Photon-photon coincidences for activity determination: I-125 and other radionuclides.

Theory and experiment of photon-photon coincidence counting with distance variation for activity standardisation of radionuclide solutions are described. The activity of an I-125 solution from an international comparison organised by the BIPM in 2004 was determined. The value obtained agrees with other methods within a limit of 0.2%. The extension of this activity standardisation method to other radionuclides such as Cd-109 with I-125 as a tracer nuclide, In-111, Am-241 and I-124 with a radionuclidic impurity of I-125 is studied. Achieved uncertainties of the activities of Cd-109 and In-111 are of the order of 1.5%. The activity values of sources of Am-241 are determined relative to each other. Am-241 is used for checking the measuring system and as a long-lived reference source. After decay of the order of several I-124 half-lives, sources of I-124 with I-125 are measured to determine the I-125 activity portion.