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Personal neutron dose meter (dosimeter). The green casing clips together around a piece of detector material, and the safety pin is used to attach the dosimeter to a person's clothing. The detector material used is poly-allyl diglycol carbonate (PADC), a plastic polymer that is damaged when exposed to radiation such as alpha particles or neutrons. The amount of damage is proportional to the dose of radiation received. Neutrons, one of the two particles found in atomic nuclei, can be emitted by the spontaneous fission of an unstable nucleus, such as californium-252. This detector is from the UK's Health Protection Agency. (KEYSTONE/SCIENCE PHOTO LIBRARY/HEALTH PROTECTION AGENCY)