Summing spectra in gamma spectrometry for improving detection of low-level radionuclides in aerosol samples

Abstract

In the frame of IRSN's French radioactivity observatory, aerosol samples are collected in a ten-days basis in different locations and measured by gamma spectrometry. For some studies, it is necessary to have the average of a radionuclide in aerosol samples, i.e. monthly or annually. Gamma spectrometry measurement of artificial radionuclides such as ¹³⁷Cs is becoming more difficult due to decreasing radioactivity levels. Individual measurements are sometimes below the minimum detectable activity (MDA) even when low-background measurement procedures are used. In these cases, how can a MDA in an average computation be taken into account? In order to acquire this mean value, the analysis can be performed on the annual spectrum corresponding to a running sum of each individual spectrum. As for the sample, the summing is applied for the background spectrum. In this way, the ¹³⁷Cs concentration in aerosols corresponding to the annual mean value is obtained. For aerosols collected in La Reunion, a mean concentration of $0.029 \pm 0.010 \mathrm{\mu}$Bq.$\textrm{m}^{-3}$ for ¹³⁷Cs is obtained, when 25 measurement results on 26 samples are below the MDA. For radionuclides always detected, a good agreement between summing spectrum results and computed mean value is observed.