The CUSB-II detector

Nuclear Instruments and Methods in Physics Research A263 (1988) 35-42 North-Holland, Amsterdam

35

Section H. Lepton identification and measurement THE CUSB-II DETECTOR Juliet L E E - F R A N Z I N I S U N Y at Stony Brook, Stony Brook, N Y 11794, USA

The properties and design philosophy of the first bismuth germanate electromagnetic calorimeter used in high energy physics are discussed. Presented also are the first results on the energy resolution obtainable under different background conditions.

1. Introduction

The properties and first performance of a high resolution bismuth germanate (BGO) electromagnetic calorimeter, CUSB-II [1], is described in the following. Fig. 1 is a perspective drawing of the detector seen through partially cut away counters used for muon trigger. Shown in the inset is an enlarged view of the inner portion of the detector composed of a BGO cylinder enclosing a minijet chamber around the beam pipe. The BGO cylinder is surrounded by an 8 radiation length ()%) square array of 328 sodium iodide (NaI)

Luminosity ~

L

e

a

d

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crystals, arranged in five radial layers, 32 azimuthal sectors, and two polar halves. Between NaI crystals are four proportional chambers with x and y cathode strip readout, used for tracking noninteracting charged particles. The NaI array is surrounded by four square arrays of 8 x 8 lead glass blocks 7~ 0 thick. Outside the lead glass, plastic scintillator counters cover four sides of the detector. They provide our dimuon trigger and give time of flight information for cosmic ray rejection. The section of the detector outside of the BGO cylinder is retained from the CUSB-I [2] detector. It had poor projective geometry and many gaps and cracks, due to its square geometry and the necessity of hermetically sealing the highly hygroscopic NaI crystals which also resulted in > 0.15~ 0 of inactive material between layers of NaI. Its longitudinal segmentation provided, however, excellent particle identification (hadrons vs photons and electrons) because of the five energy deposit ionization measurements. Radioactive sources between crystal layers allowed real time calibration during data taking, making it possible to achieve the theoretical 4

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resolution of the detector: OE= 3.9%/~/Ev(GeV). CUSB-I was the first calorimeter to explore the T energy region at CESR; it has the distinction of having discovered over ten T states in the past five years [3]. For the second generation of T physics experiments, which include the precision measurements of fine and hyperfine splittings, a spectrometer with improved photon resolution and perfect projective geometry is necessary. To accomplish these purposes requires more Xo'S, no inactive materials, while retaining the longitudinal segmentation and source imbedding features, occupying the available radial space 9 cm < r < 25 cm. These can be realized using bismuth germanate scintillating crystals [41. 2. CUSB-II geometry

Fig. 1. Perspectivedrawing of the CUSB-II detector. 0168-9002/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

One might be deterred from using BGO because at its lower light output relative to NaI, hence poorer II. LEPTON IDENTIFICATION

36

J. Lee-Franzini / The C U S B - I I detector

MATERIAL : BGO DIMENSIONS IN MILLIMETERS TOLERANCES : + .025rnm (-+.005 INCH)

DETAIL

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resolution for low energy 7's. However, its better material properties, including that of being nonhygroscopic, allow us to fit 122~0's of B G O with ideal geometry into the available ,space. Each element has a trapezoidal cross section' and subtends 10° in q~ and 45 ° to 90 ° (or 9 0 ° t o 135 ° ) in 0. Five such elements, of increasing size, form one ~ - 0 sector; the details of their mechanical specifications are listed in fig. 2. The complete B G O detector is comprised of 72 such sectors, for a total of 360 elements. Each crystal is viewed at one end by a miniature photomultiplier tube (p.m.), permanently glued on with clear RTV. All crystals are uniform, in light response, to better than 2% across their length (some after compensation, most as delivered from the Shanghai Institute of Ceramics). Teflon and aluminized mylar wrapping (less than three mils thick) maximize light output and remove optical cross talk. Fig. 3 shows one polar half of the B G O assembly (180 elements) viewed from the free crystal end. One notes 5 free standing concentric rings, constructed in a time

Fig. 2. Mechanical specifications of CUSB-II BGO crystals.

Fig. 3. One polar half of the BGO assembly, viewed from the free crystal end.

37

J. Lee-Franzini / The CUSB-I1 detector

Fig. 4. One polar half of the BGO assembly, viewed from the photomultiplier end.

honored (Roman Arch) method. Fig. 4 shows the crystal assembly viewed from the phototube end.

charge is -- 0.5 fC, or 3100 electrons, equivalent to 50 keV energy deposit (i.e. < 1 / 2 0 t h of the calibration source signal). At the line driver output the full scale differential signal is -- 1 V.

3. CUSB-II electronics Noise considerations require the readout electronics to be placed in close proximity to the detector elements. This requires a miniature " v o l t a g e divider, preamp, differential line driver" assembly which runs at very low power levels to reduce local heating. The physical size of this package relative to the phototubes and crystals is shown in fig. 5. A module composed of 11 q~ sectors, complete with electronic readout, is shown in fig. 6. Fig. 7 gives a schematic diagram of the signal flow and typical values, from phototube and charge preamplifier, through differential sending and receiving amplifiers. Photomultiplier voltages are typically 600 V and divider currents ~ 100 I~A. Peak and average anode full scale signal ( 2 - 4 GeV) currents are respectively 100 and 30 /IA. Long term anode average current is 10-100 pA. The equivalent amplifier noise input

4. CUSB-II calibration All C U S B calibration is done in real time during data taking with sources which are imbedded between crystal layers. In fig. 8 we show 13VCs (0.66 MeV) and 65Zn (1.11 MeV) spectra in one of our BGO crystals. The online calibration is accomplished by having a dual signal path, as shown in fig. 9, such that two different sensitivity channels measure collision events and source signals. The complete calibration cycle for all crystals is _< 30 min. We use for this purpose a (286/310) stand alone Intel microcomputer. All programs are written in Fortran, but the histograms are accumulated in memory by an 8089 Intel I / O processor. We have achieved _< 0.1% channel to channel calibration during a test run at C E S R (see next section) and can achieve -~ 1-2% II. LEPTON IDENTIFICATION

38

J. Lee-Franzini / The C U S B - I I detector

Fig. 5. One q~ sector, BGO unwrapped.

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GeV )

39

J. Lee-Franzini / The CUSB-11 detector

Fig. 6. Eleven assembled sectors, showing attached electronic readout. absolute calibration as checked with B h a b h a events a n d shower leakage calculations.

5. CUSB-II resolution 5.1. Bhabha resolution

In fig. 10 we show a first c o m p a r i s o n of the relative B h a b h a energy resolution between C U S B - I a n d C U B S II for --- 5 G e V electrons. Indeed the B G O array provided the expected i m p r o v e m e n t of a factor of two. A fit to this preliminary B G O s p e c t r u m yields a O E / E ~1.2% for the u p p e r half of the peak.

5.2. Photon resolution in light background environment

The Xb(2P) states were discovered by C U S B - I in 1982 [5] from observations of the p h o t o n s (71, 72) resulting from transitions between the T triplet S and triplet P states: 33S1 ~ 7123p2a,0 ---, 7xyzl(2)3Sa, The 71's were observed in our inclusive p h o t o n spectra, a n d b o t h "/1 a n d "/2 were observed in events where the final 3S 1 states decayed into a pair of m u o n s or electrons ( - = " e x clusive" events). W i t h C U S B - I we did not have sufficient resolution to resolve the J = 2 a n d J = 1 X b ( 2 P ) lines in either the inclusive p h o t o n spectrum or in the exclusive events. N e i t h e r were we able to see the 72's in the inclusive p h o t o n spectrum. II. LEPTON IDENTIFICATION

40

J. Lee-Franzini

/ The C U S B - I I detector

2 - 4 GeV

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Fig. 9. Schematic diagram of dual signal path for real time calibration. F r o m our run on the T(3S) in spring '86 with CUSB-II we obtained = 60 exclusive events where the final state T decayed into a muon pair. Note that the muons, which are minimum ionizing noninteracting charged particles, are identified by their energy loss, which is sampled 5 times in BGO, 5 times in NaI, and once in lead glass, for a total of 2.5 nuclear interaction lengths. The scatter plot of the high energy photon (Evhigh) versus the low energy photon (Eylow) for/~/~'Y3' events at the T(3S) peak is shown in fig. 11. The data

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cluster around 80-100 MeV for the lower energy photon, and either around 230 or 760 MeV for the higher energy photon, confirming their origin as being due to the cascade chain T(3S) --* Xb(2P)y ---, T(2S, 1S)y'y. Fig. 12 shows the energy spectrum of Eylow (130 > Ev~ow > 65 MeV) for the T(3S) ~ Xb(2P) ~ T(2S, 1S)7~' ~ ~#yy candidates, i.e. events for which 920 > E.chigh -I- E,tlow > 800 MeV, and 375 > Evhigh + Eynow > 250 MeV. The superimposed curves are fits using two Gaussians of 4

width OE= 1 . 8 % / ~ E v ( G e V ) with a fixed background of 0.2 event/2.5 MeV. The X 2 value of the fit is excellent (4.5/22 d.o.f.). We thus confirm that the detector resolution in a clean, low multiplicity ( N = 4 for these events) environment indeed improved by a factor of two with respect to CUSB-I. F r o m a physics point of view, it is gratifying to see the J = 2 and J = 1 lines fully resolved.

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J. Lee-Franzini / The CUSB-11 detector 5.3. Photon resolution in heauy background environment

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The inclusive photon spectrum situation is complicated by the large multiplicity contained in an T(3S) event• The photon search codes used are based on the CUSB-I photon algorithms which use longitudinal segmentation for identifying electromagnetic showers• Because of the improved resolution of the new calorimeter fine tuning is necessary to optimize the somewhat orthogonal requirements of high efficiency and resolution• Acceptance and the resolution function are then obtained by adding photons of fixed energies generated with " E G S " to real hadronic events. Fig. 13 shows the recovered peak for 100 MeV Monte Carlo (MC) photons and the resolution curve obtained. The preliminary acceptance x efficiency determined for photons of energy 100-200 MeV in decays T(3S) -* ~, + Xb(2P) ~ "f + hadrons is 10 + 2%, with OE/E = 3.9%, which is = 1.2 times worse than the optimal OE/E = 3.2% obtained from low multiplicity events (see last section). For photons of energy between 70 and 1000 MeV we find the rms energy spread function well described by o J E = 4

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.

In fig. 14 we show the inclusive photon spectrum obtained at the T(3S) peak energy, plotted in constant 3% Ev energy bins. It has several definite, distinct structures, standing out on top of a large background. The data was fitted with five resolution functions of the above mentioned, fixed energy dependence, but with resolution coefficient, area and position as free parameters, plus a polynomial to account for the background. The fit yields the coefficient in o z / E tO be (2.22 _+

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42

J. Lee-Franzini 2000

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/ The C U S B - H detector sigma effect). The partial resolution of the J = 2 a n d J = 1 lines, the complete resolution of the J = 0 line, a n d Xb(2P) to T(2S) transitions, are all seen for the first time in an inclusive p h o t o n s p e c t r u m from the T(3S). T h e line positions a n d E1 transition rates o b t a i n e d from this r u n are in good agreement with the published C U S B - I results.

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In conclusion, our first r u n with C U S B - I I fully vindicates our choice of B G O for the calorimetry. We recently installed a vertex c h a m b e r for tracking. We plan to insert a layer of silicon strips at the shower m a x i m u m for 0 shower centroid localising. We anticipate that C U S B - I I will m a k e significant c o n t r i b u t i o n s to the u n d e r s t a n d i n g of the spin d e p e n d e n c e of interq u a r k forces in the c o m i n g two years.

Fig. 14. The inclusive photon spectrum from e+e ---, hadrons at the T(3S) peak. Acknowledgements

peak at Ev of = 230 MeV, is actually the merged result of two signals of equal strength which are separated 15 M e V apart, a n d is due to the p h o t o n transitions from the Xb(2P) to the T(2S). The fifth peak falls at a n energy which corresponds to the Xb(2S) to T(1S) photon transitions a n d therefore is suggestive of this source, although it is not statistically significant (a barely two

The a u t h o r wishes to emphasize that C U S B - I I is a group enterprise, albeit a small group b y today's high energy physics standards, a n d t h a n k s the co-spokesperson P. Franzini a n d all collaboration m e m b e r s for the successful realization of CUSB-II. In particular, Drs. R.D. S c h a m b e r g e r a n d P.M. Tuts especially deserve credit for their leadership during its construction a n d installation, a n d Mr. T. Z h a o for m u c h work o n this project. C U S B - I I is s u p p o r t e d b y U.S. N a t i o n a l Science F o u n d a t i o n G r a n t s Phy-8310432 a n d Phy-8315800.

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[1] CUSB-II members include: M. Artuso, P. Franzini, P.M. Tuts, S. Youssef and T. Zhao of Columbia University; U. Heintz, J. Lee-Franzini, T.M. Kaarsberg, D.M.J. Lovelock, M. Narain, S.B. Sontz, R.D. Schamberger, J. Willins and C. Yanagisawa of SUNY at Stony Brook. [2] P. Franzini and J. Lee-Franzini, Phys. Rep. 81 (1982) 239. [3] See P. Franzini and J. Lee-Franzini, Ann. Rev. Nucl. Part. Sci. 33 (1983) 1, for earlier references; and J. Lee-Franzini, Physics in Collision V, Autun, France, eds., B. Aubert and L. Montanet (Edition Frontirres, France, 1985) p. 145, for recent references. [4] P.M. Tuts and P. Franzini, Int. Workshop on Bismuth Germanate, ed., C. Newman Holmes (Princeton Univ. 1982) p. 596. [5] K. Han et al., Phys. Rev. Lett. 49 (1982) 1612; G. Eigen et al., Phys. Rev. Lett. 49 (1982) 1616; C. Klopfenstein et al., Phys. Rev. Lett. 51 (1983) 160; F. Pauss et al., Phys. Lett. 130B (1983) 439.