The use of γ-rays analysis by HPGe detector to assess the gross alpha and beta activities in waters

This paper describes an alternative method for evaluating gross alpha and beta radioactivity in waters by using γ-rays analysis performed with hyper-pure germanium detector (HPGe). Several gamma emissions related to α and β^- decays were used to provide the activity concentration data due to natural radionuclides commonly present in waters like ⁴⁰K and those belonging to the ²³⁸U and ²³²Th decay series. The most suitable gamma emissions related to β^- decays were ²¹⁴Bi (1120.29 keV, ²³⁸U series) and ²⁰⁸Tl (583.19 keV, ²³²Th series) as the equation in activity concentration yielded values compatible to those generated by the formula taking into account the detection efficiency. The absence of isolated and intense γ-rays peaks associated to α decays limited the choice to ²²⁶Ra (186.21 keV, ²³⁸U series) and ²²⁴Ra (240.99 keV, ²³²Th series). In these cases, it was adopted appropriate correction factors involving the absolute intensities and specific activities for avoiding the interferences of other γ-rays energies. The critical level of detection across the 186-1461 keV energy region corresponded to 0.010, 0.023, 0.038, 0.086, and 0.042 Bq/L, respectively, for ²²⁶Ra, ²²⁴Ra, ²⁰⁸Tl, ²¹⁴Bi and ⁴⁰K. It is much lower than the WHO guideline reference value for gross alpha (0.5 Bq/L) and beta (1.0 Bq/L) in waters. The method applicability was checked by the analysis of groundwater samples from different aquifer systems occurring in the Brazilian states of São Paulo, Minas Gerais and Mato Grosso do Sul. The waters exhibit very different chemical composition and the samples with the highest radioactivity levels were those associated with lithotypes possessing enhanced uranium and thorium levels. The technique allowed directly discard the ⁴⁰K contribution to the gross beta activity as potassium is an essential element for humans.