3. The Gas Imaging Spectrometers

There are two Gas Imaging Spectrometers (GIS: 1991). Their layout is shown in Figure 3.3. There is a ground mesh just above the quartz window. The beryllium entrance window is held at a potential of -7 kV relative to this. The cloud of photoelectrons created by an incident X-ray drifts to the accelerating grid which is at a potential of -6 kV. The cloud is then accelerated by the strong field in the emission region, collisionally exciting xenon atoms. These emit ultraviolet scintillation photons which are detected with a position-sensitive phototube. The GIS energy resolution is 7.8% $\sqrt{5.9{\rm\ keV}/E}$. Its spatial resolution is 0.5 mm which is better than that of the XRT. The charged-particle background is rejected by pulse rise-time discrimination.

Figure 5: Layout of the ASCA GIS (1995).