An adapted methodology to characterize the neutron emission of californium neutron sources

Californium neutron sources are fairly common and appreciated in ionizing radiation metrology. Due to the concomitant presence of different neutron-emitting radionuclides, the isotopic composition of the source ought to be known for an accurate estimation of neutrons emitted by the source in time. This becomes more important with the ageing of the source, because the 252Cf predominant contribution reduces in favour of other neutron-emitting radionuclides. This work illustrates a methodology to estimate the relative neutron emission of the most important nuclides. The advantage of the suggested approach lies in the use of a set of dose-rate measurements spanning several years, typically available at laboratories, since periodical checks of the field are performed. The method illustrated was applied to a californium source present at Paul Scherrer Institute calibration laboratory, yielding an estimation of the ratio of the neutrons emitted by 252Cf and 250Cf. Since for calibration laboratories any alteration of the neutron energy distribution emitted by the source could influence the response of detectors, available data on neutron spectra emitted by these nuclides have also been compared.