Gas Imaging Spectrometer

GIS instrument from the ASCA satellite
Photo of the ASCA GIS detectors before they were installed on the satellite

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.