Low-level germanium gamma-ray spectrometry at the μBq/kg level and future developments towards higher sensitivity

Abstract

Abstract. GeMPI, a highly sensitive germanium gamma spectrometer operated at the Gran Sasso underground laboratory is described. Its complete cryostat system and shield was made from highly selected low activity materials. Radon suppression, also during sample insertion is achieved by an airlock system combined with an airtight steel casing around the shield, which is pressurised with nitrogen gas. The achieved background level in combination with a large sample capacity of up to 15 l around the 2.2 kg Ge-crystal allows measuring concentrations of the gamma active chain members down to 10 -12 g/g of U/

Search for 2βdecay of116Cd with the help of enriched116CdWO4crystal scintillators

“…The radiopurity of Cu has been derived from the fit of the background spectrum and it is comparable with the results of the low background measurements with the Cu samples (see e.g. [24]). Table 1 contains radiopurity level of the main sources of the background.…”

Lowering the radioactivity of the photomultiplier tubes for the XENON1T dark matter experiment

“…We have employed some of the world's most sensitive highpurity germanium (HPGe) spectrometers, the GeMPI [28][29][30] and Gator [31] detectors. They are located at the LNGS laboratory, under an average overburden of 3 600 m. w. e., where the muon flux is reduced to ∼1 m −2 h −1 .…”

Material radioassay and selection for the XENON1T dark matter experiment

“…These spectrometers have an excellent energy resolution over the energy range of interest (∼ 50−2650 keV with, e. g. < 3 keV FWHM at 1332 keV). The achieved sensitivities are in the range of 10-100 μBq/kg for primordial U and Th isotopes, as observed using typical sample masses (∼10-100 kg) and counting times of 30-100 days [20,21]. All detectors are p-type, intrinsically pure germanium crystals in a coaxial configuration, with masses between 2.2 and 2.3 kg and enclosed in a lowbackground cryostat housing.…”

“…The XENON collaboration utilizes several of the world's most sensitive germanium spectrometers, the Gator [20] detector and the four GeMPI detectors [21], that are located in ultra-low background facilities at LNGS at the same depth as the XENON1T detector. These spectrometers have an excellent energy resolution over the energy range of interest (∼ 50−2650 keV with, e. g. < 3 keV FWHM at 1332 keV).…”

“…The efficiencies were used in combination with the sample mass, measurement time, and branching ratio of each characteristic gamma-ray line to determine the specific activities or detection upper limits of each radioisotope. Further details on analysis procedures for the HPGe detectors can be found in [20,21].…”