The quenching of Penning mixtures in a cylindrical proportional counter

Nuclear Instruments and Methods in Physics Research A256 (1987) 561-566 North-Holland, Amsterdam

561

T H E Q U E N C H I N G O F P E N N I N G M I X T U R E S IN A CYLINDRICAL P R O P O R T I O N A L C O U N T E R J.P. S E P H T O N i), M . J . L T U R N E R i) a n d J.W. L E A K E 2) I) X-ray Astronomy Group, Physics Department, Leicester University, Leicester, UK 2) Atomic Energy Research Establishment, Harwell, Oxfordshire, UK

Received 30 December 1986

The Penning mixture neon-argon has previously been shown to give improved proportional counter energy resolution over conventional gas fillings such as argon-methane. The lack of a quenching agent leads to degradation of resolution at high values of gas gain. In this paper an account is given of an investigation into the effect of methane on the performance of neon-argon. It is shown that methane can improve resolution at high values of gas gain but not to the level obtained at low values of gas gain with neon-argon. The Penning mixtures argon-acetylene and argon-xenon are also investigated. The latter filling is suitable for use at high values of gas gain and appears to merit further study.

1. Introduction The proportional counter is widely used for soft X-ray detection and spectroscopy. Its large area capabilities and simplicity of construction have led to the use of the proportional counter in medicine, astronomy and nuclear physics. Applications of this nature have encouraged research work aimed at improving the performance of the device. One of the fundamental parameters of interest is the energy resolution. This is typically about 16% at 6 keV with a gas filling of argon-methane. The resolution of the gas scintillation proportional counter (GSPC) is roughly twice as good as this. The GSPC however, does not have the advantages of robustness and simplicity offered by the proportional counter. The resolution of a proportional counter is determined fundamentally by statistical variations in (1) the number of ion pairs produced in the primary ionisation and (2) the size of the subsequent electron avalanche. The work of Alkhazov et al [1,2] indicates that fluctuations in both processes can be reduced by increasing the efficiency of the ionisation mechanisms involved. High ionisation efficiencies can be obtained by using Penning mixtures. In a Penning mixture excited atoms in the majority gas are able to ionise the minority gas. Sipila [3-5] has shown that the use of Penning mixtures does lead to improved energy resolution. Using neon-argon he has obtained a resolution of about 11.5% at 5.89 keV. The work of Sipila and Sephton et al. [6] indicates, however, that the gas gain has to be kept low to achieve such an improvement. At gas gains above about 100, secondary avalanching becomes significant 0168-9002/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publ.ishing Division)

due to the lack of a quenching agent. Secondary avalanches increase the statistical variation in the charge induced on the anode and so lead to degraded resolution. Operation at low values of gas gain causes problems at low X-ray energies as the electronic noise becomes significant. In this paper an account is given of an experimental investigation into the operation of certain Penning mixtures at high values of gas gain. The effect of adding methane to neon-argon is investigated. The main parameters of interest are ionisation efficiency, secondary avalanche suppression and energy resolution. The Penning mixtures argon-acetylene and argonxenon are also investigated as they both have admixture gases which can suppress secondary avalanche production. Some previous investigations into the quenching of Penning mixtures have been carried out. Pawlowski [7] found" that a 5% addition of quench gases (such as methane, nitrogen and butane) to the Penning mixture argon-acetylene (992 : 8) did not affect the mean ionisation energy W by more than ± 0.3 eV. Sipila [8] found that the Penning mixture argonxenon yielded a resolution of 6.2% at 22.1 keV when operated at a gas gain of 100. The mixture ratio used, 80 : 20 gives the lowest value of mean ionisation energy [14]. Fuzesy et al. [9] investigated the effect of methane and carbon-dioxide on argon-xenon in a multiwire proportional chamber. They found that the maximum gas gain at which proportional operation could be obtained rose from 10 5 (without the addition of CH 4 or COz) to 6 × 10 6. They also found that Penning ionisation was still significant.

562

J.P. Sephton et a L / The quenching of penning mixtures

2. Experimental arrangement

i

I

i

i

I

2I

10 4 .--The measurements were made with a cylindrical geometry proportional counter manufactured by A E R E (Harwell). The beryllium cathode had an effective length of 10 cm and an internal diameter of 2.34 cm. A 25.4 # m diameter wire formed the anode. When using Penning mixtures in proportional counters it is particularly important that a high level of gas purity is maintained. U H V techniques were therefore employed in the evacuation and filling of the counter. Following ion pumping (and baking when necessary) the counter was filled with BOC research grade gas. A low noise preamplifier of the type described by Smith and Cline [10] was employed. A noise charge of about 200 electrons (rms) was obtained with this design. A n Ortec 450 main amplifier and a Harwell ® 6000 series pulse height analyser were also used.

tort CH4

1.14 t o r r 0.41 torr CH4 CH4 0 torr CH

/j/

103 -

/

0

102

/

O (.9 '

3. Results and discussion

101

neon

~

3.1. Operation with N e - A r - C h 4 The counter was filled to 1.1 atm with n e o n - a r g o n (995:5) i.e. pressures of about 832 and 4 Torr, plus various admixtures of methane. The methane pressure ranged from 0.41 to 2 Tort. It seems reasonable to expect that the addition of methane to n e o n - a r g o n will reduce the predominance of the Penning effect. The counter was therefore also filled with pure neon to provide a standard of comparison. In fig. 1 the effect of methane concentration on the gas gain is indicated. The addition of methane extends the range of linearity of the gas gain curves. As more methane is added though, the gas gain, at the same anode voltage, decreases (in deriving the values of gas gain it was assumed that the methane does not alter the value of W - Pawlowski [7]). This indicates that the ionisation efficiency is being reduced by the methane. It is suggested that some of the metastable neon atoms are de-exciting by collision with methane molecules. The 3.5 eV energy difference between the metastable level of neon and the ionisation level of methane is too large for the efficient production of methane ions. The presence of methane will tend to reduce the energy gained by an electron per cm as electrons collide inelastically with methane molecules. The overall ionisation efficiency can be expected to fall. The quenching effect of the methane can be inferred from the intrinsic resolution results shown in figs. 2 and 3. The intrinsic resolution is the resolution that would be obtained in the absence of electronic noise. It is thus a more useful indication of physical processes in the gas than the directly observed resolution. As the methane level is increased, the resolution at high values of gas

100 ~ 200

300

400 500 600 Anode v o l t o g e (volts)

700

800

Fig. 1. Values of gas gain obtained as a function of anode voltage for neon-argon (995.5) with various admixture concentrations of methane. In all cases the total gas pressure is 1.1 atmospheres. The curves are identified by the corresponding methane pressure. The curve labelled 0 Torr CH 4 refers to the basic mixture of neon-argon (995.5) at 1.1 atmospheres (i.e. 832 Torr N e + 4 Tort At'). Results obtained with pure neon are also shown for comparison. All the results were obtained at 5.89 keV.

gain improves. The resolution does not, however, improve to the value obtained at very low values of gas gain with the basic n e o n - a r g o n mixture. This is preTable 1 Values of average intrinsic resolution at gas gains of 10 to 30 for various methane pressures. The standard deviation limits are

g~ven.

Methane pressure (Tort)

Intrinsic resolution at 5.89 keV (%)

Intrinsic resolution at 22.1 keV (%)

0 0.41 1.14 2.0

11.4±0.1 11.9±0.6 11.9±0.3 13.1±0.4

5.9±0.6 5.8±0.3 5.9±0.1 6.4±0.5

J.P. Sephton et al. / The quenching of penning mixtures 28

i

i

i

i i iii

I

i

i

I

[

Illll

I

I

I

llll[

563 I

I

I

I

I III

26

2/+

-

22

-

20

-

18

-

0

E

•

O

-6

•

o

•

V3

*0

.u_

o

tn

EL .

16

-

14

--

12

--

o

o o

c-

•0

~

•A e

e

ee

f

I

10

•

I

I

I I I I I I

, ,,,ill

i

I

,

i

i

iiiii[

i

102

101

10 o

Gas

i

i

i ill

103

10/4

gain

Fig. 2. Values of intrinsic resolution obtained as a function of gas gain for neon-argon (995:5) with various admixture concentrations of methane. Results obtained with pure neon are also shown. The measurements were made at 5.89 keV. • N e o n - a r g o n (995 : 5)(832 Torr Ne + 4 Torr Ar), © N e o n - a r g o n (995 : 5), + 0.41 Tort CH4, • N e o n - a r g o n (995 : 5) + 1.14 Torr CH, • N e o n - a r g o n (995 : 5), + 2 Tort CH4, • Pure neon.

neon-argon at low values of gas gain arises partly because, for theoretical reasons, the avalanche size variation decreases with gas gain (Alkhazov [2]). It is inevi-

sumably due to the reduced ionisation efficiency and only partial quenching arising from the addition of methane. The good intrinsic resolution obtained with 22

i

20

--

18

--

16

--

1/+

--

12

--

10

--

i

i

i

i

llll

i

i

i

i

i

iiii

i

i

i

i

i llll

i

i

i

i

i t i

tO 3

o

O tn g-

•

u tn E L-

•

c-

8

--

6

--

O

o

o.~ °

~

•

t /+ 10 o

,

,

,',.,I;,F

. 101

, ......

, 102

........

, 103

.

.

.

.

.

,,

10 L.

Gas gain

Fig. 3. Values of intrinsic resolution obtained as a function of gas gain for neon-argon (995:5) with various admixture concentrations of methane. The measurements were made at 22.1 keV. Pure neon was found to be very unstable at this energy. The various gas fillings are denoted by the same symbols as fig. 2.

564

J.P. Sephton et aL / The quenching of penning mixtures

10 3

table therefore that this contribution to improved resolution will be lost at high values of gas gain. Note that in figs. 2 and 3, values of intrinsic resolution at low values of gas gain, are only shown for the basic neon-argon mixture. This is to improve graphical clarity. Table 1 shows the average resolution obtained over the gas gain range 10 to 30 for the various methane levels. It can be seen that the intrinsic resolution at low values of gas gain is not degraded significantly until the methane level is greater than at least 1.14 ToE.

10 2

.E

3.2. Operation with A r - C 2 H 2 o

The Penning mixture argon-acetylene is an interesting gas filling for a number of reasons. Heylen [11] has shown that the ionisation efficiency of such a mixture is very high. This should lead to good energy resolution. Heylen suggested that the organic admixture gas could also act as a quenching agent. Like other organic quench gases, acetylene will eventually become degraded by the action of the electron avalanche. An advantage of using argon as opposed to neon as the majority gas is the much higher photon absorption efficiency which results. In the energy range 6-30 keV for example, the absorption efficiency of 1 / 2 atm cm of argon is 10 times higher than for neon. The counter was operated at 1.1 arm with argon-acetylene (995 : 5) as this mixture ratio is close to

101

~00 ' 120

~ 160

200

240 280 320 360 Anode voltage (volts)

400

440

480

Fig. 4. Experimental variation of gas gain with anode voltage for the gas mixture argon-acetylene (995:5). The measurements were made at 22.1 keV. L,¢L,~

40

I

i

i

i

iiii

i

i

i

t

i

i

i i i

i

i

I

I

I

I

I

I

i

I

I i

i

--

36 --

R°

32 - -

28

-

-

-

".,7.

= -6

24

--

20

--

t/1

® rr

÷

16

12

-

RI

-

--

8

'

100

I

'

I

i ,JiI

I

I

101

I

I

I

I

I li

10 2 Gas

l

103

gain

Fig. 5. R 0 and R] are observed and intrinsic resolutions, respectively. Experimental variation of resolution with gas gain for argon-acetylene (995 : 5) is shown. The measurements were made at 5.89 keV.

565

J.P. Sephton et aL / The quenching of penning mixtures

the optimum suggested by Heylen for most efficient ionisation (997 : 3) and was readily available. In fig. 4 the gas gain is shown as a function of anode voltage. By comparing the linearity of this curve with the corresponding curve for neon-argon one can see that, with this concentration of acetylene, there does not appear to be a significant amount of quenching. This is also indicated by the resolution results (fig. 5) which show degradation of resolution at gas gains above about 50. The intrinsic resolution is similar to that obtained by Sipila [12] - about 11.5% at 5.89 keV. Increasing the concentration of methane may result in more effective quenching. If the admixture concentration is too high, however, the ionisation efficiency will fall [11] and the resolution will deteriorate.

10 4

,

,

,

,

,

,

I

I

l

I

,

10 3

._c

~ 102 o

3.3. Operation with A r - X e 101

The Penning mixture argon-xenon has been studied by Yamane [13] and Kubota [14]. Yamane has shown that the mixture ratio which gives greatest enhancement of charge multiplication in the avalanche (and hence highest ionisation efficiency) is argon-xenon (98:2). This mixture was therefore chosen for the measurements reported here. Fig. 6 shows the variation of gas gain with anode voltage. Higher voltages are needed than was the case with argon-acetylene. The ionisation efficiency is lower but the range of linearity is much greater. In fig. 7 the 21

I

I

I

i III

~

i

t

I

1oo

I

400

500

600 700 800 900 Anode voltage(volts)

I

I

1000

1100

I I Illl

1

I

I

I

I III

I

Key

I

I

I

t i II

-

/ I 19

--

18

-

R0

o

R z

x

o° 17 -C:

.9 "~

16

-

15

--

14

--

°

10 0

1

101

1200

Fig. 6. Experimental variation of gas gain with anode voltage for argon-xenon (98 : 2). The measurements were made at 22.1 keV.

\

20 - -

I

300

10 2 Gas gain

10 3

10 4

Fig. 7. Experimental variation of resolution with gas gain for argon-xenon (98 : 2). The measurements were made at 5.89 keV.

566

J.P. Sephton et al. / The quenching of penning mixtures

resolution obtained with the counter is shown. In can be seen that the resolution does not become degraded until rather high values of gas gain are reached i.e. greater than about 500. Secondary avalanches appear to have been suppressed. The xenon will tend to absorb U V radiation produced by the argon and thus act as a quenching agent. The optimum resolution measurements obtained are similar to those for argon-acetylene.

Acknowledgements

4. Conclusions

References

The operation of Penning mixtures at high values of gas gain has been investigated. The addition of methane to n e o n - a r g o n reduces secondary avalanches but also reduces ionisation efficiency. The resolution is not therefore as good as that obtained at low values of gas gain with an unquenched n e o n - a r g o n mixture. With the gas mixture argon-acetylene (997:3) the admixture concentration seems to be too low for significant quenching to take place. The Penning mixture a r g o n - x e n o n does allow operation at high values of gas gain with good energy resolution. The xenon appears to act as a quenching agent. Xenon also increases the absorption efficiency and does not suffer the degradation problems associated with the use of organic admixture gases. The Penning mixture a r g o n - x e n o n appears to merit further study.

[1] G.D. Alldaazov, A.P. Komar and A.A. Vorob'ev, Nucl. Instr. and Meth. 48 (1967) 1. [2] G.D. Alkhazov, Nucl. Instr. and Meth. 89 (1970) 155. [3] H. Sipila, Nucl. Instr. and Meth. 133 (1976) 251. [4] H. Sipila, Acta Polytech. Scan& Appl. Phys. Ser. no. 116. Helsinki (1976). [5] H. Sipila and E. Kiuru, Adv. X-ray Anal. 21 (1978) 187. [6] J.P. Sephton, M.J.L. Turner and J.W. Leake, Nucl. Instr. and Meth. 219 (1984) 534. [7] Z. Pawlowski, Nucleonika 15 (1970) 51. [8] H. Sipila, Nucl. Instr. and Meth. 140 (1977) 389. [9] R.Z. Fuzesy, J. Jaros, J. Kaufman, J. Marriner, S. Parker, V. Perez-Mendez and S. Redner, Nucl. Instr. and Meth. 100 (1972) 267. [10] K.F. Smith and J.E. Cline, IEEE Trans. Nucl. Sci. NS13 (1966) 468. [11] A.E.D. Heylen, J. Phys. D3 (1970) 789. [12] H. Sipila, IEEE Trans. Nucl. Sci. NS26 (1979) 181. [13] M. Yamane, J. Phys. Soc. Jpn. 15 (1960) 1076. [14] S. Kubota, J. Phys. Soc. Jpn. 29 (1970) 1017.

The authors wish to thank Dr. E. Mathieson (at Leicester University) and Dr. R. Stewart (at Strathclyde University) for helpful discussions. Financial support was provided by the Science and Engineering Research Council and the underlying programme of the U K Atomic Energy Authority (HarweU).