The effect of radiation on human tissue or any other material can be studied as the biological effects of radiation. The radiation produced by nuclear reactions includes high-energy, charged particles (alpha and beta), neutrons of various energies, and photons (gamma and x-rays).
In addition to this primary radiation, nuclear fission also produces radioactive isotopes of many elements, which in turn can emit particles and photons, known as secondary radiation. This high energy radiation has an ionizing effect due to its high energy. Radiotherapy uses ionizing radiation to interact with biological tissue to produce free radicals and cause biological damage.
The biological effect begins with the ionization of atoms. Living tissue contains large amounts of water and the light elements, hydrogen, carbon, nitrogen, and oxygen, with lesser quantities of phosphorus and sulfur.
The abundance of these light nuclei affects the interaction of nuclear radiation with living tissue. Alpha and beta particles, protons, and fission products of sufficient energy, as well as photons (gamma and x-rays), can remove the valence electrons of the elements in living materials. This process, known as ionization, changes the chemical reactivity of the affected atoms. Molecules containing ionized atoms may react to form substances that are detrimental to life.
The ionizing particles and fission products may undergo thousands of collisions with the atoms before stopping thus they may cause extensive ionization in the target material. This results in destruction of the molecular structure and the molecule falls apart.
Ionization process can be of two types: direct ionization or indirect ionization. When a high energy photon interacts with matter, instead of losing its energy through large numbers of collisions in a relatively short distance, a photon loses its energy in a single interaction with an atom of the target material.
The outcome of this interaction depends on the energy of the photon; it candirectly transfer its energy to the target material or it can produce secondary electrons and photons that can continue the transfer of energy to nearby atoms.
The radiation produced by alpha particles and protons is known as direct ionization because its energy is transferred directly to the target material. Because gamma and x-ray photons, as well as neutrons, transfer energy to neighboring atoms by means of secondary electrons and photons, the radiation they produce is known as indirect ionizing radiation.