REFERENCE NEUTRON FIELDS IN CALIBRATION LABORATORY; SIMPLE DOSIMETRIC PARAMETERS AND THEIR CHANGES IN TIME

The calibration laboratory revises periodically the dosimetric properties of its calibration fields. For reference neutron fields based on radionuclide sources, it is due to presence of isotopic impurities in the source ingredients. Consequently, in long term, the admixtures and their decay products become significant. Neglecting the impact of such effect leads to inconsistencies between the neutron emission rate observed during the measurements and the emission rate derived from the decay curve of the main isotope. In the Radiation Protection Measurements Laboratory at the National Centre for Nuclear Research the neutron fields of the bare sources of Cf-252, (AmBe)-Am-241 and (PuBe)-Pu-239 have been examined for nearly 30 years, regularly, in fixed geometry. Additionally, at the particular point of a calibration bench, determination of neutron fluence rate, ambient dose equivalent rate and its scattered component, total neutron and gamma dose rate, gamma to total dose ratio, radiation quality factor and total tissue kerma are occasionally determined. In this article, we would like to present recently achieved results and their comparison with the measurement data formerly presented. The growth of the neutron emission of (PuBe)-Pu-239 source, as well as discrepancies between the decay curves of relevant isotopes and the emission rate of Cf-252 and (AmBe)-Am-241 will be presented.