The ASCA GIS (Gas Imaging Spectrometer) is an imaging gas scintillation proportional counter (IGSPC). X-ray photons, focused by the mirrors, enter the detector through the thin entrance window (made from beryllium) into the ``drift region'' containing the detector gas: xenon with ten per cent of helium. The xenon atoms are ionized by the X-rays, and the photoelectrons produced then drift into the ``scintillation region'' which contains the same mixture of gases as the drift region. Here, the electrons are accelerated by a strong electric field the voltage of which is such that the electrons gain enough energy to excite xenon atoms but not to ionize them. When the atoms de-excite, UV scintillations are produced with intensity proportional to the energy of the original X-rays. These scintillations are then detected by the position sensitive phototube (PSPT). The electric field is parallel to the entrance window so the two-dimensional position of the scintillation corresponds to the position of the incident X-ray photon.
The structure of the focal plane assembly of the flight-model GIS is shown in Figure 6.2a. The total weight of the focal plane assembly is 3.8 kg, and the X-ray sensitive area has a diameter of 50 mm. It consists of the detector housing and the high-voltage supply units. The IGSPC--comprising the gas cell, the position-sensitive phototube (PSPT), and the front-end electronics--is placed in a housing cast from magnesium alloy with a diameter of 140 mm and a length of 580 mm, including a small radiation-belt monitor (RBM). The detector housing is placed on the optical axis of the X-ray reflective mirror which has a focal length of 3.5 m. The top section is a hood which prevents background X-rays and particles from entering along paths parallel with the mirror axis but outside the field of view of the GIS. The middle section accommodates the gas cell and the phototube, and the bottom section holds the front-end electronics. To improve the mechanical strength against vibration, a number of ribs run along the housing for support.
Two high voltage units are used for GIS. One produces a maximum of
+1400 V for the phototube, and the other applies a negative
potential of -8000 V to the gas cell. Since the photocathode of the
IPMT is held at the ground potential, the potential of the X-ray
entrance window, which is in principle exposed to open space, needs
to be set at -7000 V. At an altitude of 
km, the orbit of
ASCA is in the Earth's ionosphere where space plasma entering
the
GIS system could cause a high-voltage breakdown. To assess this
effect, a simulation experiment was carried out in the space chamber
at ISAS. Based on this study, the GIS has a thin (
m)
plastic window with a conductive coating placed within the hood.
Also, the electric current running in the high voltage network will be
monitored continuously, giving a quantitative measure of the safety
level.
The Radiation Burst Monitor (RBM) is a small solid-state detector, similar to the one used for Ginga, which continuously monitors the strength of the radiation environment and automatically controls the high voltage of the GIS.
The structure of the IGSPC in the GIS system is shown in Figure 6.2b.
The gas cell is made from a ceramic tube with a beryllium entrance
window and a quartz exit window. The volume is divided by a
mesh electrode into two parts: the drift region at the top and the
scintillation region at the bottom. X-rays entering through the
window, the electric potential of which is about -8,000 V, are
absorbed in the drift region. The electron cloud created by the
photo-ionization slowly drifts to the first mesh (-7000 V) and is
then accelerated by the strong field in the scintillation region toward
the ground mesh placed just above the quartz window. In this
process, the electrons excite the xenon atoms which then produce UV
scintillations. Seen through the quartz window, the UV scintillations
are detected with the PSPT and their position and intensity (
, the energy of the original X-ray photon) determined.
For the X-ray entrance window, a newly developed 
m-thick
beryllium foil, produced by the Yamaha Corporation, has been
employed for the first time in space. With this window, 10 per cent
transmission can be achieved at an energy as low as 
keV--the lowest energy achieved by a sealed-off gas detector. The
helium leak rate of the window is 
torr l
s
through an area 66 mm in diameter, an acceptable level for
sealed-off gas detectors. The X-ray transmission of the beryllium foil
has been tested at various positions on the surface with a narrow
X-ray beam at different energies, and proved to be uniform (within
experimental error) over the surface.
The structure of the window support, developed specially for the GIS,
is shown in Figure 6.2c. It consists of a thin metal grid and a mesh,
supporting the entrance area of 50 mm diameter. The molybdenum
grid has 7 mm pitch and 3.5 mm height, with a wall thickness of 0.1
mm. The grid itself can withstand a pressure of several atmospheres
and causes negligible shadowing on the image since the position
resolution of IGSPC is 
mm. The grid also supports the
mesh underneath against rupture. The stainless steel mesh has 1.2
mm pitch, 87 per cent transmission, and 0.3 mm thickness. Since the
rigidity against deflection is a strong function of thickness, the
supporting mesh used for the GIS is actually made from two
identical meshes bonded together. The loss of the effective area due
to the welding is negligible because the wire patterns of the two
meshes overlap each other smoothly. The beryllium foil is then
bonded onto a kovar flange using a high-temperature epoxy resin.
The window support restricts the linear expansion of the beryllium
to less than 0.1 per cent under a pressure of 1.5 atm, inside the limit
of damaging the foil.
The quartz window is 2.5 mm thick with a diameter of 78 mm, and is
bonded to the cupro-nickel flange using indium and a small amount
of high-temperature epoxy resin as an additional coating. The
sealing is successful over a temperature range from 
C to
C under a pressure of 1.5 atm and causes negligible stress
to the quartz plate.
The prime content of the gas is xenon with the addition of 10 per cent of helium to improve the rise-time performance for X-ray signals (the helium increases electron drift velocities and can be used to check leakage). A small getter is attached to the gas cell and keeps the gas free from contamination. The gas mixture and the getter material are essentially same as those used for the Tenma SPC, therefore their performance is already known.
The position-sensitive phototube (PSPT) is a Hamamatsu Photonics
R4268 type, equipped with a 3-inch quartz window and 10-stage
dynodes. The dynodes in the PSPT have a plane geometry, and in
each dynode plane runs a number of parallel grids of triangular cross
section. The direction of the grids is rotated by 
for each
successive dynode stage. This dynode configuration keeps the
electron path from the photocathode to the anode plane straight and
the cloud size small. The anode is a cross-wire grid with 16 wires
running in each X and Y direction at a separation of 3.75 mm. The
position information is obtained from the anode signals, and the
pulse-height is measured with the last dynode. A pencil-beam input
to the PSPT shows the distribution of the output charge to be
mm FWHM. The size of the sensitive area and the
uniformity of the output signal have been measured by mapping the
whole cathode area with a blue LED (
nm).
The peak-to-peak gain variation is
less than 10 per cent over the 60 mm length
of the photocathode diameter, except for the dynode
support 10 mm from the center. Such a small gain variation does not
influence position determination since the light distribution from the
gas cell has a large positional spread (
mm FWHM).
The 2-dimensional X-ray position is calculated from the signals of the
32 anode wires. The front-end electronics attached to the phototube
accommodate 33 charge-sensitive preamplifiers, including the one
for the last dynode. For the anodes, using operational amplifiers
saves weight and space. Since, at 
s, the
rise time of X-ray signals is rather slow, the op-amps work well as
pre-amplifiers. These 32 anode signals are handled in four groups,
i.e., even and odd numbered anodes in the X and Y directions. Each
group carrying eight signals is handled by an
analog multiplexer which picks up the signal and transfers it to an
8-bit flash ADC in the main electronics. Thus, only four flash ADCs
can handle all the anode signals. The decay time of the
preamplifier--an e-folding time of 100 
s--is set long enough
to hold the signal level during the switching process which lasts
s. The switching at the multiplexer is controlled by
the main electronics, and the duration, 1.2 s for each anode, is
long enough for the flash ADCs to convert the signal. By contrast, the
pulse-height, rise-time, and lower-discriminator information are
obtained from the last dynode, which produces the total-charge
signal with inverse polarity. The use of the last dynode is simpler
than adding all the anode outputs. The charge-sensitive preamplifier
for the last dynode consists of discrete components to assure high
signal-to-noise.
Signals from the IGSPC are first processed in the front-end electronics and then analyzed in the main electronics of the GIS. The pulse-height signal from the last dynode is first examined by the upper and the lower discriminators and then processed by the pulse-height and rise-time ADCs. The rise-time discriminator, the same as the one employed for the Tenma SPC, efficiently removes most of the background events. After filtering in this way, events are then analyzed by the on-board CPU, a 16-bit processor 80C286 operated at a clock rate of 10 MHz with a memory of 32 kB. The position signals sent from the multiplexers are converted with 8-bit flash ADCs, before transfer to the CPU for position determination. The two main functions of the CPU are the calculation of event position and background rejection.
Since the photons from
the gas cell spread radially according to an inverse square law while
the dynodes have a plane geometry, the distribution shows a long
extended tail. In addition, when an event occurs near the edge of the
window, the profile becomes asymmetric. A simple calculation of the
``center of gravity'' of the light distribution thus does not yield the
true incident position. We find, however, that a Lorentzian
distribution, with only three parameters, can describe the light
distribution adequately. For the actual CPU algorithm, we employ a
linearized approximation of the Lorentzian function to avoid iteration
and to quicken the calculation. A simulation of the position
determination method has been carried out by generating an
approximate light distribution based on the measured distribution of
6-keV X-rays at various incident positions. The calculated position is
linear out to a radius of 30 mm, and the position resolution is better
than 0.5 mm within a radius of 20 mm and scales approximately
with 
. The processing time is 
ms per event.
Considering the image size produced by the ASCA mirror (HPD
mm), the position resolution of IGSPC thus obtained is
satisfactory for an instrument in the focal plane of the XRT.
For the observation of strong X-ray sources (up to Crab),
faster position calculation may be required. A simple alternative
method is to take the square of the light intensity in each anode and
then to calculate the mean. This procedure emphasizes the peak
position and makes the result relatively insensitive to the extended
tail. Although the position linearity and the usable area are not as
good as for the Lorentzian fit, the calculation is three times faster.
The method of background rejection is briefly reviewed here. First, a background particle which deposits more energy in the gas cell than can be associated with an X-ray photon can be immediately rejected by the upper discriminator. In addition, non X-ray background can be distinguished from the proper X-ray signal in various ways, even when inexcessive energy is released in the gas cell. In the IGSPC, the characteristics of a background event can be recognized by any of the following four features:
Co background count rate.
The first method used is the
rise-time discriminator (RTD) which proved successful in
Tenma. The discrimination is effected by a hard-wire analog circuit
that can operate even under very high counting rates (
count
s). RTD is particularly good at rejecting background types 1, 2
and 4 (above). The background rate within the pulse-height range of
1-10 keV is reduced to about 10 per cent after the RTD. The second
method, carried out by the CPU, is to remove events occurring
outside the X-ray sensitive region (50 mm diameter). Even though
RTD removes most of the events near the detector wall, the residual
image still indicates that the events tend to concentrate in the outer
region. The removal of the outer region
results in a further reduction by a factor of 2-3 in the background
rate. Finally, type-3 background can be rejected using the width of
the light distribution. This results in a 20 per cent reduction of the
final background level. Therefore, the use of CPU results in the
background reduction by a factor of 3 compared with the case of
hard-wired electronics only. We also note that the background
rejection does not seriously affect the X-ray efficiency. The counting
rate for 6-keV X-rays decreases only by 5 per cent within 20 mm
from the center after background rejection.
The GIS has four different modes: pulse height (PH) mode, multi-channel pulse count (MPC) mode, position calibration (PCAL) mode, and memory-check mode. In practice, only PH and MPC mode are used for Guest Observations:
In almost all cases, PH mode will be used. Although MPC mode may be used for studying short time variations of bright sources, proposers must be aware that a precise spectral response is not available for the MPC mode.
The energy resolution of the GIS as derived from in-flight calibration
matched almost exactly what was expected from ground
measurements. That is, about 
per cent
(FWHM).
The internal background rate measured from night Earth
observations is 
count s cm
keV
count sensor with on-board background
discriminators effective. This is not very different from the
measured laboratory background rate of 
count
s cm keV and the Ginga LAC in-flight
background rate of 
count s cm
keV. The diffuse X-ray background is 
count
sensor, hence the 5
detection limit is 
count sensor
Crab for an exposure of 
seconds. On the other hand, the confusion limit for the XRT half
power diameter (3 arcmin) is 
count s (XRT
HPD)
Crab.
Due to the nature of the detector, the performance of the GIS
depends on radial position. When an X-ray is captured near the edge
of the gas cell, some of the resultant UV photons cannot be detected
by the phototube. Consequently, the energy gain becomes lower and
the light distribution on the anode wires becomes asymmetric near
the rim of the detector. The latter effect results in the radially
``shrunk'' position response. These effects are taken into account in
the GIS response function and are correctable. According to
current calibrations the GIS performance does not vary
within 64 per cent (
15 arcmin) of the full 25 mm radius of the
phototube. Data taken in the range 15-20 arcmin can still be used
for most purposes, but beyond 20 arcmin the calibrations are
unreliable and beyond 22 arcmin the particle background completely
dominates the signal. Note that it
is estimated that the GIS gain is uncertain by 
within a radius of 15 arcmin; by 
between 15 and 20 arcmin;
and up to 
outside 20 arcmin. Note that only events inside the
22 mm arcmin radius (as a default) are tagged wth the corresponding
sky coordinates derived from the attitude solution. Energy resolution
is flat within a radius of 20 arcmin.
The other effect on the useable field of view is the presence of the supporting grid. The supporting grid structure yields non-uniformity of the efficiency over the FOV. The efficiency can be degraded by as much as by 30% on the grid. Figure 6.4 shows the grid superimposed on the field of view.
The material in this chapter has been drawn almost entirely from Imaging Gas Scintillation Proportional Counters for ASCA by Ohashi et al. (1991, SPIE Proceedings Series, 1549, 9). Further information can be found in the following references.
Tashiro, M. et al. 1995, "In-orbit performance of the GIS instrument on board ASCA (Astro-D)", SPIE, 2518, 2
Kohmura, Y. et al. 1993, "Calibration of imaging gas scintilation proportional counters on Astro-D", SPIE, 2006, 78
Figure 6.2a: The focal-plane assembly of the GIS. The total length is 580 mm.
Figure 6.2b: The structure of the IGSPC for the GIS. The depth of the drift and scintillation regions are 100 mm and 150 mm, respectively, with an inner diameter of 92 mm.
Figure 6.2c: The structure of the beryllium window flange for the IGSPC. The flange body is made of kovar.
Figure 6.4: A bright earth image taken with GIS2. The data were accumulated over a period of one year. The calibration source is included (upper-right). The support grid structure is clearly seen. The plot has a linear scale from 50 to 170 cts/pixel. This image can be accessed at ftp://heasarc.gsfc.nasa.gov/asca/gis_information/earth2.img.