Performance of a BGO calorimeter below 1.6 GeV

Nuclear Instruments and Methods in Physics Research A248 (1986) 309-316 North-Holland, Amsterdam

PERFORMANCE

OF A BGO CALORIMETER

BELOW

309

1.6 G e V

J u n ' i c h i r o I W A H O R I l), T a k u y a K A M I T A N I 2), M a s a a k i K O B A Y A S H I R o b e r t E. L A N O U 2) , , Y o r i k i y o N A G A S H I M A 2), T s u n e h i k o O M O R I M a s a h i k o U E D A 2) ** a n d H a j i m e Y O S H I D A 1)

3), S h i n ' i c h i K U R O K A W A 3), 2), S h o j i r o S U G I M O T O 2,3),

1) Faculty of Engineering, Fukui University, Fukui, 910Japan 2) Department of Physics, Osaka University, Toyonaka, 560 Japan k~)"KEK, National Laboratory for High Energy Physics, Oho-machi, Tsukuba, Ibaraki, 305 Japan Received 21 October 1985 and in revised form 24 February 1986

An array of 7 bismuth germanate (BGO) modules, each being 17 or 20 cm long and having a hexagonal cross section with 50 mm across the parallel edges, has been tested with 0.2-1.6 GeV/c electrons and pions. The fwhm energy resolution for 1 GeV electrons was 4.1% in agreement with the EGS calculations. The e/~r separation for momentum analyzed particles at 1 GeV/c was 1/500 with only the energy cut, and was improved to 1/2000 with an additional cut in the lateral energy spread. Finally a calorimeter has been constructed of 10 BGO modules surrounded with Nal slabs and successfully employed in KEK Experiment E68 to measure the "r ray energy spectrum in ~p annihilation at rest. A fwhm resolution of 10% was obtained for 129 MeV y rays.

1. Introduction In recent years, calorimeters made of bismuth germanate (BGO: Bi4Ge3012) crystals have shown great advances [1] in several areas; for example, in growing transparent crystals, photodiode readout, understanding of radiation damage, etc. A large scale B G O calorimeter is under construction by the L3 collaboration for the use in a detector at LEP. Although many B G O detectors have been tested, the actual usage of them in high energy physics experiments has been few. In preparation to use a B G O calorimeter in an actual high energy experiment, we have constructed a small scale B G O calorimeter as sketched in fig. 1. The calorimeter consists of 10 B G O modules, each having a hexagonal cross section. The B G O occupies a total cross sectional area of a b o u t 14 × 20 cm 2 and is surrounded with 6.5 cm thick NaI walls which should measure the leakage energy from the BGO. The B G O calorimeter was used at the K E K 12 GeV Proton Synchrotron in Experiment E68 (PPC) [2], designed to search for baryonia by detecting monochromatic 3' rays or ~r°s in ~p annihilation at rest. Additionally, another large scale modular array of NaI was placed [3] at the opposite position to B G O with respect to the liquid hydrogen target. * Visitor from Physics Department, Brown University, Providence, RI, 02912 USA. ** Present address: Matsushita Electric Industrial Co., Moriguchi, Osaka, 570 Japan. 0168-9002/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

Prior to E68, a system of 7 B G O modules was tested with and without N a I shielding, using both electrons and pions below 1.6 G e V / c . The construction of the final BGO calorimeter is briefly described in the next section, followed by the test results which are presented in sections 3 and 4 for the energy resolution and the e/~r separation, respectively. The performance of the final calorimeter during its use in E68 is presented in section 5 and finally a summary is given.

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Fig. 1. BGO calorimeter constructed for E68 to measure the inclusive ~ ray and, in combination with another modular array of NaI, the ¢r° spectrum in ~p annihilation at rest [1].

310

J. Iwahori et aL / Performance of a BGO calorimeter

2. The BGO calorimeter

We have constructed a close-packed assembly of 10 BGO modules surrounded by 11 NaI modules as sketched in fig. 1. Each BGO crystal [4] is 17 (H1-6) or 20 cm (H7-10) long and has a hexagonal cross section with 50 mm across the parallel edges (see fig. 2). The crystal was polished to optical specification on all surfaces. A 1.5 in. photomultiplier of bialkali photocathode (Hamamatsu, R580) was cemented on one of the ends with silicone RTV (Shin-etsu, KE45RTV). The crystal was wrapped with an aluminized Mylar foil and then with black tape. For gain monitoring, a N a I ( T l ) - A m light pulser [5] was mounted on the end of the crystal opposite to the photomultiplier again with silicone RTV. The optical quality of the BGO varied from crystal to crystal because the crystals were obtained sequentially during a development period of longer than one year. The fwhm energy resolution for 0.662 MeV 7 rays from ]37Cs ranged from 25% (in the module H10) to 40%. The longitudinal uniformity measured with a 137Cs source or with a 1 G e V / c qr- beam is presented in fig. 3 for all the 10 modules. In the best module (H10), the nonuniformity, defined as :h ( m a x . - min.)/(max. + min.), was within + 3% between 20 to 190 mm from the photomultiplier. Longitudinal scans with the 137Cs source and 1 G e V / c ~r-s were compared in some modules~ The uniformities obtained with both probes were similar to each other in agreement with ref. [6]. Each NaI crystal [7] is encapsulated in a 0.5 mm thick stainless-steel envelope, which has the outer dimensions of 65 x 65 x 300 mm 3. A 2 in. photomultiplier (Hamamatsu, R329) was optically coupled to the glass window at one of the crystal ends through a 10 mm thick disk of transparent silicone RTV rubber. This scheme of optical coupling degraded the fwhm energy resolution at low energies: 20% fwhm for 0.662 MeV "y rays (137Cs source) and 6% for 6.13 MeV y rays (23Spu-]3C source). This occurred because these NaI modules were originally constructed to measure higher energy "r rays. At high energies, the energy resolution is predominantly determined by the fluctuation in the shower leakage rather than the photon statistics, and Nal (TL)

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3. Energy resolution

An assembly of 7 BGO modules was tested with electrons and pions below 1.6 GeV in test beam T1 at

J. lwahori et al. / Performance of a BGO calorimeter

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the K E K 12 GeV Proton Synchrotron. The B G O assembly was placed close to the focus approximately 3.8 m from the downstream end of the final beam line magnet (see fig. 4). Electrons were selected by a gas Cherenkov counter and injected along the central axis of the B G O assembly. The signal from each B G O module was clipped at the photomultiplier output with a 20 m long cable (characteristic impedance: 50 D) terminated with a 20 D resistor, and pulse-height analyzed during 400 ns by an A D C (Lecroy 2285A) in the charge mode. Before summing the pulse heights of the 7 modules, each module was exposed to the electron beam for intercalibration. The absolute energy scale was determined by using the prediction of Monte Carlo calculations based on the EGS program [8] for the energy

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deposit in both the single and the sum of the 7 B G O modules. All the modules except the central one showed no saturation up to 1.6 GeV. After a small saturation in the central module was corrected, the sum of the pulse heights over the 7 modules was sufficiently linear as is shown in fig. 5. Typical energy spectra for 1 GeV electrons in the central module and in the sum of 7 modules are presented in fig. 6(a) and (b), respectively. The energy resolution was derived from a Gaussian fit to the obtained spectrum. The obtained fwhm has contributions from the beam m o m e n t u m width, which consisted of two different sources; the intrinsic width of 0.44% fwhm for the 5 mm wide beam, and the multiple scattering mainly in S1 and in the air (see fig. 4), which amounted to 1.64% fwhm for 1 G e V / c electrons. The correction of the energy resolution for the beam m o m e n t u m width was small at 1 GeV (from 4.5 to 4.1% fwhm in the sum of 7 modules), and larger at lower energies (from 8.1 to 7.0% at 0.4 GeV). The fwhm resolution after correction for the beam m o m e n t u m width is plotted in fig. 7.

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J. lwahori et a L / Performance of a BGO calorimeter

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The fwhm energy resolution at 1 GeV was 4.1% in the sum of 7 modules and 7.3% in the single central module. The measured points follow reasonably well the E G S Monte Carlo calculations, which are calculated by assuming all 20 cm long crystals and also shown in fig. 7. A small difference between experiment and calculation for the sum of 7 modules may be due to the fact that only 3 modules (B8-10) are 20 cm long while the others are 17 cm long. In fact, an EGS calculation gives a fwhm of 3.7 ___0.5% at 1 GeV if all modules are 17 cm long. The observed energy resolution is similar to the values in previous measurements [6,9]. The effect of the NaI surrounding the BGO was examined in another combination of 7 B G O modules as sketched in the inset of fig. 8. Electrons were injected along the axis of the central module H7. When the energy deposit in the NaI was added to that in the BGO, the fwhm energy resolution was improved typically from 12 to 8% at 0.2 GeV, 6.1 to 5.6% at 0.6 GeV and 5.3 to 5.0% at 1 GeV. The obtained energy resolution in this configuration were slightly poorer than those in fig. 7. This is not a serious problem because the central module H7 in the present assembly was optically less good than the corresponding module H10 in the measurement of fig. 7.

4. Separation of electrons from pions (e/~r) I

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Besides the energy resolution for showering particles, the degree to which electrons can be separated from pions is another important quantity because much interest exists in detecting individual electrons in very large pion background during colhding beam experiments. The e/~" separation was examined by comparing energy deposition resulting from injection of electrons and pions into the arrays at fixed momentum. The beam was injected along the axis of the central module of the system of 7 B G O modules, which is sketched in the inset of fig. 7. The energy sum in the 7 B G O modules and the transverse energy spread are compared for incident electrons (fig. 9) and pions (fig. 10) at 0.6 G e V / c . For electrons, the energy sum (Esum) is principally contained in a sharp peak around 588 MeV (see fig. 9(a)). Most of the energy is deposited in the central module. Correspondingly, the distribution of Eper/E . . . . where Epe r is the energy sum in the peripheral 6 modules, has a well defined peak around 0.1 as shown in fig. 9(b). For pions, on the contrary, Esu m has a sharp peak plus a broad tail (see fig. 10(a)). The sharp peak arises from the ionization loss mechanism, while the tail results from nuclear interaction in the BGO. Correspondingly, the distribution of Eper/Esum is peaked around zero, dropping monotonically as Eper/Esum increases (see fig. lO(b)).

J. lwahori et aL / Performance of a BGO calorimeter

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0.6 GeV/c T~We take as a criterion for acceptance of the incident particle as an electron if Esu m lies, for example, within 2o ( e = 13.7 MeV) of the peak (588 MeV). The small amount of pions depositing an energy in the above region are mistaken as electrons. F r o m fig. 10(a), such fake electrons amount to O(10 -2) of total pions. This number is, however, an overestimate because the " p i o n " beam was contaminated with true electrons. The small peak at 588 MeV in the pion energy spectrum, fig. 10(a), is evidence of this contamination. Such contamination by electrons may have occurred as a result of a slight inefficiency in the gas Cherenkov counter which tagged electrons. The contaminating electrons in the " p i o n " beam can be d e a r l y seen in the logarithmic plot, fig. 11. By ignoring the electron peak at 588 MeV, the level of fake electrons can be estimated as 1 / 3 0 0 of the total pions from the area under the dotted line in the region of the electron peak. The e/~r separation can be further improved by a cut in the lateral energy spread. As Ep=r/Es, m is less than 0.2 for most of the electrons (see fig. 9(b)), we may

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ENERGY DEPOSIT I GeM Fig. 11. Logarithmic plot of the number of event versus the energy sum in the ? BGO modules (see the inset of fig. 7). 0.6 GeV/c pions were injected along the axis of the central module. The total number of events is 6423. The peak at 588 MeV is due to contamination by true electrons (see the text).

J. lwahori et aL / Performance of a BGO calorimeter

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7 BGO modules (see the inset of fig. 7). 0.6 Gev/c pions were injected along the axis of the central module. Esum (or Eper) is the energy sum in the total 7 (or the peripheral 6) modules. The total number of points is 2281.

require this additional criterion for electrons besides the previous cut in EsumThe additional e/,r rejection factor is given by the ratio of the pions with Epcr/E,u m < 0.2 to the total pions having Esum in the electron peak. A scatter plot between Esum and Eper/Esu m is useful to estimate the above factor. Fig. 12 gives such a scatter plot for 0.6 G e V / c pions. The energy range corresponding to the electron peak (0.56-0.62 GeV) should not be used due to the contamination by true electrons as mentioned before. Therefore, we utilize the energy range (0.5-0.56 GeV) to find the distribution of Er~r/Esu m. Counting the points in the energy range of 0.5-0.56 GeV below and above Er~r/E~u m = 0.2, we find that the cut in Eper/Esu m reduces the fake electrons by a further factor of 1/3. Consequently, the e/~r rejection factor at 0.6 G e V / c is about 1/1000 if both cuts in E~um and E ~ J E , u , ~ are employed. A similar measurement at 1 G e V / c gives the e/~r rejection factor of 1/500 with only the cut in E~=m and 1/2000 with an additional cut in Ep~r/Esu m. The e/~r rejection factor of 1/500 at 1 GeV/c, with only the cut in the energy sum, is similar to the values in previous measurements [9,10].

the inclusive *r° spectrum in ~p annihilation at rest. The physics results will be described elsewhere [2]. The energy calibration of each BGO or NaI module was carried out by injecting a 200 MeV electron beam along the module axis. The electronic circuits employed in the calibration were the same as in E68 including the clipping cable, the signal cable, the preamplifier, the signal divider, the ADC and the high voltage power supply. To facilitate the above, the K4 beam for E68, optimized for low momentum antiprotons, was temporarily tuned to transport electrons. The excitation curve of each BGO or NaI module had previously been measured in the T1 beam with electrons ranging from 100 to 1000 MeV, which were injected along the module axis. The saturation was negligibly small below 1000 MeV in any BGO module. The energy calibration of the BGO modules was checked a few times during 3 months of running in E68 by measuring the 129 MeV gamma rays which arise from ~r-p at rest into 7 n (the Panofsky V rays). During the calibration, the K4 beam was temporarily changed to 167 MeV/c negative pions, which were stopped in the liquid hydrogen target [2]. As the Panofsky V rays uniformly illuminated the front end of the calorimeter, the signals from neighbouring modules were added to get the gamma energies. The calibration constants of the BGO modules were determined by solving a set of linear equations so 100

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5. Application of a B G O calorimeter in a physics experiment

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J. lwahori et al. / Performance of a B GO calorimeter 41

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An array of 7 BGO modules has been tested with 0.2-1.6 GeV/c electrons and pions. Each module is 17 or 20 cm long and has a hexagonal cross section with 50 mm across the parallel edges. The optical quality varied from module to module. The fwhm energy resolution at 0.662 MeV ranged between 25% and 40%. The longitudinal nonuniformity scattered from _+3% up to more than _+20%. The fwhm energy resolution for 1 GeV electrons was 4.1% in the sum of 7 modules, while attaining 7.3% in a single module. If the energy deposit in the 6.5 cm thick NaI shield surrounding the BGO was added to the sum of 7 B G O modules, the fwhm resolution was improved further by 0.3% at 1 GeV. The e/,r separation for momentum analyzed particles at 0.6 G e V / c was 1/300 with only the energy cut. With an additional cut in the lateral energy spread, the e/~r separation was 1/1000. At 1.0 GeV/c, the above two numbers were improved to 1/500 and 1/2000, respectively. Finally, a practical BGO calorimeter has been constructed by surrounding 10 BGO modules with 11 NaI modules. It has been successfully used in E68 to measure the inclusive y ray spectrum in ~p annihilation at rest. The fwhm energy resolution of 10% was obtained

that the rms deviation of the measured energies from 129 MeV should be minimized. If a sizable energy was deposited in the NaI, such an event was rejected from the analysis. For the events with only small fractional deposits of energies in the NaI, the calibration constants of the NaI modules are less important, and therefore they were fixed for simplicity at the values determined with 200 MeV electrons. The calibration constants determined as above for 129 MeV y rays agreed within 2% for most modules with the values determined with 200 MeV electrons. A typical energy spectrum of y rays arising from ~r-p at rest is presented in fig. 13. The 129 MeV line has a fwhm resolution of 10.3%. Fig. 14 shows a typical pulse height spectrum of the NaI(T1)-Am light pulser mounted on a BGO module. The light intensity is equivalent to the energy deposit of 30 MeV in the BGO. The Am-a peak has a fwhm resolution of 10.1%, which is about twice the value obtained when the light pulser is mounted directly on the cathode window of the photomultiplier. A typical gain drift in a BGO module (H4) during

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J. lwahori et al. / Performance of a BGO calorimeter

which come from ~r-p at rest into within ± 1% was stably done at 30 an epoxy-free N a I ( T 1 ) - A m light crystal.

Acknowledgements The authors would like to express their sincere thanks to Professors T, Nishikawa and S. Ozaki for their support and encouragement throughout the work. They are thankful to Professor H. Hirabayashi and the staffs of Proton Synchrotron for providing a stable beam, and to all their colleagues in E68 for aid and helpful discussions.

References [1] See for example, E. Lorentz, Nucl. Instr. and Meth. 225 (1984) 500. [2] For the results, see M. Chiba et al, to be reported elsewhere. For a preliminary result, see M. Chiba, K. Doi, T. Fujitani, J. Iwahori, M. Kawaguchi, M. Kobayashi, M. Koike, T. Kozuki, S. Kurokawa, H. Kusumoto, H. Nagano, Y. Nagashima, T. Omori, S. Sugimoto, M. Takasaki, F. Takeutchi, M. Tsuchiya, M. Ueda, Y. Yamaguchi aaad H. Yoshida, in: Proc. Meeting on Few-Body Problems in High and Medium Energy Physics, held at KEK, Tsukuba, KEK-Report 85-13 (1985) p. 40.

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