Characterization of SABRE crystal NaI-33 with direct underground counting

Abstract

Ultra-pure NaI(Tl) crystals are the key element for a model-independent verification of the long standing DAMA result and a powerful means to search for the annual modulation signature of dark matter interactions. The SABRE collaboration has been developing cutting-edge techniques for the reduction of intrinsic backgrounds over several years. In this paper we report the first characterization of a 3.4 kg crystal, named NaI-33, performed in an underground passive shielding setup at LNGS. NaI-33 has a record low $$^{39}$$ 39 K contamination of 4.3 ± 0.2 ppb as determined by mass spectrometry. We measured a light yield of 11.1 ± 0.2 photoelectrons/keV and an energy resolution of 13.2% (FWHM/E) at 59.5 keV. We evaluated the activities of $$^{226}$$ 226 Ra and $$^{228}$$ 228 Th inside the crystal to be $$5.9\pm 0.6~\upmu $$ 5.9 ± 0.6 μ Bq/kg and $$1.6\pm 0.3~\upmu $$ 1.6 ± 0.3 μ Bq/kg, respectively, which would indicate a contamination from $$^{238}$$ 238 U and $$^{232}$$ 232 Th at part-per-trillion level. We measured an activity of 0.51 ± 0.02 mBq/kg due to $$^{210}$$ 210 Pb out of equilibrium and a $$\alpha $$ α quenching factor of 0.63 ± 0.01 at 5304 keV. We illustrate the analyses techniques developed to reject electronic noise in the lower part of the energy spectrum. A cut-based strategy and a multivariate approach indicated a rate, attributed to the intrinsic radioactivity of the crystal, of $$\sim $$ ∼ 1 count/day/kg/keV in the [5–20] keV region.

High sensitivity characterization of an ultrahigh purity NaI(Tl) crystal scintillator with the SABRE proof-of-principle detector

“…In this paper, we report a detailed study of the background components of an ultrahigh radiopurity crystal grown for the SABRE experiment. While preliminary measurements on the same crystal had been performed on the surface [20] and underground inside a passive shielding [21], this study was carried out underground in the SABRE proof-of-principle (PoP) liquid scintillator active veto shielding (Fig. 1) in the Hall C of Laboratori Nazionali del Gran Sasso, Italy [19].…”

“…The crystal has a mass of 3.4 kg and is wrapped with ten layers of polytetrafluoroethylene (PTFE) tape (about 1 mm) and optically coupled directly to 3-in Hamamatsu R11065 photomultiplier tubes (PMTs) that feature high quantum efficiency and low radioactivity [20]. The crystal-PMT assembly is sealed inside a 2-mm-thick, high-purity copper enclosure [21], whose inner volume is constantly flushed with nitrogen gas to purge moisture and radon. The enclosure is deployed vertically via a 2-mm-thick, 121cm-long blind-end copper tube into a 1.3-m-diameter, 1.5m-long stainless steel veto vessel [19].…”

“…The light yield and FWHM resolution of NaI-33 crystal scintillator, measured by fitting the 59.5-keV photopeak of an 241 Am source, are determined to be 12.1 AE 0.2 phe=keV and 13.5%, respectively. The light yield is slightly higher than that measured in [21]. To determine the background rate in the 1-6 keV region of interest (ROI), data were taken between August 9, 2020 and September 5, 2020 for a total exposure of 26.4 days.…”

“…Cuts 3-6 aim at selecting pulselike waveform by imposing constraints on the shape of the waveform: Cut 3 and Cut 4 require the amplitudeweighed mean time and the total area over maximum amplitude to be within a certain range, while Cut 5 and Cut 6 impose constraints on the ratios of pulse areas at different times. These variables are further described in detail in [21]. Figure 2 shows the final acceptance rate as a function of energy after these cuts.…”

“…In the spectral fit, a Gaussian penalty was applied to 40 K with mean equal to 0.14 AE 0.01 mBq=kg corresponding to the prediction from inductively coupled plasma mass spectrometry measurements [20]. Assuming secular equilibrium for 226 Ra and 232 Th chain segments, Gaussian penalties [ð5.9 AE 0.6Þ μBq=kg for 226 Ra and ð1.6 AE 0.3Þ μBq=kg for 232 Th] were implemented based on measurements from 214 Bi-Po and 212 Bi-Po time-correlated events, respectively [21].…”