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Radiation Protection Glossary

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Fallout
Fallout is a general term applied to Radioactive materials, produced by detonation of a nuclear device in the atmosphere, which fall back to earth. Fallout can sometimes be thought of as a secondary (radiation / contamination) hazard from a nuclear weapon once the explosion, prompt Gamma and Neutron radiation has passed. Where as the initial radiation doses delivered will be fixed by the physical distance between the explosion and those exposed, fallout can be carried great distances by atmospheric conditions and thus deliver significant radiation doses to those not effected by the initial explosion. Recently, and particularly in the light of the Chernobyl accident, fallout can also be applied to any nuclear accident (event) which results in atmospheric dispersion of radioactive material.
Fast Neutrons
Neutrons which have been ejected from a fissioning Nucleus . (Also see Thermal Neutrons ).
Film Badge
The Film Badge is a type of Passive Dosimeter . It consists of a photographic emulsion film, which responds to Ionising Radiation by changing optical density, in a light tight wrapping (contained in a holder). The holder, which also contains various filters (for measuring tissue equivalent quantities), is usually worn on the trunk of the body.
Fission
(Nuclear) Fission is the process where a heavy nuclei (e.g. U-235) decays by splitting into two equal fragments (fission fragments). This process proceeds with the emission of Neutrons and Gamma Rays , the neutrons being available to initiated further fissions and thus a nuclear chain reaction. Some nuclides such as Cf-252 can undergo spontaneous fission, although fission is induced in other nuclides such as U-235 by incoming neutrons. The fission fragments can undergo further Radioactive Decay producing a host of fission products.
Fluence Rate
Fluence can be defined as the total number of particles (typically Gamma Ray Photons ) crossing over a sphere of unit cross section which surrounds a Point Source of Ionising Radiation . The Fluence rate is the number of particles crossing per unit time (which is numerically equal to the product of number of particles and their average speed). This is a useful quantity in Radiation Protection when calculating Dose Rates from point sources or other geometries (e.g. Line Sources, plane source or Volume Sources). The dose delivered by a fluence at a point in space will be related to the energy of the photons.
Free Radical
Free radicals can be formed in biological materials (e.g. DNA ) when they undergo Ionisation (by interaction with Ionising Radiation ). The free radical can be thought of as a reactive charged molecule which will readily combine with other cell constituents. A typical example is the ionisation of water which will produce H+ and OH- ions which can further react to produce Hydrogen Peroxide, which is highly oxidizing and potentially very damaging to DNA.
Fusion
A process in which two or more light nuclei are formed into a heavier Nucleus releasing large amounts of energy. This process can be achieved by using extremely high temperature plasmas and some believe this will eventually lead to a new readily available source of energy.