Measurements of low-energy nuclear recoil quenching factors for Na and I recoils in the NaI(Tl) scintillator

TL;DR

This study measures the low-energy nuclear recoil quenching factors for Na and I in NaI(Tl) crystals, crucial for dark matter and neutrino detection, achieving improved accuracy at lower energies through enhanced experimental techniques.

Contribution

The paper introduces improved measurement methods for low-energy quenching factors in NaI(Tl), enabling more precise detection of nuclear recoils relevant to dark matter and neutrino experiments.

Findings

Measured sodium recoil QF at 3.8 keVnr with 11.2% ± 1.7%

Achieved increased light yield of 26 photoelectrons/keVee

Reevaluated previous QF results with waveform simulation

Abstract

Elastic scattering off nuclei in target detectors, involving interactions with dark matter and coherent elastic neutrino nuclear recoil (CE$\nu$NS), results in the deposition of low energy within the nuclei, dissipating rapidly through a combination of heat and ionization. The primary energy loss mechanism for nuclear recoil is heat, leading to consistently smaller measurable scintillation signals compared to electron recoils of the same energy. The nuclear recoil quenching factor (QF), representing the ratio of scintillation light yield produced by nuclear recoil to that of electron recoil at the same energy, is a critical parameter for understanding dark matter and neutrino interactions with nuclei. The low energy QF of NaI(Tl) crystals, commonly employed in dark matter searches and CE$\nu$NS measurements, is of substantial importance. Previous low energy QF measurements were constrained by contamination from photomultiplier tube (PMT)-induced noise, resulting in an observed light yield of approximately 15 photoelectrons per keVee (kilo-electron-volt electron-equivalent energy) and nuclear recoil energy above 5 keVnr (kilo-electron-volt nuclear recoil energy). Through enhanced crystal encapsulation, an increased light yield of around 26 photoelectrons per keVee is achieved. This improvement enables the measurement of the nuclear recoil QF for sodium nuclei at an energy of 3.8 $\pm$ 0.6 keVnr with a QF of 11.2 $\pm$ 1.7%. Furthermore, a reevaluation of previously reported QF results is conducted, incorporating enhancements in low energy events based on waveform simulation. The outcomes are generally consistent with various recent QF measurements for sodium and iodine.