Study of cosmogenic radionuclides in the COSINE-100 NaI(Tl) detectors
Introduction
There are a number of experiments that search for direct evidence for dark matter particles in the halo of our Galaxy by looking for nuclei recoiling from dark matter nucleus scattering [1], [2] and report null results. One notable exception is the DAMA/LIBRA experiment that has consistently reported the observation of an annual event-rate modulation, that could be interpreted as dark-matter signal, in an array of NaI(Tl) crystal detectors with a statistical significance that is now more than 12.9 σ [3], [4]. Although this signal has persisted for over two decades and for three different configurations of the detector, it remains controversial because it is in conflict with the bounds from other direct detection experiments using different target materials[5], [6], [7], [8], [9], [10] and indirect searches [11]. However, since these conflicts depend on the details of the models for dark matter-nucleus scattering [12] and the properties of the galactic dark matter halo[13], [14], [15], a conclusive statement about the DAMA/LIBRA signal can only be made by conducting an independent experiment using the same NaI(Tl) target material. This is the prime motivation of COSINE-100 and a number of other NaI(Tl)-crystal-based experiments [16], [17], [18], [19], [20].
COSINE-100 is a dark matter direct detection experiment [21], [22] that uses a 106 kg array of eight low-background NaI(Tl) crystals situated in a 2000 l liquid scintillator veto counter. The experiment is located 700 m underground at the Yangyang Underground Laboratory (Y2L), where it has been operating since September 2016. The search for an annual modulation signal requires a complete understanding of background sources and their time dependence. To accomplish this, a complete simulation that accurately models the background energy spectra measured in the detector is required [23]. In addition to backgrounds from long-lived radioactive contaminations in the crystal bulk and surfaces, we have to deal with time-dependent backgrounds from short-lived cosmogenically activated radionuclides. These are isotopes that are created by interactions of cosmic rays with stable nuclides in the detector material. In COSINE-100, almost all of the cosmogenic isotopes come from cosmic ray interactions with either Na or I nuclei.
This paper is organized as follows. The COSINE-100 detector is described in Section 2. In Section 3, the cosmogenic isotopes that are produced in NaI(Tl) are listed and the determination of the activity levels at the time of their initial deployment underground at Y2L is described. The use of these initial activity levels to infer production rates for cosmogenic isotopes at sea level and their comparison with ACTIVIA and MENDL-2 calculations [24], [25] and with experimental data are discussed in Section 4. The fitted activities of 3H and 129I from the background modeling are evaluated in Section 5 and conclusions are provided in Section 6.
Section snippets
The COSINE-100 experimental setup
The experimental setup of COSINE-100, shown in Fig. 1(a), is described in detail in Ref. [21]. Eight NaI(Tl) crystals, arranged in two layers, are located in the middle of a four-layer shielding structure. From outside inward, this comprises plastic scintillator panels, a lead-brick castle, a copper box, and a tank of scintillating liquid. The eight encapsulated NaI(Tl) crystal assemblies and their support table are immersed in the scintillating liquid that serves both as an active veto and a
Cosmogenic radionuclides and initial activities
Although the eight NaI(Tl) crystals had underground radioactivity cooling times that range from several months to three years, there are still backgrounds from the long-lived cosmogenic isotopes that were activated by cosmic rays while they were on the surface.
To understand these backgrounds, we first considered the list of cosmogenic radioactive isotopes that are produced in NaI(Tl) reported in Refs. [30], [31], [32], [33]. In Table 2, we list the contributing cosmogenic isotopes with their
Results and comparisons for production rates
In Section 3 we describe the determination of the crystals’ cosmogenic isotope activities at the time they were first deployed underground at Y2L. However, since we do not know the details of their previous exposure conditions, such as times, locations, and altitudes, these cannot be directly related to production rates or saturation activity levels. But an attempt to extract sea level production rates has been made from a simplified mathematical model for production and decay of radionuclides.
Discussion on tritium 3H and iodine 129I
It is generally difficult to measure activity levels of long-lived cosmogenic isotopes, directly from the data due to their long half-lives. This is especially the case for 3H, which has no distinguishing γ/X-ray peak that can be exploited. Therefore, we simulated background spectra from 3H in the six NaI(Tl) crystals and used the extracted spectral shapes in the data fitting, while floating their unknown fractions [23]. We determine the initial activity A0 of 3H by using the average activity
Conclusion
We have studied background contributions from cosmogenic isotopes activated by cosmic rays in the COSINE-100 detectors. To understand their time-dependent energy spectra we simulated responses to decays of the most abundantly produced cosmogenic isotopes in NaI(Tl) crystals and identified the energy regions where they make strong contributions to the crystals’ background spectra. Based on these simulation studies we measured decay rates of the cosmogenic isotopes using the time-dependent
Acknowledgments
We thank the Korea Hydro and Nuclear Power (KHNP) Company for providing underground laboratory space at Yangyang. This work is supported by: the Institute for Basic Science (IBS) under project code IBS-R016-A1 and NRF-2016R1A2B3008343, Republic of Korea; UIUC campus research board, the Alfred P. Sloan Foundation Fellowship, NSF Grants Nos. PHY-1151795, PHY-1457995, DGE-1122492, WIPAC, the Wisconsin Alumni Research Foundation, United States; STFC Grant ST/N000277/1 and ST/K001337/1, United
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Present address: Department of Physics, Carleton University, Ottawa, Ontario K1S 5B6, Canada.