How will gamma rays be used to detect water ice on the Moon?
You should think of gamma rays as particles that can enter into interactions with matter. Gamma rays striking different kinds of matter interact with different strengths depending on the detailed opacity of the material which depends on things like its atomic weight and density. It is this difference between ice and local rock which is used by gamma ray ‘back scatter’ spectrometers. A source of gamma rays is brought up close to the lunar material and the reflected gamma rays are detected and measured. Depending on the substance, a specific absorption feature will appear at a specific energy of the reflected gamma rays, and this is a finger print of the properties of the material. Future lunar prospectors will use such devices to do running assays on lunar material they travel over.
