Proportional counters for the localisation of ionising radiation

NUCLEAR

INSTRUMENTS

PROPORTIONAL

AND

METHODS

IO 3

(I972) 365-372,

©

NORTH-HOLLAND

PUBLISHING

CO

COUNTERS FOR THE LOCALISATION OF IONISING RADIATION J

HOUGH

and R W P

DREVER

Department of Natural Philosophy, Glasgow University, Glasgow, Scotland Received 17 A p r i l 1972 Gas p r o p o m o n a l counters which can be used for the l o c a h s a t l o n of iOmslng r a d l a t m n m one or two & m e n s m n s are described The technique o f charge d l w s l o n on a resistive electrode is used to o b t a i n position l n f o r m a t m n , thJs electrode being the anode in the linear detector In two d i m e n s i o n s the c o u n t e r signal ~s o b t a i n e d m a new way - fro m two m d u c t m n grids p l a c e d between the anode and the cathode, each grid p r o v i d i n g positron i n f o r m a t m n in one

1. Introduction In recent years there has been considerable interest in detectors for the locahsatlon o f lOmslng radiation One dimensional position sensitive counters have a n u m b e r o f uses in nuclear physics, and counters giving information m two dimensions have apphcatlons in medicine and other fields where images o f radiation &strlbuttons are required However, there are few detectors which are capable o f p r o w d i n g position informat10n of high resolution in either one or two dimensions, and which, at the same time, have a high count rate capability and a s~mple readout system Instruments such as magnetostrictlve spark chamberst), acoustic spark chambers2), scintillation hodoscopes 3) and semiconductor counters 4) have been built, but all of these have fmrly severe hmltatlons, in repetition rate (spark chambers), physical size (semic o n d u c t o r counters) or electronic complexity (hodoscopes) However, recent research s-7) has indicated that gas proportional counters have fewer llrmtatmns than the others m e n t m n e d and th~s c o m m u n i c a t i o n outlines a design o f proportional counter which is position sensmve in one dimension and describes how this design m a y be extended to a second &menslon

direction A p p h c a t l o n of the two d l m e n s m n a l c ount e r for X-ray i m a g i n g Is dl us t ra t e d These counters c o m b i n e ease of r e a d o u t w~th good spatial re s ol ut i on, in the one d l m e n s m n a l detector the s t a n d a r d devmt mn of p o s i t i o n m e a s u r e m e n t s can be 0 04% of the s e ns mve length

with a separate electronic channel connected to each wire 7) This latter method is rather comphcated and expenswe and, in the design of the detector to be described, the simple approach of processing the amplitude o f signal pulses at each end of a resistive electrode xs used When an avalanche takes place in a proportional counter, a charge pulse is induced on all the electrodes in the counter as a result o f the motion of the positive ions close to the anode If a particular electrode has a high resistance and is effectively earthed at both ends, the charge will leak off as a current in both directions The counter with this resistive electrode behaves as a diffusive transmlsslon line, and the position o f the avalanche can be deduced from the charges flowing to each end In the simple case when the capacity of the detector can be neglected, the &stance o f the event from one end is proportional to the fraction of the total charge reaching that end FRONTCATHODE/ (-3 5 kV) /

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2 1 POSITIONDETERMINATIONIN ONE DIMENSION A p r o p o m o n a l counter relies for ItS operation on the locahsed multiplication of the primary lonlsatlon released by the particle or p h o t o n to be detected There are different ways o f determining the position o f this avalanche, e g by using an electrode of high resistance in the detector and observing the amphtude 5 8) or rise time 6) o f the pulses at the ends o f the electrode, or by using a multlwlre anode in the counter 365

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F o r position detection m one dimension, a simple cylindrical c o u n t e r w~th an a n o d e o f resistive wire can be used 2 2

POSITION DETERMINATION IN TWO DIMENSIONS

This m e t h o d can be extended to a second & m e n s l o n if an a n o d e structure which covers an extended area is used The a n o d e m a y consist o f a grid o f parallel wires or m a y be f o r m e d by the e v a p o r a t i o n o f multiple points 9) on a c o n d u c t i n g sheet or the stacking up o f m a n y fine needles to form an a r r a y There are several possible experimental a r r a n g e m e n t s for a p r o p o r t t o n a l detector for two dimensions ~°-12) F o r example, an a n o d e o f parallel wtres m a y give p o s m o n i n f o r m a t i o n for one &rect~on a n d a c a t h o d e o f s l m d a r structure, b u t with its wires at right angles to those o f the anode, in the other~2), o r a c a t h o d e consisting o f a high resistance sheet m a y be used to gtve i n f o r m a t t o n m b o t h &mens~ons In either o f these cases the s e p a r a t i o n o f the a n o d e wires must be small to gwe g o o d posiUon resolution W i t h the resistive c a t h o d e sheet the construction o f the a n o d e is simpler than w~th the multlwlre c a t h o d e since tt IS unnecessary to take p o s m o n i n f o r m a t i o n f r o m the a n o d e itself However, g o o d hnearlty is not easy to o b t a i n &rectly by th~s m e t h o d 13) A rather different m e t h o d is used in the present w o r k Separate electrodes are placed between the a n o d e a n d the cathode, one for each & m e n s l o n , a n d the p o s a ] o n mformat~on IS o b t a i n e d from these N o signals are taken from the a n o d e or the c a t h o d e The

INDUCTION

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electrodes are planes o f high resistance wire, with the wires o f one plane p e r p e n d i c u l a r to the wires o f the other, a n d the m o t i o n o f the positive ions in the avalanche induces signals on these The a r r a n g e m e n t is shown d i a g r a m m a t i c a l l y m fig t Since a counter avalanche Induces charge on all the wires in each plane it can be expected that a relatively coarse wire spacing wdl allow r e a s o n a b l y precise poslt~on detection Thls ' t e t r o d e ' d e t e c t o r also has o t h e r a d v a n t a g e s The signal electrodes are electrostatlcally shielded f r o m external interference by the other electrodes, a n d b o t h s~gnal o u t p u t s are at the same potential

3. Construction of the detectors In o r d e r to d e m o n s t r a t e the potential o f the one d i m e n s i o n a l detectors, counters o f length 10 cm a n d 1 0 0 c m were e m p l o y e d Both are constructed like c o n v e n t i o n a l cylindrical p r o p o m o n a l counters, with cathodes o f brass t u b i n g o f internal d i a m e t e r 3 cm The resistive electrode ts the a n o d e w~re which Is o f stainless steel a n d has a d i a m e t e r o f t 0 # m a n d a resistance o f 85 D/cm The counter for two dimensions is constructed f r o m perspex frames w~th apertures 23 cm by 23 cm on which the electrodes are m o u n t e d As shown in fig 2 the a n o d e ts placed 1 9 cm f r o m each c a t h o d e a n d the two induction grids are m o u n t e d halfway between the a n o d e a n d one o f the cathodes The lower half o f the structure was not used as a detector in these experiments a l t h o u g h it could have been m a d e into a separate

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L O C A L I S A T I O N OF I O N I S I N G R A D I A T I O N

signal, obtained by summing the signals from both ends, is computed to gwe position information A schematic diagram of the arrangement is shown in fig 3 In the case of the two dimensional counter, there are two ratios to be t a k e n - o n e for each dimension However, tt was found that the ratio of the total charge signals on both grids was nearly constant and close to 1, and so the same total charge stgnal IS used

counter by fitting another pair of induction grtds* The induction grids are constructed in a simple way Each of them consists of a smgle length o f stainless steel wire, diameter 20 #m which is w o u n d in a zlg-zag fashion round nylon pegs in a perspex frame to give a spacing between adjacent wires of 8 m m The total resistance of each grid is 10kf2 Both grids are supported on the same frame, one being raised 0 3 m m above the other by thin strips o f mylar on the frame. It had been planned originally to use very fine wire mesh as the anode o f this counter However, prelimmary experiments showed that with standard commercial mesh, the aperture to web ratio was too small to locahse the gas multiplication sufficiently close to the anode The observations reported here were therefore carried out with an anode of tungsten wtre mounted on a frame in the same way as the grid wires The wire spacmg was 3 m m and the diameter 20/~m

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4. The electronics of the detecting system For the one dimensional counter the charge reaching each end of the resistive anode is integrated by means of a charge sensitive preamphfier These amplifiers have very low input impedances and so effectively earth the ends of the electrode The signals f r o m the preamplifiers are then filtered (with lntegratton and differentiation time constants of 2 ira) and amplified The ratio of the signal from one end o f the anode to the total

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Fig 3 Block & a g r a m of the counter system for posltlon detection m one &menslon r represents the shaping network VA and VF represent the potentials apphed to the anode and field correcting tubes respectively

* With such a double counter the &rectlon, as well as the position, of the track of an lOmslng particle could be determined, perhaps automatically by an analogue c o m p u t e r

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for position d e t e r m i n a t i o n m b o t h dimensions This simplified the design o f the ratio circuit as only one total stgnal had to be processed A design o f ways in which m e a s u r e d - by niques which

ratio clrcuzt There are a n u m b e r o f the ratto o f two signal pulses can be digital c o m p u t e r , or by a n a l o g u e techm a y use logartthmtc 14) or hnear 15)

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effects Thts pulse height was f o u n d to be equivalent to a b o u t 5 × l07 Ion pairs in the avalanche To enable the counters to be used for the detectton o f soft X-rays thin m y l a r windows were fitted, a n d thus, to m a m t a i n the purity o f the filling gas, the counters were o p e r a t e d as flow counters with the filling gas at one a t m o s p h e r e pressure

circuits

5 1 ONE DIMENSIONAL COUNTERS

A digital c o m p u t e r was used early in the present work, but the finite n u m b e r o f channels in the analogueto-digital converters m a d e quantlsatxon effects sometimes a p p a r e n t in the ratios recorded with uniform i r r a d i a t i o n o f the detector These effects are a v o i d e d by taking the ratio by an analogue m e t h o d , and a suitable ratio circuit o f high accuracy was developed during the course o f thts w o r k The principle o f this ratio circuit, which m a k e s use o f the linear charging o f condensers, is shown in fig 4 Positive pulses Va a n d Vb from the m a i n amplifiers enter the circuit and both are stretched a n d integrated if the d e n o m i n a t o r pulse Vb lies within a certain range set on a pulse height selector The o u t p u t o f the d e n o m i n a t o r integrator is p r o p o r t i o n a l to Vb t where t is the m t e g r a t l o n time This signal is fed to a dtscrlm l n a t o r o f threshold V, The o u t p u t o f this discrimin a t o r is used to stop the m t e g r a t t o n o f both n u m e r a t o r and d e n o m i n a t o r voltages, the resulting integration time being p r o p o r t i o n a l to Vs/V b The o u t p u t signal from the n u m e r a t o r m t e g r a t o r at that time is

Wtth a gas flow o f 9 0 % a r g o n , 10% methane t h r o u g h the detectors, the position resolution for a collimated b e a m o f 5 5 M e V a l p h a particles from americium-241 was 0 42 m m full width half m a x i m u m (fwhm) corres p o n d i n g to a s t a n d a r d deviation (a) o f 0 18 m m in the 10 cm counter, a n d 1 m m fwhm in the 100 cm counter This resolutton was i n d e p e n d e n t o f the percentage o f m e t h a n e in the mixture and m the case o f the small c o u n t e r was hmtted by the width o f the incident b e a m o f a l p h a parttcles In the 100 cm counter the h m l t a t l o n was mainly a result o f the J o h n s o n noise from the a n o d e wtre and the noise from the preamplifiers In o r d e r to test the p e r f o r m a n c e of such a detector with lower energy r a d i a t i o n and at higher counter gam, a collimated b e a m o f 8 keV X-rays, 0 15 m m wide, was used The resolution o f the 10 cm counter was f o u n d to vary with gas mixture, in this case from 0 6 m m fwhm for 50% argon, 50% m e t h a n e to a b o u t 2 m m for 90% argon, 10% methane The reason for this variation m resolution m a y be the p r o p a g a t i o n along the wtre o f the counter avalanche This might be expected to be greatest at low c o n c e n t r a t i o n s o f the

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where K is a constant Th~s signal is therefore a measure o f the required ratio The actual circutt used ~s more complex than shown in the figure a n d has a n u m b e r o f refinements to minimise errors at a high d u t y cycle The accuracy a n d llnearaty o f the final circuit are high Total errors are less than 0 3% o f m a x i m u m o u t p u t for all possible ratios less than one, gtven by values o f Va in the range 0 2 to 5 V a n d Vb in the range 2 to 5 V, a n d for c o u n t rates up to 4 × 104/s 5. Performance of the counter systems A n u m b e r o f m e a s u r e m e n t s were m a d e to determine the m a i n charactertstlcs such as position resolution, integral hnearity a n d count rate c a p a b i h t y o f the detectors In o r d e r to reduce the effect o f electromc norse in the system, the counters were o p e r a t e d to provide the m a x i m u m pulse height o b t a i n a b l e without spurious

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LOCALISATION

369

OF 1 O N I S I N G R A D I A T I O N

m e t h a n e quench gas, which is consistent with the observations The position resolution o f the counter system for the 8 keV X-rays was tested at c o u n t rates f r o m a b o u t 1/s to 10~/s a n d f o u n d to decrease by a factor o f two in this range A t least some o f this loss o f resolution was caused by pale-up o f pulses in the amplifiers The integral hnearlty o f the 100 cm counter is illustrated in fig 5 This is the relationship between the o u t p u t o f the ratio circuit a n d the p o s i t i o n o f the r a d i a t i o n detected A n y nonlinearity is less t h a n 0 2 % o f the full scale o u t p u t 5 2. THE TWO DIMENSIONAL COUNTER The charge pulses i n d u c e d on the grid wires are different in shape f r o m the n o r m a l type o f pulse from a c o u n t e r a n o d e when observed with an amplifier o f very long time c o n s t a n t - a s shown In fig 6 The rise o f the pulse has two distinct c o m p o n e n t s - a fast p a r t lasting a b o u t 40 ns during which time the positive ions are in the high field region near the a n o d e p l a n e , followed by a slower p a r t lasting a b o u t 200/~s as the positive ions a p p r o a c h the grid wires The charge pulse falls to zero again in a b o u t 200 ps as the positive ions move a w a y from the grid t o w a r d s the c a t h o d e There is some variation in wave form from pulse to

a) Anode

pulse Occasionally pulses with the shape shown d o t t e d in fig 6(b) were observed It is t h o u g h t that these occur when the positive ions f r o m the avalanches pass very close to a grid wire, the c o n c a v e - u p w a r d shape being a result o f the potential d i s t r i b u t i o n a r o u n d the wire

W i t h a flow o f 90% argon, 10% methane t h r o u g h the counter the spatial resolution o f a b e a m o f a l p h a particles f r o m a collimated a m e r i c i u m p o i n t source was a b o u t 1 25 m m fwhm in each dimension over an area o f 23 c m x 2 3 cm The resolution was limited partly by the width o f the a l p h a particle b e a m and p a r t l y by the electronic noise f r o m the grid wires a n d the amplifiers, and, as expected, it was c o n s i d e r a b l y finer t h a n the grid wire spacing It was expected t h a t the a n o d e wire spacing o f 3 m m w o u l d be large enough to have an effect on the position i n f o r m a t i o n in one dimension a n d in fact it was f o u n d t h a t there was a slight nonlinearity in position, which h a d a p e r i o d equal to the a n o d e wire spacing However, the maxim u m error in position resulting f r o m this was only _ 0 3 m m It is p r o b a b l e that the smallness o f this effect was a result o f the finite spatial distribution, a n d the t h e r m a l diffusion, o f the p r i m a r y l o m s a t i o n released by the incident r a d i a t i o n - s i n c e b o t h these factors could have caused the counter avalanches to be shared by m o r e t h a n one a n o d e wire The position response o f the two d i m e n s i o n a l counter is shown in fig 7 A n a l p h a particle llne source was used to test each d i m e n s i o n separately, b u t in b o t h cases the d e n o m i n a t o r signal was t a k e n from the grid

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p a r a l l e l to the a n o d e wires The curve labelled X in fig 7 c o r r e s p o n d s to the case where the n u m e r a t o r signal is taken f r o m this same grid M e a s u r e m e n t s o f integral hnearlty, a l t h o u g h they are an i m p o r t a n t test o f a position sensitive detector, are difficult to m a k e with sufficient precision to show the small periodic effects which might be expected to result from the finite spacing o f the grid wires In o r d e r to allow these periodicities to be measured, a m o r e sensitive technique was developed This consists o f mechanically scanning a c o l l i m a t e d a l p h a particle point source over a length o f c o u n t e r in either d i m e n sion at a constant rate and a c c u m u l a t i n g the pulses from the ratio circuit In a pulse height analyser The stored d a t a indicate variations in dtJJerenttal hnearlty F o r the dimension in which b o t h the n u m e r a t o r and d e n o m i n a t o r signal for the ratio circuit came from the same grid, the differentml hnearity o b t a i n e d is shown in fig 8 This curve has a p e r i o d equal to the grid wire spacing a n d the m a x i m u m error in position is a p p r o x i mately + 0 2 m m F o r the d i m e n s i o n where the signal from one end o f one o f the grids was divided by the total signal f r o m the other the periodic error was larger, being a b o u t _+0 7 m m There was also some v a r i a t i o n in the a p p a r e n t position in one dimension o f a source which was being moved in the other dimension The difference between (o) .10~

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the a p p a r e n t position and the actual position was nowhere greater than 1 m m This counter system was o p e r a t e d at count rates up to 104/s but as was the case for the one d i m e n s i o n a l detectors there was some loss o f resolution at high count rates as a result o f pile-up effects in the a m p h tiers Two o f the factors which hmit the spatial resolution o f this detector are the spacing o f the grid wires a n d the J o h n s o n noise from the grids The effect o f the first o f these could be reduced by using m o r e closely spaced grid wires, and o f the second by the wiring o f the grids with a material o f higher resistance A n i m p r o v e m e n t in the position resolution should be o b t a i n a b l e in this way U n f o r t u n a t e l y no material o f higher resistance t h a n the stainless steel used was a~allable when this w o r k was carried out, however it seems p r o b a b l e that a suitable material would be fine quartz or nylon thread c o a t e d with a resistive layer It should be noted that the resolution o f the one dimensional detectors should also be i m p r o v e d by using a n o d e wires o f higher resistance 53

T H E F O R M A T I O N OF IMAGES W I T H T H E T W O DIMENSIONAL DETECTOR

One possible a p p l i c a t i o n o f the b l d l m e n s l o n a l r a d i a t i o n detector described here is the f o r m a t i o n of images o f radiation patterns Typical s h a d o w images o f a cross a n d a spanner irradiated with X-rays o f 40 keV energy from an X-ray machine are shown in fig 9 In this case the i n f o r m a t i o n was stored and displayed on a storage oscilloscope The radiation dose used to obtain these images was low, being a b o u t 2 y R a d in each case The spatial resolution of the images is a b o u t 6 m m fwhm m each dimension, this being d e t e r m i n e d to some extent by the range o f the photoelectrons from the X-rays in the a r g o n - m e t h a n e mixture It might he noted that the use o f a filling gas at higher pressure or o f higher a t o m i c weight (such as k r y p t o n or xenon) could be expected to i m p r o v e the spatial resolution for X-rays and to reduce the X-ray dose required to obtain an image 6. Conclusion

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FOSITION (cm)

Fag 8 a) Dlfferent~al h n e a n t y o ~ e r a s h o r t l e n g t h m o n e d i m e n s i o n o f t h e t w o & m e n s m n a l c o u n t e r b) T h e r e s u l t i n g v a n a t m n m the posmon error over the same length of counter

The p r o p o r t i o n a l counters outlined here have a very satisfactory p e r f o r m a n c e They are able to locahse lOnlslng r a d i a t i o n with resolutions up to 0 1% fwhm o f their sensitive lengths a n d can operate at c o u n t rates o f 104/s The construction, and signal processing a n d r e a d o u t circuits are all relatively simple and the performance c o m p a r e s f a v o u r a b l y with most other position sensitive detectors

LOCALISATION

OF IONISING

RADIATION

371

F i g 9 a) S h a d o w i m a g e o f a c r o s s s h a p e d a p e r t u r e m a l e a d s h e e t e x p o s e d t o X - r a y s o f 4 0 k e V p e a k e n e r g y f r o m a n X - r a y m a c h i n e T h e t o t a l w~dth o f t h e c r o s s is 11 c m b) S h a d o w i m a g e o f a s p a n n e r e x p o s e d to t h e s a m e X - r a y s T h e d x a m e t e r o f t h e s p a n n e r h e a d as 8 c m

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The low doses required for the formation of radiation images make these devices competitive with many other lmagmg systems, and tt is envisaged that counters of this type may displace photographic methods and comphcated multi-counter arrays from some of this work

We are greatly indebted to Mr A McKellar who carried out a large part of the development and bmldang of the electronic circuits for this work, and to our workshop staff for the fabrication of the detectors We also wish to thank Prof P I Dee for his interest and encouragement One of us (J H ) wishes to acknowledge financial support from the University of Glasgow for the period when this work was carried out

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P

DREVEN

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