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Time structure of gas scintillation proportional counter signals
NUCLEAR
INSTRUMENTS
AND
METHODS
134
(I976) 2 5 - - 2 7 ;
©
NORTH-HOLLAND
PUBLISHING
CO.
T I M E S T R U C T U R E OF GAS S C I N T I L L A T I O N P R O P O R T I O N A L C O U N T E R SIGNALS R.D. ANDRESEN,
L. K A R L S S O N a n d B. G. T A Y L O R
Space Science Department, European Space Research and Technology Centre, European Space Agency, Noordwijk, The Netherlands Received 4 F e b r u a r y 1976 T h e time structure o f the anode pulses obtained f r o m a photomultiplier viewing the light o u t p u t f r o m a gas scintillation proportional counter has been investigated. With fast electronics a n d photomultiplier tubes it is observed that the anode signal is in the f o r m o f a burst o f pulses generated by the sequential arrival o f light p h o t o n s at the p h o t o c a t h o d e o f the photomultiplier tube. The inherent possibilities for direct digital energy recording a n d pulse " s h a p e " discrimination are discussed.
1. Introduction Recent investigation ~-3) on gas scintillation proportional counters have demonstrated that this type of detector has excellent inherent capabilities for X-ray detection. With xenon-filled counters an energy resolution, fwhm, of 8-9% has been obtained with 5.9 keV X-rays from an 55Fe source. This alone renders t]ae xenon gas scintillation proportional counter a very good competitor to conventional proportional counters which, at their very best, have provided an fwhm of 17% with 5.9 keV photons. One disadvantage in connection with the use of gas scintillation counters was that until recently they had to be operated with a gas purification system that on a more or less continuous basis purified the gas as the performance of the counter would otherwise degrade on a time scale of days. It has been shown 4"s) that the exploitation of ultra-high vacuum techniques, highly purified gas filling and the incorporation of a cesium getter can render the gas purification system superfluous, at least on a time scale of half a year. With the intention to use gas scintillation counters for X-ray
astronomy on free flying satellites this is an important improvement. When using this type of counter for X-ray astronomy, another important factor is the possibility to be able to discriminate against unwanted signals such as those generated by charged particles traversing the active detector volume as well as those generated by photons penetrating and interacting into the scintillation region, see fig. 1. A well proven technique with conventional proportional counters has been the pulse shape discrimination technique, i.e. where discrimination against charged particle induced signals is performed on the basis of the risetime of the charge pulse on the sensitive wire. This method cannot be used with a gas scintillation counter, primarily because of the necessary absence of charge multiplication in the counter which implies that the charge signal on the grid terminating the scintillation region caused by e.g. a 5.9 keV X-ray event or a particle event with a similar energy deposit is orders of magnitude too small to be analysed by
98% TRANSPARENT LITHIUMFLUORIDE WIRE MESH ELECTRODES WINDOW
~/~
RCA C31000M laM. TUBE
I XENON i ~AT760 TORRI
ENTRANCE V¢INDOW.~, 5,urn ALUMINUM A
TO CESIUM GETTER
C
i
D
~
/
OUARTZ WINDOW
Fig. 1. Simplified drawing s h o w i n g the main features o f the gas scintillation counter. In the present work section A was used as drift region a n d B as scintillation region.
Fig. 2. Oscilloscope p h o t o g r a p h s h o w i n g the burst structure o f the photomultiplier tube signal when 5.9 keV X-rays are absorbed in the drift region o f the gas scintillation counter. T h e time scale is 500 ns per division.
26
R.D.
ANDRESEN
existing amplifier techniques. However a particle-versus X-ray identification technique can be developed on the basis of the temporal structure of the light signal. In this paper some preliminary results of investigations on the time structure of the light signals obtained from a xenon-filled gas scintillation proportional counter are presented.
2. Experimental results Although some results on the temporal structure of the light output from gas scintillation counters have been presented 6"7) none of the measurements have been made with electronics and photomultipliers possessing sufficient bandwidth to reveal the finer details of the light output pulse shape. The present measurements are made with a UV-sensitive, linear focussed type RCA C 31000 M development type photomultiplier having a single electron response fwhm of typically 5 ns. The electronics employed have a matching bandwidth capability. In fig. 2 is shown the photomultiplier anode signal generated by a 5.9 keV X-ray absorbed in the drift region of the gas scintillation counter. It is seen that the non-integrated anode signal is in the form of a burst of single, two, three etc. electron response pulses each corresponding to the arrival of one or more light photons at the photocathode of the photomultiplier tube. The finite single clectron response fwhm time limits the number of possible individual pulses within a burst. Within the energy range from 4 to 8 keV, the lower limit being due to availability of sources, we have observed a nearly linear relationship between the number of pulses within a burst and the energy of the absorbed X-ray photon. Fig. 3 shows an expanded view of the burst signal in order to reveal the individual pulses in the burst more clearly. From these observations the
Fig. 3. Expanded view of the same type of burst signal as in fig. 1, but with a time scale of 50 ns/div.
et al.
conclusion can be drawn that with planar gas scintillation proportional counters, a direct digital energy recording can be implemented. The maximum upper X-ray energy that can be determined in this manner is limited by the temporal fwhm of the photomultiplier single electron response and the bandwidth of the counting system following the photomultiplier tube. With the gas scintillation counter and photomultiplier used in the present investigations a 5.9 keV X-ray absorbed in the drift region generates on the average about 250 individual pulses at the photomultiplier tube anode, within a time interval of about 4.5/~s. In the case of a photoelectric absorption event like this the length of the burst is primarily determined by two parameters, the length of the scintillation region and the electron drift velocity in it. The length will also be determined by diffusion in the drift region and will be dependent on the position of the conversion point. This adds as a secondary effect. In contrast to the X-ray absorption event type signal, fig. 4 shows the photomultiplier tube anode signal obtained from the absorption of an :~-particle, with an energy of about 2 MeV, in the drift region. It can be demonstrated that for such events, the length of the burst is determined by two additional parameters, namely by the length of the ~-particle track in the drift region and the electron drift velocity in this region. This implies that charged particle events in the drift region giving rise to an extended ionisation trail nonparallel to the grids can be identified on the basis of the longer burst lengths which are produced by such events. The upper trace in fig. 4 is triggered by the primary scintillation light pulse generated in the drift
Fig. 4. Oscilloscope photograph showing the photomultiplier tube anode signal arising when 2 MeV ~-particles are absorbed in the drift region. The primary scintillation signal is used as trigger pulse for the oscilloscope. The lower trace represents the charge collection on the last grid of the gas scintillation counter. The time scale is 2/ts per division.
TIME STRUCTURE
OF C O U N T E R
Fig. 5. OsciUogram o f the integrated photomultiplier tube a n o d e signals obtained with a 2°7Bi source.
region, which can be seen at the very left end of the trace. This signal precedes the scintillation signal from the scintillation cell by 4/~s; being the time taken by the electrons from the last part of the a-particle track to reach the scintillation region. The very significant difference in terms of light output from the primary and the secondary scintillation is also easily observed on this oscilllogram. The lower trace in fig. 4 displays the charge signal collected from the grid terminating the scintillation region. In fig. 5 is shown an oscillogram displaying the integrated photomultiplier anode pulses obtained for a series of events using a 2°7Bi source which emits X-rays, gamma-rays and conversion electrons, the latter two radiations with energies up to about 1 MeV. For the sake of clarity the signals shown here are integrated burst signals with a risetime equivalent to the length of the burst lengths. From the oscillogram it is seen that a wide spread in risetimes is present in this case, including signals with a two-component risetime. Signals having a shorter risetime than that given by the length of the scintillation region and the electron drift velocity in this region, i.e. about 4 ps in the present case, are signals generated by absorption events in the scintillation region and can be rejected c,n the basis of a (burst-length) risetime discrimination. Signals having a risetime longer than about 4.5/~s are generated by two different types of events. The first type of long-risetime events are those generated by an extended track contained within the drift region, li~ e
SIGNALS
27
shown in fig. 4 in the case of an e-particle absorption. When integrating the burst generated by such an event a two-component risetime signal is generated with a low slew rate interval followed by a faster rate of rise determined by the characteristics of the scintillation region. The second type of long-risetime events are those generated by charged particles penetrating into the scintillation region and leaving an ionisation track in the drift region. The free electrons generated in the scintillation region start instantaneously to produce light and later, as the rest of the electrons from the drift region arrive at the scintillation region, they produce an extended-track type of rate of rise signal. 3. Conclusions
This preliminary work on the temporal structure of the signals obtained from a xenon-filled planar gas scintillation proportional counter demonstrates that a positive, event identification-discrimination is feasible with a counter of this type. Further investigations are under way. The authors are grateful to acknowledge the useful discussions with Dr A. Peacock of their laboratory and the technical expertise and assistance provided by Mr E.-A. Leimann. This work is part of a joint effort with the Mullard Space Science Laboratory and Dr P. Sanford is thanked for his comments. Dr. L. Karlsson acknowledges the receipt of an ESA fellowship. References x) A. J. P. L. Policarpo, M. A. F. Alves, M. C. M. D o s Santos a n d M. J. T. Carvalho, Nucl. Instr. and Meth. 102 (1972) 337. 2) H . E . Palmer and L . A . Braby, Nucl. Instr. a n d Meth. 116 (1974) 587. a) A. J. P . L . Policarpo, M . A . F . Alves, M. Salete, S . C . P, Leite a n d M. C. M. Dos Santos, Nucl. Instr. a n d Meth. 118 (1974) 221. 4) R . D . Andresen, B . G . Taylor and P. Sanford, 14th Int. Cosmic R a y Conf. (M~inchen, 1975) vol. 9, p. 3107. 5) R. D. Andresen, L. Karlsson a n d B. G. Taylor, to be published in IEEE Trans. Nucl. Sci. (February 1976). 6) C. A. N. C o n d e and A. J. P. L. Policarpo, Nucl. Instr. a n d Meth. 53 (1967) 7. 7) A. J. P. L. Policarpo, M. A. F. Alves and C. A. N. Conde, Nucl. Instr. and Meth. 55 (1967) 105.
INSTRUMENTS
AND
METHODS
134
(I976) 2 5 - - 2 7 ;
©
NORTH-HOLLAND
PUBLISHING
CO.
T I M E S T R U C T U R E OF GAS S C I N T I L L A T I O N P R O P O R T I O N A L C O U N T E R SIGNALS R.D. ANDRESEN,
L. K A R L S S O N a n d B. G. T A Y L O R
Space Science Department, European Space Research and Technology Centre, European Space Agency, Noordwijk, The Netherlands Received 4 F e b r u a r y 1976 T h e time structure o f the anode pulses obtained f r o m a photomultiplier viewing the light o u t p u t f r o m a gas scintillation proportional counter has been investigated. With fast electronics a n d photomultiplier tubes it is observed that the anode signal is in the f o r m o f a burst o f pulses generated by the sequential arrival o f light p h o t o n s at the p h o t o c a t h o d e o f the photomultiplier tube. The inherent possibilities for direct digital energy recording a n d pulse " s h a p e " discrimination are discussed.
1. Introduction Recent investigation ~-3) on gas scintillation proportional counters have demonstrated that this type of detector has excellent inherent capabilities for X-ray detection. With xenon-filled counters an energy resolution, fwhm, of 8-9% has been obtained with 5.9 keV X-rays from an 55Fe source. This alone renders t]ae xenon gas scintillation proportional counter a very good competitor to conventional proportional counters which, at their very best, have provided an fwhm of 17% with 5.9 keV photons. One disadvantage in connection with the use of gas scintillation counters was that until recently they had to be operated with a gas purification system that on a more or less continuous basis purified the gas as the performance of the counter would otherwise degrade on a time scale of days. It has been shown 4"s) that the exploitation of ultra-high vacuum techniques, highly purified gas filling and the incorporation of a cesium getter can render the gas purification system superfluous, at least on a time scale of half a year. With the intention to use gas scintillation counters for X-ray
astronomy on free flying satellites this is an important improvement. When using this type of counter for X-ray astronomy, another important factor is the possibility to be able to discriminate against unwanted signals such as those generated by charged particles traversing the active detector volume as well as those generated by photons penetrating and interacting into the scintillation region, see fig. 1. A well proven technique with conventional proportional counters has been the pulse shape discrimination technique, i.e. where discrimination against charged particle induced signals is performed on the basis of the risetime of the charge pulse on the sensitive wire. This method cannot be used with a gas scintillation counter, primarily because of the necessary absence of charge multiplication in the counter which implies that the charge signal on the grid terminating the scintillation region caused by e.g. a 5.9 keV X-ray event or a particle event with a similar energy deposit is orders of magnitude too small to be analysed by
98% TRANSPARENT LITHIUMFLUORIDE WIRE MESH ELECTRODES WINDOW
~/~
RCA C31000M laM. TUBE
I XENON i ~AT760 TORRI
ENTRANCE V¢INDOW.~, 5,urn ALUMINUM A
TO CESIUM GETTER
C
i
D
~
/
OUARTZ WINDOW
Fig. 1. Simplified drawing s h o w i n g the main features o f the gas scintillation counter. In the present work section A was used as drift region a n d B as scintillation region.
Fig. 2. Oscilloscope p h o t o g r a p h s h o w i n g the burst structure o f the photomultiplier tube signal when 5.9 keV X-rays are absorbed in the drift region o f the gas scintillation counter. T h e time scale is 500 ns per division.
26
R.D.
ANDRESEN
existing amplifier techniques. However a particle-versus X-ray identification technique can be developed on the basis of the temporal structure of the light signal. In this paper some preliminary results of investigations on the time structure of the light signals obtained from a xenon-filled gas scintillation proportional counter are presented.
2. Experimental results Although some results on the temporal structure of the light output from gas scintillation counters have been presented 6"7) none of the measurements have been made with electronics and photomultipliers possessing sufficient bandwidth to reveal the finer details of the light output pulse shape. The present measurements are made with a UV-sensitive, linear focussed type RCA C 31000 M development type photomultiplier having a single electron response fwhm of typically 5 ns. The electronics employed have a matching bandwidth capability. In fig. 2 is shown the photomultiplier anode signal generated by a 5.9 keV X-ray absorbed in the drift region of the gas scintillation counter. It is seen that the non-integrated anode signal is in the form of a burst of single, two, three etc. electron response pulses each corresponding to the arrival of one or more light photons at the photocathode of the photomultiplier tube. The finite single clectron response fwhm time limits the number of possible individual pulses within a burst. Within the energy range from 4 to 8 keV, the lower limit being due to availability of sources, we have observed a nearly linear relationship between the number of pulses within a burst and the energy of the absorbed X-ray photon. Fig. 3 shows an expanded view of the burst signal in order to reveal the individual pulses in the burst more clearly. From these observations the
Fig. 3. Expanded view of the same type of burst signal as in fig. 1, but with a time scale of 50 ns/div.
et al.
conclusion can be drawn that with planar gas scintillation proportional counters, a direct digital energy recording can be implemented. The maximum upper X-ray energy that can be determined in this manner is limited by the temporal fwhm of the photomultiplier single electron response and the bandwidth of the counting system following the photomultiplier tube. With the gas scintillation counter and photomultiplier used in the present investigations a 5.9 keV X-ray absorbed in the drift region generates on the average about 250 individual pulses at the photomultiplier tube anode, within a time interval of about 4.5/~s. In the case of a photoelectric absorption event like this the length of the burst is primarily determined by two parameters, the length of the scintillation region and the electron drift velocity in it. The length will also be determined by diffusion in the drift region and will be dependent on the position of the conversion point. This adds as a secondary effect. In contrast to the X-ray absorption event type signal, fig. 4 shows the photomultiplier tube anode signal obtained from the absorption of an :~-particle, with an energy of about 2 MeV, in the drift region. It can be demonstrated that for such events, the length of the burst is determined by two additional parameters, namely by the length of the ~-particle track in the drift region and the electron drift velocity in this region. This implies that charged particle events in the drift region giving rise to an extended ionisation trail nonparallel to the grids can be identified on the basis of the longer burst lengths which are produced by such events. The upper trace in fig. 4 is triggered by the primary scintillation light pulse generated in the drift
Fig. 4. Oscilloscope photograph showing the photomultiplier tube anode signal arising when 2 MeV ~-particles are absorbed in the drift region. The primary scintillation signal is used as trigger pulse for the oscilloscope. The lower trace represents the charge collection on the last grid of the gas scintillation counter. The time scale is 2/ts per division.
TIME STRUCTURE
OF C O U N T E R
Fig. 5. OsciUogram o f the integrated photomultiplier tube a n o d e signals obtained with a 2°7Bi source.
region, which can be seen at the very left end of the trace. This signal precedes the scintillation signal from the scintillation cell by 4/~s; being the time taken by the electrons from the last part of the a-particle track to reach the scintillation region. The very significant difference in terms of light output from the primary and the secondary scintillation is also easily observed on this oscilllogram. The lower trace in fig. 4 displays the charge signal collected from the grid terminating the scintillation region. In fig. 5 is shown an oscillogram displaying the integrated photomultiplier anode pulses obtained for a series of events using a 2°7Bi source which emits X-rays, gamma-rays and conversion electrons, the latter two radiations with energies up to about 1 MeV. For the sake of clarity the signals shown here are integrated burst signals with a risetime equivalent to the length of the burst lengths. From the oscillogram it is seen that a wide spread in risetimes is present in this case, including signals with a two-component risetime. Signals having a shorter risetime than that given by the length of the scintillation region and the electron drift velocity in this region, i.e. about 4 ps in the present case, are signals generated by absorption events in the scintillation region and can be rejected c,n the basis of a (burst-length) risetime discrimination. Signals having a risetime longer than about 4.5/~s are generated by two different types of events. The first type of long-risetime events are those generated by an extended track contained within the drift region, li~ e
SIGNALS
27
shown in fig. 4 in the case of an e-particle absorption. When integrating the burst generated by such an event a two-component risetime signal is generated with a low slew rate interval followed by a faster rate of rise determined by the characteristics of the scintillation region. The second type of long-risetime events are those generated by charged particles penetrating into the scintillation region and leaving an ionisation track in the drift region. The free electrons generated in the scintillation region start instantaneously to produce light and later, as the rest of the electrons from the drift region arrive at the scintillation region, they produce an extended-track type of rate of rise signal. 3. Conclusions
This preliminary work on the temporal structure of the signals obtained from a xenon-filled planar gas scintillation proportional counter demonstrates that a positive, event identification-discrimination is feasible with a counter of this type. Further investigations are under way. The authors are grateful to acknowledge the useful discussions with Dr A. Peacock of their laboratory and the technical expertise and assistance provided by Mr E.-A. Leimann. This work is part of a joint effort with the Mullard Space Science Laboratory and Dr P. Sanford is thanked for his comments. Dr. L. Karlsson acknowledges the receipt of an ESA fellowship. References x) A. J. P. L. Policarpo, M. A. F. Alves, M. C. M. D o s Santos a n d M. J. T. Carvalho, Nucl. Instr. and Meth. 102 (1972) 337. 2) H . E . Palmer and L . A . Braby, Nucl. Instr. a n d Meth. 116 (1974) 587. a) A. J. P . L . Policarpo, M . A . F . Alves, M. Salete, S . C . P, Leite a n d M. C. M. Dos Santos, Nucl. Instr. a n d Meth. 118 (1974) 221. 4) R . D . Andresen, B . G . Taylor and P. Sanford, 14th Int. Cosmic R a y Conf. (M~inchen, 1975) vol. 9, p. 3107. 5) R. D. Andresen, L. Karlsson a n d B. G. Taylor, to be published in IEEE Trans. Nucl. Sci. (February 1976). 6) C. A. N. C o n d e and A. J. P. L. Policarpo, Nucl. Instr. a n d Meth. 53 (1967) 7. 7) A. J. P. L. Policarpo, M. A. F. Alves and C. A. N. Conde, Nucl. Instr. and Meth. 55 (1967) 105.