Absolute calibration of a well-type NaI-detector to an accuracy of 0.3-0.1%

A simple and accurate absolute calibration procedure of a well-type 5ins x 5ins NaI(Tl)-detector is described. The interaction probability with gammas is calculated by integration over the well-known volume of detector with gamma absorption coefficients determined very precisely in a previous experiment. The effect of interactions outside of the real volume of detector, in the housing, is determined experimentally. The uncertainty of the obtained total efficiency is 0.3% for mono-energetic gamma sources and will be further reduced by a factor of 2-3 in measuring sources with coincident gamma transitions. Based on this coincidence behaviour the special applicability of the detector in accurately measuring radionuclides of complex decay is shown. In addition, the correctness of the present work was checked by means of standard reference gamma sources calibrated absolutely by other methods. (Auth.)