Induced radioactivity is when a previously stable material has been made
radioactiveby exposure to specific radiation. Most radioactivity does not induce other material to become radioactive. Neutron activationis the main form of induced radioactivity, which happens when free neutrons are captured by nuclei. This new heavier isotope can be stable or unstable (radioactive) depending on the element involved. Because free neutrons disintegrate within minutes outside of an atomic nucleus, neutron radiationcan be obtained only from nuclear disintegrations, nuclear reactions, and high-energy reactions (such as in cosmic radiationshowers or accelerator collisions). Neutrons that have been slowed down through a neutron moderator( thermal neutrons) are more likely to be captured by nuclei than fast neutrons.
A less common form involves removing a neutron with the
photoneutron effect. This is where a high energy photon ( gamma ray) strikes a nucleus with an energy greater than the binding energyof the atom, releasing a neutron. This starts at energies at 2 MeV (for hydrogen) and most radionuclides do not produce gamma rays that powerful.The isotopes used in food irradiation( Cobalt-60, Caesium-137) both have peaks below this. Caesium-137 at 662 keV and Cobalt-60's two peaks are 1.2 and 1.3 MeV.
Some induced radioactivity is produced by
background radiation, which is mostly natural. However, since natural radiation is not very intense in most places on Earth, the amount of induced radioactivity in a single location is usually very small.
The conditions inside certain types of
nuclear reactors with high neutron flux can cause induced radioactivity. The components in those reactors may become highly radioactive from the radiation they are exposed to. Induced radioactivity increases the amount of nuclear wastethat must eventually be disposed, but it is not referred to as radioactive contaminationunless it is uncontrolled.
* [http://www.physlink.com/Education/AskExperts/ae636.cfm PhysLink.com - Ask the Experts "Gamma ray food irradiation"]
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