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Physics

The environmental dose rate is calculated using conversion factors from measurements of radionuclides (40 K, 238 U, 235 U, 232 Th and 87 Rb) within the sample and its surroundings and the radiation dose rate from cosmic rays. The dose rate is usually in the range of 0.5 - 5 Gy /1000 years. The total absorbed radiation dose is determined by exciting, with light, specific minerals (usually quartz or potassium feldspar) extracted from the sample, and measuring the amount of light emitted as a result. The photons of the emitted light must have higher energies than the excitation photons in order to avoid measurement of ordinary photoluminescence. A sample in which the mineral grains have all been exposed to sufficient daylight (seconds for quartz; hundreds of seconds for potassium feldspar) can be said to be of zero age; when excited it will not emit any such photons. The older the sample is, the more light it emits, up to a saturation limit.