[Paper Review] Background modeling for dark matter search with 1.7 years of COSINE-100 data

Prabal Adhikari|arXiv (Cornell University)|Jan 1, 2021

Dark Matter and Cosmic Phenomena23 references33 citations

TL;DR

This paper presents a comprehensive background model for the COSINE-100 dark matter experiment using 1.7 years of data from 106 kg of NaI(Tl) crystals. The model incorporates internal and external background sources—including surface and bulk 210Pb contamination and cosmogenic isotopes—using Geant4 simulations with empirically fitted nonlinear energy response and depth profiles for 210Pb. The key result is a measured average background level of 2.85±0.15 counts/day/keV/kg in the 1–6 keV region, dominated by 210Pb and 3H.

ABSTRACT

We present a background model for dark matter searches using an array of NaI(Tl) crystals in the COSINE-100 experiment that is located in the Yangyang underground laboratory. The model includes background contributions from both internal and external sources, including cosmogenic radionuclides and surface $^{210}$Pb contamination. To build the model in the low energy region, with a threshold of 1 keV, we used a depth profile of $^{210}$Pb contamination in the surface of the NaI(Tl) crystals determined in a comparison between measured and simulated spectra. We also considered the effect of the energy scale errors propagated from the statistical uncertainties and the nonlinear detector response at low energies. The 1.7 years COSINE-100 data taken between October 21, 2016 and July 18, 2018 were used for this analysis. Our Monte Carlo simulation provides a non-Gaussian peak around 50 keV originating from beta decays of bulk $^{210}$Pb in a good agreement with the measured background. This model estimates that the activities of bulk $^{210}$Pb and $^{3}$H are dominating the background rate that amounts to an average level of 2.85$\pm$0.15 counts/day/keV/kg in the energy region of (1-6) keV, using COSINE-100 data with a total exposure of 97.7 kg$\cdot$years.

Motivation & Objective

• To develop a precise background model for dark matter searches in the low-energy region (1–6 keV) using 1.7 years of COSINE-100 data.
• To accurately account for internal and external background contributions, especially surface and bulk 210Pb contamination and cosmogenic isotopes.
• To model the nonlinear energy response of NaI(Tl) crystals at low energies to improve spectral simulation accuracy.
• To validate the background model by comparing simulated spectra with measured data from five low-noise NaI(Tl) crystals.
• To estimate the total background level in the dark matter WIMP search region (1–6 keV) for future direct detection analyses.

Proposed method

• The background model is built using Geant4-based Monte Carlo simulations with G4 version 10.4.2, incorporating detailed detector geometry and shielding layers.
• A nonlinear energy response function f(E) = p₀·ln[(E−p₁)/p₂] / [(E−p₁)/p₂]³ + p₃ is empirically fitted to low-energy calibration data to model nonlinearity in NaI(Tl) crystals.
• Depth profiles for surface 210Pb contamination are derived from dedicated Y2L test measurements and applied to simulate surface decay contributions.
• Background contributions from internal isotopes (40K, 210Pb, 3H, 113Sn, 109Cd, 22Na) and external sources are simulated and compared with measured spectra.
• The model includes corrections for event selection efficiency and accounts for energy scale uncertainties propagated from statistical errors.
• Spectra are fitted to measured data for single- and multiple-hit events, with the 1–6 keV region extrapolated from modeled low-energy behavior.

Experimental results

Research questions

• RQ1 What is the dominant background contribution in the 1–6 keV energy region for the COSINE-100 experiment?
• RQ2 How accurately can the nonlinear energy response of NaI(Tl) crystals be modeled at low energies (1–60 keV) using empirical fitting?
• RQ3 What is the contribution of surface 210Pb contamination to the total background, and how does its depth profile affect the simulated spectrum?
• RQ4 How well does the Geant4-based simulation reproduce the measured background spectra across both single- and multiple-hit events?
• RQ5 What is the total background level in the dark matter WIMP search region (1–6 keV) after full modeling of internal and external sources?

Key findings

• The background model achieves good agreement with measured spectra across both single- and multiple-hit events, particularly in the 1–6 keV region.
• The simulated spectrum shows a non-Gaussian peak around 50 keV due to beta decays from bulk 210Pb, which matches the measured background distribution.
• The dominant background contributions in the 1–6 keV region are from bulk 210Pb and 3H, with a total average level of 2.85±0.15 counts/day/keV/kg.
• The model accurately reproduces the measured activities of 40K, 210Pb, 3H, 22Na, 109Cd, and 113Sn within ~20% agreement for the five crystals analyzed.
• The surface 210Pb contribution varies significantly between crystals, with shallow and deep depth profiles providing better fit than a single uniform distribution.
• The model extrapolates the 1–6 keV region with confidence, showing good agreement between simulated and efficiency-corrected measured spectra.