Development of 226Ra Activity Analysis for Live Fish Using Gamma-Ray Spectrometry

Abstract

Radium contamination in the lakes near uranium mines and its intake by fish and other aquatic species are of great concern in environmental radiation protection. As an alternative technique to Liquid Scintillation Counting, we present a gamma-ray spectrometric method for low level 226Ra analysis, particularly in live fish. The HyperPure Ge and a 4π NaI(Tl) array were employed as gamma-ray spectrometers. The 226Ra spectra for each spectrometer were collected in order to compare their analytical performance. From the HPGe 226Ra spectra, a detection limit of 6.81 Bq with 99% confidence was determined for an hour counting with the three strongest peaks combined. For the 4π NaI(Tl) 226Ra spectra, in order to optimize the spectral analysis, a method applied to nine different energy regions was attempted and it turned out that the detection limit is best when the total integral count subtracted by the corresponding background is used. The detection limit of the 4π NaI(Tl) was 0.99 Bq with 99% confidence for one hour counting. A benchmark simulation for point source position dependence on relative peak efficiency showed a good agreement with the experimental data. To extend 226Ra analysis to volumetric aqueous samples, MCNP Monte Carlo simulations showed that for three cylindrical volume sources, as the simulated volume source size increased from 60 ml to 125 ml to 250 ml, the full-energy peak efficiency in the energy range of interest for 226Ra decreased by approximately 3% for each size increase, due to attenuation in the source volume. A method has been proposed in order to demonstrate this technique for a live fish, whereby a fish injected with 226Ra would be kept in its own special aquarium and its gamma-ray spectrum collected.