The various ways in which high-energy photons interact with matter are introduced—photoelectric effect, Compton scattering, pair production, and nuclear reactions. Energetics of each are discussed so one can read a gamma spectrum like we saw in the NRL lab last week. We also explain how the gamma spectrometer detector works. The ashes of 1000 bananas are counted and interpreted. Prof. Short irradiates his cell phone to demonstrate “digital snow” noise in the camera’s detector.