A Comparison of True Alpha Activities in Air Filter Samples with Values Obtained from Radioactivity-in-air Monitors

A fundamental problem with measurements of alpha activity on air filters from the nuclear workplace is that the calibration sources used with radioactivity-in-air monitors are not in the same physical form as actual samples. The samples suffer from alpha absorption due to effects such as self-attenuation within the radioactive particle, entrapment of the particle within the filter, and the accumulation of dust layers on the filter. This can lead to underestimation of alpha activity in air and therefore underestimation over time of intake and hence committed internal dose. To obtain a first indication of the magnitude of the problem, NPL obtained a set of nineteen contaminated air filters from three UK nuclear sites and measured them using a number of typical workplace monitors. The same filters were then subjected to radiochemical analysis to measure the activities of Am, Pu and U nuclides. The ratio of true to monitored activities was typically in the range 1 – 5, but in a few cases was of the order of 20. The results are compared with ‘correction factors’ used at UK nuclear sites and from the literature. The study points to the possibility of significant underestimation of airborne alpha activity in some circumstances and that plant-by-plant determination of ratios may be required to ensure that workplace monitoring of alpha particulate in air is accurate.