2244 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 42, NO. 6, DECEMBER 1995

Kr Electromagnetic D. Yu. Akimov, A. J. Bolozdynya, D. L. Churakov, V. N. Afonasyev, S. G. Belogurov, A. D. Brastilov,

A. A. Burenkov, L. N. Gusev, V. F. Kuzichev, V. N. Lebedenko, T. A. Osipova, I. A. Rogovsky, G. A. Safronov, A. Simonychev, V. N. Solovov, V. S. Sopov, G. N. Smirnov, V. P. Tchernyshev, M. Chen, M. M. Smolin, W. Turchinetz, R. A. Minakova, V. M. Shershukov, and V. H. Dodohov

Abstract-A scintillating LXe/LKr electromagnetic calorimeter has been built at the ITEP and tested at the BATES @UT) accelerator. The detector consists of a PMT matrix and 45 light collecting cells made of aluminized Mylar partially covered with p-terphenyl as a wavelength-shifter (WLS). Each pyramidal cell has ( 2 . 1 ~ 2 . 1 ) ~ 4 0 ~ ( 4 . 1 5 ~ 4 . 1 5 ) cm dimensions and is viewed by an FEU-85 glass-window photomultiplier. The detector has been exposed to the 106-348 MeV electron beam. The energy resolution is a ~ f E % 5 % / a at 100-350 MeV range in LXe; the coordinate resolution is ax N 0.7 cm; the time resolution is U, % 0.6 LIS for a single cell. Possible ways to improve energy resolution are discussed.

I. INTRODUCTION

XeLKr electromagnetic calorimeters are proposed as precise, fast, radiation stable detectors for future accel-

erators [l]. Some recent attempts to build LXe scintillating calorimeters were not successful because of difficulties of effective UV light collection (150 nm for Kr and 170 nm for Xe) [ 2 ] , [ 3 ] . The standard approach needs UV sensitive pho- todetectors, reflectors with high effectivity for W light, and extremely UV-transparent scintillating liquids. Simulations show that even 1 m attenuation length is not enough to reach precise resolution. Our approach is based on using visible light sensitive PMT's and aluminized Mylar reflectors covered with wavelength shifter (WLS) to convert the scintillation light to the visible range. In this way, we found that desirable response functions can be obtained for a calorimeter cell of a needed size [4].

The scintillating calorimeter LIDER (Liquid Detector for Electromagnetic Radiation) has been built and was previously tested at ITEP with LKr as a working medium [5]. The calorimeter has been investigated in detail at the BATES (MIT) electron accelerator with LXe filling.

11. EXPERIMENTAL SETUP

The scintillating calorimeter LIDER (Fig. 1) consists of a cellular Mylar reflector structure viewed with the matrix of photomultipliers immersed in a liquid noble gas.

Manuscript received January 12, 1995; revised May 13, 1995. D. Yu. Akimov, A. I. Bolozdynya, D. L. Churakov, V. N. Afonasyev,

S. G. Belogurov, A. D. Brasilov, A. A. Burenkov, L. N. Gusev, V. F. Kuzichev, V. N. Lebedenko, T. A. Osipova, I. A. Rogovsky, G. A. Safronov, S. A. Simonychev, V. N. Solovov, V. S. Sopov, G. N. Smirnov, and V. P. Tchernyshev arc with ITEP, Moscow, Russia 117259.

M. Cheu, M. M. Smolin, and W. Turchinetz are with MIT, Cambridge, MA 02139 USA.

R. A. Minakova and V. M. Shershukov are with Monocrystal, Khar'kiv, Ukraine 310141.

V. H. Dodohov is with JINR, Dubna, Russia 141980. IEEE Log Number 9415495.

6

Fig. 1. Schematic drawing of LKdLXe scintillating calorimeter: 1-Mylar reflector, 2-LKrLXe vessel, 3-heater, 4-support for Mylar reflector, 5-PMTs in magnetic screen, &liquid nitrogen input, 7-KrXe gas input, 8 P M T ' s connectors, 9-multipin metal-glass feedthroughs, 10-PMT's divider, 1 1-support, 12-plastic scintillator, 13-fibcr, 14-monitoring pho- tomultiplier, A-LKrLXe volume, B-gas jacket, C-liquid nitrogen jacket, D-vacuum isolation.

The photomultipliers are divided into 5 groups.(7-11 PMT's per group). Each group has a common active divider placed outside of the cryostat. The electrical inputs enter through multipin metal-glass feedthroughs. The vessel with liquid (A. Fig. 1) is placed inside the cryostat including three jackets: (B) filled with nitrogen gas, (C) filled with liquid nitrogen, (D) vacuum isolation. Aluminum supplementors surround the reflector structure to reduce the volume of LXe to 35 1. The heaters ( 3 ) provide thermostabilization with f 0 . 5 K accuracy. The temperature is measured by copper-constantan thermocouples. The LIDER is placed on a moving remote- controlled table at the BATES electron beam.

The light collecting structure is glued together from 45 pyramidal cells (Fig. 2). Each cell of (2.1 x 2.1) x 40 x (4.15 x 4.15) cm dimensions is made of 50 ,um aluminized Mylar and covered with p-terphenyl strips inside. One strip per wall covering is chosen for a home-made reflector structure. The shape of the p-terphenyl strips is experimentally found to provide uniformity of light collection along a cell. It has been shown that the response functions are similar when

0018-9499/95$04,00 0 1995 IEEE

YU. AKIMOV et al.: SCINTILLATING LXeLKr ELECTROMAGNETIC CALORIMETER 2245

3

Fig. 4. Transverse response function MC simulated for 50% U V reflective index and UV attenuation length of 5 cm for one strip per wall covering.

Fig. 2. pyromidal reflector; and reflector structure.

Single calorimeter cell: 1-PMT, 2-p-terphenyl strip, 3-Mylar

2ol 10

0 I 0 5 10 15 20 25 30 35 40

Distonse from PMT, cm

Fig. 3. Longitudinal response function measured for two different reflector cells.

filling the same cell with LKr and LXe [5]. The longitudinal response functions for two different cells filled by LKr are shown in Fig. 3, which demonstrates good reproducibility of the response function for a given shape of WLS strip. The transverse response function is Monte Carlo simulated (Fig. 4).

Some types of PMT’s were tested specially to be used in LKrLXe. Photomultiplier type FEU-85 (Table I) has demon- strated the best properties; the magnitude of signal does not depend on temperature; the noise of the PMT at the LKr temperature is 2 times lower than noise of the PMT at room temperature; it is mechanically stable up to 10 atm extra pressure.

Alpha-sources produced by 237Pu implantation in the Mylar film 1 x 1 mm2 are placed in some cells:

to test the uniformity of light collection; to calibrate a spectrometric channel;

TABLE I GENERAL CHARACTERISTICS OF FEU-85 PHOTOMULTPLIER

to test the LXeLJSr transparency; and to determine a position of the level of liquid during the

detector filling. The stability of each spectrometric channel is also tested

by a pulsed nitrogen laser (333 nm). Laser radiation activates the plastic scintillator (12). The scintillating light is sent to each PMT through a plastic fiber (13) and to the monitoring photomultiplier (14). The stability of the PMT in time is measured by comparison of signals from each cell and from the monitoring PMT. Variation of signal magnitude of <2% was found for FEU-85 during 7-h run in LXe.

The gas system is shown in Fig. 5. Cooled stainless steel high pressure cylinders (HPC) of 40 L volume are used for Xe gas storage. Xenon was purified in a gas phase passing through the purifiers- during the detector filling. UV transparency of LXe is measured in a 1 L control chamber with a PM tube and two a-sources placed at different distances from the PMT- window, which is covered by p-terphenyl. Liquid xenon taken from the LIDER after beam tests shows an attenuation length of 3 cm when purifying only with “Monotor” and “Hydrox- 801,” and of 5 cm using all purifiers.

2246 EEl E TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 42, NO. 6, DECEMBER 1995

PvmP

Fig. 5. Gas system: CC-control chamber, HF'C-high pressure cylinders, F-gas filters, CP-circulating pump, VG-vacuum gauge, M-manometer, EM-erupting membrane.

The read-out system is based on CAMAC electronics and an IBM/F'C-486 computer (Fig. 6). The 2048 channels (11 b) charge-to-digital converter (QDC) LeCroy 4300B F E U are used for PMT pulse digitizing. The mean magnitude of signals form the central cell corresponds to about 500-1000 channels of QDC (200 keV per channel). The signals from the beam counters CE1 and CD2, muon counters CM1, and CM2, and laser monitoring counter CL are also digitized. A 150 ns gate is used because 95% of the scintillation light is collected within this time (Fig. 7).

The triggers generated with this system are: e beam events (for electrons passing through CE1, CE2

e pedestal events (from pulse generator), m a-events (5 independent triggers for one cell with a-

0 muon events (CM1 and CM2 coincidence from cosmic

e laser events.

counters,

source in each group),

muons),

111. MONTE CARLO SlMULAnONS

The influence of the LIDER configuration on energy resolu- tion was investigated with the GEANT Monte Carlo program. The fluctuation of energy deposited in the entrance windows, in reflector walls, in other elements, and the fluctuation of energy leakage out of the sensitive volume are taken into consideration. The measured longitudinal response function and the calculated transversal response function are included in the program.

Response functions for light collecting cells are simulated by the Monte Carlo program. The transparency of the liquid, mirror and diffuse reflective indexes for aluminized Mylar and for p-terphenyl strips for visible and for UV light are taken into consideration. Simulations show that the response function is quite sensitive to the attenuation in a liquid and to the mirror reflective index for UV light. LXe is considered to be transpar- ent for visible light. The 85% reflective index of aluminized Mylar for visible light and 50% for UV at normal incident light are used for calculations. The last value has been measured

PC-486 ii'

U

Fig. 6. Read-out electronics: CEI, CE2-beam counters, CM1, CM2-muon counters, CL-laser monitoring counter, Miscriminator, CC-Coincidence circuits, GEN-pulse generator, QDC-charge-to-digital converters LeCroy 4300B FERA, TDC-time-to-digital converter LeCroy 2228A.

Fig. 7. activated by @-particles (pictured by digital scope).

Scintillation signal from FEU-85 PMT covered by WLS in LXe

with a 170 nm UV source and a UV-sensitive photomultiplier. The reflective indexes provide a quite good correspondence of the simulated longitudinal response function to the measured one (Fig. 3). The WLS covered Mylar is considered as a total drffuse reflector with 70% efficiency for visible light and as a total absorber for UV light. The diffuse reflective index of 10% for aluminized Mylar is used for visible and for UV light. The transverse response function for 30 cm distance from the PM is presented at Fig. 4 for 5 cm attenuation length of UV light in LXe.

The influence of leakage fluctuations on energy resolution is presented in Fig. 8. The fluctuations of energy deposited in the sensitive volume are mainly caused by forward (l), backward and lateral (2) leakage fluctuations. The fluctuations of absorbed energy in "dead" matter of the Mylar walls (2 x 50 mm) and possible "dead" LXe layer between cell walls (0.5 mm) and between the inner window of A volume and the entrance walls of cells (5 mm) are shown by curves (3) and (4). Curve (5) corresponds to the fluctuation of the energy loss in all entrance windows (total of 3 mm of stainless steel). Comparison of the above simulations shows that the

YU. AJSIMOV et al. : SCINTILLATING LXe/LKr ELECTROMAGNETIC CALORIMETER 2241

100 1000 Energy, MeV

Fig. 8. Energy dependence of energy fluctuation deposited in the LIDER due to leakages (see text).

SigmaM, x

zo j

a 100 1000

Energy. YaV

Fig. 9. experiment, curves-MC simulation (see text).

Dependence of energy resolution versus electron energy. Stars-

energy resolution of the LIDER built with “ideal” (absolutely uniform) cells is limited generally by fluctuations of deposited energy in the lateral direction at the 100-400 MeV energy range (1- curve, Fig. 8).

The Monte Carlo calculated dependence of the detector energy resolution on electron beam energy is presented in Fig. 9. The curve (1) corresponds to detector with the “ideal” light collecting cells, are curves (4) and (5) correspond to a detector with one-strip covering cells for 5 cm and 3 cm attenuation length.

IV. EXPERIMENTAL DATA

The LIDER detector has been exposed at 106, 174.3, 260, and 348 MeV electrons at the BATES accelerator.

A. Calibration Procedure

To obtain the calibration coefficients, the central part of the LIDER ( 5 x 5 cells) has been scanned by a beam in steps of 20 mm. The calibration coefficients for all cells are calculated by minimization of the width of the energy distribution (Fig. 10). For this purpose, the following function is minimized by means of MINUIT code

I N

F ( K . . . , k,) = C ( E , - (Eo - N 2=1

where E, = C;=,K,A,, is the energy for ith event, k, is a calibration coefficient for j th channel, A,, is a pulse amplitude of ith events for j th channel, N is number of events, Eo is the electron beam energy, EL is a leakage energy. E t depends on beam coordinates and is calculated with the GEANT simulation program. The validity of this procedure was checked for simulated events.

B. Energy Characteristics The experimental data on energy resolution for different

electron energies are presented in Fig. 9 (stars). The data for 106 MeV, 174.3 MeV, and 260 MeV electrons have been

350

300

250

200

150

100

50 O O L L 100 200 300 400 500

Energy, MeV

Fig. 10. The energy distribution for 348 MeV electrons.

obtained with the detector filled with liquid xenon purified by the “Monotor” and “Hydrox-801”, (UV Latt is about 3 cm).

The data for 348 MeV electrons were obtained with xenon purified by all purifiers (L,tt is about 5 cm). The energy spectrum for 348 MeV electrons reconstructed with obtained calibration coefficients is shown in Fig. 10. The best energy resolution achieved at this energy is a E / E 2 8% (the depen- dence on energy may be estimated as aE/E S 5%/@).

C. Coordinate Characteristics The coordinate of an incident is defined as a center-of-

gravity position of the distribution of energy deposited in the detector. The distribution of the events on X for 348 MeV energy obtained by means of such procedure is presented in Fig. 11. A spread of the beam electron is determined by a 0.5 x 0.5 cm scintillating counter. The measured coordinate

2248 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 42, NO. 6, DECEMBER 1995

Fig. 11. The shower centers-of-gravity distribution on X .

Time. n6.c

Fig. 12. Time-of-flight spectrum for 174.3 MeV electrons.

resolution of ux 2 0.7 cm includes the spread due to the finite size of the trigger scintillator.

D. Time Characteristics The TOF measurements were performed by means of

LeCroy 2228A time-to-digital converter (TDC) at 174.3 MeV beam energy. A start signal for TDC was generated by the beam counter, a stop signal was generated by the cell with maximum deposited energy (the central cell). Signals were shaped by constant fraction discriminators with 0.25 level. The time resolution of cT 2 0.6 ns (Fig. 12) has been obtained taking into consideration the time resolution of the start beam counter. The total pulse duration (95% of charge) from a single cell is less than 150 ns (see Fig. 7) and is defined by decay time of scintillation.

S”€. x

0 ‘ ! 8 I 0 5 10’ 15 20 21

Lam, om

Fig. 13. resolution on the LXe attenuation length (see text).

Simulated for 348 MeV electrons dependences of the energy

V. DISCUSSION The presented experimental data demonstrate good time

properties, sufficiently good coordinate resolution of the de- tector, and are in agreement with Monte Carlo simulations. The energy resolution is still 2.5-3 times worse than GEANT predicted for “ideal” light collection. It is caused by the transversal nonuniformity of the “one-strip per wall” light- collecting structure. As the value of such nonuniformity and energy resolution depend on LXe transparency (curve 1, Fig. 13), the achieved parameters of the detector can be improved by increasing the LXe transparency.

Another way is to use a finer covering which is more difficult to manufacture. Curve 2 in Fig. 13 and curve 3 in Fig. 9 demonstrate about 2 times better energy resolution for the detector with 4 WLS strips per wall of light collect- ing cell covering relatively to one with one-strip per wall covering.

Better resolution can also be achieved by using cells covered by small strips perpendicular to the cell axis direction. Curve 2 on Fig. 9 is simulated for 5-mm strip spacing.

Such cells with fine covering have already been tested at the ITEP beam facility, and a good longitudinal uniformity has been achieved.

Further improvement of resolution is possible using the cell with a surface totally covered by WLS, which is transparent to visible light. The p-terphenyl covering used is a diffuse reflector for reemitted light and partially absorbs it. Prelim- mary tests with a cell covered with WLS developed by the Monocrystal Institute (Khar’kiv) have shown only 10-15% transversal nonuniformity in the vicinity of the walls. This way, the longitudinal uniformity may be provided by the variation of WLS concentration in the covering along the cell. But a new WLS has to be investigated additionally for stability in liquids, radiation hardness, and reproducibility of longitudinal uniformity for mass-produced cells.

VI. CONCLUSION

The first full-scale scintillating calorimeter has been built and tested. In LXe, the measured energy resolution is aE/E 2

W. AKIMOV et al.: SCINTILLATING LXe/LKr ELECTROMAGNETIC CALORIMETER 2249

5 % / 0 . The developed method of “visible sensitive pho- todetectors + Mylar reflector with WLS-covering” opens a real way to build precision calorimeters using inexpensive photodetectors and the required level of LXe punty which easy to be achieved (Latt = 5-10 cm).

Spatial resolution of the calorimeter is 8~ %’ 0.7 cm for single electron showers in LXe.

Good time properties-time resolution of o7 E 0.6 ns and utilized signal width of 150 ns-allow use of the detector at high luminosity accelerators.

ACKNOWLEDGMENT

We thank Prof. R. P. Redwine, Prof. S . Kowalski, Dr. L. Stinson, and others of MIT Laboratory for Nuclear Science and the Bates Linear Accelerator Center for their great support during these investigations. We also thank the International Science Foundation for supporting our attendance at the IEEE Nuclear Science Symposium.

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