An instrument for detecting high-energy X-rays and gamma rays. Crystalline sodium iodide (NaI) converts an X-ray or gamma-ray photon into a pulse of visible light, the brightness of which is proportional to the energy of the incoming photon. The optical pulse is detected by a photomultiplier tube. To obtain images, a two-dimensional array of photomultipliers is used.
Subjects: Astronomy and Astrophysics.