Ground surface ultralow background spectrometer: Active shielding improvements and coincidence measurements for the Gamma3 spectrometer.

The ultralow background versatile spectrometer GAMMA3 has been optimized with the following shielding improvements: (i) optimized nitrogen injection flux of 300Lh-1, and (ii) cosmic veto configuration with 9 scintillating plates. These improvements allow a reduction of 39% of the normalized integral background count rate down to 2.7±0.2min-1kgGe-1 (40-2500keV energy range). Minimum Detectable Activities when performing direct γ-ray spectrometry or γ-γ coincidence spectrometry are compared.