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Test of iron-liquid argon and lead-liquid argon sampling calorimeters for detection of electromagnetic showers
Nuclear Instruments and Methods 174 (1980) 357-362 © North-Holland Publishing Company
TEST OF IRON-LIQUID ARGON AND LEAD-LIQUID ARGON SAMPLING CALORIMETERS FOR DETECTION OF ELECTROMAGNETIC SHOWERS Y. ASANO, M. MIYAJIMA, Y. NAGASHIMA, K. OGAWA, T. TSURU Nattonal Laboratory for High Energy Phystcs, Tsukuba, Ibarald-ken, 300-32, Japan T. DOKE, Y. FUJITA, Y. HOSHI, T. KAGA, K. MASUDA Science and EngmeeringResearch Laboratory, Waseda University, Shinjuku-ku, Tokyo 162, Japan and H. MURAKAMI Department of Physics, Rikkyo University, Nishi-lkebukuro, Toshima-ku, Tokyo 171, Japan Received 26 November 1979
Iron-liquid argon and lead-liquid argon sampling calorimeters were constructed and tested, The obtained energy resolutions due to the samphng fluctuation ~s/E for 1 GeV electrons were 6.1% and 9.6%, respectively. Our results are compared to those of scintillation sampling calorimeters and what the energy resolution depends on is discussed.
1. Introduction
formulas proposed so far for energy resolution due to sampling fluctuation.
The hquid argon ionization calorimeter, which was proposed by Alvarez [1] in 1968, was tested as a multi-plate sampling ionization calorimeter, at first, by Willis and Radeka [2] and others [3] in 1974 and, since then, has been developed by several groups [4,5] as a large size electromagnetic or hadronic shower detector in particle physics experiments. At present, it is evaluated to be useful as a high energy resolution calorimeter with good spatial information and good electron/hadron &scriminatlon over a wide range of high energies (several to hundred GeV). On the other hand, its application for low energy ( a few hundred MeV to a GeV) gamma rays or neutral particles 7r°, 7/° or co° has not been made so often. To make such a neutral particle calorimeter, it is necessary to make clear the mechanism that the energy resolution depends on. From the viewpoint as mentioned above, we constructed and tested liquid argon sampling calorimeters with multi-iron electrodes and with multi-lead electrodes. In this paper, we describe the construction and test results of the calorimeters and discuss the dependence of the energy resolution on the material of the electrodes, comparing our results with some
2. Design and construction 2.1. Calorimeters and cryogenics A schematic view of the calorimeter is shown in fig. 1. In order to investigate the dependence o f the energy resolution on sampling materials, two kinds of electrode material as passive converters for gamma rays were tested under the same geometrical conditions. The electrodes in the first calorimeter (henceforth A) are made o f 62 unit ceils, consisting of a 2 mm Fe plate (high voltage electrode), 2 mm of liquid argon, a 2 mm Fe plate (signal electrode) and 2 mm of liquid argon. The total depth and sampling thickness (half a unit cell) are 15.5 and 0.128 radiation lengths, respectively. In the second calorimeter (henceforth B), both electrodes for high voltage and signal m the first 16 unit cells were replaced by 2 mm Pb plates and the remaining 46 unit cells were retained as in A. The total depth and sampling thickness for the replaced cells are 11.9 and 0.371 radiation lengths, respectwely. The electrodes are sup357
Y. Asano et al. / Test o f sampling calorimeters
358
x I ,_~SAFETY VALVE
I.IL <~ o
LIQUID N2
I ARGON GAS
LIQUID N2 -60,
;u HEATEXCHANGE~
X
X
DIFFUSION
I
PUMP[
< -HV BEAM
.~ SIGNALS
Fig. 1. Schematic view of the detector and cryostat.
ported b y four stainless steel bars with a diameter of 10 m m covered with 5 mm thick teflon tube. They are fixed 2 mm apart from each other b y teflon spacers. The characteristic parameters are summarized in table 1. The stack o f electrodes is contained in a cylindrical vessel o f 50 cm long and 30 cm in diameter. The top and the b o t t o m flange o f the vessel can be taken off and the stack is slid in or out along the guide attached to the inside.wall o f the vessel. The center o f the flange is made thin so as to reduce the material in
Table 1 The characteristic parameters of the calorimeters Calorimeter
A
B
Converter Gap of hqmd argon Sampling thickness Total depth Detector capacitance Energy deposit m liq. Ar at 1 GeV Noise in r.m.s. Omeas/E
Fe 2 mrn 2 mm 0.128 r.1. 15.5 r.1. 40 nF 142 MeV
Pb 2 mm/Fe 2 mm 2 mm 0.371/0.128 r.1 11.9/11.8 r.1. 10/30 nF 88 MeV a)
2 5 MeV [7.9 + 37.1/ E (GeV)]l/2% 6.1%
1.5 MeV a) [10 + 92/ E (GeV)]l/2% 9.6%
(as/E) X x/E (GeV)
a) For the first 16 unit cells (Pb coverter only)
front of the active area. The net material before the detector is about 0.25 radiation lengths. A schematic view o f the cryostat is also shown in fig. 1. The liquid argon temperature is maintained by cooling with liquid nitrogen. To get an efficient cooling, the liquid nitrogen reservoir is placed just above the calorimeter and it directly cools the calorimeter via the Cu-plate heat exchanger at the b o t t o m o f the reservoir. To reduce the heat flow into the calorimeter and liquid nitrogen reservoir from the atmosphere the whole cryogenic system and the calorimeter are covered with 10 cm thick styro-foam. This cooling system maintained the liquid argon temperature sufficiently well. The consumption rate o f the liquid nitrogen was about 4 1/h. The purity o f liquid argon is crucial to the operation o f the calorimeter. Argon gas of research grade was introduced through the vacuum pipe and liquefied with the Cu-plate heat exchanger into the calorimeter vessel, which was evacuated to less than 2 X 10 -6 torr prior to liquefaction. To avoid distortion o f the electrodes due to rapid cooling, the argon gas was intermittently introduced for the first 30 min. It took about four hours to fiU the vessel and about 24 1 o f liquid argon was necessary. The amount of impurity was far less than 1 p p m b y ttus gas fiUing system. The entire collection o f cells o f the calorimeter is
Y. Asano et al. / Test of sampling calorimeters divided into several sections in order to observe shower profiles. A signal from each section is separately extracted from the calorimeter via a feedthrough (a commercial 22-pin hermetic seal type connector) and fed to a preamplifier.
359
(MeV) 40
~030 E
A
o 2O z i,i
2.2. Preamplifier In our calorimeter, a preamplifier is required to have wide dynamic range for input capacitance, because the capacitance of the calorimeter widely varies from 1 nF to 50 nF. Therefore, we used a preamplifier with six high gm FETs (2SK 147-V) connected in parallel as shown in fig. 2. The rise time of the preamplifier is about 1/as for the calorimeter capacitance Ca = 50 nF and the feedback capacitance Cf = 56 nF, which is optimized for the calorimeter. The measured ENC (equivalent noise charge) of the amplifier was 1.2 × 103 + 1.0 X Cd(pF) electrons in r.m.s, for a 2/as quasl-Gaussian shaping time constant. The result is shown as a straight line in fig. 3, where the ENC is converted to the energy by using the effective W-value in liquid argon for multi-plate sampling calorimeter (23.6 eV [6] × 2 = 47.2 eV). The ENCs measured under actual operation were 2.5 MeV and 1.5 MeV for the calorimeters A (Ca = 40 nF) and B (Ca = 10 nF), respectively, at a counting
2SKI47 x 6
E304
0
I
I0
I
20 Input
I
30 40 Capoclfonce
I
50 (nF)
60
Fig. 3. Measured equivalent noise charge (ENC) o f the preamplifier as a function o f input capacitance (solid line). The d o s e d circle A and the open circle B are the measured ENCs under operation for detectors A and B, respectively.
rate of less than 104 cps and are also plotted in the figure. Under operation, the ENCs were somewhat degraded due to current leakage in the calorimeter, rf noise of the accelerator and so on.
3. Experimental procedure and results
Tests of the calorimeters were done by using pions and electrons produced at the internal target of the
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,,~ 2SC980
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360
Y. A sano et al. / Test o f samphng calorimeters
KEK proton synchrotron. The incident beam was defined by the three-fold coincidence of the scintdlatlon counters, which are of the same size, 5 cm in width, 5 cm in height and 2 mm in thickness. Electrons were selected by a freon gas Cherenkov counter. The ratio of electrons to pions varied from a few percent to fifty percent, depending on the energy. The data were obtained at eight or seven energy points from 0.25 GeV to 2 GeV for the calorimeters A and B. The results are shown in figs. 4 - 7 . Fig. 4 shows the saturation curves for the calorimeters A and B. They rise sharply and reach nearly saturated level at a field of higher than 6 kV/cm. The operating high electric fields were set at 10 kV/cm and 8 kV/cm for A and B, respectively. The sharp rise of the saturation curves guarantees the high purity of the hquid argon used in this experiment. We did not observe any pulse height deterioration during two weeks of operation. The dependence of the mean pulse height on the incident energy is shown in fig. 5. The linearity is excellent for both colarimeters. Fig. 6 shows the variations of r.m.s, energy resolution divided by incident energy E as a function of E. The curves follow the functional form (a+b[E) 1/2 except at lower energy (around 250 MeV). The fitted values for the constants a and b are 7.9 and 37.1 for calorimeter A and 10 and 92 for B, respectively, where E is read in units of GeV. In the following, we consider three basic effects which give major contributions to the energy resolution m the calorimeter: (a) sampling fluctuation, (b) fluctuation due to the leakage of energy out of the
~ 5oo
•
A
*
B
L
I OOC
•
A
o
B
A "7-
500
-5 13.-
Off
I ~0 Incident
[I5 Energy
210
(GeV)
Fig. 5. Relation between the mean pulse height and the incident energy. The closed circles are for A and the open ctrcles for B.
calorimeter and (c) electromc noise. The total amount of the effects can be obtained through the quadratic summation of the above-mentioned effects. In practical cases, however, we must further add the contribution due to the energy spread of the incident beam. We assume that the contribution is also to be added in quadrature. Since the effect due to the energy spread of the beam gives a constant contribution to the energy resolution, the value is obtained from the constant term in the curve of the resolution. We thus obtained about 3% for this effect. The energy leakage from the back of the calorimeter can be estimated from fig. 7, where the energy resolution and the pulse height for incident electrons of 1 GeV are plotted as a function of material thickness. The
• A --kt32C
~B
b
* B
4oo v
"5
-~300
"5 200
gc B
IC
hA
O..
I00 '
015
i10 Incldenf
Elecfrlc
Field
(KV/cm)
Fig. 4. Saturation curves for 1 GeV electrons. The closed circles are for A and the open ctrcles for B.
115 Energy
210
(GeV}
Fig. 6. Energy resolution cr/E fitted to the form of (a ÷ b[E) :/2 The closed ctrcles are for A and the open ctrcles for B. The parameters, a and b, are given in table 1
361
Y. Asano et al. / Test of sampling calorimeters
curve shows saturation at a depth of 10-15 radiation lengths, which agrees with the commonly accepted value. Since we always used a depth of larger than 12 radiation lengths, the energy leakage from the back is negligible. The energy leakage from the side is also negligible, since we have a sufficiently large active area (25 cm in diameter) to cover the lateral spread of the shower. As mentioned in section 2.2, the electronic noise of the preamphfier is quite small. We obtained 2.5 MeV and 1.5 MeV for A and B, respectively, in terms of the ENC. Both values correspond to about 1.7% for an incident energy of 1 GeV. Here, we used 142 MeV for A and 88 MeV for B as the total energy deposited in the liquid argon. These were obtained by comparing both pulse heights of electrons and pions. The energy resolution due to the sampling fluctuation a s is thus calculated by sub-
5OO
g
~>4'0C
/
-A
.A
oB
30C B
20C
tracting these contributions from the measured r.m.s. width. The energy resolutions os/E for 1 GeV were 6.1% and 9.6% for calorimeters A and B, respectively.
4. Discussion Liquid argon calonmeters with multi-lead electrodes have been mainly used to measure electromagnetic showers. On the other hand, few studies on the use of multi-iron electrodes have been made so far. In addition, the dependence of the energy resolution on converter materials has not been investigated strictly. The present experiment has attempted to elucidate such a dependence. We found a large difference in the energy resolution between a calorimeter with lead converters and that with iron converters under the same geometrical conditions. Here, let us compare our results with some formulas for energy resolutxon due to the sampling fluctuation, which have often been used. On the basis of the Monte Carlo calculation [7] for electromagnetic showers in a multi-lead converter system, the following formula [8] of the energy resolution has been quoted, Us
E IOC
~s Depth (r.l.) {e) %
-g ao ~- 2o
ILlI0 ;
,b
,;
2'0
~'~
Depth ( r . t ) (b) Fig. 7. (a) Dependence of the pulse height for incident electrons of 1 GeV on the detector thickness. The closed circles are for A and the open circles for B. (b) Dependence of the energy resolution in incident electrons of 1 GeV on the detector thxckness The closed circles are for A and the open circles for B
(1)
? 52E(GeV) '
where t(r.1 .) is the sampling ttuckness in radiation lengths. It gwes good agreement with experimental results for calorimeters with multi-lead converters. To apply the formula to calorimeters with multi-iron converters, Cobb et al. [5] derived the following modified formula, taking the critical energy of converter material into consideration,
E
o
= 1/'- t(rl!
"
[ E(GeV)
'
(2)
where AE is the energy loss per samphng thickness in MeV. This formula is based on the same consideration as that assumed by Willis and Radeka [2] for the estimation of the energy resolution in the calorimeter with multi-iron plates, and predicts almost the same resolution for both calorimeters with iron converters and with lead converters, when the thicknesses of lead converter and that of iron in both calorimeters are equal to each other [1.e. os(Pb)/os(Fe)= 1.05]. However, this prediction is inconsistent with the present result [os(Pb)/os(Fe) = 1.57]. Stone et al. [9] summarized their results of scln-
362
Y. Asano et al. / Test o f sampling calorimeters
Table 2 The values of R for the scintillation sampling calorimeters and the Iiquld argon ionization calorimeters Detector
R(Pb)
R(Fe)
R(Pb)/R(Fe)
Plastic scmt. [9] Liquid argon
14.8 15.8
16.9 17.1
1.14 1.08
tlllation sampling calorimeters with the formula
-Os (~)= R |/ E
t(r.1.)
VE(-(~eV) '
(3)
and investigated the variation of the proportional constant R for the change of the thickness of the scintillator or for various converter materials. Table 2 shows the values of R obtained by Stone et al. as well as ours. Each value of R obtained by both methods of scintillation and ionization calorimeters is nearly equal and their values are not different so much between the multi-lead converter and the multi-Iron converter either. This fact shows that the energy resolution of the calorimeter with multi-converters strongly depends on x/t rather than on the converter material.
5. C o n d u ~ o n Two liquid argon calorimeters with multi-iron converters and with multi-lead converters, whose configuration is identical except for the converter material, were tested for an electron beam of 0 . 2 5 - 2 . 0 GeV. A large difference in the energy resolution due to sampling fluctuation between both calorimeters
eters was found. This shows that the formula (1) or (3) rather than (2) gives a better expression for the energy resolution in the sampling calortmeter. The tendency of the energy resolution in the liquid argon ionization calorimeter with multi-converters is similar to that in the scintillation sampling calonmeter. The energy resolution is strongly dependent on x/t rather than on converter material. We are grateful to Prof. Y. Yoshimura and Dr. K. Tsuchiya for their kind advice and support in the preparation of the detector. We also appreciate the help of Drs. E. Shibamura and T. Tamae who participated m the operation of the detector.
References [1] L.W. Alvarez, LBL Phys. Notes No. 672 (26 Nov. 1968). [2] W.J. Willis and V. Radeka, Nucl. Instr. and Meth. 120 (1974) 221. [3] G. Knles and D. Neuffer, Nucl. Instr. and Meth. 120 (1974) 1, J. Engler et al., Nucl. Instr and Meth. 120 (1974) 157. [4] D. Hitlin et al., Nucl. Instr. and Meth. 137 (1976) 225; C.W Fabjan et al., NucL Instr. and Meth. 141 (1977) 61; C. Cerrl and F. Serglampietri, Nucl. Instr. and Meth. 141 (1977) 207, G S. Abrams et al., IEEE Trans. Nucl. Sct. NS-25 (1978) 309, A. Delfosse et al., Nucl. Instr. and Meth. 156 (1978) 425; A. Babaev et al., Nucl. Instr. and Meth. 160 (1979) 427. [5] J.H. Cobb et al., Nucl. Instr. and Meth. 158 (1979) 93. 16] M. Miyajima et al., Phys. Rev. A9 (1974) 1438. [7] H.H. Nagel, Z. Physik 186 (1965) 319; U Vorker, DESY Report 65/6. [8] D.R. Nygren et al., Proposal for a PEP facility based on the time projection chamber, PEP-4 (30 Dec. 1976). [91 S.L Stone et al., Nucl. Instr. and Meth. 151 (1978) 387.
TEST OF IRON-LIQUID ARGON AND LEAD-LIQUID ARGON SAMPLING CALORIMETERS FOR DETECTION OF ELECTROMAGNETIC SHOWERS Y. ASANO, M. MIYAJIMA, Y. NAGASHIMA, K. OGAWA, T. TSURU Nattonal Laboratory for High Energy Phystcs, Tsukuba, Ibarald-ken, 300-32, Japan T. DOKE, Y. FUJITA, Y. HOSHI, T. KAGA, K. MASUDA Science and EngmeeringResearch Laboratory, Waseda University, Shinjuku-ku, Tokyo 162, Japan and H. MURAKAMI Department of Physics, Rikkyo University, Nishi-lkebukuro, Toshima-ku, Tokyo 171, Japan Received 26 November 1979
Iron-liquid argon and lead-liquid argon sampling calorimeters were constructed and tested, The obtained energy resolutions due to the samphng fluctuation ~s/E for 1 GeV electrons were 6.1% and 9.6%, respectively. Our results are compared to those of scintillation sampling calorimeters and what the energy resolution depends on is discussed.
1. Introduction
formulas proposed so far for energy resolution due to sampling fluctuation.
The hquid argon ionization calorimeter, which was proposed by Alvarez [1] in 1968, was tested as a multi-plate sampling ionization calorimeter, at first, by Willis and Radeka [2] and others [3] in 1974 and, since then, has been developed by several groups [4,5] as a large size electromagnetic or hadronic shower detector in particle physics experiments. At present, it is evaluated to be useful as a high energy resolution calorimeter with good spatial information and good electron/hadron &scriminatlon over a wide range of high energies (several to hundred GeV). On the other hand, its application for low energy ( a few hundred MeV to a GeV) gamma rays or neutral particles 7r°, 7/° or co° has not been made so often. To make such a neutral particle calorimeter, it is necessary to make clear the mechanism that the energy resolution depends on. From the viewpoint as mentioned above, we constructed and tested liquid argon sampling calorimeters with multi-iron electrodes and with multi-lead electrodes. In this paper, we describe the construction and test results of the calorimeters and discuss the dependence of the energy resolution on the material of the electrodes, comparing our results with some
2. Design and construction 2.1. Calorimeters and cryogenics A schematic view of the calorimeter is shown in fig. 1. In order to investigate the dependence o f the energy resolution on sampling materials, two kinds of electrode material as passive converters for gamma rays were tested under the same geometrical conditions. The electrodes in the first calorimeter (henceforth A) are made o f 62 unit ceils, consisting of a 2 mm Fe plate (high voltage electrode), 2 mm of liquid argon, a 2 mm Fe plate (signal electrode) and 2 mm of liquid argon. The total depth and sampling thickness (half a unit cell) are 15.5 and 0.128 radiation lengths, respectively. In the second calorimeter (henceforth B), both electrodes for high voltage and signal m the first 16 unit cells were replaced by 2 mm Pb plates and the remaining 46 unit cells were retained as in A. The total depth and sampling thickness for the replaced cells are 11.9 and 0.371 radiation lengths, respectwely. The electrodes are sup357
Y. Asano et al. / Test o f sampling calorimeters
358
x I ,_~SAFETY VALVE
I.IL <~ o
LIQUID N2
I ARGON GAS
LIQUID N2 -60,
;u HEATEXCHANGE~
X
X
DIFFUSION
I
PUMP[
< -HV BEAM
.~ SIGNALS
Fig. 1. Schematic view of the detector and cryostat.
ported b y four stainless steel bars with a diameter of 10 m m covered with 5 mm thick teflon tube. They are fixed 2 mm apart from each other b y teflon spacers. The characteristic parameters are summarized in table 1. The stack o f electrodes is contained in a cylindrical vessel o f 50 cm long and 30 cm in diameter. The top and the b o t t o m flange o f the vessel can be taken off and the stack is slid in or out along the guide attached to the inside.wall o f the vessel. The center o f the flange is made thin so as to reduce the material in
Table 1 The characteristic parameters of the calorimeters Calorimeter
A
B
Converter Gap of hqmd argon Sampling thickness Total depth Detector capacitance Energy deposit m liq. Ar at 1 GeV Noise in r.m.s. Omeas/E
Fe 2 mrn 2 mm 0.128 r.1. 15.5 r.1. 40 nF 142 MeV
Pb 2 mm/Fe 2 mm 2 mm 0.371/0.128 r.1 11.9/11.8 r.1. 10/30 nF 88 MeV a)
2 5 MeV [7.9 + 37.1/ E (GeV)]l/2% 6.1%
1.5 MeV a) [10 + 92/ E (GeV)]l/2% 9.6%
(as/E) X x/E (GeV)
a) For the first 16 unit cells (Pb coverter only)
front of the active area. The net material before the detector is about 0.25 radiation lengths. A schematic view o f the cryostat is also shown in fig. 1. The liquid argon temperature is maintained by cooling with liquid nitrogen. To get an efficient cooling, the liquid nitrogen reservoir is placed just above the calorimeter and it directly cools the calorimeter via the Cu-plate heat exchanger at the b o t t o m o f the reservoir. To reduce the heat flow into the calorimeter and liquid nitrogen reservoir from the atmosphere the whole cryogenic system and the calorimeter are covered with 10 cm thick styro-foam. This cooling system maintained the liquid argon temperature sufficiently well. The consumption rate o f the liquid nitrogen was about 4 1/h. The purity o f liquid argon is crucial to the operation o f the calorimeter. Argon gas of research grade was introduced through the vacuum pipe and liquefied with the Cu-plate heat exchanger into the calorimeter vessel, which was evacuated to less than 2 X 10 -6 torr prior to liquefaction. To avoid distortion o f the electrodes due to rapid cooling, the argon gas was intermittently introduced for the first 30 min. It took about four hours to fiU the vessel and about 24 1 o f liquid argon was necessary. The amount of impurity was far less than 1 p p m b y ttus gas fiUing system. The entire collection o f cells o f the calorimeter is
Y. Asano et al. / Test of sampling calorimeters divided into several sections in order to observe shower profiles. A signal from each section is separately extracted from the calorimeter via a feedthrough (a commercial 22-pin hermetic seal type connector) and fed to a preamplifier.
359
(MeV) 40
~030 E
A
o 2O z i,i
2.2. Preamplifier In our calorimeter, a preamplifier is required to have wide dynamic range for input capacitance, because the capacitance of the calorimeter widely varies from 1 nF to 50 nF. Therefore, we used a preamplifier with six high gm FETs (2SK 147-V) connected in parallel as shown in fig. 2. The rise time of the preamplifier is about 1/as for the calorimeter capacitance Ca = 50 nF and the feedback capacitance Cf = 56 nF, which is optimized for the calorimeter. The measured ENC (equivalent noise charge) of the amplifier was 1.2 × 103 + 1.0 X Cd(pF) electrons in r.m.s, for a 2/as quasl-Gaussian shaping time constant. The result is shown as a straight line in fig. 3, where the ENC is converted to the energy by using the effective W-value in liquid argon for multi-plate sampling calorimeter (23.6 eV [6] × 2 = 47.2 eV). The ENCs measured under actual operation were 2.5 MeV and 1.5 MeV for the calorimeters A (Ca = 40 nF) and B (Ca = 10 nF), respectively, at a counting
2SKI47 x 6
E304
0
I
I0
I
20 Input
I
30 40 Capoclfonce
I
50 (nF)
60
Fig. 3. Measured equivalent noise charge (ENC) o f the preamplifier as a function o f input capacitance (solid line). The d o s e d circle A and the open circle B are the measured ENCs under operation for detectors A and B, respectively.
rate of less than 104 cps and are also plotted in the figure. Under operation, the ENCs were somewhat degraded due to current leakage in the calorimeter, rf noise of the accelerator and so on.
3. Experimental procedure and results
Tests of the calorimeters were done by using pions and electrons produced at the internal target of the
2SC594
E400
2SC574 2SC594
AD518
+24V
i 82K HT
T 2 2,4/. 56K
l'-,,,'r; I
IOK I
I J --o 5/,=
I Co I
15K
1501(
I I
7
E304
3 9K
lOP P'~EST ,tOM ~ ~50
l
'
;I;
:
y, lOp
2
Rf IOM
o4rja ~ _ _ _ _
0
K
IOK
c*
J8.2K
TZ 2/., •
E~5
1
~
E507
2SC594
Fig. 2. Circmt diagram o f the preamplifier.
,,~ 2SC980
2SA594 2SA594
- 24V
360
Y. A sano et al. / Test o f samphng calorimeters
KEK proton synchrotron. The incident beam was defined by the three-fold coincidence of the scintdlatlon counters, which are of the same size, 5 cm in width, 5 cm in height and 2 mm in thickness. Electrons were selected by a freon gas Cherenkov counter. The ratio of electrons to pions varied from a few percent to fifty percent, depending on the energy. The data were obtained at eight or seven energy points from 0.25 GeV to 2 GeV for the calorimeters A and B. The results are shown in figs. 4 - 7 . Fig. 4 shows the saturation curves for the calorimeters A and B. They rise sharply and reach nearly saturated level at a field of higher than 6 kV/cm. The operating high electric fields were set at 10 kV/cm and 8 kV/cm for A and B, respectively. The sharp rise of the saturation curves guarantees the high purity of the hquid argon used in this experiment. We did not observe any pulse height deterioration during two weeks of operation. The dependence of the mean pulse height on the incident energy is shown in fig. 5. The linearity is excellent for both colarimeters. Fig. 6 shows the variations of r.m.s, energy resolution divided by incident energy E as a function of E. The curves follow the functional form (a+b[E) 1/2 except at lower energy (around 250 MeV). The fitted values for the constants a and b are 7.9 and 37.1 for calorimeter A and 10 and 92 for B, respectively, where E is read in units of GeV. In the following, we consider three basic effects which give major contributions to the energy resolution m the calorimeter: (a) sampling fluctuation, (b) fluctuation due to the leakage of energy out of the
~ 5oo
•
A
*
B
L
I OOC
•
A
o
B
A "7-
500
-5 13.-
Off
I ~0 Incident
[I5 Energy
210
(GeV)
Fig. 5. Relation between the mean pulse height and the incident energy. The closed circles are for A and the open ctrcles for B.
calorimeter and (c) electromc noise. The total amount of the effects can be obtained through the quadratic summation of the above-mentioned effects. In practical cases, however, we must further add the contribution due to the energy spread of the incident beam. We assume that the contribution is also to be added in quadrature. Since the effect due to the energy spread of the beam gives a constant contribution to the energy resolution, the value is obtained from the constant term in the curve of the resolution. We thus obtained about 3% for this effect. The energy leakage from the back of the calorimeter can be estimated from fig. 7, where the energy resolution and the pulse height for incident electrons of 1 GeV are plotted as a function of material thickness. The
• A --kt32C
~B
b
* B
4oo v
"5
-~300
"5 200
gc B
IC
hA
O..
I00 '
015
i10 Incldenf
Elecfrlc
Field
(KV/cm)
Fig. 4. Saturation curves for 1 GeV electrons. The closed circles are for A and the open ctrcles for B.
115 Energy
210
(GeV}
Fig. 6. Energy resolution cr/E fitted to the form of (a ÷ b[E) :/2 The closed ctrcles are for A and the open ctrcles for B. The parameters, a and b, are given in table 1
361
Y. Asano et al. / Test of sampling calorimeters
curve shows saturation at a depth of 10-15 radiation lengths, which agrees with the commonly accepted value. Since we always used a depth of larger than 12 radiation lengths, the energy leakage from the back is negligible. The energy leakage from the side is also negligible, since we have a sufficiently large active area (25 cm in diameter) to cover the lateral spread of the shower. As mentioned in section 2.2, the electronic noise of the preamphfier is quite small. We obtained 2.5 MeV and 1.5 MeV for A and B, respectively, in terms of the ENC. Both values correspond to about 1.7% for an incident energy of 1 GeV. Here, we used 142 MeV for A and 88 MeV for B as the total energy deposited in the liquid argon. These were obtained by comparing both pulse heights of electrons and pions. The energy resolution due to the sampling fluctuation a s is thus calculated by sub-
5OO
g
~>4'0C
/
-A
.A
oB
30C B
20C
tracting these contributions from the measured r.m.s. width. The energy resolutions os/E for 1 GeV were 6.1% and 9.6% for calorimeters A and B, respectively.
4. Discussion Liquid argon calonmeters with multi-lead electrodes have been mainly used to measure electromagnetic showers. On the other hand, few studies on the use of multi-iron electrodes have been made so far. In addition, the dependence of the energy resolution on converter materials has not been investigated strictly. The present experiment has attempted to elucidate such a dependence. We found a large difference in the energy resolution between a calorimeter with lead converters and that with iron converters under the same geometrical conditions. Here, let us compare our results with some formulas for energy resolutxon due to the sampling fluctuation, which have often been used. On the basis of the Monte Carlo calculation [7] for electromagnetic showers in a multi-lead converter system, the following formula [8] of the energy resolution has been quoted, Us
E IOC
~s Depth (r.l.) {e) %
-g ao ~- 2o
ILlI0 ;
,b
,;
2'0
~'~
Depth ( r . t ) (b) Fig. 7. (a) Dependence of the pulse height for incident electrons of 1 GeV on the detector thickness. The closed circles are for A and the open circles for B. (b) Dependence of the energy resolution in incident electrons of 1 GeV on the detector thxckness The closed circles are for A and the open circles for B
(1)
? 52E(GeV) '
where t(r.1 .) is the sampling ttuckness in radiation lengths. It gwes good agreement with experimental results for calorimeters with multi-lead converters. To apply the formula to calorimeters with multi-iron converters, Cobb et al. [5] derived the following modified formula, taking the critical energy of converter material into consideration,
E
o
= 1/'- t(rl!
"
[ E(GeV)
'
(2)
where AE is the energy loss per samphng thickness in MeV. This formula is based on the same consideration as that assumed by Willis and Radeka [2] for the estimation of the energy resolution in the calorimeter with multi-iron plates, and predicts almost the same resolution for both calorimeters with iron converters and with lead converters, when the thicknesses of lead converter and that of iron in both calorimeters are equal to each other [1.e. os(Pb)/os(Fe)= 1.05]. However, this prediction is inconsistent with the present result [os(Pb)/os(Fe) = 1.57]. Stone et al. [9] summarized their results of scln-
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Y. Asano et al. / Test o f sampling calorimeters
Table 2 The values of R for the scintillation sampling calorimeters and the Iiquld argon ionization calorimeters Detector
R(Pb)
R(Fe)
R(Pb)/R(Fe)
Plastic scmt. [9] Liquid argon
14.8 15.8
16.9 17.1
1.14 1.08
tlllation sampling calorimeters with the formula
-Os (~)= R |/ E
t(r.1.)
VE(-(~eV) '
(3)
and investigated the variation of the proportional constant R for the change of the thickness of the scintillator or for various converter materials. Table 2 shows the values of R obtained by Stone et al. as well as ours. Each value of R obtained by both methods of scintillation and ionization calorimeters is nearly equal and their values are not different so much between the multi-lead converter and the multi-Iron converter either. This fact shows that the energy resolution of the calorimeter with multi-converters strongly depends on x/t rather than on the converter material.
5. C o n d u ~ o n Two liquid argon calorimeters with multi-iron converters and with multi-lead converters, whose configuration is identical except for the converter material, were tested for an electron beam of 0 . 2 5 - 2 . 0 GeV. A large difference in the energy resolution due to sampling fluctuation between both calorimeters
eters was found. This shows that the formula (1) or (3) rather than (2) gives a better expression for the energy resolution in the sampling calortmeter. The tendency of the energy resolution in the liquid argon ionization calorimeter with multi-converters is similar to that in the scintillation sampling calonmeter. The energy resolution is strongly dependent on x/t rather than on converter material. We are grateful to Prof. Y. Yoshimura and Dr. K. Tsuchiya for their kind advice and support in the preparation of the detector. We also appreciate the help of Drs. E. Shibamura and T. Tamae who participated m the operation of the detector.
References [1] L.W. Alvarez, LBL Phys. Notes No. 672 (26 Nov. 1968). [2] W.J. Willis and V. Radeka, Nucl. Instr. and Meth. 120 (1974) 221. [3] G. Knles and D. Neuffer, Nucl. Instr. and Meth. 120 (1974) 1, J. Engler et al., Nucl. Instr and Meth. 120 (1974) 157. [4] D. Hitlin et al., Nucl. Instr. and Meth. 137 (1976) 225; C.W Fabjan et al., NucL Instr. and Meth. 141 (1977) 61; C. Cerrl and F. Serglampietri, Nucl. Instr. and Meth. 141 (1977) 207, G S. Abrams et al., IEEE Trans. Nucl. Sct. NS-25 (1978) 309, A. Delfosse et al., Nucl. Instr. and Meth. 156 (1978) 425; A. Babaev et al., Nucl. Instr. and Meth. 160 (1979) 427. [5] J.H. Cobb et al., Nucl. Instr. and Meth. 158 (1979) 93. 16] M. Miyajima et al., Phys. Rev. A9 (1974) 1438. [7] H.H. Nagel, Z. Physik 186 (1965) 319; U Vorker, DESY Report 65/6. [8] D.R. Nygren et al., Proposal for a PEP facility based on the time projection chamber, PEP-4 (30 Dec. 1976). [91 S.L Stone et al., Nucl. Instr. and Meth. 151 (1978) 387.