Ionizing radiation which can damage living tissue is emitted as the unstable atoms (radionuclides) change ('decay') spontaneously to become different kinds of atoms.
The principal kinds of ionizing radiation are: These are helium nuclei consisting of two protons and two neutrons and are emitted from naturally-occurring heavy elements such as uranium and radium, as well as from some man-made transuranic elements.
Radiation is energy in the process of being transmitted.
It may take such forms as light, or tiny particles much too small to see.
Many scientists then undertook study of these, and especially their medical applications.
This led to the identification of different kinds of radiation from the decay of atomic nuclei, and understanding of the nature of the atom.
Gamma activity in a substance ( rock) can be measured with a scintillometer or Geiger counter.
X-rays are also electromagnetic waves and ionizing, virtually identical to gamma rays, but not nuclear in origin.
Fast neutrons can be very destructive to human tissue.
Neutrons are the only type of radiation which can make other, non-radioactive materials, become radioactive.
They are more penetrating than alpha particles, but easily shielded – the most energetic of them can be stopped by a few millimetres of wood or aluminium.
They can penetrate a little way into human flesh but are generally less dangerous to people than gamma radiation.
Neutrons were identified in 1932, and in 1939 atomic fission was discovered by irradiating uranium with neutrons.