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An imaging gas scintillation proportional counter coupled to a channel multiplier array for application in cosmic X-ray spectroscopy
0273—1177/83/O~0225O4$03.OO/O
28, 1983 Adv. Space Res. Vol.2, No.4, pp.225—2 Printed in Great Britain. All rights reserved.
Copyright © COSPAR
AN IMAGING GAS SCINTILLATION PROPORTIONAL COUNTER COUPLED TO A CHANNEL MULTIPLIER ARRAY FOR APPLICATION IN COSMIC X-RAY SPECTROSCOPY B. G. Taylor, M. R. Sims, J. Davelaar and A. Peacock European Space Agency, SSD, ESTEC, Noordwijk, Netherlands ABSTRACr The develo~nt of an imaging gas scintillation proportional counter which utilises a channel xi-ultiplier array as the readout elemant is discussed. Preliminary experin~ntal results and theoretical considerations indicate that spatial and energy resolutions of below 500 pm and 8% respectively slould be achievable at an X-ray energy of 6 key.
INTI~)DUCTION Imaging gas scintillation proportional counters (GSPC) have recently been developed based on either a niulti—photanultiplier Anger canera readout system or a plotoiordsation detector (1), (2). Both readout systems are currently limited to spatial and energy resolutions of 1 ma and 9% respectively at an X—ray energy of 6 key. Fran a technological point of view the multi-pi-otanultiplier tube approach has a drawback in the need for a fine gain ountrol system for the photaiultiplier tubes. The long term integrity of a photoionisation detector readout has yet to be denonstrated, particularly in a space environmant. In an endeavour to improve the spatial resolution, enabling sub-arc minute angular resolution, yet minimising the technical difficulties, a channel nultiplier array (c4~) readout system has been investigated. The introduction of a CNA as the position and energy sensing el~nt of an imaging GSPC was suggested by Hailey et al. (1981). Their theoretical results indicate that spatial resolutions of below 500 pm for 6 key X-rays should be achievable whilst still retaining the good energy resolution of the GSPC (3). In this paper we present the experirental results fran an imaging GSPC coupled to a Q’IA using non-optiniised but “off-the-shelf” elemants, while optimisations, suggested through Monte Carlo sinulatioris are discussed. EXPERIMENTAL RESULTS The experinental results have been obtained fran the detector illustrated scheniatically in Figure 1. The GSPC gas cell was essentially similar to that discussed in reference 1, whilst the readout elen~nt consisted of a 5 an dianeter chevron—pair of channel plates, ‘~hich are coupled to the gas cell. Adequate quantum efficiency (- 10% peak) in the UV waveband 16502000 ~ was achieved by coating the front plate with CsI. The a.1P~was operated in a vacuum of < io—5 tort, arid at a charge gain of - 5 i0~. EdhAO 4~~~OVkmMICROCHANREL PLATES CHEVRON PAIR
1,
2S~rn MYLAR X-RAY ENTRANCE WINDOW •
CSJNTER Z-AXIS-—--—
.
GASXENON Ii SARI
5TORT ~VACUUM
I
.
-
•
I
.
I
DRIFT PHOTO ABSORPTION REGION
RESSTIVE DISC READOUT
.
1, PICK-UP ELECTRODESI
CaF
2 WINDOW\ PLATE SURFACE COATED WITH CsI I Z~O
SCINTILLATION RESOW —Ti
—~
SCALE ~1~iss
Fig. 1 JASU 2:4 — P
A sch~natic of the gas cell coupled to the CP1P~. 225
226
B. G. Taylor et al.
The principle of operation of such a detector can be surnnarised as follows: an incident X-ray photon will prcduce on average - 45 ohotoelectrons keV~which turn, the via the 1 sr1 as they will, drift in through gas scintillation process, create 1O~TN photons m scintillation region. These TN photons will be observed by the CI~as a burst with a burst
tire geverned primarily by the cell depth z 0-z1. Each UV photon detected by the GIA will produce an electron avalanche that can be recorded by a variety of readout systems; in this case the readout system is based on a resistive anode, similar to that described by StUnpel et al. (1973) (4). The charge distrib.ition on the resistive anode is continuously integrated throughout the IN photon burst by the disc impedance which results in a typical tire constant of - 15 pS for the centre of the disc. This charge distribution is sensed by four pick-up electrodes, located synmetrically on the edge of the disc. The amplitude ratios of these signals provides the ‘centroid’ of the charge distrib..ition and thus the X-ray çhotoabsorption position in the detector’s X,Y plane. The X-ray photon energy is directly
proportional to the sum of these four signals for a given position
(4).
Previous experimoental results using a photanultiplier tube array which operates as an Anger camera have indicated that, for a given X-ray photon energy, both the spatial and energy resolution are inversely proportional to the square root of the total TN light yield (1). The spatial resolution is defined here as the full width half maxixonxn of the point spread function. The current results have been obtained for the 6 key X-rays collimated onaxis. SCINTILLATION FIELD E5 (V/Cs) 200
2750
1750
2000
ENERGY RES~UTION
C
Fig. 2 The variation of the inverse of the 04A sum signal with the
-<
100
-
1000
spatial and energy
POS~~CN RESOLUTION
000
,‘
—
cc~p~:’icFOR COLLIMATION) ~O1 002
-
(AMPLITUDE OF SLM~1E READOuT SIGNAL ii
Figure 2 illustrates the variation of the square of both the position and energy resolution as a function of the inverse of the G~1Asun~1 output signal. The variation in the sum signal was obtained by changing the scintillation field E5. A minflinim position and energy resolution of - 5 rim arid 24% respectively was obtained for a scintillation field of 2750 V/an for the experirental configuration described here. In the limit, the spatial resolution will be constrained by diffusion effects, whilst the energy resolution is limited by the Fano factor. SThJ1ATIC~SAND OPTIMISATICtI The dependency of the spatial and energy resolution on the characteristics of both the gas cell and Q4P. have been investigated by means of a Monte Carlo procedure with a view to optinising the detector system. The following factors s~re included: (i)
Fluctuations in the initial
number of electrons produced in the photoabsorption
orocess. (ii)
Poissonian statistics on the TN light produced as the electrons drift through the scintillation region which is of a finite depth (zo—zi) and at a distance z1 fran the photocathode.
(iii)
The variation in the EN tranADnission through the CaF2 exit window as a function of incident angle.
(iv)
The variation of the quantum efficiency of the GI7E as a functimn of incident angle.
(v)
The statistics on the charge gain/collection process in the GIA. (It is assumed that only one photon triggers a channel, and that the single electron response is - 80% FT*~N).
Civ
photon
Gas Scintillation Proportional Counter in Cosmic X—Ray SpectroscopY (iv)
The readout non-linearity of the resistive disc based on the ~rk et al. (1981) (5).
227
of Frazer
The predicted spatial and energy resolution for the experirental configuration illustrated schematically in Figure 1 was found to be - 4 mu and 23% respectively in good agreement with the preliminary experirental data. The single major contribution to the position resolution arises fran the large non-linearity towards the edge of the resistive disc. This distortion leads to large signal amplitudes on the edge of the disc compared to the centre. If the readout system were linear the position and energy resolution wuld markedly improve to 2 rim and 20% respectively. The optimisat ion of the gas cell coupled to the reduction in the strong deoendence of the C’IA quantum efficiency with TN photon incident angle can lead to a further improven~nt in both energy and spatial resolution. The reduction of the angular dependence of the quantum efficiency can be achieved by depositing the Cal on the outer surface of the Civ exit window rather than on the plates themselves. It is caripated that this s~uld lead to a spatial resolution of - 1 inn arid an energy resolution of — 10%. The variation of the on-axis spatial and energy resolution, for 6 key X-rays, with the position of the scintillation cell zi relative to the channel plate is shown in Figure 3 for the case of a 5 an diameter GIA with a linear readout system and no angular dependence of the quantum efficiency (10%). In addition the GIlL was assumed to be directly coupled to the CaF2 exit window, and the scintillation cell depth (zo—z 1) was taken as - 10 inn. 5
DISTANCE FRSI PHOTOCATHOOE 26 10 15 20 25
(ISIS)
30 11
2200
CaF7W1NDOW
2000
- - -
1000
_-\ — — ENERGY
1600 S
~
(‘.00
~
1200
—
1000
C
P
400
10
~
9
~, ~ 0
RESOWTK)I
— — —
8
AKeVINTCNSIC ELECTRON RESOLIJOON DIFFUSION ~F~TS ~ -
-n
16
DISTANCE FROM PHOTOCATHOOC 21 (mml
Fig. 3 The predicted spatial arid energy resolution as a function of scintillation cell position z1 for the case of a 10 ma deep scintillation cell. The best spatial and energy resolution clearly occurs when the scintillation cell is directly coupled to the ~‘2 exit window. A spatial and energy resolution of - 500 pm and 8% respectively is predicted. It has been denonstrated that such a scintillation cell can be coupl~ directly to the exit win&M provided the gas cell is operated in a negative potential configuration with the exit window on ground. In addition energy resolutions of 7.5% for 6 key X-rays have been achieved in such configurations (6). Needless to say quantum efficiencies in excess of 10%, which might be achieved with say Caesium Telluride or other photocathodes, wnuld provide an additional bonus. It should be noted that for the case of the scintillation cell directly coupled to the exit window the spatial arid energy resolution are relatively insensitive to the depth of the cell (zo—zi) provided it is larger than - 5 rim. CONCLUSION It has been denonstrated experirentally that a G4Pi can replace the classical photanultiplier tube array as the energy and position sensing element of an imaging GSPC. With the appropriate optimisation of the gas cell, GIlL and readout system, spatial and energy resolutions of - 500 ~in and 8% at 6 key should be achievable. Such a capability s~ouldlead to the possibility to perform broad-band spectral mapping of cosmic X-ray sources at the sub-arc minute level. The spectral and ireging capabilities of the GSPC can be ccnpared with other imaging detectors that could be used as cameras in the focal plane of an X—ray telescope. The key parameter for the instrument’ s performance as a spectraaeter is the energy resolution over the dynamic range, determined by the response of the optics (typically below several keV). Figure 4 shows the energy resolution as a function of X—ray photon energy for a GSPC and other instrumerits. The data from the negative affinity detector (NEAl)), the charge coupled device (CCD) and the position sensitive proportional counter PSPC are taken fran references 7, 8, 9
228
B. G. Taylor et al.
100 50
I
I
I
\\
-
~
~
-
\
—
I
O~’.
Z
10
\.\ -
Q
—
0.1psPc
0_s
2
-
GSPC
101
0.2
—
0.5
CCO I 2,1 2cCINEAD 25~25cm _______
1.0
20
50
10.0
0002
—P ENERGY (key) Fig. 4
\~~2
—.—
The energy resolution of various focal
plane instruments for an X—ray telescope as a function of X-ray photon energy.
0.5
10
2.0
510
10
—PENERGY (key) Fig. 5
The spatial resolution versus X-ray
photon energy for various instrurrerits.
respectively. The (Xl) and GSPC offer an improvement of a factor 2-4 over the PSPC in the energy range 0.28—6 key. Below - 1.5 key the GSPC appears to be the rrost promising because of the CQ) dark current noise limitations. The imaginu capability of these instruments can be described by three quantities: spatial resolution, field of view (FOV) and energy range. Figure 5 illustrates the spatial resolution of these instrtutents as a function pf X-ray energy both in a linear scale and in arc seconds assuming an P~XAFtype telescope. The GSPC results are for the predicted optimised spatial resolution on-axis. The highest angular resolutions are obtained with the COD over a relatively small field coupled to a reasonable energy resolution. However the GSPC provides a better energy resolution below - 1.5 key and a larger field of view and bandwidth acccrapanied by medium spatial resolution. It should be noted that the quality and off-axis performance -f the optics may be the determining factors in telescope resolving power rather than detector spatial resolution. For example at - 7 key the P~XP.F optics deposit - 70% of the energy within 10 arc sec on—axis while at- 3Oarcminoff—axis, the RI’lSblur circle radiusis 1 arcmin (10). Finally the excellent background rejection capability of the GSPC achieved through burst discrimination should also be noted; a figure of 95% at 6 key being obtained by Taylor et al. (11) with an imaging GSPC with Anger camera readout.
~c}~Thq1EEx3PZ4~NrS The engineering support ~rk of Mr. E. -A. Leiniann is gratefully acknowledged. Olr. W. Vleeshhouwer is thanked for his ~rk on the vacuum and filling systems for these detectors, and the data acquisition software, whilst Mr. A. van Dordrecht is thanked for the design and testing of the system. Stimulating discussions with R.D. Andresen and G. Manzo and other colleagues of the Space Science Department are appreciated. Dr. K. R. Sims and Dr. J. Davelaar acknowledge the receipt of ESA fellowships.
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
J. Davelaar, A. Peacock and B.G. Taylor, IEEE Trans. Nuci. Sci. NS-29, 142 (1982). C.J. Hailey, W.H.-M. Ku and M.H. yartanian, IEEE Trans. Nucl. Sci. NS-29, 138 (1982). C.J. Hailey, T.T. Hamilton and W.H.-M. Ku, Nucl. Instr. & Meth. 184, 543 (1981). J.W. Stümpel, P.W. Sanford and H.F. Goddard, Journal of Phys. H. 6, 397 (1973). G. Frazer and W. Mathieson, Nucl. Instr. & Meth. 184, 537 (1981). A. Peacock, R.D. P,ndresen, A. Long, G. Manzo and B.G. Taylor, Nucl. Instr. & Meth., 169, 613 (1980). P.W. Sanford, I.M. Mason, K. Kixmock, J.C. Ives, IEEE Trans. Nucl. Sd. NS-26, (1978). D. Dardas, E. Kellogg, S. Murray and R. Ench Jr., Rev. Sci. Instrum. 49, 9, 1273 (1978). P. Burstein, R. Hall, B. Ho]res and D. Harrison, Paper presented at 17th Aerospace Sciences Meeting, New Orleans (1979). M.V. Zcn±eck (private ca!rnunication). B.G. Taylor, J. Develaar, G. Manzo and A. Peacock, IEEE Trans. Nuci. Sci. NS-28, 857 (1981).
28, 1983 Adv. Space Res. Vol.2, No.4, pp.225—2 Printed in Great Britain. All rights reserved.
Copyright © COSPAR
AN IMAGING GAS SCINTILLATION PROPORTIONAL COUNTER COUPLED TO A CHANNEL MULTIPLIER ARRAY FOR APPLICATION IN COSMIC X-RAY SPECTROSCOPY B. G. Taylor, M. R. Sims, J. Davelaar and A. Peacock European Space Agency, SSD, ESTEC, Noordwijk, Netherlands ABSTRACr The develo~nt of an imaging gas scintillation proportional counter which utilises a channel xi-ultiplier array as the readout elemant is discussed. Preliminary experin~ntal results and theoretical considerations indicate that spatial and energy resolutions of below 500 pm and 8% respectively slould be achievable at an X-ray energy of 6 key.
INTI~)DUCTION Imaging gas scintillation proportional counters (GSPC) have recently been developed based on either a niulti—photanultiplier Anger canera readout system or a plotoiordsation detector (1), (2). Both readout systems are currently limited to spatial and energy resolutions of 1 ma and 9% respectively at an X—ray energy of 6 key. Fran a technological point of view the multi-pi-otanultiplier tube approach has a drawback in the need for a fine gain ountrol system for the photaiultiplier tubes. The long term integrity of a photoionisation detector readout has yet to be denonstrated, particularly in a space environmant. In an endeavour to improve the spatial resolution, enabling sub-arc minute angular resolution, yet minimising the technical difficulties, a channel nultiplier array (c4~) readout system has been investigated. The introduction of a CNA as the position and energy sensing el~nt of an imaging GSPC was suggested by Hailey et al. (1981). Their theoretical results indicate that spatial resolutions of below 500 pm for 6 key X-rays should be achievable whilst still retaining the good energy resolution of the GSPC (3). In this paper we present the experirental results fran an imaging GSPC coupled to a Q’IA using non-optiniised but “off-the-shelf” elemants, while optimisations, suggested through Monte Carlo sinulatioris are discussed. EXPERIMENTAL RESULTS The experinental results have been obtained fran the detector illustrated scheniatically in Figure 1. The GSPC gas cell was essentially similar to that discussed in reference 1, whilst the readout elen~nt consisted of a 5 an dianeter chevron—pair of channel plates, ‘~hich are coupled to the gas cell. Adequate quantum efficiency (- 10% peak) in the UV waveband 16502000 ~ was achieved by coating the front plate with CsI. The a.1P~was operated in a vacuum of < io—5 tort, arid at a charge gain of - 5 i0~. EdhAO 4~~~OVkmMICROCHANREL PLATES CHEVRON PAIR
1,
2S~rn MYLAR X-RAY ENTRANCE WINDOW •
CSJNTER Z-AXIS-—--—
.
GASXENON Ii SARI
5TORT ~VACUUM
I
.
-
•
I
.
I
DRIFT PHOTO ABSORPTION REGION
RESSTIVE DISC READOUT
.
1, PICK-UP ELECTRODESI
CaF
2 WINDOW\ PLATE SURFACE COATED WITH CsI I Z~O
SCINTILLATION RESOW —Ti
—~
SCALE ~1~iss
Fig. 1 JASU 2:4 — P
A sch~natic of the gas cell coupled to the CP1P~. 225
226
B. G. Taylor et al.
The principle of operation of such a detector can be surnnarised as follows: an incident X-ray photon will prcduce on average - 45 ohotoelectrons keV~which turn, the via the 1 sr1 as they will, drift in through gas scintillation process, create 1O~TN photons m scintillation region. These TN photons will be observed by the CI~as a burst with a burst
tire geverned primarily by the cell depth z 0-z1. Each UV photon detected by the GIA will produce an electron avalanche that can be recorded by a variety of readout systems; in this case the readout system is based on a resistive anode, similar to that described by StUnpel et al. (1973) (4). The charge distrib.ition on the resistive anode is continuously integrated throughout the IN photon burst by the disc impedance which results in a typical tire constant of - 15 pS for the centre of the disc. This charge distribution is sensed by four pick-up electrodes, located synmetrically on the edge of the disc. The amplitude ratios of these signals provides the ‘centroid’ of the charge distrib..ition and thus the X-ray çhotoabsorption position in the detector’s X,Y plane. The X-ray photon energy is directly
proportional to the sum of these four signals for a given position
(4).
Previous experimoental results using a photanultiplier tube array which operates as an Anger camera have indicated that, for a given X-ray photon energy, both the spatial and energy resolution are inversely proportional to the square root of the total TN light yield (1). The spatial resolution is defined here as the full width half maxixonxn of the point spread function. The current results have been obtained for the 6 key X-rays collimated onaxis. SCINTILLATION FIELD E5 (V/Cs) 200
2750
1750
2000
ENERGY RES~UTION
C
Fig. 2 The variation of the inverse of the 04A sum signal with the
-<
100
-
1000
spatial and energy
POS~~CN RESOLUTION
000
,‘
—
cc~p~:’icFOR COLLIMATION) ~O1 002
-
(AMPLITUDE OF SLM~1E READOuT SIGNAL ii
Figure 2 illustrates the variation of the square of both the position and energy resolution as a function of the inverse of the G~1Asun~1 output signal. The variation in the sum signal was obtained by changing the scintillation field E5. A minflinim position and energy resolution of - 5 rim arid 24% respectively was obtained for a scintillation field of 2750 V/an for the experirental configuration described here. In the limit, the spatial resolution will be constrained by diffusion effects, whilst the energy resolution is limited by the Fano factor. SThJ1ATIC~SAND OPTIMISATICtI The dependency of the spatial and energy resolution on the characteristics of both the gas cell and Q4P. have been investigated by means of a Monte Carlo procedure with a view to optinising the detector system. The following factors s~re included: (i)
Fluctuations in the initial
number of electrons produced in the photoabsorption
orocess. (ii)
Poissonian statistics on the TN light produced as the electrons drift through the scintillation region which is of a finite depth (zo—zi) and at a distance z1 fran the photocathode.
(iii)
The variation in the EN tranADnission through the CaF2 exit window as a function of incident angle.
(iv)
The variation of the quantum efficiency of the GI7E as a functimn of incident angle.
(v)
The statistics on the charge gain/collection process in the GIA. (It is assumed that only one photon triggers a channel, and that the single electron response is - 80% FT*~N).
Civ
photon
Gas Scintillation Proportional Counter in Cosmic X—Ray SpectroscopY (iv)
The readout non-linearity of the resistive disc based on the ~rk et al. (1981) (5).
227
of Frazer
The predicted spatial and energy resolution for the experirental configuration illustrated schematically in Figure 1 was found to be - 4 mu and 23% respectively in good agreement with the preliminary experirental data. The single major contribution to the position resolution arises fran the large non-linearity towards the edge of the resistive disc. This distortion leads to large signal amplitudes on the edge of the disc compared to the centre. If the readout system were linear the position and energy resolution wuld markedly improve to 2 rim and 20% respectively. The optimisat ion of the gas cell coupled to the reduction in the strong deoendence of the C’IA quantum efficiency with TN photon incident angle can lead to a further improven~nt in both energy and spatial resolution. The reduction of the angular dependence of the quantum efficiency can be achieved by depositing the Cal on the outer surface of the Civ exit window rather than on the plates themselves. It is caripated that this s~uld lead to a spatial resolution of - 1 inn arid an energy resolution of — 10%. The variation of the on-axis spatial and energy resolution, for 6 key X-rays, with the position of the scintillation cell zi relative to the channel plate is shown in Figure 3 for the case of a 5 an diameter GIA with a linear readout system and no angular dependence of the quantum efficiency (10%). In addition the GIlL was assumed to be directly coupled to the CaF2 exit window, and the scintillation cell depth (zo—z 1) was taken as - 10 inn. 5
DISTANCE FRSI PHOTOCATHOOE 26 10 15 20 25
(ISIS)
30 11
2200
CaF7W1NDOW
2000
- - -
1000
_-\ — — ENERGY
1600 S
~
(‘.00
~
1200
—
1000
C
P
400
10
~
9
~, ~ 0
RESOWTK)I
— — —
8
AKeVINTCNSIC ELECTRON RESOLIJOON DIFFUSION ~F~TS ~ -
-n
16
DISTANCE FROM PHOTOCATHOOC 21 (mml
Fig. 3 The predicted spatial arid energy resolution as a function of scintillation cell position z1 for the case of a 10 ma deep scintillation cell. The best spatial and energy resolution clearly occurs when the scintillation cell is directly coupled to the ~‘2 exit window. A spatial and energy resolution of - 500 pm and 8% respectively is predicted. It has been denonstrated that such a scintillation cell can be coupl~ directly to the exit win&M provided the gas cell is operated in a negative potential configuration with the exit window on ground. In addition energy resolutions of 7.5% for 6 key X-rays have been achieved in such configurations (6). Needless to say quantum efficiencies in excess of 10%, which might be achieved with say Caesium Telluride or other photocathodes, wnuld provide an additional bonus. It should be noted that for the case of the scintillation cell directly coupled to the exit window the spatial arid energy resolution are relatively insensitive to the depth of the cell (zo—zi) provided it is larger than - 5 rim. CONCLUSION It has been denonstrated experirentally that a G4Pi can replace the classical photanultiplier tube array as the energy and position sensing element of an imaging GSPC. With the appropriate optimisation of the gas cell, GIlL and readout system, spatial and energy resolutions of - 500 ~in and 8% at 6 key should be achievable. Such a capability s~ouldlead to the possibility to perform broad-band spectral mapping of cosmic X-ray sources at the sub-arc minute level. The spectral and ireging capabilities of the GSPC can be ccnpared with other imaging detectors that could be used as cameras in the focal plane of an X—ray telescope. The key parameter for the instrument’ s performance as a spectraaeter is the energy resolution over the dynamic range, determined by the response of the optics (typically below several keV). Figure 4 shows the energy resolution as a function of X—ray photon energy for a GSPC and other instrumerits. The data from the negative affinity detector (NEAl)), the charge coupled device (CCD) and the position sensitive proportional counter PSPC are taken fran references 7, 8, 9
228
B. G. Taylor et al.
100 50
I
I
I
\\
-
~
~
-
\
—
I
O~’.
Z
10
\.\ -
Q
—
0.1psPc
0_s
2
-
GSPC
101
0.2
—
0.5
CCO I 2,1 2cCINEAD 25~25cm _______
1.0
20
50
10.0
0002
—P ENERGY (key) Fig. 4
\~~2
—.—
The energy resolution of various focal
plane instruments for an X—ray telescope as a function of X-ray photon energy.
0.5
10
2.0
510
10
—PENERGY (key) Fig. 5
The spatial resolution versus X-ray
photon energy for various instrurrerits.
respectively. The (Xl) and GSPC offer an improvement of a factor 2-4 over the PSPC in the energy range 0.28—6 key. Below - 1.5 key the GSPC appears to be the rrost promising because of the CQ) dark current noise limitations. The imaginu capability of these instruments can be described by three quantities: spatial resolution, field of view (FOV) and energy range. Figure 5 illustrates the spatial resolution of these instrtutents as a function pf X-ray energy both in a linear scale and in arc seconds assuming an P~XAFtype telescope. The GSPC results are for the predicted optimised spatial resolution on-axis. The highest angular resolutions are obtained with the COD over a relatively small field coupled to a reasonable energy resolution. However the GSPC provides a better energy resolution below - 1.5 key and a larger field of view and bandwidth acccrapanied by medium spatial resolution. It should be noted that the quality and off-axis performance -f the optics may be the determining factors in telescope resolving power rather than detector spatial resolution. For example at - 7 key the P~XP.F optics deposit - 70% of the energy within 10 arc sec on—axis while at- 3Oarcminoff—axis, the RI’lSblur circle radiusis 1 arcmin (10). Finally the excellent background rejection capability of the GSPC achieved through burst discrimination should also be noted; a figure of 95% at 6 key being obtained by Taylor et al. (11) with an imaging GSPC with Anger camera readout.
~c}~Thq1EEx3PZ4~NrS The engineering support ~rk of Mr. E. -A. Leiniann is gratefully acknowledged. Olr. W. Vleeshhouwer is thanked for his ~rk on the vacuum and filling systems for these detectors, and the data acquisition software, whilst Mr. A. van Dordrecht is thanked for the design and testing of the system. Stimulating discussions with R.D. Andresen and G. Manzo and other colleagues of the Space Science Department are appreciated. Dr. K. R. Sims and Dr. J. Davelaar acknowledge the receipt of ESA fellowships.
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
J. Davelaar, A. Peacock and B.G. Taylor, IEEE Trans. Nuci. Sci. NS-29, 142 (1982). C.J. Hailey, W.H.-M. Ku and M.H. yartanian, IEEE Trans. Nucl. Sci. NS-29, 138 (1982). C.J. Hailey, T.T. Hamilton and W.H.-M. Ku, Nucl. Instr. & Meth. 184, 543 (1981). J.W. Stümpel, P.W. Sanford and H.F. Goddard, Journal of Phys. H. 6, 397 (1973). G. Frazer and W. Mathieson, Nucl. Instr. & Meth. 184, 537 (1981). A. Peacock, R.D. P,ndresen, A. Long, G. Manzo and B.G. Taylor, Nucl. Instr. & Meth., 169, 613 (1980). P.W. Sanford, I.M. Mason, K. Kixmock, J.C. Ives, IEEE Trans. Nucl. Sd. NS-26, (1978). D. Dardas, E. Kellogg, S. Murray and R. Ench Jr., Rev. Sci. Instrum. 49, 9, 1273 (1978). P. Burstein, R. Hall, B. Ho]res and D. Harrison, Paper presented at 17th Aerospace Sciences Meeting, New Orleans (1979). M.V. Zcn±eck (private ca!rnunication). B.G. Taylor, J. Develaar, G. Manzo and A. Peacock, IEEE Trans. Nuci. Sci. NS-28, 857 (1981).