Radiation is composed of energetic particles or photons that can pass through a medium (i.e., human tissue) and deposit energy. Radiation emission occurs when an unstable nucleus decays to a more stable state.
Radiation is divided into two categories: ionizing and non-ionizing. What distinguishes the two is that ionizing radiation has sufficient energy to remove an electron from atoms and produce ions. Types of ionizing radiation include:
Another form of radiation that indirectly causes ionization is neutrons. Neutrons are released from a few unstable nuclei and also during the fission process. Neutrons have no charge but are very massive. As a results, they interact with the nucleus of a target material, not the electron shell. The also transfer more energy per collision to light nuclei such as the hydrogen in water. As neutrons interact in a material they can cause interactions in nuclei that release alpha particles and protons. These massive, charged particles slow down in the surrounding materials causing massive ionization. Considering that the body is comprised of nearly 70% water, a high neutron field can cause a substantial dose.
Radiation passing through human tissue will remove electrons from atoms, causing the covalent bonds between atoms in a molecule to break apart. This may result in damage to the cells that make up the tissue. The radiation could damage critical parts of the cell, such as chromosomes which contain DNA, and cause the cell to die or function abnormally. Abnormal cells may not function properly or may cause damage to other cells. They may also fail to reproduce or reproduce at an uncontrolled rate, and such conditions are the underlying cause of cancer.
If the body absorbs a low amount of radiation over a long period of time, the cells' ability to repair and replace can outpace the damage caused by radiation. However, if higher doses are received over a short time (seconds to minutes above 100,000 mrem) cell death starts occurring faster than cell replacement, so tissues may fail to function. At this point, a person will start experiencing nausea, vomiting, and diarrhea, which are symptoms of radiation sickness, also known as Acute Radiation Syndrome. If the acute dose is high enough, general weakness, burns on the skin, loss of consciousness and death may occur.