Low-level gamma ray counting on environmental samples

One of the major demands in gamma spectrometry of environmental samples is the accurate determination of activity concentration of present radionuclides (naturally occurring and those of artificial origin), due to the fact they are commonly of relatively low content. Thus, all these measurements have in common that the detection limit, in the spectral region of interest should be as low as possible. For this reason, the construction of a good passive, as well as active shield requires a detailed knowledge of the origin of the background events in the absence of an environmental sample. In addition, an analysis of the impact on detection limits due to the presence of the sample itself is also important. Also, the knowledge of the statistical basics for low-level counting is helpful to enable the best choice of detector characteristics (relative efficiency, peak to Compton ratio, resolution), measuring time, and required level of precaution against the different background contributions.

In this paper, the background spectra of several gamma spectroscopy systems (with passive and active veto shields) are analyzed and discussed, regarding their capabilities for measurements of environmental samples. Furthermore, various environmental samples are analyzed by low-level gamma spectrometry, including the sample measurements in the presence of an active veto shield against cosmic-ray muons. The disturbance of radioactive equilibrium between members of radioactive series in the samples is commented on, together with the possibility of use of certain gamma lines (including their interference and the corresponding intensities) for radionuclide activities determination.