The CUSB-II BGO calorimeter

Nuclear Instruments and Methods in Physics Research A265 (1988) 243-251 North-Holland, Amsterdam

243

THE CUSB-II BGO CALORIMETER P. M i c h a e l T U T S Columbia University, New York, New York 10027, USA

A description of the CUSB-II bismuth germanate calorimeter is presented. Details of the design philosophy are given, together with the performance characteristics. Energy resolution results obtained with 5 GeV Bhabha scattering events and with radiative photons from electric dipole transitions from the third excited upsilon state to the second excited P state are presented.

1. Introduction

In this report, I will describe the properties and performance of the CUSB-II detector [1], a bismuth germanate, or B G O (Bi3Ge4012), electromagnetic calorimeter. This high precision calorimeter has been in operation since late 1985 in the North Area of the Cornell Electron Storage Ring (CESR), where it is used in an ongoing program of high resolution studies of the upsilon meson started in 1979 [2]. The commissioning of this new detector marks the first use in high energy physics of B G O in an electromagnetic calorimeter. A1-

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though the original C U S B sodium iodide and lead glass calorimeter was very successful in its studies of upsilon spectroscopy (bound states of the b quark and its antiquark), the discovery of the second excited P state in the upsilon system pointed out the need for improved resolution in order to carry out precision spectroscopic studies of the fine and hyperfine structure in the bound state upsilon system. The three spin states of the 2P state were seen but not explicitly resolved in that first observation [3]. The general design philosophy behind CUSB-II was to eliminate the weaknesses of the original C U S B detector, while incorporating those features that made the detector so successful in upsilon spectroscopy. The original C U S B detector [4] consists of a 328 crystal array of thallium-doped sodium iodide, NaI(TI), which is 8 radiation lengths thick at normal incidence and arranged in a square array of five radial layers, 32 azimuthal sectors, and two polar halves. The upper half

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244

P.M. Tuts / The C U S B - I I BGO calorimeter

of fig. 1 shows the arrangement. In addition, there are four p r o p o r t i o n a l c h a m b e r s with x a n d y c a t h o d e strip r e a d o u t that are located between N a I layers. Surrounding the N a I are four square 8 x 8 arrays of lead glass (7 r a d i a t i o n lengths thick), which are used to capture a n d measure energy leakage from the N a I array. The outermost regions of the detector are occupied by an array of plastic scintillators that are used to trigger on d i m u o n events in upsilon decay, as well as provide timing inform a t i o n for cosmic ray b a c k g r o u n d rejection in d i m u o n events.

2. The B G O calorimeter T h e heart of C U S B - I I is a cylindrical array of 360 b i s m u t h g e r m a n a t e ( B G O ) crystals. This array is shown in the inset in fig. 1 in a partially cut away view. The reasons for choosing B G O in the array as shown are manyfold: (1) The B G O array overcomes the poor geometry of the N a I array; there are n o large cracks at 45 ° in the cylindrical geometry adopted for the B G O array. (2) There is m u c h less dead material s u r r o u n d i n g

Fig. 3. A single 10 o sector of the BGO array.

P.M. Tuts / The C U S B - l l BGO calorimeter

each BGO crystal vs the NaI crystals, since BGO is, unlike NaI, nonhygroscopic and thus crystals do not need individual hermetic seals. This dramatically reduces the amount of inactive material surrounding the BGO crystals and consequently improves the resolution. (3) The depth of the calorimeter (in radiation lengths) was increased for superior shower containment. (4) BGO has twice as short a radiation length (l.1 cm) as NaI, allowing the insertion of a new array between the beam pipe ( r = 8 cm) and the first Nal layer ( r = 25 cm) with minimum disturbance to the ongoing physics program. In addition, the important features of the Nal-lead glass detector were retained: (1) The radial segmentation which provides excellent particle identification (i.e. hadrons vs photons or electrons) is maintained at five layers. (2) The embedded radioactive sources on each crystal, which are crucial to the crystal energy intercalibration system. All of the above requirements can be met by using bismuth germanate scintillating crystals [5].

245

Each BGO crystal has a trapezoidal shape which subtends a 10 ° angle in the q~ direction, and covers from 45 ° 90 ° in 0 (or equivalently from 90 ° to 135°). There are a total of five different shapes, as shown in fig. 2. The 360 crystals have a total volume of 38.0 liter; the majority were manufactured by the Shanghai Institute of Ceramics (with others from Japan Optical (NKK), Crismatec, and Harshaw Chemical). A group of five crystals comprise a 10 ° sector (see fig. 3) where nominal 2 mm gaps accommodate the 5 rail crystal tolerances, as shown in fig. 5. Note the enlarged gap (4 ram) after the inner layer of BGO; it is reserved for a shower centroid detector made of silicon pads. The crystals are then assembled in position, around the beam pipe, by building up each layer into free standing concentric rings (much as one would build a Roman arch). A view showing half of the full array (180 crystals) from the perspective of the interaction point is shown in fig. 4, and from the photomultiplier end in fig. 6. Readout of the crystals is accomplished with photomultiplier tubes. The outermost two layers (planes 3 and 4) are read out with a 1 in. diameter, 10 stage tube

Fig. 4. The BGO array viewed from the point of view of the interaction region. VI. CALORIMETRY

246

P.M. Tuts / The CUSB-11 BGO calorimeter

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(Hamamatsu R1924), the next two layers (planes 1 and 2) with a 3 / 4 in., 6 stage tube (Hamamatsu R1666), and the inner layer (plane 0) with a 1 / 2 in., 6 stage tube (Hamamatsu R1665). The tubes are permanently attached to the crystals with clear RTV, and the electronics (voltage divider, preamp, and cable driver) is mounted on the phototube, as shown in figs. 3 and 7. Each crystal is wrapped in teflon tape (2 mil) and aluminized mylar (½ mil) for increased light collection, optical isolation and uniformity control. The crystal pulse height nonuniformity ranges up to 5% as received from the manufacturer (typically peaked at both ends of the crystal), but is individually compensated to 1% by adjusting the teflon wrapping. The result of this compensation is shown in fig. 8. In order to achieve the optimal resolution possible with the CUSB-II detector, we require a precision cross calibration of all crystals. This is achieved by placing radioactive 6SZn (1.1 MeV) sources on each crystal, and monitoring the peak position of the source peak in each crystal. The signal path for this calibration system is shown schematically in fig. 9; the phototube signals are transmitted to the "normal" data taking ADCs, while they are simultaneously "picked off" through a non-

Fig. 6. The BG0 array as viewed from the photomultiplier tube end.

P.M. "ruts / The C U S B - H BGO calorimeter

247

Fig. 7. Part of the BGO array being cabled up. loading high gain path (about × 100) and multiplexed into a single reference 16 channel "calibration" ADC. The data are then processed in a stand-alone Intel 286 based system (histograms are accumulated in memory via an Intel 8089 I / O processor). A typical source calibration peak for a BGO crystal is shown in fig. 10a and b, where an average of 5000 source events are collected in the peak, with a 0.1% channel to channel calibration. The absolute energy calibration has been checked with Bhabha events and is found to be a few percent. The gain variations observed by the calibration system are typically in the order of 1-2% over a period of days. These are readily tracked by the calibration system, providing a relative crystal normalization good to less than 1%.

We have also used the measured calibration peak widths to monitor the possible effects of radiation on the performance of the crystals. In the 1.5 years that the crystals have been installed, we have observed no degradation in light output, or equivalently resolution, in any crystals. This is illustrated in fig. 10 b, which shows the calibration spectrum for the same crystal over a 6 month period. No significant change in resolution is observed.

3. B G O readout electronics

As can be seen in figs. 3 and 7, we have tried to miniaturize the electronics and minimize possible noise VI. CALORIMETRY

248

P.M. Tuts / The CUSB-II BGO calorimeter

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pickup by placing the electronics as close as possible to the PMT. The close-packed electronics requires low power in the voltage divider, preamp, and cable driver. A schematic view of the electronics chain is shown in fig. 11. The photomultipliers are run at 600 V with typical divider currents of about 100 #A. The equivalent

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The energy resolution at high energies (approximately 5 GeV) can be measured with Bhabha scattering events. In addition, the energy resolution is a stringent test of the relative crystal cross calibration at high energies. The relative energy resolution obtained for these Bhabha events in the original C U S B N a l - l e a d glass detector has an rms of 2.6% and is shown in fig. 12. Over a factor of 2 improvement in resolution is realized in the BGO detector, where an rms resolution of 1% is observed (see fig. 13), using the upper half of the energy distribution. The effects of the relative crystal cross calibration are evident in fig. 14, where, instead of using all Bhabha events, we limit ourselves to Bhabhas that point into a single direction in the detector (i.e., many fewer crystals are used in the resolution calculation). In this case the resolution is further improved to about 0.8%; we continue to work on and improve the cross calibration measurements. A check of the energy resolution at lower energies (around 100 MeV) is provided by the electric dipole

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photon transitions from the T(3S) state to the 2P state. We observe these photons in the exclusive decay mode T(3S) --* y + T(2P) ~ 7 + + yT(1S or 2S) -+ 7 + 7 +/~+ + / t - . These photons were first seen with the CUSB-I detector in 1982 [3], but they were not explicitly resolved. In the first preliminary analysis of our last run on the T(3S), we have observed about 100 such events. The scatter plot of the two photon energies is shown in fig. 15; note the clear clustering of events that have low energy photons at about 100 MeV, corresponding to the E1 photons from the T(3S) to the T(2P); the high energy photons cluster at two values corresponding to El photon transitions from the T(2P) to the T(1S) (with energies of about 760 MeV) or to the T(2S) (with

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energies of about 230 MeV). Projecting the distribution onto the low energy axis (for events where the sum of both photon energies lies between 800 and 920 MeV or between 250 and 375 MeV) yields the result shown in fig. 16. A very clear separation of the two photon lines is observed, with one at 85 MeV and the other at 100 MeV. (The third spin state line, corresponding to transitions to the J = 0 line, is expected to be suppressed because of its large hadronic width.) The curves shown correspond to Gaussians with widths of 1.8%/[ E ( G e W ) ] 1/4 added to a fixed background of 0.2 events per 2.5 MeV. The chi-squared value of the fit is excellent (4.5/22

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5. Conclusions In summary, our first run with CUSB-II fully justifies the use of B G O for calorimetry. We have achieved over a factor of 2 improvement in energy resolution over the original C U S B NaI and lead glass calorimeter, and thus have been able to explicitly measure the fine structure of the upsilon system• Our future plans include the addition of a new tracking chamber, with a reduced beam pipe diameter of about two inches, and about six layers of tracking. We are also going to install a shower centroid detector after the first layer of BGO, using silicon pads which will provide the necessary z information on the shower centroid. Finally, we are investigating the possible addition of end caps to recover the solid angle•

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CUSB-II is the result of a dedicated small group of individuals that make up the C U S B collaboration. I would particularly like to thank our spokespersons J. Lee-Franzini and P. Franzini, without whose efforts there would be no CUSB-II, and R.D. Schamberger and T. Zhao without whose efforts there would be no B G O array.

P.M. Tuts / The CUSB-11 BGO calorimeter

References [1] The present memebers of the CUSB-II collaboration are: M. Artuso, P. Franzini and P.M. Tuts 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] For a list of references see P. Franzini and J. Lee-Franzini, Ann. Rev. Nucl. Part. Sci. 33 (1983) 1;

251

J. Lee-Franzini, Physics in Collision V, Autun, France, eds., B. Aubert and L. Montanet (Editions Frontirres, France, 1985) p. 145. [3] K. Han et al., Phys. Rev. Lett. 49 (1982) 1612; G. Eigen et al., Phys. Rev. Lett. 49 (1982) 1616. [4] P. Franzini and J. Lee-Franzini, Phys. Rep. 81 (1982) 239. [5] P.M. Tuts and P. Franzini, Int. Workshop on Bismuth Germanate, ed., C. Newman-Holmes (Princeton University Press, Princeton, NJ, 1982) p. 596.

VI. CALORIMETRY