Quenching factors for Na recoils as a function of Tl dopant concentrations in NaI(Tl) crystals

Thallium-doped sodium iodide (NaI(Tl)) scintillation detectors play an important role in the field of direct dark matter (DM) searches. The DAMA/LIBRA experiment stands out for its reported observation of an annually modulating DM-like signal, which is in direct contrast with other results. To accurately calibrate the energies of nuclear recoil signals with electron recoils, precise measurements of the quenching factor of the NaI(Tl) crystals are essential, as the two processes have different scintillation light yield. In this article, we present results of a systematic study carried out by the COSINUS collaboration and Duke University to measure the quenching factor of sodium (Na) recoils as a function of nuclear recoil energy and for differing Thallium (Tl) dopant concentrations in the bulk crystal. Five ultrapure NaI(Tl) crystals, manufactured by the Shanghai Institute for Ceramics, were irradiated with a quasi-monoenergetic neutron beam at the Triangle Universities Nuclear Laboratory, North Carolina, USA. The quenching factor for nuclear recoils off Na with low energies of 6–26keV ₙᵣ nr was extracted for all crystals, using two proportional calibration schemas with lines from a ²⁴¹ 241 Am source and a ¹³³ 133 Ba source. A Tl-dependence could be deduced in both cases, where a lower Tl concentration leads to smaller observed QF.