Characterization of SABRE crystal NaI-33 with direct underground counting
M. Antonello, I. J. Arnquist, E. L. Barberio, Ti. Baroncelli, Jay B. Benziger, L.J. Bignell, I. Bolognino, F. Calaprice, S. Copello, I. Dafinei, D. D’Angelo, G. D’imperio, M. D’Incecco, G. Di Carlo, M. Diemoz, A. Di Giacinto, A. Di Lüdovico, W. Dix, Alan R. Duffy, E. W. Hoppe, Aldo Ianni, M. Iannone, L. Ioannucci, S. Krishnan, G. J. Lane, I. Mahmood, A. Mariani, Silvia Milana, J. R. Mould, F. Nuti, D. Orlandi, V. Pettinacci, L. Pietrofaccia, S. Rahatlou, F. Scutti, Michael Souza, A. E. Stuchbery, B. Suerfu, C. Tomei, P. Urquijo, C. Vignoli, A. Wallner, M. Wada, Anthony G. Williams, A. Zani, M. J. Zurowski
Abstract
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 $$^{39}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mrow/> <mml:mn>39</mml:mn> </mml:msup> </mml:math> 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 $$^{226}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mrow/> <mml:mn>226</mml:mn> </mml:msup> </mml:math> Ra and $$^{228}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mrow/> <mml:mn>228</mml:mn> </mml:msup> </mml:math> Th inside the crystal to be $$5.9\pm 0.6~\upmu $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mn>5.9</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.6</mml:mn> <mml:mspace/> <mml:mi>μ</mml:mi> </mml:mrow> </mml:math> Bq/kg and $$1.6\pm 0.3~\upmu $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mn>1.6</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.3</mml:mn> <mml:mspace/> <mml:mi>μ</mml:mi> </mml:mrow> </mml:math> Bq/kg, respectively, which would indicate a contamination from $$^{238}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mrow/> <mml:mn>238</mml:mn> </mml:msup> </mml:math> U and $$^{232}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mrow/> <mml:mn>232</mml:mn> </mml:msup> </mml:math> Th at part-per-trillion level. We measured an activity of 0.51 ± 0.02 mBq/kg due to $$^{210}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mrow/> <mml:mn>210</mml:mn> </mml:msup> </mml:math> Pb out of equilibrium and a $$\alpha $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>α</mml:mi> </mml:math> 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 $$\sim $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mo>∼</mml:mo> </mml:math> 1 count/day/kg/keV in the [5–20] keV region.