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

TL;DR

This study systematically measures the quenching factor of NaI(Tl) crystals for sodium recoils across different Tl dopant concentrations, crucial for calibrating dark matter detection signals.

Contribution

It provides the first detailed analysis of how Tl dopant levels affect the sodium recoil quenching factor in NaI(Tl) crystals.

Findings

Quenching factor varies with Tl concentration at certain energies.

No Tl-dependent effect observed with low-energy calibration line.

Results aid in improving calibration accuracy for dark matter detectors.

Abstract

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 low nuclear recoil energies of 5-26keV$_{nr}$ was extracted for all 5 crystals. A Tl-dependence could be deduced with a proportional response calibration schema using a $^{241}$Am source. However, this effect was not observed when using a low-energy calibration line from $^{133}$Ba.