Spectral interference corrections for the measurement of ²³⁸U in materials rich in thorium by a high resolution γ-ray spectrometry

In this study, the spectral interferences are investigated for the analytical peaks at 63.3 keV of ²³⁴Th and 1001.0 keV of ^234mPa, which are often used in the measurement of ²³⁸U activity by the γ-ray spectrometry. The correction methods are suggested to estimate the net peak areas of the γ-rays overlapping the analytical peaks, due to the contribution of ²³²Th that may not be negligible in materials rich in natural thorium. The activity results for the certified reference materials (CRMs) containing U and Th were measured with a well type Ge detector. The self-absorption and true coincidence-summing (TCS) effects were also taken into account in the measurements. It is found that ignoring the contributions of the interference γ-rays of ²³²Th and ²³⁵U to the mixed peak at 63.3 keV of ²³⁴Th (²³⁸U) leads to the remarkably large systematic influence of 0.8-122% in the measured ²³⁸U activity, but in case of ignoring the contribution of ²³²Th via the interference γ-ray at 1000.7 keV of ²²⁸Ac to the mixed peak at 1001 keV of ^234mPa (²³⁸U) results in relatively smaller systematic influence of 0.05-3%, depending on thorium contents in the samples. The present results showed that the necessary correction for the spectral interferences besides self-absorption and TCS effects is also very important to obtain more accurate ²³⁸U activity results. Additionally, if one ignores the contribution of ²³²Th to both ²³⁸U and ⁴⁰K activities in materials, the maximum systematic influence on the effective radiation dose is estimated to be ~6% and ~1% via the analytical peaks at 63.3 and 1001 keV for measurement of the ²³⁸U activity, respectively.