Electromagnetic calorimeter with thin gap wire chambers

Nuclear Instruments and Methods in Physics Research A 351 (1994) 330-335

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SectionA

ELSEVIER

Electromagnetic calorimeter with thin gap wire chambers C. Chen a,*, H.S . Chen X.W. Tang a, K.L. Tung a

a,

a,

M. Fukushima b, P. LeCoultre c, H.T. Li a, J .B . Liu a, J .H. Wang a, R.J. Wu a, Y. Yoshimura b, H.L. Zhuang

a

Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100039, China

b KEK, National Laboratory for High Energy Physics, 1-1 Oho, Tsukuba, Ibaraki 305, Japan Eidgenossische Technische Hochschule, ETH Zurich, CH-8093 Zurich, Switzerland

Received 21 February 1994 ; revised form received 27 June 1994 Abstract In this paper we report about the experimental results obtained in a beam test of a gas sampling EM calorimeter . The module consists of 20 thin gap (30 mm) multiwire chambers as active elements, interleaved with brass absorber plates of 12 mm thickness or 0.84 radiation length . The KEK electron beam was tuned to energies of 0.5, 1.0, 2.0, 3.0, 4.0 GeV. The construction and performances of the chamber, the assembling of the module, the experimental setup and data acquisition system are briefly described. The energy resolution of the module is found to be 25%/ 1E (E in GeV) . In the range of measured energies, good linearity of the energy response is obtained . The longitudinal development of EM shower, the relation between energy resolution and thickness of the absorber as well as the high voltage of the chamber are studied. The effect of incident position of the beam on the performance of the module is also given. 1. Introduction Currently new types of sampling calorimeters are being developed in view of experiments to be performed at highest energies, e.g. at the LHC. Very tight constraints are imposed on the detectors, like fast response, fine granularity, compact volume, stable and reliable run conditions, minimizing the maintenance because of difficult access . In Refs . [1-4] a thin gap chamber working with a CO, n-pentane mixture to be used in such calorimeters has been described and results of its performance have been given. The performance of our thin gap multiwire chambers has been reported in Ref. [5] and a calorimeter module with brass plates has been tested with cosmic rays [6]. These detectors can work in the limited streamer mode of operation giving large output signals, about 100 mV into 50 dl load, therefore simplifying the readout and the associated electronics. The time response is fast (< 5 ns rise time and about 10 ns width) . In order to reduce the overall length of a calorimeter module we intend to use the brass absorber plates as cathodes . In this paper we report about the experimental results of the test calorimeter obtained at an accelerator beam . The calorimeter module has been tested with the sec-

* Corresponding author.

ondary Tr-2 beam of the 12 GeV proton synchrotron at KEK, Japan. We firstly describe the structure and main performance of the chambers, the construction of the calorimeter module, the experimental setup and data acquisition system . We then disscus our calibration technique of the chamber, the identification of electron events and pion events, the response and finally the energy resolution of the calorimeter module . The longitudinal development of the electromagnetic showers, the energy resolution with respect to the beam position, high voltage and thickness of absorber are also presented.

2. Chamber structure and main characteristics The general structure of the thin gap gas multiwire chamber used as sampling layer of the calorimeter module is shown in Fig. 1. The size of the chamber is 16 .0 X 16 .0 cm z and the effective area is 12.0 X 12 .0 cmz. The cathodes of the chamber body are copper foils (50 wm) which are glued to G10 plates (1 .5 mm thickness) . The central anode plane consists of 59 goldplated tungsten wires of 50 p.m diameter with a spacing of 2 mm . Both ends of the anode wire were welded on the printed circuit plate which was glued to an insulating frame of 1 .5 mm thickness in order to keep a gap of 3 mm. The wire tension is 200 g. The cathode plate, anode frame and gap spacer were glued and sealed with Araldite glue .

0168-9002/94/$07 .00 © 1994 Elsevier Science B.V . All rights reserved SSDI0168-9002(94)00871-X

C. Chen et al. I Nucl. Instr. and Meth. to Phys. Res. A 351 (1994) 330-335 HV Beam

d wunter r-

St 52

S3 . SO

331 Module of EM cal

j-790---+-305-+-470-1 .-250 1 j

Po S5

S6 660--+- 500 -j

Fig. 3. Experimental layout of test beam and calorimeter module. Fig. 1. Schematic view of the small gap gas chamber.

lators S1, S2, S3 and S4 after the

The chambers were operated with a mixture of CO,

bubbling through liquid n-pentane . We summarize the main characteristics of our chamber as described in Ref. The amplitude of the non-amplified signals (with 50 SZ load) is 80 mV at 3.5 kV for an incident 55 Fe X-ray . The rise time of the output pulses is smaller than 5 ns and the base width is about 10 ns .

55

(2) Pulse charge characteristics were measured using Fe 106Ru X-ray and ß-rays. As shown in Ref. [5] the ratio of the chamber response to X-rays and ß-rays demon-

strates that the pulse charges are proportional to pri-

mary ionization at low voltage (about 2.7 V) and are practically independent of the type of primary ionization at higher voltage (3 .8 kV).

3. Experimental setup and data acquisition Fig. 2 shows the structure of the calorimeter module.

The calorimeter consists of 20 chambers (0 .03Xo radiation length each) interleaved with 19 pieces of brass plates (0 .84Xo each).

The wires of neighbouring chamber planes are perpen-

dicular to each other. In our experiment 59 wires in one chamber were connected together as one output channel.

The total thickness of the absorber and chambers amounts to 16 .5X, . The -rr-2 test beam covers a momentum region between

0.5 and 4.3 GeV/c and contains pions and several percent

electrons. The momentum spread is 1% . The layout of our

C

counter were used as a

beam telescope . The sizes of Sl-S4 scintillators were 60 X 10 X 5 mm ;. They were installed in a crossed way, in

order to define a beam size of 10 X 10 mm2. Electrons

were identified by the fivefold coincidence of C and S1-S4; pions by the anticoincidence of C and S1-S4.

The calorimeter module was placed behind this beam

telescope and in front of a 200 X 200 mm 2 scintillator with

two phototube readout, each one was at one end and called S5 and S6 respectively . Most of the showers produced by electrons are absorbed by the module . The pions mostly

reach S5 and S6 after having passed through the calorimeter module . Electron and pion events have different time characteristics. Using the time of coincidence

C X S1

X S2

X S3 X S4 as a start signal to the TDC and S5 and S6 as a

stop allows the separation of electrons and pions. Electrons

do not produce a stop signal, whereas pions do, because they reach S5 and S6. Sometimes pions could be misidentified as electrons by the C counter due to limited resolution . The time information was therefore used for further e/ ,rr identification . The signals from the sampling chambers were digitized via CAMAC ADCs and readout . Amplifiers (eight fold) were used in runs with beam momenta of 0 .5, 1 .0 and 2.0

GeV/c, but not at momenta 3.0 and 4 .0 GeV/c. A VAX workstation is taken as on-line computer and a KSC-2922 is used as interface to the on-line computer and CAMAC crate controller KSC-3922. The chamber high voltage was fixed to 3.70 kV . The n-pentane container was in a thermal bath to keep the temperature stable in a range of ± LOT at 15°C . The temperature changes were monitored.

setup is shown in Fig. 3. The electrons were identified by a Cherenkov counter

C,

which was filled with Freon-12 at 1

atm for beam momenta under 2.0 GeV/c and 0.5 atm at beam momenta 3.0 or 4 .0 GeV/c. Four small size scintil-

4. Data analysis The chamber calibration was carried out with thoroughgoing -rr (nonshowering as at 0 .5 GeV/c are considered as minimum ionizing particles) . The pulse height distribution of each chamber was recorded. Fig. 4 shows the data of chamber no . 1. Fitting the data with a Landau distribu-

160.160

tion, the most probable signal could be found for each chamber. The average over the most probable values of the 20 sampling chambers has been used as normalization value for each chamber.

N1 %

N- i

1,

We have performed measurements with electrons at 3.0

0639Xa(cu)

N-20

Fig. 2. Schematic view of the calorimeter module .

and 4 .0 GeV/c without amplifiers and at 0.5, 1.0 and 2.0 GeV/c with amplifiers . The compatibility between the two situations has been studied. Data at 3.0 GeV/c with

332

C. Chen et al. /Nucl. Instr. and Meth. to Phys. Res. A 351 (1994) 330-335 C Ó

300

250 -

100

50

0

Fig. 4. Signal distribution for events without interaction in first sampling chamber. High voltage: 3.70 kV. Beam momentum : 0.5 GeV/c. The curve is a fit with a Landau-function.

amplifiers and without have been compared . It has been found that multiplying by a factor 8 the mean peak channel of the data collected without amplifiers, corresponds to the peak position of the data collected with amplifier. In the experiment a Cherenkov counter was used to identify electrons and pions. Pions which were misidentified as electrons could be identified via their signal in the calorimeter . Overflow events were eliminated being supposed to correspond to occasional discharges in the chambers or to other reasons. 5. Experimental results

2000

4000

6000

11000

10000 12000 14000 16000 18000 20000

Pulse Amplitude

Fig. 5. Distribution of the summed ADC signals of all 20 sampling chambers. Beam momentum : 1 .0 GeV/c. High voltage: 3.70 kV.

Table 1 Energy resolution and linearity of energy response E [GeV] 0.5 1.0 2.0 3.0 4.0

o- IE [%] 34±2 25+1 17±2 15±1 13+1

(Q IE) X ~E 24±2 25+1 24±2 26±1 26+1

[MIP]' s'

32 .8±0 .3± 2.5 66 .7 ± 0.5 ± 5.2 129.8±1 .5±10.1 187.0±2 .0+14.5 246.9±2.2±19.1

' First error is statistical and second is systematical

300

5.1 . Energy response and resolution Fig. 5 shows as an example the signal sum spectra of the 20 sampling chambers at 1 .0 GeV/c. We have fitted data at 0.5 GeV/c with a Landau function and took the FWHM/2 .36 as the standard deviation o, of a Gaussian . For the other momenta data were fitted with a Gaussian . Table 1 (column 4) and Fig. 6 show the energy response in MIP. By fitting the data of Fig. 6, we found a value of 65 MIP/GeV. The calorimeter shows good linearity in the energy interval studied. The quoted systematic errors in Table 1 are taken from the measured deviations from the MIP value (defined at 0.5 GeV/c) compared to the most probable value for particles at the particular momentum . Table 1 lists the energy resolution Q/E at different energies . Fig. 7 shows the dependence of the energy Our results resolution on energy . Q/E is about 25%/ of energy resolution is consistent with the results reported

r.

Fig. 6. Linearity of energy response .

C. Chen et al. /Nucl. Instr. and Meth. In Phys. Res. A 351 (1994) 330-335

w

45

W

b

40

O Expenmental Data

35

" MC Dam

333

*

05G-1 Exp dam

"

4 G,vl Exp dora

30

}_

25

10

d

20

4

10

2

05

1

L., 15

2

25

I 3

35

4

-iiL . 2 3 4

45

E(Gev)

Fig. 7. Energy resolution of calorimeter .

`* :n± i

f"

I 5

6

I 7

I 8

I ~3 9 10 11

I I 12 13 14

_

* * * * 15 16 17 18 19 20

No. of CHAMBER

Fig. 9. Normalized longitudinal development of the electron-induced EM shower. The histograms represent the Monte Carlo simulation, while the data points are the experimental results.

in Ref. [2], or/E = 27%/ v/E _ at electron energy of 1-4 GeV. A Monte Carlo calculation using the EGS program [9]

has been performed for our specific calorimeter. The obtained energy resolution is 30%D better than what we got from our measurement. Again this observation is con-

firmed in Ref. [2]. This can be explained by the relatively high energy cuts used in the program namely 0.1 MeV for photons and 1 .5 MeV for electrons. 5.2.

Longitudinal development of the electromagnetic

shower

0

1

2

3

4

5 6

7

8

9 1011 12 1314 151617181920

No . of CHAMBER

Fig. 8. Normalized longitudinal development of EM showers for several beam momenta. The curves for 0.5 and 1 .0 GeV/c are Landau-function fits to the distribution, while the ones for 2.0, 3.0 and 4.0 GeV/c are F-function fits to the distribution . The abscissa represents the number of samplings, while the ordinate indicates the normalized energy deposit . Table 2 Maximum longitudinal development tin_ of EM shower )a E [GeV] tmáx (Xo 0 .5 1.0 2.0 3.0 4.0 ' The error is statistical only .

2.18+0.09 3.24+0.11 4.28+0.01 0.01 4.59+0.02 0.02 5 .04+0.02

Fig. 8 shows the normalized longitudinal development of electromagnetic showers at different energies . Fitting the data with a F-function, the maximum tm;x (in units of radiation length) of the shower development could be extracted. As mentioned in Refs . [7,8], the suitable energy

region for fitting the longitudinal development with a 1'-function is between 1 and 100 GeV. Indeed the F-function does not fit the data at 0.5 GeV/c (for absorbers like Table 3 Energy resolution for different incident beam positions a

E [GeV]

Ll'

L3

0.5 1.0 2.0

37+3 26±1 17±1

34±1 27±1 17±2

b

U2 b

U4

35+1 26±1 17+1

35±1 26±1 17±2

a Ll and L3 represent the calorimeter positions 1 cm, respectively 3 cm horizontally off the beam line . b U2 and U4 represent the calorimeter positions 2 cm, respectively 4 cm vertically off the beam line.

C. Chen et al./Nucl. Instr. and Meth . in Phys. Res. A 351 (1994) 330-335

334

Table 4 Characteristics of calorimeter module as function of high voltage of chamber High Voltage [kV] 3 .2 3 .3 3 .4 3 .5 Peak position (ADC counts)

3 .6

3 .7

3.8

2420 +16

3240 +23

3967 +28

5064 ±34

6021 ±50

7581 ±39

8384 ±77

(ADC counts)

704.7 +16.9

1011 +27

1133 +35

1361 ±41

1697 +61

1985 ±47

2210 ±94

Q/E [%]

29

31

29

27 ±1

28 ±1

26 ±1

26 ±1

0'

(stat. only)

±1

±1

±1

carbon to uranium) . In fact the data can be well fitted with a Landau function. Table 2 presents the measured t e, . These values do not differ much from the t.*ax according to Refs . [7,8]. t*ax=1 .0X[ln(E/Ee)+ Cel,

Ce = -0 .5 .

Here E, is the critical energy . E is the energy of the incident particle . Ce = -0 .5 for EM shower induced by electrons. Fig . 9 shows both the longitudinal developments of electron showers and the Monte Carlo simulation at 0.5 and 4.0 GeV/c. The experimental results are in good agreement.

20

10

32

34

36

38

HV (Kv)

Fig. 10 . Relationship between energy resolution and high voltage at 1.0 GeV/c. Table 5 Relation between energy resolution and absorber thickness at 1 GeV/c. t(X") 0 .87 1 .74 2 .61

I(

0 .93 1 .32 1 .62

Xo )

o- 1E [%]

Associated chambers (no.)

25 .4±0.7 40.2±1 .3 50.5±1 .5

1+2+ 1+3+ 1+4+

+18+19 +17+19 +16+19

5.3 . Energy resolution and shower development as a function of position of the incident beam, the high voltage and the absorber thickness The above discussed data were taken with a beam incident on the center of the calorimeter module . We have also studied the effects when the beam positions were off center . Ll and L3 represent the calorimeter positions 1 cm, respectively 3 cm horizontally off the beam line . U2 and U4 represent the calorimeter positions 2 cm, respectively 4 cm vertically off the beam line . Table 3 shows the energy resolution obtained for the four positions at 0.5, 1 .0 and 2.0 GeV/c . The data show that the energy resolution does not change for the mentioned position variations . The cross section of the calorimeter is 16 .0 X 16 .0 cm 2 . Even at position U4 the leakage is negligible. Also the longitudinal shower development as a function of incident position has been checked. No obvious change in the longitudinal development has been observed . Fig. 10 shows the energy resolution at 1 GeV/c for different high voltage settings . The energy resolution improves as high voltage increases, because the chambers work more and more in streamer mode . Table 4 shows the energy resolution as a function of high voltage. In order to study the dependence of the energy resolution on the absorber thickness, we summed up the signals of each chamber, of each second or each third chamber. Table 5 shows the results. As expected the resolution is found to be proportional to v~_t, t being the sampling absorber thickness in units of the radiation length . 6. Conclusion The beam test for the calorimeter module with this new kind of sampling chambers shows the expected energy resolution of calorimeters with gas wire chambers : o-/E is about 25%/ ~E . In the energy region tested the calorimeter shows a good linear response . These results are in agreement with the ones obtained in Ref. [2]. The many advantages of this new calorimeter type, such as the thin thickness, the low cost, easy mass production, large output pulse and fast time response, makes it a good candidate for calorimeters in experiments at the next generation hadron collider if aging is demonstrated not to be a problem.

C. Chen et al. /Nucl. Instr. and Meth . in Phys. Res. A 351 (1994) 330-335

Acknowledgements We acknowledge the support of IHEP, Chinese Academy of Sciences and KEK, Japan . We thank Mr . K.S . Yang, Mr . Y . Kodama, Drs . X.F . Yang, G .M. Chen, J .Y. Zeng and Mrs . G .J . Zhou, X.L. Dong for their help . We also thank the test beam group of IHEP, Chinese Academy of Sciences . Our members of IHEP, Chinese Academy of Sciences acknowledge the support of the National Natural Scientific Foundation of China .

335

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