Dose rate conversion parameters: Assessment of nuclear data

The dating of materials using stored dose methods requires accurate determination of the environmental dose rate. The calculation of dose rates from radionuclide concentrations requires conversion parameters derived from nuclear data (half life, decay energies and intensities, and branching ratios). With the substantial body of primary data, it is convenient to use data from evaluated libraries. These libraries show variations reflecting both newer data unavailable to earlier evaluations and the relative importance given to different data sets by the evaluators. Commonly used conversion parameters derive from the Evaluated Nuclear Structure Data File (ENSDF), either directly or from secondary publications, with new tabulations produced in recent years following revisions to this library. Other international evaluations of nuclear data include the NEA/OECD supported JEF2.2 and JEFF3.11 evaluations, and the Decay Data Evaluation Project (DDEP). A technique comparing different evaluations to identify data that can not be confidently used has been developed. These differences have been investigated with an evaluation of underlying nuclear data. Particular radionuclides of interest are discussed; 214Bi where recent evaluations depend on a single high precision data set, 228Ac where the decay scheme is incomplete and further measurements are required, and 40K where the mean beta energy has been calculated in the evaluations using an incorrect shape factor. Revised dose rate conversion factors have been produced, which are largely consistent with earlier values with the exception of the 40K beta parameter which is 4% higher than recent values but consistent with earlier calculations.