Influence of the thorium decay series on the background of high-resolution gamma-ray spectrometers

The background induced by the members of the thorium decay sequence in six high-resolution, gamma-ray spectrometers was analyzed. For the analysis, the count rates in the peaks of the background spectra, normalized to the unit of emission probability and detection probability, were used. The energy dependence of these normalized count rates carries information about the location of the sources of contamination. The contributions of the detector contamination, the contamination of the shielding material and the radiation penetrating the shield were calculated. The comparison of these contributions among the spectrometers pointed to the weaknesses of some shields, making such a comparison a useful tool for assessing the effectiveness of the shields.

► Background due to the thorium decay sequence of gamma-ray spectrometers was measured.
► Peak count rates at energies 239, 583, 911 and 2615 keV were determined in six spectrometers.
► A spherically symmetric model of the spectrometer was used in the analysis.
► Contributions of the detector, shield and ambient radiation to the background were assessed.
► Weaknesses of the shields due to non-optimal construction and voids were found.