Enhanced electron transfer in Penning gases

Nuclear Instruments and Methods in Physics Research 225 (1984) 325-329 North-Holland, Amsterdam

ENHANCED

ELECTRON

H.E. SCHWARZ,

TRANSFER

J. T H O R N T O N

IN PENNING

325

GASES

and I.M. MASON

Mullard Space Science Laboratory, University College London, Holrnbury St. Mary, Dorking~ Surrey RH5 6NT, England Received 27 February 1984

We present the discovery of the enhanced transfer of electrons produced in avalanches through metal grids in Penning gas mixtures. Measurements are presented showing the enhancement which is particularly dramatic at very low transfer fields. With electric fields on either side of the grid in the ratio of 0.015 the transfer ratio is 0.11, representing an increase of a factor of 7 over that predicted and observed in a conventional gas by Bunemann et al. [1]. We propoae a model for this phenomenon and we show that both a uniform electric field and the Penning effect [2] are necessary for this transfer to occur. We also identify a possible reason why other workers have not obtained similar results in Penning mixtures. The application of this "Penning transfer" to a position sensitive X-ray detector (the Penning Gas Imager or PGI [3]) is discussed.

1. Introduction In a p a p e r p u b l i s h e d in 1949, B u n e m a n n et al. [1] use c o n f o r m a l r e p r e s e n t a t i o n theory to show t h a t a mesh of parallel wires forms a n efficient shield against the ind u c e d charge effect of positive ions drifting t h r o u g h a gas in a n electric field. They show that the inefficiency (or field leakage) of such meshes is typically less t h a n a few percent. This indicates that the loss of electrons drifting from a high field region to o n e with a low field (i.e. a high field ratio) is small. However, at low field ratios transfer occurs only in the ratio of the fields. In recent years electroformed grids or wire meshes h a v e b e e n used to separate the a b s o r p t i o n a n d avalanche regions in parallel plate p r o p o r t i o n a l counters [3,4] a n d to split the gas multiplication process in particle detectors into two parts [5,6]. In any scheme to transfer electrons t h r o u g h grids or meshes, the goal is to transfer the largest p o s s i b l e fraction of the total n u m b e r of electrons at a given field ratio. In their counters, Breskin et al. [6] o b t a i n e d n o transfer at all in c o n v e n t i o n a l gases a n d transfer as predicted b y B u n e m a n n et al. was only o b t a i n e d b y the use of a P e n n i n g mixture. In this p a p e r we present m e a s u r e m e n t s of the transfer of electrons in a conventional c o u n t e r gas ( A r / 1 0 % C H 4 o r P10) a n d in a P e n n i n g gas ( A r / 0 . 5 % C2H2). T h e experimental set-up is s h o w n schematically in fig. 1. X-rays from a n 55Fe source enter t h r o u g h the w i n d o w a n d produce localised ionisation clouds in the drift region. The electrons from these clouds drift t h r o u g h grid 1 into region A where they avalanche in a n electric fie.ld o n t o grid 2. By applying a n o t h e r electric field which can b e varied between the a n o d e a n d grid 2, the 0 1 6 7 - 5 0 8 7 / 8 4 / $ 0 3 . 0 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)

transfer of electrons from region A to B c a n b e controlled a n d m e a s u r e d as a function of the field ratio. W e used electroformed nickel grids with square holes, a pitch of 2 0 0 / t i n a n d an o p e n ratio of 72%.

2. Transfer in conventional gases B u n e m a n n et al. developed a theoretical model for the transfer of electrons t h r o u g h a mesh of parallel wires. F o r wires of radius r, spaced at equal distances d w h i c h separate two regions A a n d B whose electric

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H.E. Schwarz et al. /' Enhanced electron tran.ffer in Penning gases

326

fields are E 1 and E 2 respectively, the transfer of electrons from A to B can be written as (but see appendix) T( R

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(I)

where R = E 2 / E 1 and p = 2~rr/d. Since p is a parameter describing the effective opacity of the mesh, eq. (1) should be applicable to our electroformed grids too. For a given grid pitch (equivalent to d for wire meshes) a higher value of p is needed, which is related to the open ratio of the grid. Bunemann et al. measured transfer ratios as a function of field ratio in argon and they found that the experimental results were in agreement with their theoretical predictions. W e have measured transfer ratios in P10 gas and found that Bunemann-type transfer occurs. Fig. 2 shows the data points and a best fit (p = 0.89) curve obtained from eq. (1). Again the agreement between data and theory is good, confirming the validity of our assumption that the Bunemann model applies to both parallel meshes and crossed grids.

well known technique for the prevention of secondary avalanching and in some cases is also used to reduce the diffusion of the primary ionisation cloud [7]. By choosing the admixture to have an ionisation potential which is lower than the excitation energy of the main filling gas, a dramatic increase in ionisation efficiency is obtained. Due to this increase in the first T o w n s e n d coefficient the field needed to produce a given gas gain is reduced significantly. Also bolh the Fano factor and the avalanche statistics factor are reduced, giving improved energy resolution in proportional counters [3,8,9]. This ionisation of an admixture was first reported by Penning [2] in 1934 and is called the Penning effect. Our measurements of the transfer ratio as a function of field ratio in the Penning gas are shown in fig. 3. By comparison with the P10 results it can be seen that the transfer of electrons through grids is changed significantly by the Penning effect. Most important is the creation of electrons below the grid by photo-ionisation resulting in a significant number of electrons being collected at the anode, even at field ratios close to zero. This appears as an offset added to the Bunemann curve (clearly present in fig. 3) and represents a major improvement in low field transfer (e.g. a factor 7 at R = 0.015). We continue to use the word "transfer"

3. Transfer of electrons in Penning gases The addition of an admixture to a noble gas to quench the U V photons produced in avalanches is a

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H.E. Schwarz et al. / Enhanced electron transfer in Penning gases a l t h o u g h strictly speaking it is not electron transfer b u t electrons p r o d u c e d b y p h o t o - i o n i s a t i o n being swept up by the (low) electric field. Also shown in fig. 3 is a set of measures of the P e n n i n g gas with 2% C O 2 added. T h e observed e n h a n c e m e n t is reduced significantly in this gas due to the q u e n c h i n g of U V p h o t o n emission by t h e C O 2. Both curves show a n u p t u r n at field ratios above 0.35 which is due to the onset of a v a l a n c h i n g in region B. T o model this situation, consider a grid separating two regions A a n d B with their respective electric fields E 1 a n d E 2 as defined above. A n avalanche with a gas gain of G, produces n electrons a b o v e the grid a n d m electrons below it. This is shown schematically in fig. 1. T h e r e is a region Be, below the grid, into which field lines from region A p e n e t r a t e before ending on the grid. Fig. 4 shows an example of this effect for a wire mesh [6]. Let a fraction p of the electrons p r o d u c e d in region B b e below region Bc, leaving a fraction ( 1 - p ) in region Bc. T h e n we have for the fractional transfer of the n electrons above the grid

T, = nT( R ) / G = nT( R ) / ( m + n ),

(2)

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327

Let the transfer from region Bc be a function of the field ratio g ( R ) , then the transfer of electrons from this region is given by T3 = ( l - p ) m g ( R ) / ( m

+ n).

(4)

T h e total transfer is just the sum of eqs. (2), (3), a n d (4) viz. Ttot = [ n T ( R ) + ( 1 - p ) m g ( R )

+ p m ] / ( m + n).

(5)

T h e overall relative electron collection o n the a n o d e as given b y eq. (5) consists of a field d e p e n d e n t term, p a r t of which is the B u n e m a n n transfer function a n d an additional field i n d e p e n d e n t term which gives the offset discussed above. It can _be seen that for a conventional gas, with m = 0 in eq. (5), the total transfer simply reduces to r,o , = T ( R ) , which, as expected, describes B u n e m a n n transfer. By measuring b o t h the P e n n i n g transfer a n d the B u n e m a n n transfer for a given grid it is possible to d e t e r m i n e the ratio m / n from which the relative n u m ber of electrons p r o d u c e d by p h o t o - i o n i s a t i o n can be calculated. By subtracting the P10 d a t a from b o t h Penn i n g transfer curves, the m / n ratio can b e d e t e r m i n e d in the following m a n n e r . Subtracting eq. (1) from eq. (5) we o b t a i n

T'=m[(1-p)g(R)'-T(R)+p]/(m+n).

(6)

T h e experimental curves o b t a i n e d b y subtracting the P10 data from the P e n n i n g data are s h o w n in fig. 5.

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328

H.E. Schwarz ez al. / Enhanced electron transfer in Penning gases

Ignoring the upturn above R -- 0.35 both data sets can be represented by a straight line. Least-squares fitting gives Penning curve: = 0.43 _+ 0.02R + 0.11 _+ 0.01; Penning + C O 2 curve: = 0.31 _4-0.02R + 0.04 _+ 0.01; where the subscripts p and c indicate Penning and Penning + C O 2 respectively. The correlation coefficient in both cases is > 0.98, indicating a good fit. In principle, equating each of these lines with expression (6) allows us to calculate m / n and p for both data sets. Lack of knowledge about g ( R ) prevents this. However, by assuming g = 1 at the highest value of R for which the data is represented by a straight line we can obtain lower limits for m / n and upper limits for p. Evaluation at R = 0.32 with T(0.32)= 0.23 (from the best fit Bunemann curve) gives pp < 0.34 and ( m / n )p > 0.47, Pc < 0.22 and ( m / n ) c >=0.22.

(7)

The values for p suggest a mean free path for the photons of a few hundred /,tm under the assumption that the depth of region Bc is of the order of the grid pitch. This compares with a value of 200 /tm obtained by Breskin et al. in an A r / a c e t o n e mixture. The n electrons in region A are produced by both photo-ionisation and other processes such as collisional ionisation. Let n = m ' + k, where m" is the number of photo-electrons produced and k is the number of electrons produced by all other processes. By assuming an isotropic photon production process and taking the relative transparency t, of the grid into account by putting m' = m / t we have Q = 2m'/k = 2/(tn/m

- 1),

(8)

which is the ratio of photon produced electrons to all other electrons. Using expressions (7) and (8) we obtain

Qp>_ 31

and

Qc--> 1.6.

(9)

Qp is equivalent to the relative increase in the first Townsend coefficient and our value of > 31 fits in with the data of Druyvesteyn and Penning [10], who obtained values between 5 and 100 for different field strengths in a N e / A r mixture. N o t e the reduction by a factor of >= 15 due to the addition of C O 2 to the gas. It is interesting that the Penning effect is still present and its advantages of improved X-ray energy resolution [3,8,9] ~nd enhanced transfer can be exploited. The added advantage of reduced electron diffusion and secondary avalanching due to the quenching effect of CO 2 could make this a useful filling gas for imaging proportional counters. We have described [3] a new imaging X-ray detector,

the PGI, in which we have obtained an energy resolution of 24% fwhm and a position resolution of 100 # m fwhm at 1.5 keV. The detector makes use of two avalanche regions and a wedge and strip position readout system [11]. The transfer of electrons to the second avalanche region is accomplished by using the enhanced transfer in a Penning gas. The overall voltage is kept below 2500 V because the high value of Qp allows high gas gains to be achieved ( > I0 n) with relatively small applied fields.

4. Non-uniform field avalanching To investigate the results obtained by Breskin et al., we have used a grid with a pitch of 920/~m and an open ratio of 85%. This is similar to their crossed wire mesh. Like Breskin et al. we did not observe transfer ir/ P10 gas. A possible explanation of this result is that the avalanches take place directly onto the wires (or grid bars) in the locally increased field and not in the uniform field away from the wires. This can also explain the Penning gas results. If the avalanche took place uniformly above the wires, a significant fraction of the U V photons, which have a mean free path of about 200 p m [6] would pass through the mesh and enhanced transfer would be observed. Shadowing by the wires would prevent this photon transfer. Moreover, Breskin et al. use field ratios of up to 2 without avalanching taking place in the lower region. This effect sets in at a field ratio of about 0.35 in our data. Avalanching in the higher field near the wires explains this discrepancy. Finally, the maximum gain achieved in the first region by Breskin et al. is about 103 after which breakdown occurs. We have obtained gains in excess of 2 x 105 in similar preamplification regions; comparable gains have been obtained using a continuous anode [12] at similar field strength, indicating that true parallel field avalanching takes place. In conclusion, it seems that the transfer efficiency of electrons can be improved significantly by using parallel field avalanches. This can be achieved by using grids or meshes with a small pitch. Our thanks to Prof. J.L. Culhane for his support. During the period of this work one of us (J.T.) was in receipt of a S E R C C A S E studentship. This work was supported by SERC.

Appendix In their paper Bunemann et al. derive the expression describing the transfer of electrons as a function of field

H.E. Schwarz et al. / Enhanced electron transfer in Penning gases ratio. This equation [their eq. (19)] only gives the correct results for values of R > 1. At R = 1 there is a discontinuity and for R < 1 our eq. (1) must be used. The full expression should be -

-cos-l(IAVoB)}),

.<1, R>I,

where A = R - 1 and B = R + 1. The absolute values d o appear in eq. (18) in Bunem a n n et al.

References [1] O. Bunemann, T.E. Cranshaw and J.A. Harvey, Can. J. Res. 27 (1949) 191. [2] F.M. Penning, Physica 1 (1934) 1028.

329

[3] H.E. Schwarz and I.M. Mason, Nature 309 (1984) 532. [4] J.W. Stumpel, P.W. Sanford and H.F. Goddard, J. Phys. E 6 (1973) 397. [5] G. Charpak, G. Melchart, G. Petersen, F. Sauli, E. Bourdinaud, P. Blumenfeld, J.C. Duchazeaubeneix, A. Garin, S. Majewsld and R. Walczak, CERN 78-05 (1978). [6] A. Breskin, G. Charpak, S. Majewski, G. Melchart, G. Petersen and F. Sanli, Nucl. Instr. and Meth. 161 (1979) 19. [7] O.H.W. Siegmund, S. Clothier, J.L. Culhane and I.M. Mason, IEEE Trans. Nucl. Sci. NS-30 (1983) 350. [8] G.D. Alkhazov, A.P. Komar and A.A. Vorob'ev, Nucl. Instr. and Meth. 48 (1967) 1. [9] H. Sipilit, Nucl. Instr. and Meth. 133 (1976) 251. [10] M.J. Druyvesteyn and F.M. Penning, Rev. Mod. Phys. 12 (1940) 87. [11] H.O. Anger, Instr. Soc. Am. Trans. 5 (1966) 311. [12] I.M. Mason, G. Branduardi-Raymont, J.L. Culhane, R.H.D. Corbet, J.C. Des and P.W. Sanford, IEEE Trans. Nucl. Sci. NS-31 (1984) 795.