Measurement of intrinsic radioactivity in a GSO crystal

Scintillating crystal detectors offer potential advantages in low-energy low-background experiments for particle physics and astrophysics. The GSO crystal is an interesting detector to explore in future neutrino physics experiments. The contributions to background due to the various channels of intrinsic radio isotopes from the ²³²Th and ²³⁸U series are identified and studied with time-correlation analysis and detailed fits to the spectral shape. Good agreement is achieved between measured and simulated spectra, indicating that background suppression factors to the 10^-2-10^-3 level are possible. The procedures can be adopted for background understanding and suppression in other low-count-rate experiments where the dominant source of background is from internal radioactivity. Based on 1656 h of data taking, limits on the double beta decay half-life for the various channels in ¹⁶⁰Gd are derived. The limits for the neutrinoless and the Majoron modes are T_1/2^0νββ > 2.0(0.8) × 10²⁰ yr and T_1/2^χββ > 3.0(1.9) × 10¹⁹ yr, respectively, at 68(90)% confidence level.