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Gas scintillation proportional counters for Japanese astronomical satellites
Nuclear Instruments and Methods 196 (1982) 69-72 North-Holland Publishing Company
69
GAS SCINTILLATION P R O P O R T I O N A L C O U N T E R S FOR J A P A N E S E A S T R O N O M I C A L SATELLITES H. I N O U E , K. K O Y A M A , T. MAE, M. M A T S U O K A , T. O H A S H I , Y. T A N A K A a n d I. W A K I
Institute of Space Astronautical Science, Komaba, Meguro-ku, Tokyo 153, Japan
Two types of sealed gas scintillation proportional counters (GSPC) have been developed for the observation of cosmic X-rays on board satellites [l]. One of the counters was designed for the wide band non-dispersion spectroscopic measurement of solar flares. It has an effective area of 1.7 c m 2 and is being used for the observation of solar X-rays in the energy range 1.5-30 keV. The detector is on board the Japanese solar observatory ASTRO-A launched on February 21, 1981. Preliminary results are presented here. The other type of GSPC has been developed for the detailed spectral and temporal study of galactic and extragalactic X-ray sources. Ten GSPCs each of 100 cm2 area are being fabricated and will be on board the second Japanese X-ray astronomy satellite, ASTRO-B, scheduled for launch in 1983. In this paper we present the design and the performance of two different types of satellite-borne GSPCs.
AI 50/u absorber with1.3 mm(~pinhole
1. Gas scintillation proportional counter for ASTRO-A
a. ~
Collimator(13,5"FWHM~
The configuration of the sealed gas scintillation proportional counter designed for the observation of solar flare X-rays on board ASTRO-A is shown in fig. I. The detector consists of a cylindrical ceramic tube with an X-ray entrance window on one side. The window consists of a 200 # m thick berylium foil and is 15 mm in diameter. The berylium foil is fixed to a stainless steel support ring by fused glass and thus can withstand a temperature of upto 300°C in the baking process. Two wire mesh plates A and B represented by the dotted lines in fig. 1 define the drift and accelerating regions of the counter. High voltages of 230V and 6100V are supplied to the mesh electrodes A and B, respectively. A Spectrosil quartz plate is used for the scintillation-light outlet window. The quartz window is compression-sealed with a kovar ring by using transition glass, which again allows high temperature baking. The counter is filled with 1.2 arm of pure Xe after a baking at 300°C for three days. A small SAES Zr-A1 getter is mounted within the gas volume, in order .to maintain gas purity against out-gassing, throughout a long term operation. The Spectrosil quartz is highly transparent to the UV emission of the scintillation gas. The UV light is converted to visible light near the wavelength of 4500,A by a p-quarterphenyl wavelength shifter which is deposited on the outer surface of the quartz window. The wavelength-shifted emission is quite compatible with photomultipliers of S-11 wavelength-response. Silicone gel was used for the optical coupling between the window and the photomultiplier. An energy resolution of 10.5% at 5.9 keV of a 55Fe
radio-active source was obtained with this counter under normal operation. The uniformity of the pulse height over the entire entrance area (15 mm diameter) was found to be excellent. A slight variation of the pulseheight with temperature and a small long-term drift both probably due to the gain variation of the photomultiplier were observed. The long term variation of the flight unit is shown in fig. 2. The figure shows the variation of the pulse height and the energy resolution as a function of time of 22 keV X-rays during the pre-flight test of the satellite from August 1980 to February 1981. The measured pulse-height variation with temperature during the thermal and the thermal-vacuum tests is noticeable. Since
0029-554X/82/0000-0000/$02.75 © 1982 North-Holland
II. PROPORTIONAL COUNTERS / LIQUID DETECTORS
~
Getter
EMI
~P-quaterphenyl 9637R ] SiliconeGel I
Fig. I. The configuration of the sealed gas scintillation proportional counter on board ASTRO-A.
70
]]. hlol¢C t'Z J/.
( olOl[Ut'.~ [t~t" ,:lSlFY)IIg)D11(gI[ ~IIt'//IIU.~
HLIGHT(cH) RLS(%)
i
,
10(]
IO
80
8
60
6
40
4-
PULSE
I I
I
I
iiiI
~ I t I lit!
2
0
0
~
ENERGY
THERMAL TEST
20
~
AUG
SEP
FIEI']111
}
I
Ill
RESOLUTION
THERMAL VACUUM TEST
OCT
NOV
[
DEC
1980
JAN
FEB
1981
Fig. 2. Variation of tile pulse height and the energy resolution for 22 keV X-rays during the pre-flight test of the (;SPC aboard ASTRO-A.
the completion of the counter no appreciable degradation in energy resolution has been observed. Since the slope of the spectrum of solar X-rays is very steep and the photon flux at low energies is extremely high, it is essential to suppress the flux at lob energies to avoid pulse pile-up and a large dead time problem, It is also necessary to have a larger area for collecting the high energy photons. This condition was achieved by using a 50/~m thick aluminum mask with a pin hole of 1.3 mm diameter. The effective area versus energy for the ASTRO-A GSPC is shown in fig. 3(a). For the time being, the GSPC on board ASTRO-A is working at a reduced high voltage level for the drift and accelerating regions 130 V and 4700 V, respectively. In this operating condition, the energy resolution at 5.9 keV is -- 12%. Fig. 3(b) shows the time evolution of the X-ray energy spectra observed with the GSPC during a solar flare on March 4, 1981. The solar flare started at 1720 U T and attained a maximum X-ray flux at 1728 UT. These spectra are obtained from the pulse height mode with the integration time of 64s each. Pulse height channels in the abscissa correspond to X-ray energies as indicated. From this figure we can clearly identify various emission-lines from highly ionized Si, S, Ca, Fe and Ni atoms produced in the flare. We can determine the respective line intensities and also the plasma temperature from the continuum. The GSPC on board ASTRO-A was fabricated by Hamamatsu TV Co., Ltd.
2. Gas scintillation proportional counter for ASTRO-B A schematic diagram of the other kind of gas scintillation proportional counter is shown in fig. 4. This type is designed for the detailed spectral and temporal study of X-ray sources. Ten units each having --100 cm2 effective area will be on board the second X-ray astronomy satellite ASTRO-B scheduled for launch in early 1983. This detector has a spherical dome-shaped berylium window of 100 /~m thickness which is electron-beam welded to an aluminum ring. Two focusing electrodes of spherical shape are used. The spherical electrodes were first introduced by the ESTEC group [2]. A set of five ring electrodes are used to correct the field in the drift region. These are bonded to the inner surface of the funnel part of the ceramic bowl. In order to determine the optimum potentials for the field correcting ring electrodes we carried out a numerical computation of the field structure and compared the result with the actual measurements for the pulse height uniformity. For the best conditions an energy resolution of -- 10% at 5.9 keV was achieved over the entire area of 115 cm 2. The background rejection is one of the most important aspects in the study of weak X-ray sources. A pulse-shape discrimination can be used to distinguish between the X-rays and the non-X-ray background. However, for a pure Xe filled GSPC, the rise time distribution for X-rays of uniform incidence spreads out widely because the entrance window and the electrodes
©
~J
¢3 © C Z t'n
Z
Q ©
7~
.
.
.
.
.
.
.
Is. .64
. - ~,- ~ - ~ .
6400
c/64~
, ~ I~o: k~v
~
.
XIX
~
17-~:29
0
. . . . . .
~
I
15. . . .
r I
o
(a)
~
Fig. 3. (a) Effective area versus X-ray energy. (b) Solar flare spectra observed on March 4, 1981 by GSPC aboard ASTRO-A.
o~
(b)
6400
8000
c/64s
XXll
,~
,..
5 10 15 X-RAY ENERGY
. . . . . . . . . . . . .
1~'::.I
1 i ..................
,,.Ni
' ~ Fe XXV
17:27:49
keY
-..J
r3
m~
BERYLIUM ENTRANCE WINDOW
72
f XE + HE (1,2 ATM) FOCUSING RINGS
/L/•
SPHERI CAL GRIDS
r
CERAMIC DETECT
4.
"~
%
! ARTH GRID
QUARTZ WINDt 3W
SAES GETTER
PHOTOMULTIPLIER TUBE (BOROSILICATE GLASS WINDOW) R1307 (HAMAMATSU)
WAVE LENGTH SHIFTER (P-QUATERPHENYL)
,Sc. Fig. 4. The mechanical arrangement of the GSPC having 100 cm 2 areas for ASTRO-B.
Time Slleclra let
are not quite in spherical symmetry. The measured rise time distribution for the 22 keV monoenergetic line from ]°9Cd is shown in fig. 5(a). In order to increase the electron drift velocity, 20% He was mixed with Xe. This enhances the drift velocity b y an order of magnitude. Fig. 5(b) shows the rise time distribution for the 22 keV X-rays for the gas mixture. A large i m p r o v e m e n t over the results shown in fig. 5(a) is obvious. The flight units are presently being fabricated.
Illl
References
g
[1] H. Inoue, K. Koyama, T. Mae, M. Matsuoka, T. Ohashi, Y. Shinkai, Y. Tanaka and H. Tsunemi, Bulletin of ISAS. University of Tokyo, 14 (1978) 1289. [2] A. Peacock, R.D. Andresen, E-A. Leimann, A. Long, G. Manzo and B.G. Taylor, Nucl. Instr. and Meth. 169 (1980) 613.
T
) Rial Time
t
Fig. 5. Rise time distributions (25--75% of signal peak) for the 22 keV X-rays from 1°9Cd. (a) Pure Xe gas at 1.2 atm. (b) Mixture of 80% Xe and 20% He at a total pressure of 1.2 atm.
69
GAS SCINTILLATION P R O P O R T I O N A L C O U N T E R S FOR J A P A N E S E A S T R O N O M I C A L SATELLITES H. I N O U E , K. K O Y A M A , T. MAE, M. M A T S U O K A , T. O H A S H I , Y. T A N A K A a n d I. W A K I
Institute of Space Astronautical Science, Komaba, Meguro-ku, Tokyo 153, Japan
Two types of sealed gas scintillation proportional counters (GSPC) have been developed for the observation of cosmic X-rays on board satellites [l]. One of the counters was designed for the wide band non-dispersion spectroscopic measurement of solar flares. It has an effective area of 1.7 c m 2 and is being used for the observation of solar X-rays in the energy range 1.5-30 keV. The detector is on board the Japanese solar observatory ASTRO-A launched on February 21, 1981. Preliminary results are presented here. The other type of GSPC has been developed for the detailed spectral and temporal study of galactic and extragalactic X-ray sources. Ten GSPCs each of 100 cm2 area are being fabricated and will be on board the second Japanese X-ray astronomy satellite, ASTRO-B, scheduled for launch in 1983. In this paper we present the design and the performance of two different types of satellite-borne GSPCs.
AI 50/u absorber with1.3 mm(~pinhole
1. Gas scintillation proportional counter for ASTRO-A
a. ~
Collimator(13,5"FWHM~
The configuration of the sealed gas scintillation proportional counter designed for the observation of solar flare X-rays on board ASTRO-A is shown in fig. I. The detector consists of a cylindrical ceramic tube with an X-ray entrance window on one side. The window consists of a 200 # m thick berylium foil and is 15 mm in diameter. The berylium foil is fixed to a stainless steel support ring by fused glass and thus can withstand a temperature of upto 300°C in the baking process. Two wire mesh plates A and B represented by the dotted lines in fig. 1 define the drift and accelerating regions of the counter. High voltages of 230V and 6100V are supplied to the mesh electrodes A and B, respectively. A Spectrosil quartz plate is used for the scintillation-light outlet window. The quartz window is compression-sealed with a kovar ring by using transition glass, which again allows high temperature baking. The counter is filled with 1.2 arm of pure Xe after a baking at 300°C for three days. A small SAES Zr-A1 getter is mounted within the gas volume, in order .to maintain gas purity against out-gassing, throughout a long term operation. The Spectrosil quartz is highly transparent to the UV emission of the scintillation gas. The UV light is converted to visible light near the wavelength of 4500,A by a p-quarterphenyl wavelength shifter which is deposited on the outer surface of the quartz window. The wavelength-shifted emission is quite compatible with photomultipliers of S-11 wavelength-response. Silicone gel was used for the optical coupling between the window and the photomultiplier. An energy resolution of 10.5% at 5.9 keV of a 55Fe
radio-active source was obtained with this counter under normal operation. The uniformity of the pulse height over the entire entrance area (15 mm diameter) was found to be excellent. A slight variation of the pulseheight with temperature and a small long-term drift both probably due to the gain variation of the photomultiplier were observed. The long term variation of the flight unit is shown in fig. 2. The figure shows the variation of the pulse height and the energy resolution as a function of time of 22 keV X-rays during the pre-flight test of the satellite from August 1980 to February 1981. The measured pulse-height variation with temperature during the thermal and the thermal-vacuum tests is noticeable. Since
0029-554X/82/0000-0000/$02.75 © 1982 North-Holland
II. PROPORTIONAL COUNTERS / LIQUID DETECTORS
~
Getter
EMI
~P-quaterphenyl 9637R ] SiliconeGel I
Fig. I. The configuration of the sealed gas scintillation proportional counter on board ASTRO-A.
70
]]. hlol¢C t'Z J/.
( olOl[Ut'.~ [t~t" ,:lSlFY)IIg)D11(gI[ ~IIt'//IIU.~
HLIGHT(cH) RLS(%)
i
,
10(]
IO
80
8
60
6
40
4-
PULSE
I I
I
I
iiiI
~ I t I lit!
2
0
0
~
ENERGY
THERMAL TEST
20
~
AUG
SEP
FIEI']111
}
I
Ill
RESOLUTION
THERMAL VACUUM TEST
OCT
NOV
[
DEC
1980
JAN
FEB
1981
Fig. 2. Variation of tile pulse height and the energy resolution for 22 keV X-rays during the pre-flight test of the (;SPC aboard ASTRO-A.
the completion of the counter no appreciable degradation in energy resolution has been observed. Since the slope of the spectrum of solar X-rays is very steep and the photon flux at low energies is extremely high, it is essential to suppress the flux at lob energies to avoid pulse pile-up and a large dead time problem, It is also necessary to have a larger area for collecting the high energy photons. This condition was achieved by using a 50/~m thick aluminum mask with a pin hole of 1.3 mm diameter. The effective area versus energy for the ASTRO-A GSPC is shown in fig. 3(a). For the time being, the GSPC on board ASTRO-A is working at a reduced high voltage level for the drift and accelerating regions 130 V and 4700 V, respectively. In this operating condition, the energy resolution at 5.9 keV is -- 12%. Fig. 3(b) shows the time evolution of the X-ray energy spectra observed with the GSPC during a solar flare on March 4, 1981. The solar flare started at 1720 U T and attained a maximum X-ray flux at 1728 UT. These spectra are obtained from the pulse height mode with the integration time of 64s each. Pulse height channels in the abscissa correspond to X-ray energies as indicated. From this figure we can clearly identify various emission-lines from highly ionized Si, S, Ca, Fe and Ni atoms produced in the flare. We can determine the respective line intensities and also the plasma temperature from the continuum. The GSPC on board ASTRO-A was fabricated by Hamamatsu TV Co., Ltd.
2. Gas scintillation proportional counter for ASTRO-B A schematic diagram of the other kind of gas scintillation proportional counter is shown in fig. 4. This type is designed for the detailed spectral and temporal study of X-ray sources. Ten units each having --100 cm2 effective area will be on board the second X-ray astronomy satellite ASTRO-B scheduled for launch in early 1983. This detector has a spherical dome-shaped berylium window of 100 /~m thickness which is electron-beam welded to an aluminum ring. Two focusing electrodes of spherical shape are used. The spherical electrodes were first introduced by the ESTEC group [2]. A set of five ring electrodes are used to correct the field in the drift region. These are bonded to the inner surface of the funnel part of the ceramic bowl. In order to determine the optimum potentials for the field correcting ring electrodes we carried out a numerical computation of the field structure and compared the result with the actual measurements for the pulse height uniformity. For the best conditions an energy resolution of -- 10% at 5.9 keV was achieved over the entire area of 115 cm 2. The background rejection is one of the most important aspects in the study of weak X-ray sources. A pulse-shape discrimination can be used to distinguish between the X-rays and the non-X-ray background. However, for a pure Xe filled GSPC, the rise time distribution for X-rays of uniform incidence spreads out widely because the entrance window and the electrodes
©
~J
¢3 © C Z t'n
Z
Q ©
7~
.
.
.
.
.
.
.
Is. .64
. - ~,- ~ - ~ .
6400
c/64~
, ~ I~o: k~v
~
.
XIX
~
17-~:29
0
. . . . . .
~
I
15. . . .
r I
o
(a)
~
Fig. 3. (a) Effective area versus X-ray energy. (b) Solar flare spectra observed on March 4, 1981 by GSPC aboard ASTRO-A.
o~
(b)
6400
8000
c/64s
XXll
,~
,..
5 10 15 X-RAY ENERGY
. . . . . . . . . . . . .
1~'::.I
1 i ..................
,,.Ni
' ~ Fe XXV
17:27:49
keY
-..J
r3
m~
BERYLIUM ENTRANCE WINDOW
72
f XE + HE (1,2 ATM) FOCUSING RINGS
/L/•
SPHERI CAL GRIDS
r
CERAMIC DETECT
4.
"~
%
! ARTH GRID
QUARTZ WINDt 3W
SAES GETTER
PHOTOMULTIPLIER TUBE (BOROSILICATE GLASS WINDOW) R1307 (HAMAMATSU)
WAVE LENGTH SHIFTER (P-QUATERPHENYL)
,Sc. Fig. 4. The mechanical arrangement of the GSPC having 100 cm 2 areas for ASTRO-B.
Time Slleclra let
are not quite in spherical symmetry. The measured rise time distribution for the 22 keV monoenergetic line from ]°9Cd is shown in fig. 5(a). In order to increase the electron drift velocity, 20% He was mixed with Xe. This enhances the drift velocity b y an order of magnitude. Fig. 5(b) shows the rise time distribution for the 22 keV X-rays for the gas mixture. A large i m p r o v e m e n t over the results shown in fig. 5(a) is obvious. The flight units are presently being fabricated.
Illl
References
g
[1] H. Inoue, K. Koyama, T. Mae, M. Matsuoka, T. Ohashi, Y. Shinkai, Y. Tanaka and H. Tsunemi, Bulletin of ISAS. University of Tokyo, 14 (1978) 1289. [2] A. Peacock, R.D. Andresen, E-A. Leimann, A. Long, G. Manzo and B.G. Taylor, Nucl. Instr. and Meth. 169 (1980) 613.
T
) Rial Time
t
Fig. 5. Rise time distributions (25--75% of signal peak) for the 22 keV X-rays from 1°9Cd. (a) Pure Xe gas at 1.2 atm. (b) Mixture of 80% Xe and 20% He at a total pressure of 1.2 atm.