Gamma Ray

Gamma rays ($\gamma$) are electromagnetic waves with the highest frequency and shortest wavelength in the electromagnetic spectrum.

They are the most penetrating type of nuclear radiation but the least ionising. They are reduced (but never completely stopped) by several centimetres of lead or thick concrete.

Gamma emission does not change the proton number or nucleon number of the nucleus. It often accompanies alpha or beta decay as the nucleus releases excess energy.