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Position sensitive detector with wedge-and-wedge readout
Nuclt~ar Instruments and Methods in Physics Research A31I)(1991) 344-347 North-Holland
N~L~ IN$~UME~S & METHODS IN ~ Y $ ~ S RESEA~H Sect,on A
Position sensitive detector with wedge-and-wedge readout M.W. Trow, J.S. Lapington and R.D. Bentley Alullard Space St'Jr'net' Laboratory, Departnu'nt of Physics and Astronomy, Unil'ersity C'olh,gt' London, Lomlon, UK
A one-dimensional position sensitive proportional counter has been developed. It incorporates a wedge-and-wedge cathode as tile position sensitive element, which operates by measuring the charge induced from a nearby anode wire. The detector is sealed and contains a Xe-Ar-CO 2 gas mixture. Tests of a number of detectors of this type are described. Position resolutions of ~ 3511 i~m are demonstrated, as well as good energy resolution and gain characteristics. These detectors will form part of a soft X-ray spectrometer on board Solar-A, a solar studies satellite to be launched in 1991.
I. The Solar-A mission Solar-A is a small spacecraft which will be launched by ISAS, the Japanese Institute of Space and Astronautical Science. The spacecraft is designed to provide coordinated observations of solar flare X-rays over a range of wavelengths. The spacecraft has a mass of ~ 400 kg. The average power available for all instruments on board will be 200 W. it will be launched from the Kagoshima Space Centre in Southern Japan during the period August-September 1991. It will be placed in a 600 km quasi-circular orbit. The spacecraft and instruments are expected to opcratc for at least one year from launch.
2. Bragg crystal spectrometer A Bragg crystal spectrometer [1] (BCS) is being built by a consortium of groups from the UK and the USA, led by the Mullard Space Science Laboratory (MSSL) of University College London, and including the Rutherford Appleton Laboratory (RAL) and the US Naval Research Laboratory (NRL). The purpose of this instrument is to study the dynamics and heating of solar flare plasmas. X-rays having energies in the range 2.4 keV to 6.97 kcV arc of interest. In a Bragg crystal spectrometer such as the Solar-A BCS, solar X-rays arc dispersed along the Icngth of a position sensitive detector by curved Bragg crystals, being Germanium in this case. Each wavelength is directed to a particular part of the detector, since the Bragg relationship is satisfied for a different angle at each point of the crystal. The crystal cut used and its curvature determines the wavelength range for the spectrometer.
The BCS consists of two crystal/detector structures, and a separate electronics box. Both structures house two crystals, one position sensitive detector and a high voltage unit. Each detector observes radiation from two adjacent crystals. To fulfil the requirements of this mission, the detector must have the following properties: A high detector quantum efficiency, intrinsic position resolution of better then 400 I~m, energy resolution and low relative gain variation sufficient to allow discrimination of 9.89 keV X-rays. The latter condition is imposed because high energy solar X-rays incident on the Bragg crystals will cause them to fluoresce, emitting Ge K radiation (9.89 keV). These photons should be identified prior to position encoding, otherwise the recorded spectra will be superimposed on an undesirable background. Thc mass budget of the Solar-A mission precludes the use of a gas replenishment system, therefore the detector should have a leak tight window, and have no possible source of contamination from either the gas filling or internal components. The gas filling should not be susceptible to charge-induced ageing.
3. Position sensitive detector The U~L',..'..t'.,, A,~,.,.,,.~ h.,.. ........ . . . . A. W l,r C S . s e p a ,,t~,'J ,tWV,~ U t : t;l.l a. . .o t ¢ atlt.~uv,,. by a cathode screen. The anodes arc 15 Ixm diameter and the central cathode screen consists of nine equally spaced 25 I~m diameter wires. The anodes of each detector half are electrically connected, and each pair is connected to a separate preamplifier. The detector can function as two separate counters, due to the presence of the cathode screen. The readout cathode is sharcd between the detector halves. This reduccs the number of electronics channels required, rated
0168-91]02/91/${}3.50 ,~ 1991 - Elsevier Science Publishers B.V. All rights reserved
M.~ 7~n'etaL /Det~'torwithwedge.and-w~lgereadout Entrance Uindow
Anode wire
.L15
Detector wall
Cathode wire screen
Wedge and wedge cathode
Fig. I.Schematiccrosssectionofthe BCSdetector, showingthe m~or~mponents. saving mass and power. X-ray transmission into the counter is via a 125 tzm beryllium foil window. A simplified representation of the detector is shown in fig. 1. The configuration described was arrived at after several alternative geometries and gas mixtures had been examined [2]. The wedge-and-wedge readout is shown schematically in fig. 2. It is manufactured on a gold coated fused silica plate. The pattern is formed photolithographically. Fused silica is chosen for the substrate due to its low dielectric constant. This ensures a minimum interelectrode capacitance and therefore lower noise in the wedge preamplifiers [3]. The counter body is of stainless steel, and is in two halves: a top assembly into which the beryllium window is brazed, and a bottom assembly onto which the anodes and cathodes are mounted. The two parts arc joined by means of an electron-beam weld. This makcs large flanges unnecessary. The detector is permanently sealed, therefore steps to prevent poisoning of the detector gas must bc taken.
Insulating gap
,/,2
] [
Materials with low outgassing properties are used throughout, and all components are vacuum baked before assembly. The gas mixture used in these detectors consists of equal parts xenon and argon, with 5e/c CO, as the quenching agent, to a pressure of 1.2 atm. The overpressure is needed because any net inward fi~rcc on the window would cause it to rupture. This gas mixture has several features which make it suitable for this application [1]. The inclusion of xenon in the gas mixture achieves the desired quantum efficiency. The combined efficiency of the window and 2 cm of the gas is 9 5 ~ at 6 keV.
4. Operation of the detector The anodc wires are held at a positive potential with respect to the cathodes, and X-ray photons entering the window cause an avalanche onto one of the wires [4,5]. Following collection of electrons on thc anode, the residual distribution of ions induces charge on the surrounding cathodes. The induced charge distribution on the wedge-and-wedge cathode provides the position determination [6]. if Qwl and Qw_~ are the induced charges on the electrodes, the x-coordinate of the avalanche is given by x -
]
Ow:
(1)
Qwl + Qw2"
[ I
L___
F x
Fig. 2. Schematic of the wedge-and-wedge cathode readout. The number of pitches shown here has been reduced for clarity. Wl and W2 are the electrodes. The anode wires run in the x-direction.
5. Testing and performance of the detectors For testing, the detectors were mounted inside a vacuum chamber equipped with a wide aperture X-ray source consisting of an interchangeable target situated near a heated filament. The results presented here were obtained with an mild steel target, giving predominantly Fe K (6.4 keV) radiation. The anode and cathode electrodes of the detector were connected to charge-sensitive preamplifiers mounted outside the chamber. Signals from these were led to pulse shaping IV. ASTRONOMY
346
M, IV, Tro., t't aL / Detector with wedge.aml.wedge readout
ampliliers, and followed by "peak and hold" circuitry before being ted into four ADCs. The two wedge signals were septm~tcly fed into an analog summing unit with single channel analyscr. This unit generated a convert signal for the ADCs fi)r events where the total wedge signal lay in a specified range. The ADC data were passed via a parallel interface to a microcomputer. The data were examined to determine which anode pair had fired, and the position encoding formula applied to the wedge data. The data wcrc stored on floppy disc, typically 45 00{I events per run, for later transfer to a VAX computer system for subsequent analysis. A pinhole array mask, consisting of 201) p.m pinholes drilled in ! mm thick AI sheet was placed in front of the detector window. This has four groups of eight pinholes, each group being a diagonal line across the detector. The pinholes are separated by 2 mm in the x-direction and equally spaced in the y-direction. The four groups are situated so as to cover the entire active area of the detector. No two pinholes have the same x-coordinate in either detector half, therefore radiation passing through each pinhole produces a separate distribution of counts when the position encoding formula is applied. The width of this distribution is the position resolution, unless this is comparable with the pinhole diameter, in which case a dcconvolution analysis must be performed to obtain the resolution. For each event, the total charge collected by the anode is also recorded, giving a second parameter for that event. The position linearity, position resolution, energy resolution and gain variation across the detec-
I
I
I
,.1:
I
I
,
,_~
E
800
_
7 O O
_
6OO
_
5 0 0
_
Solor--A
BCS D e t e c t o r
g
c~
g
400
3oo 200
1400
I
I
1450
1500 Anode
l
15.50 voltage
I
I
1600
I
1650
1700
(V)
Fig. 4. Data from tests at a number of anode voltages. showing variation of position resolution. tot window arc all obtainable from the resulting twoparameter distribution. A sample data set of this type is shown in fig. 3. This shows 45000 separate events from one anode pair, i.e. one half of the detector only. The position histogram is shown below. An automatic routine performs the data analysis. Minima in the position histogram are located to separate the data for each pinhole, and a Gaussian profile is fitted to each distribution. The typical F W H M obtained are ~ 350 p.m. These tests were performed at many anode voltage levels to obtain the optimum operating point. Fig. 4. shows a summary performance for a range of anode voltages. The plotted error bars represent the range of values seen, rather than experimental uncertainty. Larger values for position resolution were obtained for those parts of the detector lying away from the X-ray beam's axis. This is because off-axis rays enter the detector at a slight angle, and absorption takes place in a finite depth of gas, thus smearing the position of the pinhole. The position distributions at the ends of the detector show exponential "tails" duc to this effect.
,the v
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--
_
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,:
1"7 . t
I
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I
•
.
.,
, : t --
I
I
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2000
2500
3000
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1500 Position
(Arb
Acknowledgements The authors are grateful to the MSSL mechanical engineering team "t~Jt" . . . . tuu'-- atccnamcat--'- ' ' ~aes~gn' ' and manufacture of these detectors. We also thank the other participants in the BCS consortium for many helpful discussions. This work was supported through the Science and Engineering Research Council.
3500
Units)
Fig. 3. Sample data set, with anode wires at 1540 V. The top half of the figure is a two-parameter distribution of 45000 events from 6.4 keV photons. The bottom half is the position histogram, showing position resolutions of ~ 350 ~m.
References [I] L.W. Acton. J.L. Culhane, A.H. Gabriel 65 (198{I) 53.
el
al.. Sol. Phys.
M.W. Trow et al. / Detector with wedge.and-.'edge readout [2] J.S. Lapington, M.W. Trow, R.D. Bcnlluy and J.L. Culhunt, Proc. SPIE 1159 (I~89) 252. [3] H.E. Schwarz and J.S. Lapington, IEEE Trans. Nucl. Sci. NS-32 (1985) 433. [4] F. Sauli. CERN report 77-1)9 (It;79).
347
[5] D.ll. Wilkinson. hmization ('hamber, and Counter,, (Cambridge Univer~,ily Pre,,~,.Cambridge, 1~50). [¢)] B.P. Dural, J. Barth, R.D. I)eslatles. A. Ilenin5 and (i.G. [.tither, Nucl. Instr. and Meth. 222 (19x4)274.
IV. ASTRONOMY
N~L~ IN$~UME~S & METHODS IN ~ Y $ ~ S RESEA~H Sect,on A
Position sensitive detector with wedge-and-wedge readout M.W. Trow, J.S. Lapington and R.D. Bentley Alullard Space St'Jr'net' Laboratory, Departnu'nt of Physics and Astronomy, Unil'ersity C'olh,gt' London, Lomlon, UK
A one-dimensional position sensitive proportional counter has been developed. It incorporates a wedge-and-wedge cathode as tile position sensitive element, which operates by measuring the charge induced from a nearby anode wire. The detector is sealed and contains a Xe-Ar-CO 2 gas mixture. Tests of a number of detectors of this type are described. Position resolutions of ~ 3511 i~m are demonstrated, as well as good energy resolution and gain characteristics. These detectors will form part of a soft X-ray spectrometer on board Solar-A, a solar studies satellite to be launched in 1991.
I. The Solar-A mission Solar-A is a small spacecraft which will be launched by ISAS, the Japanese Institute of Space and Astronautical Science. The spacecraft is designed to provide coordinated observations of solar flare X-rays over a range of wavelengths. The spacecraft has a mass of ~ 400 kg. The average power available for all instruments on board will be 200 W. it will be launched from the Kagoshima Space Centre in Southern Japan during the period August-September 1991. It will be placed in a 600 km quasi-circular orbit. The spacecraft and instruments are expected to opcratc for at least one year from launch.
2. Bragg crystal spectrometer A Bragg crystal spectrometer [1] (BCS) is being built by a consortium of groups from the UK and the USA, led by the Mullard Space Science Laboratory (MSSL) of University College London, and including the Rutherford Appleton Laboratory (RAL) and the US Naval Research Laboratory (NRL). The purpose of this instrument is to study the dynamics and heating of solar flare plasmas. X-rays having energies in the range 2.4 keV to 6.97 kcV arc of interest. In a Bragg crystal spectrometer such as the Solar-A BCS, solar X-rays arc dispersed along the Icngth of a position sensitive detector by curved Bragg crystals, being Germanium in this case. Each wavelength is directed to a particular part of the detector, since the Bragg relationship is satisfied for a different angle at each point of the crystal. The crystal cut used and its curvature determines the wavelength range for the spectrometer.
The BCS consists of two crystal/detector structures, and a separate electronics box. Both structures house two crystals, one position sensitive detector and a high voltage unit. Each detector observes radiation from two adjacent crystals. To fulfil the requirements of this mission, the detector must have the following properties: A high detector quantum efficiency, intrinsic position resolution of better then 400 I~m, energy resolution and low relative gain variation sufficient to allow discrimination of 9.89 keV X-rays. The latter condition is imposed because high energy solar X-rays incident on the Bragg crystals will cause them to fluoresce, emitting Ge K radiation (9.89 keV). These photons should be identified prior to position encoding, otherwise the recorded spectra will be superimposed on an undesirable background. Thc mass budget of the Solar-A mission precludes the use of a gas replenishment system, therefore the detector should have a leak tight window, and have no possible source of contamination from either the gas filling or internal components. The gas filling should not be susceptible to charge-induced ageing.
3. Position sensitive detector The U~L',..'..t'.,, A,~,.,.,,.~ h.,.. ........ . . . . A. W l,r C S . s e p a ,,t~,'J ,tWV,~ U t : t;l.l a. . .o t ¢ atlt.~uv,,. by a cathode screen. The anodes arc 15 Ixm diameter and the central cathode screen consists of nine equally spaced 25 I~m diameter wires. The anodes of each detector half are electrically connected, and each pair is connected to a separate preamplifier. The detector can function as two separate counters, due to the presence of the cathode screen. The readout cathode is sharcd between the detector halves. This reduccs the number of electronics channels required, rated
0168-91]02/91/${}3.50 ,~ 1991 - Elsevier Science Publishers B.V. All rights reserved
M.~ 7~n'etaL /Det~'torwithwedge.and-w~lgereadout Entrance Uindow
Anode wire
.L15
Detector wall
Cathode wire screen
Wedge and wedge cathode
Fig. I.Schematiccrosssectionofthe BCSdetector, showingthe m~or~mponents. saving mass and power. X-ray transmission into the counter is via a 125 tzm beryllium foil window. A simplified representation of the detector is shown in fig. 1. The configuration described was arrived at after several alternative geometries and gas mixtures had been examined [2]. The wedge-and-wedge readout is shown schematically in fig. 2. It is manufactured on a gold coated fused silica plate. The pattern is formed photolithographically. Fused silica is chosen for the substrate due to its low dielectric constant. This ensures a minimum interelectrode capacitance and therefore lower noise in the wedge preamplifiers [3]. The counter body is of stainless steel, and is in two halves: a top assembly into which the beryllium window is brazed, and a bottom assembly onto which the anodes and cathodes are mounted. The two parts arc joined by means of an electron-beam weld. This makcs large flanges unnecessary. The detector is permanently sealed, therefore steps to prevent poisoning of the detector gas must bc taken.
Insulating gap
,/,2
] [
Materials with low outgassing properties are used throughout, and all components are vacuum baked before assembly. The gas mixture used in these detectors consists of equal parts xenon and argon, with 5e/c CO, as the quenching agent, to a pressure of 1.2 atm. The overpressure is needed because any net inward fi~rcc on the window would cause it to rupture. This gas mixture has several features which make it suitable for this application [1]. The inclusion of xenon in the gas mixture achieves the desired quantum efficiency. The combined efficiency of the window and 2 cm of the gas is 9 5 ~ at 6 keV.
4. Operation of the detector The anodc wires are held at a positive potential with respect to the cathodes, and X-ray photons entering the window cause an avalanche onto one of the wires [4,5]. Following collection of electrons on thc anode, the residual distribution of ions induces charge on the surrounding cathodes. The induced charge distribution on the wedge-and-wedge cathode provides the position determination [6]. if Qwl and Qw_~ are the induced charges on the electrodes, the x-coordinate of the avalanche is given by x -
]
Ow:
(1)
Qwl + Qw2"
[ I
L___
F x
Fig. 2. Schematic of the wedge-and-wedge cathode readout. The number of pitches shown here has been reduced for clarity. Wl and W2 are the electrodes. The anode wires run in the x-direction.
5. Testing and performance of the detectors For testing, the detectors were mounted inside a vacuum chamber equipped with a wide aperture X-ray source consisting of an interchangeable target situated near a heated filament. The results presented here were obtained with an mild steel target, giving predominantly Fe K (6.4 keV) radiation. The anode and cathode electrodes of the detector were connected to charge-sensitive preamplifiers mounted outside the chamber. Signals from these were led to pulse shaping IV. ASTRONOMY
346
M, IV, Tro., t't aL / Detector with wedge.aml.wedge readout
ampliliers, and followed by "peak and hold" circuitry before being ted into four ADCs. The two wedge signals were septm~tcly fed into an analog summing unit with single channel analyscr. This unit generated a convert signal for the ADCs fi)r events where the total wedge signal lay in a specified range. The ADC data were passed via a parallel interface to a microcomputer. The data were examined to determine which anode pair had fired, and the position encoding formula applied to the wedge data. The data wcrc stored on floppy disc, typically 45 00{I events per run, for later transfer to a VAX computer system for subsequent analysis. A pinhole array mask, consisting of 201) p.m pinholes drilled in ! mm thick AI sheet was placed in front of the detector window. This has four groups of eight pinholes, each group being a diagonal line across the detector. The pinholes are separated by 2 mm in the x-direction and equally spaced in the y-direction. The four groups are situated so as to cover the entire active area of the detector. No two pinholes have the same x-coordinate in either detector half, therefore radiation passing through each pinhole produces a separate distribution of counts when the position encoding formula is applied. The width of this distribution is the position resolution, unless this is comparable with the pinhole diameter, in which case a dcconvolution analysis must be performed to obtain the resolution. For each event, the total charge collected by the anode is also recorded, giving a second parameter for that event. The position linearity, position resolution, energy resolution and gain variation across the detec-
I
I
I
,.1:
I
I
,
,_~
E
800
_
7 O O
_
6OO
_
5 0 0
_
Solor--A
BCS D e t e c t o r
g
c~
g
400
3oo 200
1400
I
I
1450
1500 Anode
l
15.50 voltage
I
I
1600
I
1650
1700
(V)
Fig. 4. Data from tests at a number of anode voltages. showing variation of position resolution. tot window arc all obtainable from the resulting twoparameter distribution. A sample data set of this type is shown in fig. 3. This shows 45000 separate events from one anode pair, i.e. one half of the detector only. The position histogram is shown below. An automatic routine performs the data analysis. Minima in the position histogram are located to separate the data for each pinhole, and a Gaussian profile is fitted to each distribution. The typical F W H M obtained are ~ 350 p.m. These tests were performed at many anode voltage levels to obtain the optimum operating point. Fig. 4. shows a summary performance for a range of anode voltages. The plotted error bars represent the range of values seen, rather than experimental uncertainty. Larger values for position resolution were obtained for those parts of the detector lying away from the X-ray beam's axis. This is because off-axis rays enter the detector at a slight angle, and absorption takes place in a finite depth of gas, thus smearing the position of the pinhole. The position distributions at the ends of the detector show exponential "tails" duc to this effect.
,the v
¢,.)
--
_
¢.n
,:
1"7 . t
I
-
I
•
.
.,
, : t --
I
I
l
2000
2500
3000
_
"5 ,:5 z 500
1 OOO
1500 Position
(Arb
Acknowledgements The authors are grateful to the MSSL mechanical engineering team "t~Jt" . . . . tuu'-- atccnamcat--'- ' ' ~aes~gn' ' and manufacture of these detectors. We also thank the other participants in the BCS consortium for many helpful discussions. This work was supported through the Science and Engineering Research Council.
3500
Units)
Fig. 3. Sample data set, with anode wires at 1540 V. The top half of the figure is a two-parameter distribution of 45000 events from 6.4 keV photons. The bottom half is the position histogram, showing position resolutions of ~ 350 ~m.
References [I] L.W. Acton. J.L. Culhane, A.H. Gabriel 65 (198{I) 53.
el
al.. Sol. Phys.
M.W. Trow et al. / Detector with wedge.and-.'edge readout [2] J.S. Lapington, M.W. Trow, R.D. Bcnlluy and J.L. Culhunt, Proc. SPIE 1159 (I~89) 252. [3] H.E. Schwarz and J.S. Lapington, IEEE Trans. Nucl. Sci. NS-32 (1985) 433. [4] F. Sauli. CERN report 77-1)9 (It;79).
347
[5] D.ll. Wilkinson. hmization ('hamber, and Counter,, (Cambridge Univer~,ily Pre,,~,.Cambridge, 1~50). [¢)] B.P. Dural, J. Barth, R.D. I)eslatles. A. Ilenin5 and (i.G. [.tither, Nucl. Instr. and Meth. 222 (19x4)274.
IV. ASTRONOMY