The Fermilab E835 scintillating fiber detector

ELSEVIER

Nuclear Physics B (Proc. Suppl.) 78 (1999) 479-483

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The Fermilab E835 Scintillating Fiber Detector M.Ambrogiani ~, W.Bal(tini ~, D.Bettoni ~, R.Calab,'ese ~, E.Luppi ~, R.Mussa ", and G.Stan(:ari ~ ~INFN and Dit)artimento di Fisi(:a, Universitg di Ferrara, 44100 Ferrara(FE), Italy A cylindrical scintillating-fiber tracker for Lit(: measurement of the polar coordinate 0 has been built h)r experiment E835 at Fermilab.This detector combines Lit(: high gramflarity, flexibility and fast response of the scintillating fibers with the high quantum efficiency of the Visible-Light Photon Counters (VLPC). Complete results about the tracker t)erformance are given: tra(:king and timing resolution, photoelectron yieht per mit). detection efl3(:iency,singlc-doubh~ track discrilnination.

1.

INTRODUCTION

Expe,'iment E835 studies the dire(:t forination of (:harmonium IlleSOIIS iIl antiproton-proton annihilations ([1], [2], [3], [4]). The antiproton t)eam is stored and stochastically (:ooled in the Fermilat) Antit)rot, on A('(:nlnulator, where it, interse(:ts a gaseous hydrogen jet-target in a region of al)()llt 5 X 5 >( 5 IilIll 3. Tile E835 dete(:tor is designed to observe ('harmoniunl decays to final states containing electrons, positrons or photons. The (:entral detector is symmetrical around the beam axis and covers tile polar angle fI"oiil 15 ° to 65 °. Its main COlnt)onents are an inner tra(:king system, a tilresilold Cerenkov counter and a leadglass electromagnetic calorimeter. Part of tim inIlel" tracking system is a scintillating ilt)er detector for the ineasllrenmnt of tile pola,' angle 0. Due to tile V L P C ' s fast response this detector (:an be used in the first-level trigger logi(:. The homogeneity of tile response makes tile anah)g readout of ea(:h channel usefifl for pulse-height analysis.

2.

DETECTOR

DESIGN

This cylindrical detector is used t,o measure I;he polar coordinate 0 in tile region between 15 ° and 65 °. Details regarding the fibers (Kuraray SCSF3HF-1500), tile (:ylindri(:al sut)ports , the VLPCs (HISTE-V, EOC-h)w), the cryostat and the readout electronics can be iound in [5] and [6]. Eacil scintillating fiber (core diameter = 740 l,m) is wound around one of t,he two support cylinders (radii 144.0 m m and 150.6 inin), on tile

surfa(:e of which a set of U-shat)ed grooves has been inachined (pit(:hes 1.10 mm and 1.15 ram). Tile det)th of tile grooves varies linearly with the azimuthal coordinate qS, so that tile fibe,' overlaps itself afl;er one tllrIl. OIl oIle. end, the fibers are aluminized, to increase the light yield and improve its honmgeneity; on the other end, they are t,herlnally spliced to clear fibers, which, afte,' a fimr-Ineter path, bring the light to tile surface of the VLPCs, housed in a c,'yostat and kept at a t e m p e r a t u r e of 6.5 K. Tile electronic signal generated by the VLPCs is amplified by QPA02 cards, designed at Fermilab. After tile amplification stage, the signal is sent to discrilninator-OR-splitter modules, cusloin made in Ferrara. These inodules provide all analog and a digital output for each input (:hannel, together with the digital OR of all intmts. Tile analog signal is tilen sent to ADCs, while tile digital outtmt is read out by latches. For trigger pu,'poses the dete(:tor is subdivided into 19 polar angular regions, consisting of a set, (bundle) of adjacent fibers. Tile digital OR of t,ile signals from each bundle is sent to T D C s and to t,ile first-level trigger logic of tile experiment. The design parameters imve been derived f,'oin Monte Carlo cah:ulation. Fig. 1 simws the nuln})eI" N (.)f f i b e r s (SlliilIile(t oveI" b o t h layers) whose core is intersected by a straight tra(:k originating froill the interacti(m vertex as a flm(:tion of the polar angle 0. The dots represent tim average over an arbitrary 0 slice, while the error bars indi(:ate tile minimum and m a x i m u m possible values. In order to maximize its geometrical acceptance,

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M. Ambrogiani et al./Nuclear Physics B (Proc. SuppL) 78 (1999) 479-483

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Figure 1. (a) Detector efficiency from Monte Carlo, (b)Nuinber of hit fibers in tile detector as a flmction of the polar angle 0.

tile detector has been designed so that the efficiency is better than 99% in average and better than 90% in the backward O region, as shown in Fig.la. Tile average nulnber of hit fibers as a flmction of 0 is also shown in Fig.lb. 3. D E T E C T O R

PERFORMANCES

3.1. C a l i b r a t i o n Tile signal generated by a track crossing one fiber, as seen at the input of the discrilninatorOR-splitter modules, is typically 180 inV high and 80 ns wide, corresponding to a collected charge ~ 0.2nC. In order to get, for each VLPC channel, the one-photoelectron (piLe) equivalent in ADC counts (1 ADC count = 0.25 pC), we petgormed an LED test on all channels with the setup actually used in the experiment (VLPC cassettes, cryostat and readout electronics). The pulse charge ill ADC counts generated by a Ininimuln-ioLfizing particle (nfip), instead, ix obtained studying a high-statistics hadronic sample of trench-through tracks in tile ele(:tronlagnetic calorimeter (~ 10 a events/fiber). The distritmlion of this variable shows an excellent homogeneity (mean ~ 400 ADC/inip, standard deviation ~ 12%). Tile nlean nlLint)er of photoelectrons per Inip turns out to be distritmted around 14 phe/mip. More details can be found in [5] and [6].

3.2. E f f i c i e n c y a n d R e s o l u t i o n We measm'ed tile detection efficiency by using e+e - tracks coming fl'om J/'~/, and '¢' decays (~ 4.104 events over the whole data-taking). For each track, we look fin' an associated hit in the scintillating fibers detector above a given software threshoht (typically, 0.2 mip), within a polar window of 4-50 mrad; the results are shown in fig. 2. Variations of the efficiency over tilne are due to different run conditions (gate width of the ADCs) and to loss of channels (see below). The scintillating fibers detector is by far the one with tile best spatial resolution ill our appal'atus. For this reason, we can only Ineasure what we call 'intrinsic' tracking resolution, i. e. the standard deviation (divided by v'~) of the distribution of tile differences OINN -- OOUT, where, for a given track, OINN (OouT) is tile polar angle measured b y tile inner (outer) layer. The resolution, averaged over all polar angles, comes out to be (0.7 4- 0.1) mrad, as shown in fig. 2.

3.3. T i m i n g R e s o l u t i o n Ill order to evaluate the intrinsic tilne resolution of the detector, we select those tra(:ks that hit at least two fibers behmging to adjacent bundles, and are therefore read out by different T D C chalmels. We then define tile tilne resolution as the standard deviation, divided by v/2, of tile distribution of the variable ti - ti+l, when the salne

M. Ambrogiani et al./Nuclear Physics B (Proc. Suppl.) 78 (1999) 479-483

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track (:rosses b o t h a fiber of bmMle i and a fiber of bundle i + 1 (i = 1, 18). This intrinsic time resohltion (:omes out t<) be around 3.5 ns, as can be seen in fig. 3; it is m o s t l y due to the decay time of the s(:intillator.

3.4. First-level Trigger Given the excellent timing t)erformmme of tile dete(:tor in terms of b o t h fast rest)onse and intrinsic time resolution, a fast first level trigger logi(: based on t,he s(:intillating fiber tracker has t)een designed. Tile goal is to observe tile decay of tile rl~(liSo) and 'r/~(2iSo) into ¢ ¢ ~ K + K - K + K and tile interferen(:e [)etween resonalit and elastic PI) filial states near 90 ° in tile C M frame. Coin(:iden(:es between sets of fiber bundles can sele(:t the right polar kinemati(:s, whereas the hodos(:opes provide azimuthal information. This first-level sele(:tion yields affordable trigger rates: 20-110 Hz fl)r the ~p trigger and 600-1000 Hz for the ¢(b trigger, as the C M energy varies between 4.2 GeV and 2.9 GeV.

3.5. Reliability of the Equipment T h r o u g h o u t the run, the eryostat worked very etfi(:iently. Tlle liquid heliuln flux was constant, and the t e m p e r a t u r e stable (6.500 4- 0.004 K). Four inevitable warm-ut)s o c c u r r e d , due ix) s(:he(t-

uled slmtxh)wns or to t)ower outages; the temt)erature of the cryostat rose tyt)ically ut) to ~ 70 K in a t~w horn's. T h a t was the main (:allse of loss of channels, a t t r i b u t e d to the t)oor quality of tile nfiero-welding joining tile V L P C substrates to the eleetri(:al cables. Fore" (:hannels out of 860 were not working at the beginning of tim run; at the. end, we had almost eighty dead (:hannels.

3.6. Pulse-Height Analysis A kind of ba(:kground particularly i m p o r t a n t fin" E835 is e+e: - pairs with small opening angle, generated by t)hoton conversions or by Dalitz de(:ays of neutral i)ions, simulating a single tra(:k. Tile tools t h a t we use off-line to reje(:t this ba(:kground are tile t)ulse height in the hodos(:opes, t;he signal in tile threshoht Cerenkov counter and tile energy and shat)e of the showers in the (:eiltral (:ah)rimeter. T h e s(:intillating fiber detector can add two tools: pulse height and granularity. W h e n the ot)ening angle of t,he e:+e:- pair is so sinall t h a t just one (:luster (defined as a set, of ad.ia(:ent, hit fibers) is generated, the energy deposit is likely to be big; whereas when the pair's separation is big enough, an extra cluster in t,he dete(:t, or will appear. In fig. 4 ()tie can sue the discrimination, based on charge det)osition only,

M. Ambrogiani et al./Nuclear Physics B (Proc. Suppl.) 78 (1999) 479-483

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between a clean sainple of single tracks (electrons or tmsitrons) and a satnt)le of pairs, defined as charged tracks which, combined with a neutral deposit in the central calorimeter, form a ~.o invariant mass. Q is the sum of the charges deposited in all the clusters in a 4-2 ° window centered a r o u n d tile track.

3.7.

Rate

effects

During the characterization of the devices, rate effects on tile V L P C s were observed on the test stand, when all devices were simultaneously irradiated with a L E D som'ce. T h e final answer on all possible rate dependent effects could only come froln a real r m m i n g ext)eriment. Fig.5a shows the rate of tracks per fiber (i.e. per V L P C pixel) as a fllnction of the polar angle 0, with respect to tile beain axis, in the L A B frame. The minim u m bias rate per fiber is constants between 15 and 40 degrees, at b o t h high and low CM energy. We analyzed d a t a froin a set of runs not affected by changes in the readout chain (therefore using only 1 calibration constant), taken at b o t h high and low L. Tile average signal dependence from L of fibers in this angular range is shown in Fig.Sb, fl'onl L = 0.5 x 1031c,m,-2s -1 (upt)er curve) to

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L = 2.5 × 1031c?7~.-2.s-1 (lower curve). T h e excellent honlogeneity ahmg tile fiber length (the average signal variation from the far to tile Ileal" end - left to right in the figure - is ~ 3.5%), is due to the high reflectivity of tile mirrored end, as well as the high a t t e n u a t i o n length of the scintillating

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An estimate of tile crosstalk between neighboring pixels (:an be given extfloiting tile VLPCs (:hannels that are not directly coupled to scintillating fibers. Most of these spare pixels are adjacent to chalmels actually coupled to fibers. Sele(:ting events having one or both neighboring pixels hit by real mit)s , we estimate an average (:rosstalks level about 1%.

2.

3. 4. C O N C L U S I O N S

4. We t)resented tile results of tile first detector used in a high-energy-physics experiment which exploits scintillating fibers and VisibleLight Photon C(mnters. It, is a mfique (tete(:tor, because it combines a high tracking performan(:e (resolution < 1 mr•d, dete(:tion etficien(:y 99.5%), fast first-level trigger characteristi(:s (time spread< 5 ns) and single/double-tra(:k dis(:rimination power. REFERENCES

1. Pastrone N. (for the E835 Collaboration),

5.

6.

"New Results fi'()m E835 at Fernfilab Antiproton A(:cunmlator Study of (c~) States fornmd in Antiproton-Proton Annihilations 'l, t)resented at the 4th International Workshop on Progress in Heavy-Quark Physics, Rostook, GerInany, September 20 22, 1997. Zioulas G. (for the E835 Collaboration), "First Results on Charinoniuin Spectroscopy fi'om Fermilab E835",i)resented at the 7th International Conference on Hadron Spectroscopy, Brookhaven National Laboratory, Ut)ton, New York, August 25 30, 1997. Cester R. and Rapidis P. A., Ann. Rev. NucL Sci. 44, 329 371 (1994). Armstrong T. et al. (E760 Collaboration), Ferinilab Proposal P-835-REV (1992). Luppi E., "Tile E835 Scintillating-Fiber Tracking Dete(:tor '11 t)resented at tile 5th International Conference on Advan(:ed Te(:hnologies and Particle Piwsics, Villa Ohno (CO), Italy, October 1996. Ambrogiani M. et al., I E E E Truns. Nucl. Sci. 441 460 463 (1997).