Optimization of parameters of alpha spectrometry with silicon detector for low level measurements of actinides in environmental samples

Determination of actinides in environmental and biological samples is an important activity of radiation protection program at nuclear energy facilities. High resolution alpha spectrometry with passivated ion implanted Silicon detectors is widely used for the determination of actinides concentration. Low levels of activity concentrations in these samples often require long counting duration of a few days to obtain accurate and statistically significant data for further impact assessment. In alpha spectrometry, the chamber in which Si detector operated is a critical component and maintained at a desired vacuum for minimizing the alpha particle attenuation. Experimental evaluation of variations in energy resolution and tailing of alpha spectra was investigated under different chamber air pressures from about 6.7 Pa to more than 2700 Pa under the chamber hold mode and pump electrically switched off conditions. As part of validation, data collected on an IAEA inter-comparison exercise sample are presented under short and long counting durations with pump operating and switched off conditions respectively. It has been observed that the FWHM values do not significantly degrade, to impact the low and medium level concentration alpha spectra, for variations in vacuum chamber pressures from about 6.7 Pa to 2700 Pa.

► Several parameters relevant to low level alpha spectrometry have been investigated and appropriately optimized.
► The most important parameter has been the influence of chamber pressure on resolution when the chamber is in hold mode while the vacuum pump is electrically switched off for more than 40 h.
► Samples were counted for about 4 day for low levels of detection. Efficiency, tail length, detector size and other parameters were evaluated.