There were two Gas Imaging Spectrometers (GISs) on ASCA. These detectors are imaging gas scintillation detectors, which means that they detect incoming X-rays through interactions that turn them into visible light.
The GIS has two main parts: the gas cell, and the phototube. The gas cell is filled with a mixture of xenon gas (90%) and helium gas (10%). When an X-ray enters the gas cell, it interacts with the gas, causing a cascade of visible photons. The visible photons are then detected by the phototube.
The area of the GIS that is sensitive to X-rays is 50 mm in diameter. It can detect X-rays in the range of 0.7 to 10 keV.
