Alpha backgrounds in NaI(Tl) crystals of COSINE-100

Abstract

COSINE-100 is a dark matter direct detection experiment with 106 kg NaI(Tl) as the target material. ²¹⁰Pb and daughter isotopes are a dominant background in the WIMP region of interest and are detected via $\beta$ decay and $\alpha$ decay. Analysis of the $\alpha$ channel complements the background model as observed in the $\beta$/$\gamma$ channel. We present the measurement of the quenching factors and Monte Carlo simulation results and activity quantification of the $\alpha$ decay components of the COSINE-100 NaI(Tl) crystals. The data strongly indicate that the $\alpha$ decays probabilistically undergo two possible quenching factors but require further investigation. The fitted results are consistent with independent measurements and improve the overall understanding of the COSINE-100 backgrounds. Furthermore, the half-life of ²¹⁶Po has been measured to be $143.4\pm 1.2$  ms, which is consistent with and more precise than most current measurements.