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♤ They include cosmic rays from space and terrestrial radiations from radio-nuclides present in earth’s crust such as radium-224, uranium-238, thorium-232, potas-sium-40, carbon-14, etc.
Man - made
Atomic explosion (Nuclear fallout):
♤ The nuclear arms use uranium-235 and plutonium-239 for fission and hydrogen or lithium as fusion material. Atomic explosions produce radioactive particles that are thrown high up into the air as huge clouds. These particles are carried to long distances by wind and gradually settle over the earth as fall out or are brought down by rain. The fall out contains radioactive substances such as strontium-90, cesium- 137, iodine - 131, etc.
♤ Nuclear power plants
♤ Nuclear weapon
♤ Transportation of nuclear material
♤ Disposal of nuclear waste
♤ Uranium mining
♤ Radiation therapy