Mass internal exposure monitoring of the population in Russia after the Chernobyl accident

The organisation and carrying out of mass monitoring of radionuclide activity in the inhabitants of Russian regions contaminated with radionuclides after the Chernobyl accident are here reviewed. Methods to calibrate I-131 in the thyroid and Cs134-137 in the whole body are presented. Dependence of calibration factors for monitoring of I-131 in, thyroid, and Cs-134 and Cs-137 in a whole body on age, body mass and size are presented. The shielding of the background radiation by a human body influences significantly the measurement result. It depends on the chosen measurement geometry, the energy range applied, the spectrum of background radiation, mass and dimensions of the measured person. I-131 content in the thyroid should be calculated from the measurement results taking into account the contribution of radiation from radionuclides distributed in the body. Neglect of this factor could result in an overestimation of I-131 content in the thyroid up to a factor of 2 to 4.