with detector chamber cannot be vertical, but horizontal. However, such horizontal extension requires the cooling power of the dilution refrigerator to be transferred through a very long (about 1.7 m) copper rod—cold finger— with a flexible mechanical connection to the mixing chamber. Similarly, the thermal shields connecting the dilution refrigerator cryostat and the side access, must be also flexibly connected to each other in order to compensate for the
vertical thermal contractions of about 5 mm relative to the horizontal side access during cooling to 77 or 4 K, respectively. In addition, the 4 K connections must also be leak-tight to separate the outer vacuum space (OVC) from the inner vacuum space (IVC) as IVC contains the helium exchange gas during the cooldown to 4 K. This was achieved using 28 membrane pairs of 316 L stainless steel bellows with dimensions +65/90 for the 4 K connection and +120/140 for the 77 K connection (Comvat AG, 9469 Haag, Switzerland). The bellows provide the flexibility required but are insufficient for heat transfer. The thermal connection between the horizontal side access and the vertical 4 K cryostat thermal shield is provided by a bundle of 98 Cu wires (RRR ¼ 180, +1.2 mm, l ¼ 10 cm). Such a thermal bypass was then fastened with stainless screws to the bellows copper flanges (Fig. 3) using a thin layer of Apiezon-N grease (Apiezon Products Ltd., London, UK) to improve essentially the contact conductance [16]. Before applying the Apiezon-N grease, both contact surfaces were polished with 600-grit emery paper, then
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Fig. 3. Thermal bypass mounted on the stainless steel bellows.
ultrasonically cleaned in acetone followed by alcohol and dry-blown in clean nitrogen gas. A similar thermal bypass was used also for 77 K thermal shield. From a practical point of view, Apiezon–N grease provides a very simple method to increase thermal contact conductance at helium temperatures. Previous work with gold coating [17] showed that the improvement in the thermal contact conductance was much below the magnitude realized by Apiezon–N grease. However, new results for the copper to copper joints with gold-plated surfaces below 400 mK [18], when linearly extrapolated to 1 K, are in disagreement with data from Ref. [17]. As far as we know, the copper to copper joints coated with a thin layer of Apiezon–N grease have not been investigated yet below 1.6 K [19,20]. The thermal connection for the 1.7 K thermal shield was made of 63 Cu wires (+1.8 mm, l ¼ 6 cm). The flexible thermal connection between the vertical cold finger (OFHC copper, +30 mm, l ¼ 32 cm) and the horizontal cold finger (OFHC copper, +20 mm, l ¼ 133 cm) was made of 93 Cu wires (+1.4 mm, l ¼ 10 cm). The ends of the
copper wires were electron beam welded to the bulk copper cylinders. One end of such connection was then welded to the horizontal cold finger and the other one was fastened to the vertical cold finger with four M6 screws. Our plan to replace the conventional readout system by a SQUID system installed in the same cold box and in the same detector chamber was a decisive one for choosing a rigid support between thermal shields and for the detector chamber instead of a swinging support system used in the Cryogenic Dark Matter Search (CDMS) collaboration [21,22]. However, the problem with a rigid support is rather high heat loads in comparison with Kevlar hanger loops used in the CDMS. Our experience from the nitrogen and helium cooling runs in 1996–1997 showed, that the heat conduction via copper sideaccess tubes with thermal bypasses alone were insufficient to reach the low temperatures required (about 300 mK). To solve this problem, we developed additional liquid nitrogen and liquid helium cooling systems for the cold box. This additional cooling allowed us to achieve an operating temperature of 115 mK on the ORPHEUS detector.
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3. Detector principle The idea to use superheated superconding granules as a particle detector goes back to 1967 [23]. The detection principle is based on the phase transition of the type I superconductor from the metastable superheated state to the normal conducting state. Interaction of an incoming particle (e.g., WIMP) within a superconducting granule produces quasiparticles, which spread over the volume of the granule. The quasiparticles lose their energy via electron–phonon interaction. The resulting temperature increase can induce such a phase transition into the normal state. Disappearance of the Meissner–Ochsenfeld effect allows the magnetic field to enter the normal conducting granule, and therefore the phase transition can be detected with a pickup loop around the detector. The detected signal is proportional to the volume of the granule and to the strength of the magnetic field. The detector threshold is given by the minimum deposited energy needed to raise the granule temperature from the operating temperature to the phase transition temperature. The sudden character of phase transitions of the granules (100 ns) allows for fast time correlation between SSG signals and those of other detectors (e.g., scintillation veto counters surrounding the detector). We operated the detector at as low as 100 eV threshold. Our detector consists of many billions of small grains (e.g., 30 mm diameter) embedded in a dielectric medium (teflon, paraffin, plasticine) with a volume filling factor of 10%. The detector is operated in the superheated superconducting state at a constant temperature of 115 mK and in an external magnetic field of 285 G. We studied the response of the SSG detector to nuclear recoil energies in the keV range by irradiating Sn, Al, and Zn SSG detectors with a 70 MeV neutron beam in experiments performed at the Paul Scherrer Institute in Villigen (Switzerland) [24–26]. Recoil energies down to 1 keV were measured. These experiments confirmed that SSG detectors can also be used to detect very small recoil energies in elastic WIMP-nucleus scatterings and encouraged us to develop a large SSG detector
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for dark matter search. A review of SSG investigations can be found in Ref. [27].
4. Detector The detector chamber (Fig. 1) is made of electroformed copper (produced by Steiger S.A., Vevey, Switzerland) to guarantee a low level of radioactivity. Inside the detector chamber (+12 cm, l ¼ 54 cm) there are supports made of Delrin (Polyoxymethylene) for 56 pickup coils. A signal coil has a length of 6.8 cm, a diameter of 1.8 cm, 1500 turns of 60 mm diameter high purity Cu wire, and contains 8 g of tin grains immersed in teflon powder (Goodfellow Cambridge Ltd., Cambridge, UK) with a 10% volume filling factor. The size of the pickup coil is limited by the required signal-to-noise ratio between 10 and 20. Each pickup coil is connected to a J-FET preamplifier working at room temperature [28]. In order to investigate the sensitivity on the grain size we filled one group of signal coils with smaller tin granules (+27.774.1 mm), and the other one with larger tin granules (+3672.2 mm). The total of target material in the detector chamber amounts to 448 g. The granules were produced by gas atomization at the Technical University Clausthal, Germany, and mechanically sieved in our laboratory using a sieving machine Type TS18-IG (SWECO Europe, Nivelles, Belgium). The granules are located in a an external magnetic field with a homogeneity better than 1.6% produced by a superconducting solenoid made from multifilament NbTi wire in copper cladding (SUPERCON, USA Type MR 24, Cu:NbTi ¼ 7:1, +0.4 mm, l ¼ 9.556 km, 19109 turns). The solenoid is wound on the high purity copper body which is directly thermally contacted to the detector chamber (Fig. 1) and is operated in high vacuum. In order to provide adequate thermal contact between tin granules and the detector chamber, the free space in the detector chamber (1 l) is filled with superfluid helium. Liquid 4He as a contact agent for granules below 1 K has great advantage because of its high thermal conductivity [29–31]. The main difficulty is that the detector chamber with 114 electrical feedthroughs and two
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condensation lines for 4He (each 14 m long) must be leak tight against superfluid helium.
5. Cooling systems To cool the detector chamber below 1 K we use a Kelvinox 300 dilution refrigerator (Fig. 1). The cryostat has two side-access ports near the bottom. One of these ports connects the side-access thermal shields (77, 4.5, and 1.7 K). The second port at the opposite side is used for IVC pumping. The liquid nitrogen bath has a volume of 36 l and is refilled automatically every 12 h using a Model 180 Liquid Level Controller with solenoid-operated fill valve (American Magnetics Inc., Oak Ridge, Tennessee, USA). The liquid helium bath has a useful volume of 22 l and is refilled automatically every 8 h using a Model 136 Helium Level Controller (American Magnetics Inc., Oak Ridge, Tennessee, USA). A Kelvinox IGH gas handling system and ITC 503 temperature controller (Oxford Instruments, UK) were used for monitoring the dilution refrigerator. The main operation parameters were displayed in real time on the Internet. 3He evaporating in the still is pumped through a 10 cm diameter tube by an Alcatel Roots pump RSV 601B backed by a Alcatel 3He rotary pump 2063 H. When the backing pressure of the 3He rotary pump rises above 800 mbar, both 3He pumps are switched off. The 3He gas is purified in two successive charcoal traps at 77 K followed by the liquid helium-cooled trap installed immediately in front of a dilution refrigerator in the cryostat [32] to absorb traces of air. It was necessary to regenerate this trap only after 1 month of continuous operation. The 3He flow rate is measured by a Teledyne–Hastings flow meter HFM-200 (Hampton, VA, USA). Typically 3 He circulation rate was 450 mmol/s when applying 10 mW into the still heater. The gas charge of the dilution refrigerator is 64 l of 4He and 16 l of 3He at NTP. Additional liquid nitrogen and helium cooling systems using a continuous flow cryostat mode and a liquid bath were developed for the cold box. These cooling systems for 77 and 4.5 K thermal shields used vacuum isolated transfer lines with needle valves (Janis Transfer lines Type SF-100,
Janis Research Company Inc., Wilmington, MA 01887) and a membrane GF3 gas flow pump (Oxford Instruments) to regulate flow through heat exchangers soldered directly onto the thermal shields. The heat loads to the 4.5 K (77 K) chambers were 0.4 and 28.5 W, respectively. This arrangement allowed us to maintain the 77 K shield continuously at 77 K for 11 days without changing the 200 l liquid nitrogen storage dewar. Likewise, the 100 l liquid helium storage dewar was changed after 30 h. Changing the liquid helium and liquid nitrogen storage dewars for additional cold box cooling has practically no influence on the temperature of the detector chamber.
6. Cryogenic seals The choice of superfluid helium leaktight cryogenic seals on demountable joints was primarily governed by requirements for low radioactive background due to their proximity to the detector chamber. Commercially available aluminium Helicoflex gaskets, which are leaktight against superfluid helium [33], require stainless steel nuts and bolts and have high radioactivity due to 60Co, whose beta decay yields two high energy g-rays. The aluminium contains 40K, which decays by electron capture yielding a high-energy g. Polycarbonate-based cryogenic seals have been examined at CERN [34] down to 4.2 K using flanges made of aluminium alloys (AlMg, AlMgSi) and the bolts made of stainless steel 304 L. No experiments have been performed with polycarbonates using the OFHC copper flanges and brass bolts to achieve very low residual radioactivity and tightness against superfluid helium. An all copper seal, e.g., CF-type, with a nose profile on the copper flanges and with a copper gasket [22], is mechanically rather complicated and not very appropriate for repeated assembling and disassembling. Another candidate, the conventional cryogenic vacuum seal gasket made from indium wire, is too radioactive for our purpose because of indium beta decay. We decided to use high-purity lead wire of 1 mm diameter (Puratronic grade, 99.998%, Alfa Aesar,
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Karlsruhe, Germany) as the gasket material. The joint is tightened with brass screws, because copper screws would break easily. Since the lead wire is considerably harder than indium one, it requires more pressure to get a reliable seal. However, it is difficult to apply large pressure on the seal using the brass bolts only. Therefore, for the start, we tightened the lead wire seal using stainless steel bolts. Then the seal was tested using a leak detector. When the test was satisfactory, the stainless steel screws were replaced by the brass screws. Throughout all this procedure the leak detector signal was monitored. This procedure allowed us to make very reliable seals leaktight even against superfluid helium.
7. Electrical wiring, electrical feedthrough, and thermal anchoring of wires (1) The wiring of the signal coils and the test pulse consists of 114 Cu wires (RRR ¼ 120, +100 mm) in twisted pairs running from room temperature to 4.5 K, 114 superconducting NbTi in CuNi matrix wires (Supercon, USA, Type SWM, +80 mm) in twisted pairs running from 4.5 K to the detector chamber, 114 Cu wires (+100 mm) in twisted pairs inside the detector chamber connecting the signal coils with the electrical feedthrough into the detector chamber. The wires enter the cryostat at room temperature flange through two 61-pin feedthroughs (Framatome, France, Type 851-02H24-61P50). These serial production high-vacuum connectors are designed for room temperature applications and are not suitable for low temperature because their flange is sealed with a rubber O-ring. We have used these connectors also at 77 and 1.7 K thermal shields, because at these positions vacuum tightness is not required (77 K thermal shield is in OVC and 1.7 K shield is in the IVC). There were, however, additional requirements for leak tightness on electrical feedthroughs mounted on the 4.5 K thermal shield, which separates the OVC and IVC and also for detector chamber, which contains superfluid helium. The urgent need for high-pin density electrical feedthrough for superfluid helium applications stimulated our effort to
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test different types of many pin connectors. We have found a simple modification of the Framatome connectors which enabled us to obtain very reliable high-pin density feedthroughs leak tight against superfluid helium [35]. (2) The wiring of the solenoid consists of one twisted pair of Cu wires (+0.4 mm) from room temperature to 4.5 K followed by one pair of single filament superconducting NbTi (+0.15 mm) wires without copper matrix (Supercon, USA, Type T48B-M) from 4.5 K to the solenoid. (3) For thermometry we used eight bundles of 36 AWG phosphor-bronze quad leads (Lake Shore, USA, Type QL-36) from room temperature down to the sensors. The resistances of the thermometers were measured by the 4-wire method (see below). (4) Thermal anchoring of wires: Twisted pairs of copper and NbTi (CuNi) signal wires were glued, using General Electric 7031 varnish (General Electric Company, Schenectady, USA), onto the OFHC copper cylinders with a diameter of 26 mm.The glued tempered length was 74 cm [36]. The cylinders were greased with Apiezon N grease and fastened with M5 screws to the flanges at 77, 4.5, and 1.7 K, respectively. In the same way, the solenoid current leads were glued to the OFHC copper cylinders with a diameter of 49 mm achieving the glued tempered length of 246 cm.
8. Thermometry The basic thermometers which we use for diagnostic purposes are 12 calibrated Cernox resistors model CX-1050-CU (Lake Shore Cryotronics Inc, Westerwille, OH43082), two calibrated Ruthenium oxide resistors (Oxford Instruments) and three calibrated Ruthenium oxide resistors from Rivac Technology bv, Waalre, Netherlands. The detector chamber is fitted with three thermometers: calibrated Cernox resistor used from 1.4 to 300 K, calibrated Ruthenium oxide resistor used below 2 K and Germanium resistor thermometer Model 5He3A (Scientific Instruments Inc., Mangonia Park, FL33407) used below 3 K. The resistance values are measured using AVS-46 and
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AVS-47 AC resistance bridges (RV.Elektroniikka Oy, Vantaa, Finland) and two Lake Shore DRC91C temperature controllers.
9. Operation Filling the cryostat’s nitrogen reservoir and the cold box nitrogen thermal shield with liquid nitrogen starts precooling to 77 K. At the same time, there is 1 bar of helium gas in the detector chamber and the 3He exchange gas in the IVC. It takes 13 days for the detector chamber to reach 124 K (Fig. 4). Then we start cooling the helium thermal shield in the cold box with liquid helium using a GF3 membrane pump and gas flow controller. It takes another 2.5 days for the detector chamber to reach 14 K with a helium gas flow of 12 l/min. After checking the temperatures in the cryostat to see whether they have reached thermal equilibrium with the detector chamber, we transfer liquid helium in the cryostat’s helium reservoir. The reservoir is filled in 40 min using 35 l of LHe to cool the cryostat and fill the 22 l reservoir. During this procedure the detector chamber reaches helium temperature. Then we pump IVC for 8 h to remove the 3He exchange gas. When the 3He-signal on leak detector falls below 106 mbarl/s, we start to pump the 1 K pot and slowly condense and circulate the helium mixture with the rotary pump. After all the helium mixture condensed, we start the Roots pump and apply 10 mW into the still heater to increase the circulation. It takes 4 h for the detector chamber to reach 115 mK.
10. Conclusion We have constructed and operated a cryogenic system, which can keep a SSG detector for dark matter search below 200 mK with temperature stability better than 75 mK for more than 2 months. Our experiments have demonstrated the cryogenic feasibility of a large SSG detector for the next-generation dark matter search.
Acknowledgements This work was supported in part by the Schweizerischer Nationalfonds zur Fo¨rderung der wissenschaftlichen Forschung. We are thankful to Dr. Jan Nye´ki for the critical comments to the manuscript and to Dr. Hajime´ Yoshiki for very useful technical information. References [1] [2] [3] [4] [5]
[6] [7] [8] [9] [10] [11]
Temperature (K)
350
150
[12] [13] [14] [15] [16]
100
[17]
300 250 200
50 [18]
0 0
2
4
6
8 10 12 Time (days)
14
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Fig. 4. Cool-down curve of the detector chamber.
18 [19] [20]
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