Effective determination of the long-lived nuclide ⁴¹Ca in nuclear reactor bioshield concretes: Comparison of liquid scintillation counting and accelerator mass spectrometry

The routine application of liquid scintillation counting to ⁴¹Ca determination has been hindered by the absence of traceable calibration standards of known ⁴¹Ca activity concentrations. The introduction of the new IRMM ⁴¹Ca mass-spectrometric standards with sufficiently high ⁴¹Ca activities for radiometric detection has partly overcome this although accurate measurement of stable Ca concentrations coupled with precise half-life data are still required to correct the certified ⁴¹Ca: ⁴⁰Ca ratios to ⁴¹Ca activity concentrations. In this study, ⁴¹Ca efficiency versus quench curves have been produced using the IRMM standard, and their accuracy validated by comparison with theoretical calculations of ⁴¹Ca efficiencies. Further verification of the technique was achieved through the analysis of ⁴¹Ca in a reactor bioshield core that had been previously investigated for other radionuclide variations. Calcium-41 activity concentrations of up to 25 Bq/g were detected. Accelerator mass spectrometry (AMS) measurements of the same suite of samples showed a very good agreement, providing validation of the procedure. Calcium-41 activity concentrations declined exponentially with distance from the core of the nuclear reactor and correlated well with the predicted neutron flux.