Gas amplification in isobutane and P10 filled gas counters

L

*H @L __

NuclearInstruments

and Mrthods

in Physics

Research

A 366 (1995)

TO-.?21

NUCLEAR INSTRUMENTS a METHODS IN PHYSICS RESEARCH

-9

-53 i--

I@

._

SectIon A

ELSEVIER

Gas amplification P.P. Skakkeeb,

in isobutane

A. Joseph, Department Received

and PlO filled gas counters

A.M. Vinodkumar,

of Physics. 22 December

KM. Varier*,

B.R.S. Babu

Urlir~ersityof Calicut, Kernla 673 63. Itdin

form received 7 June 199.5

1994; revised

Abstract Gas amplification factors have been measured for isobutane and PlO gases in single wire proportional counter and parallel plate avalanche counter geometries for reduced field strength value ranging from 140 to 1000 V cm- ‘Torr ‘. Data using the SWPC were found to satisfy the Aoyama formula for gas amplification with 1~ = 0 and also the Diethorn formula. The PPAC data are satisfactorily described again, by the Aoyama formula with m = 0 as well as by the Zastawny formula. The values of the relevant parameters in the formulae used have also been extracted from the fits.

1. Introduction Ever since the first observation [l] of the ionising property of radiations traversing a gaseous medium. its potency as a powerful technique for detection of radiation has been fully appreciated. A variety of gas detectors are in use at present for different purposes. The simplicity of construction, the comparatively low cost. their flexibility to suit different experimental conditions, the large number of geometrical configurations (especially in large areas) in which they can be used and the various types of pure gases and gas mixtures that can be employed in these detectors make them a common equipment in any nuclear physics laboratory. They are being used for simple detection purposes as well as for energy loss, timing and position measurements. Excellent reviews on gas detectors are available in the literature [2]. Proportional Counter The SingleiMultiwire (SWPC/MWPC) and the Parallel Plate Avalanche Counter (PPAC) are the two main types of gas detectors being commonly used. In both these types, the operating voltages and pressures correspond to the proportional gas amplification region. where the net number of ion pairs produced is proportional to the energy deposited in the gas volume by the incident ion. The first Townsend coefficient (Y [3], defined as the mean ionisation probability per unit path length of the radiation/particle, or alternatively as the ionisation number per unit path length, is the most important parameter determining the overall performance of the

* Corresponding

author.

0168.9002/95/$09.50 @ 1995 Elsevier SSDl 016%9002(95)00623-O

Science

B.V. All rights

gas detectors operated under gas multiplication conditions. Various formulae are available in the literature [411] for the first Townsend coefficient as functions of the reduced electric field strength S = E/p where E is the electric field and p is the gas pressure. Each formula is characterised by a set of parameters. The range of values of S over which these formulae are valid. of course. vary from one formula to another. Overlapping ranges of applicability also exist for some of these. Some formulae are purely empirical but some are derived from models on the behaviour of electrons in an electric field using appropriate initial assumptions (e.g. Kowalski [9]. Aoyama [lo] etc.). Using the above formulae, one can derive expressions for the gas gain M defined by the following formula lnM=

rl -cudr, I II

where r, and r, represent the starting and terminal points of the gas amplification. The output pulse height from a gas detector can be seen to be related to M by the following relation: In M = In(hlgp) where

,

g is the overall

g = (fe/C)

(K/w).

(2) gain factor

given by (3)

In the above equation, f (in channels per V) is the net gain of the electronics used for detector pulse processing, e is the charge on the electron, C is the detector capacitance, w is the average energy for the proreserved

P.P. Shakkeeb

et al.

I Nucl.

Ittstr.

nnd Meth.

duction of one ion pair in the gas used and K is a factor such that Kp gives the energy deposited by the incident ion within the active volume of the detector. Thus. measurements of the pulse height h at a given pressure p will give the gas gain M if g is known. The gas gain formula proposed by Aoyama [lo] is a generalised one from which the others can be derived as special cases. m = 0 gives Williams and Sara’s formula [7], m = 1 gives Diethorn’s formula [6] and m = 1 gives a product of the forms proposed by Rose and Korff [4] and by Charles [S]. The Zastawny form [ll] also is derivable as another special case. With L = 0, one obtains Kowalski’s form [9] and with the additional constraint m = 0. one gets Khristov’s form [S]. In the Aoyama expression, the ratio L/K = y has the significance of an effective ionisation potential. On the experimental side, there have not been very many attempts to verify the various gas gain formulae and then determine the relevant parameters. The early measurements [12-151 refer to relatively low values of Brunner [16] and Sernicki [17] extended these for S values from 173.5940 V cm ‘Torr - ’ . Later investigators [l&25] attempted to test the validity of the various gas gain formulae and to determine the parameters used in these formulae. Gases like n-heptane, xenon. xenon + hydrogen, neon + hydrogen and argon and argon + hydrocarbon mixtures have been used in these investigations. Here we report the results of our measurements on the SWPC and PPAC geometries using PlO and isobutane gases for reduced electric field strength values in the range from 140 to 1000Vcm~‘Torr~‘. Some preliminary results have been reported elsewhere [26].

2. Experimental

details

The single wire proportional counter used in the measurements had an outer aluminum cathode in cylindrical form with an inner diameter of 4cm. The anode wire was a 10 pm isohm wire. The entrance window was of 10m mylar with an active area of 14cmXlcm. The parallel plate avalanche counter was of active area 4 cm X 4 cm. It was also enclosed in an aluminum enclosure. The entrance window in this case was also 10 km mylar foil supported by a nylon wire mesh. The electrodes used were of 6 m aluminised polypropylene foils. The electrode spacing was 1.4 mm. The counter gases used were isobutane supplied by M/s Alphagaz, France and PlO (Ar + 10% methane) supplied by Indian Oxygen, Madras. Gas pressures in the range 4-20Torr were used for the PPAC and 25-100 Torr for the SWPC. A standard gas flow setup was been used for uniform gas flow through the

in Phys.

Res. A .Wh (199.r)

320-313

321

detectors at the desired pressure, which was measured by a Leybold-Heraeus capsule vacuum gauge. The pressure control was achieved by using precision needle valves at the inlet and the outlet parts. A 2.4 PCi Am source (BARC. Bombay) was used to get 5.48 MeV alpha particles. The detector and the source were placed inside a vacuum chamber. The output signals from the detectors were fed to an ORTEC preamplifier and then to an amplifier. Detector bias has been provided by an ORTEC HV supply. Typical voltages used range from around 250V to 400 V for the PPAC and 600-800 V of the SWPC. Pulse height spectra were stored on a PC based multichannel analyser. From the recorded pulse height spectra, the peak pulse height in channels (h) were noted as function of p and V. i.e. as a function of the reduced electric field strength S. The data were analysed by means of the generalised Aoyama formula [lo] for m =0, i and + as also the Diethorn expression (special case m = 1 of the Aoyama form). In the above equation (2), g is the net gain. It was treated as a free parameter along with the constants K and L of the Aoyama formula and a least squares curve fitting was done to determine K and L for various g values. The value of g giving the minimum xz was chosen and the corresponding K and L are taken as the best fit.

3. Results and discussion The plots of the experimental values of lnln[Ml are given in Figs. 1 and 2 for the SWPC measurements (using PlO and isobutane). where ra is the anode wire radius and S, is the S value at the surface of the wire. Similar plots of In[Ml(pdS)] are given in Figs. 3 and 4 for the PPAC measurements for m = 0, $ and _{(h ere d is the electrode spacing). The Diethorn plots (m = 1) are given in Fig. 5 for SWPC. The PPAC data clearly do not satisfy Diethorn formula. Hence for these data, the modified Diethorn formula, viz. that of Zastawny has been used. The results are plotted in Fig. 6 for both the gases used. It has been observed that the variance values are minimum for m = 0 in the Aoyama expression and for the Diethorn expression. as far as the SWPC data are concerned. For the PPAC data. the variances were found minimum, again for m = 0 in the Aoyama expression and for the modified Diethorn expression (i.e. the Zastawny formula). The parameters in the relevent expressions for the gas gain, derived from the various fits to the present data, are given in Tables l-4. An overall error of about 10% has been estimated for the values of these parameters. It is seen that the V, values obtained from the SWPC data are agreeing well with those obtained (prJ,)]

322

P.P.

I\

et al. I Nucl. Instr. and Meth. in Phw.

150

130

110

-3.0

Shakkeeh

330

m = Y3

110

Res. A 366 (199.5) 20-324

’ c 0

SWPC

II

II

E

E

I

350

I

”

370

I

390

I

I

m=%

PPAC

s

I, E

P10 AOYAtlA PLOT5

PlO AOYAMA

PLOTS

_c

m=o

b

m.t2

-+-

m=t3

-

m=O

1.2-

i

% Y

fl.0. C

j-6

0.8

3LO

380

420

10

12

1L

b

Y

I

460

m ='/2

16

78

80

L I

m=O

6.0

S,m-l[xlO‘( v cme'Tori'lm"]

Fig. 1. Experimental plots of In[ln M/(pr,S,)] vs. Sy-’ for SWPC using PIO. The solid lines are the Aoyama fits for m = 0, t and i7’

50

60

10

90

80

100

I

m-93

110

82

86

88

90

92 m=O

6.5 7.0 m-l m-l 50 [Xr030' cm-' Torr-1 1

J

75

]

Fig. 3. Experimental plots of In[ln Ml( pr,Sz)] vs. Sy-’ for PPAC using PIO. The solid lines are the Aoyama fits for

m =O, f and +.

120

from the PPAC data for PlO. For isobutane the V, values agree within the experimental error. However. the L values are seen to be different. This may be attributed to the fact that in the case of SWPC, the reduced field strength varies over a very wide range, whereas in the PPAC it is a constant. The observed variation in the value of L suggests a possible residual dependence of L on S so that in the case of the SWPC data what we are deriving from the fits is an average value only.

SWPC ISOBUTANE AOYAIIA PLOTS

Acknowledgements

-3.25-

ml=

%

I

\ ‘

I

I

250

200

I

300

m=O L

L

1 6

1

S,m-'[X 10 (V

! 8 rni’

1

(

10

’

Tori'lm-'1

Fig. 2. Same as Fig. 1, for SWPC using isobutane.

i 12

The authors are grateful to Prof. V.K. Thankappan and Prof. G.K. Mehta for their encouragement and help throughout this work. Financial support from the Department of Science and Technology, Government of India, New Delhi, is gratefully acknowledged. Thanks are also due to Dr. A.K. Sinha and Mr. D.O. Kataria of the Nuclear Science Centre, Delhi, for help during part of the work which was carried out there.

P.P. Shakkeeb

et al. I Nucl.

Instr.

and

Meth.

in Phys. Res. A 366 (1995) 320-324

323

PPAC ZASTAWNY

PLOTS

-1SDBUTANE AOYAMA

PLOTS

PlO

\ .023-

,022-

,026

5: e f. c

.021-

.02-

1

1

4.0

Fig. 6. Zastawny 3.0

Sam-' [XlO'lV

Fig, 4. Same

5.0

4.0

Table 1 Aoyama parameters

using isobutane.

1 80

Isobutane

0 + ;

345.0 48.4 118.0

11.26 10.90 11.10

PlO

0 ; t

513.0 37.1 84.3

12.7 13.1 13.6

1.0 cm-’

plots for SWPC

1.5 Torr-'1

8.0

for PI0 and isobutane.

using PPAC

Gas used

L[(Vcm-‘Torr-‘)I-“]

Yl”l

Isobutane

206 12.9 32.9

9.92 9.99 10.0

PlO

307 23.7 52.8

13.83 13.01 13.39

Table 3 Diethorn I

WI

N2

i:

I

using SWPC

Gas used

Table 2 Aoyama parameters

Fig. 5. Diethorn

plots for PlO and isobutane.

cm-'T~rr-?~~]

as Fig. 3, for PPAC

In So (V

10

In=0

SWPC

6.5

8.0

6D

s-'[xldv-'cm lorr 1

parameters

using SWPC

[lo]

Gas used

S, [V cm-‘Torr-‘1

D [v-l]

Isobutane PlO

19.61 170.10

0.016 0.027

P.P. Shakkwh

324 Table 4 Zastawny

parameters

using PPPC

Gas used

S,, [Vcm

lsobutane PI0

0.018 0.025

et al. I Nucl. Instr. und Meth. irl P&s.

[IO]

‘Torr~ ‘1

D IV-‘] 1.71 1.67

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