Gamma rays are photons emitted by the nuclear decay of radioactive isotopes such as Potassium 40.  Like X rays they can pass through matter, but they are higher in energy and shorter in wavelength and so more penetrating and more dangerous. Gamma ray imagery, confined until relatively recently to physics labs, is used now in medicine (PET scans) and in astronomy (the Fermi Gamma-ray Space Telescope studying the elusive but spectacular Gamma-ray bursts associated with the birth of black holes). Like other forms of ionizing radiation, gamma rays can be detected by a Geiger-Muller Counter.

“I wouldn’t know a gamma ray if I saw it”

Stan Lee, 1922-2018