Improvements of 60 keV γγ-ray spectrum by reducing scattering effects

A considerable reduction of background and an improvement of spectra of low-energy γγ-rays, measured with a planar high-purity Ge detector, has been achieved using a small source and a careful design of collimators and shielding. The aim of the design was to reduce strongly the detection of scattered radiation. The design of the shielding was based on previous studies by the present authors of effects of near-source scattering on photon spectra. That work has shown the effects of different distribution of mass around the γγ-ray emitting atoms, seen as continuous distributions adjacent to the full-energy peaks (the “shoulders”). In the present measurement, reduction of intensity of the shoulders by as much as a factor of 10 has been achieved. The calculated shoulder spectra reproduce well the measured ones. Since a much lower background was achieved (peak-to-shoulder height ratio of about 2500), the shape of the full-energy peak could be analysed in a broader range. Deviations from the Gaussian shape have not been observed in the range ±3.4σ±3.4σ about the midpoint of the peak. The calculated shoulder spectrum, which is dominated by scattering in the source, and the Gaussian peak represent the measured spectrum in their energy range very well, so that less than 0.1% of counts are not accounted for. Present arrangements offer improved searches for lines of weak intensity and more accurate determination of peak intensities in X-ray and low-energy γγ-ray spectra.