High count rate multiwire proportional counters for Mössbauer spectroscopy

NUCLEAR INSTRUMENTS

AND METHODS

157 ( 1 9 7 8 )

2 8 7 - 2 9 3 ; (~) N O R T H - H O L L A N D

P U B L I S H I N G CO.

H I G H COUNT RATE MULTIWIRE PROPORTIONAL COUNTERS FOR MOSSBAUER SPECTROSCOPY ZDZISI2AW PAW,LOWSKI and WOJCIECH CUDNY Warsaw Technical University, Institute o f Radioelectronics, Nowowiejska 15/19, 00-665 Warszawa, Poland Received 19 January 1978 and in revised form 11 July 1978 The design and construction of multiwire proportional counters intended specifically for high count-rate M6ssbauer spectroscopy are discussed. Two ways of improving the counter performance are suggested: (1) Improvement of the field distribution in the counter through the application of field-correcting electrodes, and (2) introduction of drift spaces into the active volume of the counter. Two counters were constructed and tested, one with correction electrodes and one with drift spaces. For the multiwire counter with field-correcting electrodes counting rates of about 1.5× 106 cps were obtained for gas amplification 100 and energy range 3-15 keV. The multiwire counter with drift spaces measured in the same conditions showed a spectrum line shift about three times smaller than that for the counter with correction electrodes. From the point of view of M6ssbauer study applications, both these counters surpass the traditional single-anode counter as well as the counter of Semper and Walker.

1. Introduction In M6ssbauer spectroscopy the quality of the spectrum depends mainly on the statistical error. For a single M6ssbauer line the signal-to-noise ratio is defined 1) as a measure of the spectrum quality, namely : S

N-No

N -

+lVo)'

where No is the total number of counts per channel at the minimum of a single MiSssbauer line and N is the total number of counts per channel in the region away from the resonance. For a small M6ssbauer effect the minimum measurement period for which the statistical error S / N is obtained, is equal to: t ~ 2(s/N)2 c, --

g2 n

where e is the "M~Sssbauer effect" defined as: e = ( N - N o ) / N , n is the maximum count-rate of the detector, and C i s t h e number of channels. For M6ssbauer experiments in transmission geometry, gaseous proportional counters are commonly used. Commercially available proportional counters have been successfully applied in the 14.4keV region up to counting rates of about 10 5 cps. However, if the M~ssbauer effect is small, at such counting rates it is necessary to collect data over periods of several hundred hours. To shorten the period of measurement one should increase the count-rate of the detector. In typical proportional counters, at count-rates

exceeding 105 cps, the shift of the pulse height distribution towards lower pulse amplitudes and degradation of energy resolution become too significant for the counter to be suitable for M6ssbauer spectroscopy. High count-rate effects in proportional counters have been studied by several authors2-4). According to Hendricks2), the effect is caused by the space charge of slowly moving positive ions formed in previous discharges, located in the whole volume of the counter. Spielberg 3,4) suggests that the basic mechanism consists in the buildup on the anode wire of a loosely adherent sheath of polarizable molecules or molecular fragments of the quench gas. This sheath in turn increases the effective diameter of the wire, thus decreasing the gas gain. In flow proportional counters, for counting rates below 105 cps, Spielberg found experimentally that the magnitude of the shift depended on the hardness of the material used for the anode wire, that shift decreased with increasing anode diameter, and that shift became more serious when the counter was used over longer periods. In other papers 5'6) the influence of ion pair recombination and of electron capture by the quench gas on the magnitude of the shift are discussed. Recently, Semper and Walker have reported a count-rate limit of 106 cps obtained in a multiwire gas flow proportional counterV). In principle, this counter works in the same manner as would several single-wire counters stacked one behind the other. We have worked on improving the counter of

288

z, P A W L O W S K I

Semper and Walker, (1) by improving the field distribution around the anodes through the application of field-correcting electrodes, and (2) by introducing drift spaces into the active v o l u m e of the counter. Two counters were constructed and tested, one with correction electrodes and one with drift spaces. For the rnultiwire counter with field-correcting electrodes we obtained counting rates of about 1.5 × 106 cps, for gas amplification 100 and energy range 3 - 1 5 keV. The multiwire counter with drift spaces measured in the same conditions showed a spectrum line shift about three times smaller than that for the counter with field-wires. From the point of view of M~Sssbauer study applications, both these counters surpass the traditional singleanode counter as well as the counter of Semper and WalkerT). 2. hnprovement of the high count-rate limit in the proportional counter The influence of space charge on the change of the gas amplification factor in the proportional counter is given by the following formula [see appendix, eq, (8)]:

M(N) lepr2 E~ M(N) N ln2 - J~(N--+0) - exp - [--87rAeLW#+ Va~ × ln(rjra)]+ 11} ,

(1)

where M is the mean value of gas gaim N - mean count-rate, e - elementary charge, p - gas pressure, A is the mean potential difference between ionization events caused by that same electron, K - critical value of electrical field below which ionization does not occur,/~ + - positive ion mobility, W - ionization work for a given counter gas, E~, LEAD SHIELD7

ANODE . W .SEVEN ..... . . .I.R. .E S /

/

/

I

I/////,~'---~

~

'"

oJ. . I I I - .-

.

energy of q u a n t u m , e - dielectric permittivity, F~ - applied voltage. It follows that through a proper choice of anode and cathode radii, and of the effective length of the anode (r~, r~ and L, respectively) one is able to influence the count-rate limit of the counter. These parameters depend on the mechanical construction of the counter. The relative change of the gas amplification factor caused by the increase in count-rate is smaller for lower values of amplification. One may thus achieve an additional improvement of the high count-rate limit by using lower supply voltage and low-noise preamplifiers. Although the high count-rate operation of the proportional counter was studied in counters of cylindrical geometry, the qualitative conclusions concerning the construction of the counter are quite general. One way of improving the high count-rate limit of the proportional counter is to increase the number of anodes in the counter, a solution chosen by Semper and Walker7). A cross-section of their counter and the electrostatic field distribution around the anodes are shown in fig. 1. In the multiwire counter the beam of ionizing radiation is

\

BERYLLIUM

/WINDOW -.

~_-~\\\

WIRES

Fig. 1. Cross-section of the counter described by Semper and Walker 7) with distribution of electrostatic field around anodes.

\ .

ANODE

WINDO~

'\

a2 .

FIVE

\CORRECTION ELECTRODES

\

/

AND W. CUDNY

/

Fig. 2. Cross-section of the multiwire counter with field-correcting electrodes.

MOSSBAUER SPECTROSCOPY LEAD SHIELD

ANODE WIRES

289

SPACE A /BETWEEN GRIDS/

BERYLLIUM WINDOW

>XRAY

GRIDS /

/ ........

. .

~/

30TENTIAL WIRES

Fig. 3. Cross-section of the multiwire counter with drift spaces. perpendicular to the anodes. Anodes are connected in parallel to a common preamplifier. Here, the active length of the anode, L, is increased, decreasing therefore [according to eq. (1)] the change of gas amplification with increasing count-rate. A disadvantage of this solution is that the electrostatic field around the edge anodes is higher, therefi)re these anodes work with a higher gas amplification. In our multiwire proportional counter we introduced special electrodes which correct the electros~:atic field distribution in the counter. This enabled us to place more anodes in the counter. A crosssection of the counter with field-correcting electrodes is shown in fig. 2. Owing to the presence of these additional electrodes, all anodes work ~ith the same gas amplification. Moreover, field-correcting electrodes which work at potentials positive with respect to the cathode eliminate part of the background due to radiation scattered by the cathode walls. We have also developed another construction, namely a multiwire counter with drift spaces. The cross-section of this counter is shown in fig. 3. The volume of the counter is divided into three parts by additional grids. The central volume, A, is the actual counter. The physical principle explaining the improvement of high count-rate properties of such a counter is as follows: the space charge of positive ions, removed from the central volume by the action of the electrostatic field, does not influence the field distribution around the anodes because of screening by the grids. Due to the smallness of the volume A, the remaining space charge cannot be very large and the corre-

sponding change of gas amplification is also small. At the same time, the active volume of the counter does not change because electrons of primary ionization formed inside the drift volumes travel freely into the central multiplication region due to the presence of an electrostatic field. The flatness of the central volume enables a large number of anodes to be placed in this part. 3. Construction of the counters The life time of a sealed counter operating at count-rates of the order of 106 cps would be relatively short because of the finite stability of the quenching agent. The counters were therefore designed to operate in the flow regime. The gas-tight bodies of both counters, i.e. the multiwire counter and the counter with drift spaces, are identical, made of aluminum, and have internal dimensions 300x 175x70 mm 3. The depth of the counters, 175 ram, defining the path length of the beam of radiation in the counter, was dictated by the desire to achieve a high detection efficiency for 14.4keV radiation (57Co) in argon at atmospheric pressure. The obtained efficiency (of about 50%) could be radically increased by using other noble gases as the main components of the counter mixture, however this would lead to much higher exploitation costs. The height of the counters, 70 ram, is sufficient to diminish the interaction of the radiation beam with the upper and lower covers of the counter body, thus decreasing background due to scattered radiation. The width, 300 ram, is determined by the length of the anodes, which must be long enough for edge effects not to interfere with the field dis-

290

z.

PAW-LOWSKI

tribution around the anodes in the region of the radiation beam. Similarly as in the counter of Semper and Walker, two windows are made in the counter body, for the beam to enter and leave the counter. Rectangular-shaped windows of dimensions 40 × 50 m m 2 (input) and 50 × 70 m m 2 (output) were covered with beryllium tape of 0.3 m m thickness. Further extension of the size of the counter, although leading to an increase in detection efficiency, would demand unnecessarily long flushing periods. Two valves are attached to the counter body for flowing the gas through the counter. These valves can be closed, which enables the counter to be detached from the gas supply system for a few days, and protects the gas mixture from external contamination. Placement of the anodes and field-correcting electrodes in the multiwire counter is shown in fig. 2. All electrodes are attached to two supports made of epoxy resin. The anodes are made of gold-plated m o l y b d e n u m wires of 35/~m diameter. In order to eliminate discharges at the anode ends, anodes are threaded through steel tubes of 1 m m diameter attached to the supports. Field-correcting electrodes are supplied through a resistor chain from the anode high-voltage supply. The field distribution around the field-correcting electrodes is shown in fig. 2. The counter with drift spaces contains 8 anodes placed between 9 potential wires, and two grids. Anodes are made o f gold-plated 35/~m diameter m o l y b d e n u m wires. Potential wires and grids are made of aluminum alloy wire of 125/zm diameter. The grids consist of wires parallel to the anodes, spaced at 2 m m intervals, their separation from the anode plane being 9 m m each. In order for the counter to work correctly, proper supply voltages need to be chosen. For the P-10 mixture the grids and potential wires are usually supplied with about 300 V with respect to the cathode and the anode voltage is about 1600 V, which corresponds to gas amplification of about 150.

AND

W.

CUDNY

4. High count-rate electronics The electronic equipment used for the analysis of pulse amplitudes at high counting rates should minimize the effect of pulse overlap and baseline shift which mainly contributed to the deformation of the energy spectrum. This necessitates special pulse shaping, usually quite different from the optimum one with respect to signal-to-noise ratio. One can limit pulse overlap by shortening pulse duration, from which it follows that low rise-time and short pulse-forming time constant amplifiers should be used. The natural limit in this case is the mechanism of pulse formation in the proportional counter. A single counter pulse is usually a superposition of several pulses arising from individual primary ionization electrons. A certain fluctuation of the counter pulse rise-times is therefore present, connected with the spatial distribution of electrons of primary ionization. Differentiation of counter pulses with very short time constants would then lead to additional dispersion in the amplitude spectrum. A short time constant also causes a considerable decrease in the amplitude of the output pulse, decreasing the signal-to-noise ratio. The baseline shift is eliminated by the use of bipolar pulse shaping. A block diagram of the electronic apparatus used in our measurements is shown in fig. 4. The charge preamplifier, POLON 1002, after modification had a rise-time below 25 ns and the output pulse decay time constant decreased to 10/~s. The POLON 1101 amplifier was modified by additional pulse-forming circuits with shorter time constants, ;R[~'J lOO2

~-~+--;

901 80-

z -~

ff~ ...............

~

A -~+--oB

70. 60. 50

PREAMPLIFIER

I TEST GENERATOR

Fig. 4. Block diagram of the electronic apparatus used for measurements.

0,5'x108

1(38

1,5x108

2x108 " ~

Fig. 5. Dependence of relative gas gain on average charge generated in the counter in unit time. The source is 57Co, count rate measured above 3 keV. A - counter with drift spaces, gas amplification 150; B - counter with correction electrodes, gas amplification 100, 200, 400, 800; C - conventional single anode counter, gas amplification 200.

MOSSBAUER SPECTROSCOPY

and is able to form bipolar pulses of about 500 ns width. A single-channel analyzer was used in order to achieve smaller distortions of the amplitude spectrum. The relatively large spectrum distortion observed in multichannel analyzers working at higher counting rates is probably caused by pulse overlap in the analog-to-digital converter. The dead-time of the whole spectrometric channel is less than 500 ns at low counting rates and increases with increasing count-rates due to pulse overlap. At counting rates of the order of several hundred thousand or more pulses per second, the actual counting-rate can differ considerably from the one measured. In the presented plots we give the actual counting rates after dead-time correction. The gas amplification factor was measured by a test generator feeding pulses to the preamplifier input through a calibrated capacitance. 5. Counter performance The counters operated with the P-10 gas mixture under atmospheric pressure. After flushing the counters, the typical gas flow rate was about 2 l/h. The flow rate was set by a precision smallflow regulator placed after the pressure reducer. Measurements were performed using a 50mCi 5:'Co source. To avoid high intensity radiation scattered in the counter walls, the front of each counter was shielded by a 1 cm thick layer of lead. The counter with field-correcting electrodes had a gas amplification of about 60 at 2100 V working

t/3 F-Z Cb

6

43-

21i

614

14,4

ENERGY

Fig. 6. Change in the spectrum of 57Co'sOurce with increasing count-rate.

291

voltage. Its amplification increases twofold on increasing the supply voltage by 140 V. The counter with drift spaces had gas amplification about 150 at anode voltage 1600V and that of drift and potential wires equal to 300 V. Increase of the anode voltage by 50 V doubles the gas amplification. Results of measurements of the shift of spectrum lines with the increase in the count-rate are presented in fig. 5. The dependence of the relative spectrum line shift on the mean charge generated over unit time in the counter volume is plotted. For the multiwire counter with field-correcting electrodes the measurements of spectrum line shifts were performed for several values of gas amplification (the straight line B in fig. 5). For gas amplification 100 we obtained a count-rate of 1 . 5 × 10 6 counts per second in the energy range 3-15 keV. For larger gas amplifications the measurement points lie on the same straight line. In the same measurement conditions the counter with drift spaces shows a shift three times smaller than for our multiwire counter. For comparison, in the same figure we plot the relative spectrum line shift measured in a typical proportional counter filled with argon, type PXAr, of cathode dimensions 36 × 72 mm 2 . In all cases the width of the beam was equal to the width of the window in the lead shield, i.e. 5 cm. For measurements at very high counting rates the counting geometry had to be changed because the source had to be placed close to the counter window. For both counters a considerable decrease in their energy resolution is seen as the count-rate increases. This is illustrated in fig. 6 where changes in the spectrum of the 57Co source with increasing count-rates are evident. The decreased energy resolution of the counter is reflected in flattening of the M6ssbauer spectrum. In fig. 7 we show M~Sssbauer spectra of non-enriched iron obtained at counting rates (in the region of the 14.4 keV peak) of 2× 105 pulses/second (fig. 7a) and of 0.5 × 105 pulses/second (fig. 7b). 6. Conclusions Results obtained in our counters confirm the correctness of their design with respect to high count-rate performance. At sudden changes of the intensity of the radiation beam we did not observe any particular influence on the counter operation of the effect described by Spielberg2'3). This effect consists in the

292

z. PAW-EOWSK1 AND W. CUDNY

100- ,.'.".,'..'..'.

>Ft/3 Z IaJ l--Z t,i :>

or"

93VELOCITY Fig.To r

1001 + , , " . ,:~: ,..,

I

Appendix

80 FigI7b

ture (Ar+ 10% CH4) is an almost ideal proportional counter mixture. The C H ; a n d [CH4]~- positive ions form the voltage pulse. They are formed very rapidly through interactions with Ar + and [Ar2] + ionsS). The CHg and [ C H 4 ] ~- ions have great mobility, decreasing therefore the average density of space charge corresponding to a given count-rate [cf. eq. (1)]. For the same reason the rise-time of counter voltage pulses is short which is very important because of the necessity of forming very short output pulses. Differentiation of pulses with short rise-time causes a smaller degradation of pulse amplitude, improves the signal-to-noise ratio and, consequently, enables lower gas amplifications to be used for the operation of the counter.

VELOCITY

Fig. 7. M6ssbauer spectra of nonienriched iron obtained using the multiwire counter with field-correcting electrodes. Count-rates in the 14.4 keV peak are: 200 kcps (a) and 50 kcps

(b).

relatively slow change of the gas amplification at step-like changes of the radiation intensity, a fact explained by covering of the anode surface with products of dissociation of particles of the quench gas. It is possible that the time constant of this effect is very small in a counter operating at normal pressure. In our opinion the dominating effect causing the shift of lines in the amplitude spectrum at th~ investigated counting rates is the influence of space charge. The deterioration of energy resolution is caused by processes occurring in the counter as well as by effects taking place in the electronic circuitry, mainly the effect of pulse overlap. In the counter itself the most important factor is the increasing fluctuation of the electrostatic field around the anodes caused by space charge. At very high counting rates there is also a certain geometrical contribution due to unsatisfactory measurement geometry in cases when the source has to be placed close to the counter window. The choice of the counter gas mixture is very important if the counter is to operate correctly at high counting rates. In this respect, the P-10 mix-

Derivation of the formula describing the influence of the space charge on the change in the gas amplification factor. For cylindrical gaseous counters in the stationa r y state, assuming that all ions are formed at the anode, the mean anode current is given by the following expression:

2~zrLj, = la,

(2)

where j, =p(r) W + (r). p(r) is the mean space charge density of positive ions at the distance r from the anode surface, W + (r) is the positive ion drift velocity in their motion towards the cathode, and L is the active length of the anode. On the other hand, the mean anode current can be calculated knowing the mean charge generated in the counter in unit time: E~

I, = e ~ M N ,

(3)

where e is the elementary charge, Ey is the energy of the monoenergetic quantum, W is the ionization work for a given counter gas, M is the mean gas amplification, and N the mean count-rate. Taking into account that W + (r)=/.z + E(r)/p and E(r)= Vac/r In r~/r,, one can calculated the mean space charge p(r) using eqs. (2) and (3), namely" =

ep ln(rc/r,) Er )~(.N) 2 uLW~ l/~c

(4)

This dependence was derived in a different manner by Hendricks2). Solving the Poisson equation V O = - p / e , or the Gauss theorem, one obtains an expression describing the change of the electrostatic field in the vicinity of the anode,

MI3SSBAUER

namely:

V.o 1 E(r) - lnro/ra r

-

2

pro

1

(5)

4elnro/ra r

Decrease o f the field around the anode caused by the presence of space charge corresponds to the following decrease in the voltage applied to the anode:

A V = Pr--~2~

~'i (N)

[epr~ E~ !~ (N) N ln2 8~AsLW#+ Va° -- x

z = )~r(N~0) = exp - ~

In(r Jro)]

1]}.

(8)

This is the formula given in the main text [eq. (1)].

Our formula differs slightly from that given by Hendricks 2) by the factor re. For cylindrical s y m m e t r y of the field distribution around the anode the gas amplification of the counter has been expressed by Diethorn 9) as follows -

T h e relative change in gas amplification dependent on the count-rate can then be obtained using eqs, (4), (6) and (7):

(6)

4~"

-

293

SPECTROSCOPY

[

l/,

M -- exp [~lT~-n(r~,) In Kpr, ln(ro/r,)JJ

(7)

where K is the critical value of E/p below which ionization does not occur, and A is the m e a n potential difference between ionization events caused by that s a m e electron.

References l) E. Kankelait, Proc. Int. Conf. on M6ssbauer spectroscopy, vol. 2 (1975) p. 43. 2) R. W. Hendricks, Rev. Sci. Instr. 40 (1969) no. 9. 3) N. Spielberg, Rev. Sci. Instr. 37 (1966) 1268. 4) N. Spielberg, Rev. Sci. Instr. 46 (1975) no. 8. 5) L. Ferguson, Rev. Sci. Instr. 37 (1966) 964. 6) N. Spielberg, Rev. Sci. Instr. 38 (1967) 291. 7) R. J. Semper, C. R. Guarnieri and J. C. Walker, Nucl. Instr. and Meth. 129 (1975) 447. 8) G. R. Ricker, Rev, Sci. Instr. 40 (1969) no. 2. 9) W. Diethorn, USAEC Rep. NYO-6628 (1956).