The L3 BGO electromagnetic calorimeter

252

Nuclear Instruments and Methods in Physics Research A265 (1988) 252-257 North-Holland, Amsterdam

T H E L3 B G O E L E C T R O M A G N E T I C

CALORIMETER

T h e L3 C o l l a b o r a t i o n Richard SUMNER

*

Department of Physics, Joseph Henry Laboratories, Princeton University, Princeton, New Jersey 08544, USA

The L3 detector at LEP will contain a unique electromagnetic calorimeter, consisting of 11488 scintillating bismuth germanate crystals, with silicon photodiode readout. The expected resolution is less than 1% for energies from 5 GeV to 50 GeV, rising to 5% at 100 MeV. The first part of the final detector (3840 crystals) is being assembled at CERN, and is to be calibrated in a test beam in the summer of 1987. The design of the calorimeter and its large dynamic range electronics are discussed.

1. Introduction

2. The B G O calorimeter

The L3 detector is unique among the four LEP experiments in m a n y respects. This is a direct result of the decision to measure leptons well, and not try to be a universal detector. A cutaway view of the detector is shown in fig. 1. The interaction region is surrounded by a precision drift chamber, out to a radius of 50 cm. This is a time expansion chamber, with an expected resolution of less than 50 /~m. This is surrounded by the electromagnetic calorimeter, from 50 to 85 cm radius. The electromagnetic calorimeter is probably the most unique part of the detector, and is the primary subject of this report. This is in turn surrounded by the hadron calorimeter, which extends to a radius of 2 m. The hadron calorimeter is made of depleted uranium plates, with proportional wire readout. The resolution is modest, about 50% over square root of E. The hadron calorimeter also functions of a m u o n filter. The final component is the m u o n chamber system. The m u o n m o m e n t u m will be measured to better than 2% at 50 GeV. This requirement determines the size of the solenoid magnet, 11.2 m inside diameter. The magnetic field is 0.5 T, produced by an octagonal aluminum coil. The total size, including the coil and iron flux return, is 16 m in diameter by 14 m long. The total weight is more than 8000 tons. The detector is built without provision for a garage position. As is true of all the large detector groups, the L3 collaboration is large. Currently, there are more than 300 physicists from 37 institutions. The BGO subgroup is also quite large, with major contributions from at least 14 of these institutions [1] (see the appendix).

The electromagnetic calorimeter uses bismuth germanate (BGO) as both the showering and detecting medium. Bismuth germanate is a high density, high Z, scintillating crystal [2-4]. Its properties are summarized in table 1. In many respects, BGO is the best material available for an electromagnetic calorimeter. The short radiation length produces a very compact shower, both longitudinal and transversely. It is capable of excellent energy and position resolution. There are some disadvantages. The light output is low, only 15% as much as Nal. The index of refraction is 2.19, making it difficult to get the light out of the crystal. The material is expensive, and growing large crystals is difficult. In/ fact, when the decision was made to use BGO, it was not possible to obtain crystals of the size and quality required for this detector, and there were no commercial suppliers capable of producing the large quantity of BGO needed. For

* Presented by R. Sumner. The institutes involved in the BGO subgroup of L3 are listed in the appendix. 0168-9002/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

Table 1 Some relevant properties of BGO Density Hardness Melting point Non-hygroscopic Nuclear interaction length Radiation length Moli~re radius Energy loss, d E / d X (minimum ionizing) Wavelength at peak emission Index of refraction (480 nm) Light output compared to NaI(T1) Light output decay time Temperature coefficient of decay time Temperature coefficient of light output

7.13 g/cm 3 5 Mho 1050 ° C 22 cm 1.12 cm 2.7 cm 9.2 MeV/cm 480 nm 2.19 15% 300 ns - 6.4 n s / ° C - 1%/° C

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R, Sumner, The L3 Collaboration / The L3 BGO calorimeter

EXr~FR~Mf~NTAL

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Fig. 1. A cutaway view of the L3 detector. The experimental hall is 40 m below the surface, just north of St. Genis, France.

this and many other reasons, building a large BGO calorimeter has proved to be a very challenging task. The electromagnetic calorimeter is shown in more detail in fig. 2. There are nearly 12000 BGO crystals, arranged as a barrel with endcaps. The barrel is split transverse to the beam into two halves, to allow calibration in an external beam line. The barrel extends to 42 o in 8, the endcaps down to 13 °. Each crystal is 24 cm long, and is a truncated pyramid, about 2 × 2 cm at the Hadron

CaloPtmeter

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Fig. 2. A longitudinal section of the BGO calorimeter.

inner end, and 3 × 3 cm at the outer end. The crystals are 22 radiation lengths long. The barrel section contains 7680 crystals, 48 in 0, and 160 in ~. The endcaps will contain another 3808 crystals for a total of 11488 in a single detector. A slightly separated forward section will extend the angular acceptance down to 5°. The entire calorimeter is only 1.7 m in diameter and about as long. This small size is important, as it determines the size of the hadron calorimeter, the muon chambers, and the magnet. The BGO is produced by the Shanghai Institute of Ceramics of The Chinese Academy of Sciences, PRC, using germanium oxide provided by ITEP, Moscow, USSR. This deceptively simple statement is the end result of several years of effort on the part of many of the members of L3. Much research has been devoted to the problem of growing pure crystals, measuring their properties, and machining them to the small tolerances required. Many potential suppliers were investigated, including the possibility of growing the crystals ourselves. The mechanical structure which supports the BGO crystals is made from a carbon fiber composite material VI. CALORIMETRY

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[5]. Each crystal is held in compression (the carbon fiber walls are in tension) in its own pigeon-hole in the structure. There are no bending forces on any crystal, all the weight is carried by the support structure. The wall thickness between crystals is only 200 #m, of low Z and low density material. The wrapping material (mostly paint) adds less than 100/~m, also low Z. The readout uses a photodiode as a detector, not a photomultiplier [6]. Since the BGO is in a magnetic field, and space is at a premium, the use of conventional photomultipliers is simply not possible, The photodiodes are made by Hamamatsu, and are a 1.5 cm 2 version of their s1790. These are insensitive to the 0.5 T magnetic field, and have a quantum efficiency of 0.8. Since they have no gain (each photon detected produces one electron-hole pair), an amplifier must be added. This is extremely stable however, and we do not expect any of the variations in gain that are normally experienced when using photomultipliers. Each crystal has two photodiodes glued to its rear face, for a total active area of 3 cm 2. The total diode capacitance is 230 pF. The preamplifier [7], which is mounted directly behind the crystal, uses a low noise, high transconductance FET, the Toshiba 2SK147. The noise level of the photodiode and preamplifier is about 1 MeV (o). This is much higher than can be obtained with photomultipliers, but significantly affects the resolution only for very small signals, less than 500 MeV. The ADC units (one for each crystal) are mounted 3 m away, just outside the hadron calorimeter. A simplified schematic of the analog part is shown in fig. 3. The ADC is designed to give high resolution over a very wide dynamic range. The signal from the preamplifier in split into a high and low range channel, each with a resettable integrator, and a sample-hold circuit. The low range has an additional gain of 32, before the integrator. After each beam crossing, the signal is integrated and stored by the sample-hold. Then the in-

tegrator is reset in preparation for the next beam crossing. The sample-hold (with the stored signal) is not released until just before the next beam crossing, allowing maximum time for the external trigger to operate. The sample-hold circuits are each followed by two amplifiers, each with a gain of 4. A single-chip, masked ROM, microcomputer chooses which of the six signals available to digitize, using a 12 bit DAC and six comparators. The least count (on the most sensitive range) is 5/~V, about 100 keV. Full scale (on the least sensitive range, no gain after the preamplifier) is 10 V, about 200 GeV. The digitizing resolution is at least 1:1000 for signals greater than 100 MeV. The linearity is better than 1% over the full range. The dynamic range achieved is 200 000 : 1, from full scale to the noise level. The microcomputers complete the digitizing and store the data within 300 t~s after the trigger. The microcomputers (one for each BGO crystal) are organized in token ring networks of 60 crystals, controlled by a master, which is a single-board computer in a VME crate. These are located more than 100 m away in the counting room. Communication is via differential TTL drivers and receivers. Readout by these higher level computers is done as a background task, since the microcomputers can buffer up to 20 events internally. This system allows a peak instantaneous event rate of 3000 triggers per second, and an average rate up to 500 triggers per second. The microcomputers also allow several system features to be added at the lowest level. Each crystal has an individual sparse scanning threshold, gain constant (for the analog output to the trigger system), trim constant (to set the pedestal value), test pulse enable, etc., all downloaded from the master computer. Each ADC can also measure the leakage current of the photodiodes and, if desired, the temperature of the BGO crystal. Several test modes are avalable to ensure the integrity of the system. All data has a parity bit added before it leaves the microcomputer.

TO TRIGGER I

Pore-Zero

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TO COMPARATORS

T

X32

X4

Fig. 3. The wide dynamic range ADC used for the BGO readout.

X4

R. Sumner, The L3 Collaboration / The L3 BGO calorimeter

The strong temperature dependence of the light output of BGO, about - 1 % / ° C, is a potential limit to the resolution. The BGO temperature will be monitored at several hundred places in the calorimeter, with a resolution of 0.1 ° C. The entire calorimeter will be stabilized in temperature to the same level of accuracy by a low pressure liquid freon cooling system. Another potential limit on the resolution is the light collection nonuniformity. This is a result of the tapered shape of the crystals, made worse by the high index of refraction. The efficiency of light collection is much higher for scintillation light produced near the small end of the crystal, compared to light produced near the large end (close to the photodiodes). For the crystal with all faces polished, the ratio is nearly 2 : 1. This can have a substantial effect on the resolution, since the beginning of the shower is variable. The length of the shower is also variable and is a function of energy. This can produce both a broadening of the resolution and a nonlinearity with energy. Our solution, after many trials, is to paint all faces of the polished crystal with a diffuse white paint, NE560, to a thickness of 40 ~tm. This produces a nearly uniform response, with an efficiency approximately the average of the extremes for the unpainted crystal. The remaining small departure from uniformity will be removed by calibrating the non linear response of each crystal in an electron beam at CERN.

3. Performance We have made many test-beam measurements over the past years, as the calorimeter and its readout system have evolved, from a 100 MeV electron beam at Cornell to a 50 GeV beam at CERN [8-13]. The most im-

6Z

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A Cornell

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........ I ........ P ........ r ........ Ol 1 10 100 ENERGY [GeVl

Fig. 4. Measurements of the resolution of test calorimeters, at several energies.

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portant property of the calorimeter, the energy resolution, is summarized in fig. 4. At 100 MeV the resolution is about 5%, and for all measured energies above 5 GeV the resolution is less than 1%. At 100 MeV our measurements are dominated by the electronic noise. Our resolution at 100 MeV is about twice that presented at this conference by the CUSBII group, whose noise level (using photomultiplier) is only 50 keV per crystal. It is interesting that the lowest resolution measurements (0.5% at 40 GeV) are limited by our knowledge of the beam energy, not by the BGO. At 50 GeV, the resolution is beginning to increase, due to an increaSein the rear leakage of the shower. These measurements were made under conditions quite different from the final detector, and with different configurations of BGO crystals, wrapping materials, photodiodes, and electronics. Nevertheless, they are a reasonable representation of the performance that we confidently expect from the final detector at LEP.

4. Calibration With such impressive performance possible, calibration and maintenance of the calibration at LEP, are extremely important. It is likely that the ultimate performance of the final calorimeter will be limited by our ability to calibrate it, and not any intrinsic limitations of BGO. The initial calibration will be done in a test beam at CERN, with each crystal individual calibrated at more than one energy. This will be done with each part of the final detector, after complete assembly. To this end, a fixture has been built that is capable of rotating the detector to place any crystal on the beam line. The effort required to calibrate each crystal is substantial, and there are nearly 12000 crystals in the complete detector. Maintaining the calibration will also be difficult. Fortunately we do not have to deal with the short-term gain variations of photomultipliers. As mentioned earlier, the photodiode and amplifier are expected to be quite stable, and only very slow long-term changes are anticipated as components age. The major cause of short-term gain change is expected to be radiation damage to the BGO. This has been a major research topic within L3, and the symptoms and causes are reasonably well understood [14,15]. BGO is self-healing at room temperature, with a time constant of a few days. Radiation exposures of less than a few rad per day will not affect our calibration. The expected exposures at LEP are very low for the entire calorimeter under normal running conditions. Only in the case of a complete beam loss on a nearby collimator will any crystals get more than 10 rad, and then only the crystals closest to the beam pipe. However we must be able to detect VI. CALORIMETRY

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R. Sumner, The L3 Collaboration / The L3 BGO calorimeter

even the slightest change in the calibration, for whatever reason, if we are to keep the resolution of 1% or better. We are planning to implement four different methods of monitoring the calibration. Bhabha scattering, the standard calibration method at low-energy colliders, will be possible only for the crystals nearest the beam pipe. For most of the detector the rate in any single crystal will be too low to calibrate that crystal in a timely way. Muons from cosmic rays are another source of calibration particles. Even though the detector will be 40 m below the surface, the cosmic-ray rate will be high enough that we need a set of scintillation counters to be able to reject them in the muon analysis. This scintillator barrel, completely surrounding the BGO, will enable a cosmic-ray trigger for calibration purposes. The rate will be high enough to enable a 1% calibration within a few days, although systematic problems may limit the accuracy. The energy deposit varies from 30 MeV for a transverse muon to 200 MeV for a longitudinal muon. The muon trajectory must be known precisely enough to determine the expected energy deposit in each crystal that the muon traverses. Each crystal will have two optical fiber-inputs, at the rear face of the crystal, from two independent xenon lamp light pulsers. With substantial effort, these light pulses have been stabilized to better than 0.5% and will cover a wide range of equivalent energy. This system will monitor both the photodiode and electronics response and the transparency of the BGO crystal, since the light detected must be reflected from the far end of the crystal. It cannot monitor the scintillation response of the BGO. This is a quick test, requiring only a few minutes to check all crystals. The system will be in place during calibration, and will assist in transferring the calibration from the test beam to the LEP environment. The final method being developed uses 17 MeV photons from a nuclear reaction. This requires a 1.5 MeV proton beam from a radiofrequency quadrupole accelerator (RFQ), and a target located near the center of the detector. The proton beam must be converted to neutral hydrogen, because of the magnetic field, and must have very low emittance, because of the long drift length ( > 5 m thru the detector). The resulting low-energy photons are absorbed and produce scintillation light in the first few centimeters of the BGO, so this will be sensitive to small amounts of radiation damage. This method appears promising as a monitor of the calibration, although systematic problems will make it difficult to push the accuracy below 1%. This will be as quick as the xenon lamp monitor, and has the advantage of actually testing the scintillation of the BGO. None of these methods can really calibrate the BGO in situ, but the combination of all four will allow us to detect changes in the BGO calibration, and compensate for some of them.

5. Conclusions The first half of the barrel section, containing 3840 crystals, is now undergoing final assembly at CERN. Calibration is scheduled for this summer in the X3 beam line at CERN. We expect to have the complete barrel portion in place at the start of LEP, with the endcaps coming within the next year.

Acknowledgement The role of the Princeton group in building this detector is mostly in the readout. We have designed and are building the ADC system and most of the higher level readout. Even though the BGO calorimeter is a small part of L3, it is a major detector in its own right and could not have been built by any single group. We are indebted to our colleagues in L3 for their efforts over the past several years in making this detector in reality. Appendix The BGO subgroup of the L3 collaboration consists of physicists and engineers from the following institutions: I. Physikalisches Institut, RWTH, Aachen, FRG, California Institute of Technology, Pasadena, CA, USA, Carnegie-Mellon University, Pittsburgh, PA, USA Central Research Institute for Physics, Budapest, Hungary, CERN, Geneva, Switzerland, DPNC, Universite de Geneve, Geneva, Switzerland, LAPP, Annecy-le-Vieux, France, INFN-Sezione di Roma and Dipartimento di Fisica, Universita " L a Sapienza", Roma, Italy, Institute of High Energy Physics, Academia Sinica, Beijing, PRC, Institut de Physique Nucleaire, IN2P3, Lyon, France, Institut de Physique Nucleaire, Universit6 de Lausanne, Switzerland, Massachusetts Institute of Technology, Cambridge, MA, USA, Princeton University, Princeton, N J, USA, University of Nijmegen, and N I K H E F , Nijmegen, The Netherlands.

References [1] The L3 Collaboration, L3 Technical Proposal, CERN (1983). [2] R. Nitsche, J. Appl. Phys. 36B (1965) 2358, [3] M.J. Weber and R.R. Monchamp, J. Appl. Phys. 44 (1973) 5496.

R. Sumner, The L3 Collaboration / The L3 BGO calorimeter

[4] Proc. Int. Workshop on Bismuth Germanate, ed., C. Newman-Holmes, Princeton University (November 1982). [5] M. Lebeau and C. Girard, The BGO Mechanical Structure, unpublished L3 internal report. L31M 191 & 252 (1983). [6] J.A. Bakken et al., IEEE Trans. Nucl. Sci. NS-31 (1) (1984) 180. [7] M. Goyot, Hybrid low power and low noise charge preamplifier for large capacitance photodetector (PAC-LP), unpublished L3 internal report, L31M 312 (1984). [8] J.A. Bakken et al., Nucl. Instr. and Meth. A254 (1987) 535.

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[9] J.A. Bakken et al., Nucl. Instr. and Meth. 228 (1985) 294. [10] H. Dietl et al., Nucl. Instr. and Meth. A235 (1985) 464. [11] U. Micke, PITHA 85f18, doctoral thesis, RWTH Aachen (1985). [12] D. Braun, AC-INTERN 84/03, diploma thesis RWTH Aachen (1984). [13] P. Lebrun, doctoral thesis, Lyon University (1986). [14] C. Laviron and P. Lecoq, Nucl. Instr. and Meth. 227 (1984) 45. [15] G.J. Bobbink et al., Nucl. Instr. and Meth. 227 (1984) 470.

VI. CALORIMETRY