Yangyang Underground Laboratory Research Articles

Background study of the AMoRE-pilot experiment

We report a study on the background of the Advanced Molybdenum-Based Rare process Experiment (AMoRE), a search for neutrinoless double beta decay (0νββ) of 100Mo. The pilot stage of the experiment was conducted using ∼1.9 kg of 48deplCa100MoO4 crystals at the Yangyang Underground Laboratory, South Korea, from 2015 to 2018. We compared the measured β/γ energy spectra in three experimental configurations with the results of Monte Carlo simulations and identified the background sources in each configuration. We replaced several detector components and enhanced the neutron shielding to lower the background level between configurations. A limit on the half-life of 0νββ decay of 100Mo was found at T1/20ν≥3.0×1023 years at 90% confidence level, based on the measured background and its modeling. Further reduction of the background rate in the AMoRE-I and AMoRE-II are discussed.

Construction of Yemilab

The Center for Underground Physics of the Institute for Basic Science (IBS) in Korea has been planning the construction of a deep underground laboratory since 2013 to search for extremely rare interactions such as dark matter and neutrinos. In September 2022, a new underground laboratory, Yemilab, was finally completed in Jeongseon, Gangwon Province, with a depth of 1,000 m and an exclusive experimental area spanning 3,000 m3. The tunnel is encased in limestone and accommodates 17 independent experimental spaces. Over 2 years, from 2023 to 2024, the Yangyang Underground Laboratory facilities will be relocated to Yemilab. Preparations are underway for the AMoRE-II, a neutrinoless double beta decay experiment, scheduled to begin in Q2 2024 at Yemilab. Additionally, Yemilab includes a cylindrical pit with a volume of approximately 6,300 m3, designed as a multipurpose laboratory for next-generation experiments involving neutrinos, dark matter, and related research. This article provides a focused overview of the construction and structure of Yemilab.

Search for Boosted Dark Matter in COSINE-100.

We search for energetic electron recoil signals induced by boosted dark matter (BDM) from the galactic center using the COSINE-100 array of NaI(Tl) crystal detectors at the Yangyang Underground Laboratory. The signal would be an excess of events with energies above 4MeV over the well-understood background. Because no excess of events are observed in a 97.7 kg·yr exposure, we set limits on BDM interactions under a variety of hypotheses. Notably, we explored the dark photon parameter space, leading to competitive limits compared to direct dark photon search experiments, particularly for dark photon masses below 4MeV and considering the invisible decay mode. Furthermore, by comparing our results with a previous BDM search conducted by the Super-Kamionkande experiment, we found that the COSINE-100 detector has advantages in searching for low-mass dark matter. This analysis demonstrates the potential of the COSINE-100 detector to search for MeV electron recoil signals produced by the dark sector particle interactions.

AMoRE Neutrinoless Double Beta Decay Experiment

AMoRE is an international project searching for neutrinoless double beta decay (0νββ) of 100Mo. Highly purified molybdate crystals are utilized with heat-light detection based on magnetic micro-calorimeters operating at 10 mK. The high energy resolution is achieved with the heat sensors capable of efficient background rejection from the dual heat-light detection. The present AMoRE, composed of 18 crystal modules, has been operating at Yangyang underground laboratory with an exposure exceeding 9 kg·year, reaching the best limit for 100Mo 0νββ mode. Moreover, the collaboration builds a large-size detector for a total detector mass of about 180 kg in Yemilab, reaching the world-leading sensitivity for 0νββ search.

Radon concentration variations at the Yangyang underground laboratory

The concentration of222Rn in the air has been measured in the 700 m-deep Yangyang underground laboratory between October 2004 and May 2022. The average concentrations (spreads) in two experimental areas, called A6 and A5, were measured to be 53.4 Bq/m3(13.9 Bq/m3) and 33.5 Bq/m3(7.9 Bq/m3), respectively. The lower value in the A5 area reflects the presence of better ventilation. The radon concentrations sampled within the two A5 experimental rooms’ air are found to be correlated to the local tunnel temperature outside of the rooms, with correlation coefficientsr= 0.22 andr= 0.70. Therefore, the radon concentrations display a seasonal variation, because the local temperature driven by the overground season influences air ventilation in the experimental areas. A sinusoidal function with a period of 1 year was used to fit the radon concentration data of both underground areas finding a maximum amplitude on August, 31 ± 6 days.

New Results from a Three-Year Annual Modulation Search with COSINE-100

COSINE-100 is a direct detection dark matter experiment that is testing DAMA/LIBRA’s claim of dark matter discovery. Located in South Korea’s Yangyang underground laboratory, C0SINE-100 comprises 106 kg of sodium iodide detectors surrounded by a ∼2000 L liquid scintillator veto. In this talk, I will present new results from an annual modulation search using three years of data and the impact on DAMA/LIBRA’s discovery claim. I will also discuss improvements over our previous modulation analysis, including lowering the analysis threshold to 1 keV and the development of a more robust time-dependent background model. In addition, I will review ongoing R&D projects for, and the physics reach of future phases of the experiment.

Status of AMoRE

AMoRE is to search for neutrinoless double beta decay using Mo-100 enriched crystal scintillators at an ultra-low temperature. We have understood the major background sources and how they can be reduced from a pilot stage run in the Yangyang underground laboratory. AMoRE-I is currently running with twelve 48deplCa100MoO4 and five Li2 100MoO4 crystals with a total mass of enriched 100Mo 2.8 kg. The detector design for the main stage of the experiment, AMoRE-II, has been finalized and the detector is under construction in YemiLab, a new underground laboratory located in an iron mine with an average depth 2500 meter of water equivalent.

Background modeling for dark matter search with 1.7\xa0years of COSINE-100 data

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 2.85pm 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 kgcdot years.

Lowering the energy threshold in COSINE-100 dark matter searches

COSINE-100 is a dark matter detection experiment that uses NaI(Tl) crystal detectors operating at the Yangyang underground laboratory in Korea since September 2016. Its main goal is to test the annual modulation observed by the DAMA/LIBRA experiment with the same target medium. Recently DAMA/LIBRA has released data with an energy threshold lowered to 1 keV, and the persistent annual modulation behavior is still observed at 9.5σ. By lowering the energy threshold for electron recoils to 1 keV, COSINE-100 annual modulation results can be compared to those of DAMA/LIBRA in a model-independent way. Additionally, the event selection methods provide an access to a few to sub-GeV dark matter particles using constant rate studies. In this article, we discuss the COSINE-100 event selection algorithm, its validation, and efficiencies near the threshold.

Measurement of the cosmic muon annual and diurnal flux variation with the COSINE-100 detector

We report measurements of annual and diurnal modulations of the cosmic-ray muon rate in the Yangyang underground laboratory (Y2L) using 952 days of COSINE-100 data acquired between September 2016 and July 2019. A correlation of the muon rate with the atmospheric temperature is observed and its amplitude on the muon rate is determined. The effective atmospheric temperature and muon rate variations are positively correlated with a measured effective temperature coefficient of αT = 0.82 ± 0.10. This result is consistent with a model of meson production in the atmosphere. We also searched for a diurnal modulation in the underground muon rate by comparing one-hour intervals. No significant diurnal modulation of the muon rate was observed.

Measurement of the background activities of a 100Mo-enriched powder sample for an AMoRE crystal material by using fourteen high-purity germanium detectors

The Advanced Molybdenum-based Rare process Experiment (AMoRE) in its second phase will search for neutrinoless double-beta (0νββ) decay of 100Mo in 200 kg of molybdate crystals. To achieve the zero-background level in the energy range of the double-beta decay Q-value of 100Mo, the radioactive contamination levels in AMoRE crystals should be low. Enriched molybdenum trioxide powder, 100EnrMoO3, enriched to 95.73% abundance of the 100Mo isotope is used to grow the AMoRE crystals. A shielded array of fourteen high-purity germanium detectors with 70% relative efficiency each was used for the measurement of background activities in a sample of 9.6-kg powder. The detector system named the CAGe located at the Yangyang underground laboratory was designed for measuring low levels of radioactivity from natural radioisotopes or cosmogenic nuclides such as 228Ac, 228Th, 226Ra, 88Y, and 40K. The activities of 228Ac and 228Th in the powder sample were 0.88 ± 0.13 and 0.669 ± 0.089 mBq/kg, respectively. The activity of 226Ra was measured to be 1.19 ± 0.42 mBq/kg. The activity of 88Y was 0.101 ± 0.016 mBq/kg. The activity of 40K was found as 36.0 ± 4.1 mBq/kg.

Development of an array of fourteen HPGe detectors having 70% relative efficiency each

Searches for new physics push experiments to look for increasingly rare interactions. As a result, detectors require increasing sensitivity and specificity, and materials must be screened for naturally occurring, background-producing radioactivity. Furthermore, the detectors used for screening must approach the sensitivities of the physics-search detectors themselves, thus motivating iterative development of detectors capable of both physics searches and background screening. We report on the design, installation, and performance of a novel, low-background, fourteen-element high-purity germanium detector named the CAGe (CUP Array of Germanium), installed at the Yangyang underground laboratory in Korea.

Neutron background measurement for rare event search experiments in the YangYang underground laboratory

Several experiments have been conducted in the YangYang Underground Laboratory in the Republic of Korea such as the search for dark matter and the search for neutrinoless double beta decay, which require an extremely low background event rate due to the detector system and the environment. In underground experiments, neutrons have been identified as one of the background sources. The neutron flux in the experimental site needs to be determined to design a proper shielding system and for precise background estimation. We measured the neutron spectrum with a Bonner sphere spectrometer, with Helium-3 (3He) proportional counters. The neutron flux at the underground laboratory was so low that the radioactive decays from the radioisotopes contained in the detector created a significant background interference to the neutron measurement. Using Monte Carlo simulations, the intrinsic α background distribution due to the radioactive isotopes in the detector materials, was estimated. The neutron count rate of each Bonner sphere was measured from the pulse height spectrum of the 3He proportional counter, after subtracting the α particle background. The neutron flux and the energy spectrum were determined using the unfolding technique. The total neutron flux measured was (4.46 ± 0.66) × 10−5 cm−2 s−1, and the thermal and fast neutron flux (in the range 1–10 MeV) were (1.44 ± 0.15) × 10−5 cm−2 s−1 and (0.71 ± 0.10) × 10−5 cm−2 s−1, respectively.

AMoRE: a search for neutrinoless double-beta decay of 100Mo using low-temperature molybdenum-containing crystal detectors

The AMoRE is an experiment to search for neutrinoless double-beta decay of 100Mo in molybdate crystal scintillators using a cryogenic detection technique. The crystals are equipped with metallic magnetic calorimeter (MMC) sensors that detect both phonon and photon signals at temperatures of a few tens of mK. Simultaneous measurements of thermal and scintillation signals produced by particle interactions in the crystals by MMC sensors provide high energy resolution and efficient particle discrimination. AMoRE-Pilot, an R&D phase with six 48deplCa100MoO4 crystals and a total mass of ∼1.9 kg in the final configuration, operated at the 700 m deep Yangyang underground laboratory (Y2L). After completion of the AMoRE-Pilot run at the end of 2018, AMoRE-I with a ∼6 kg crystal array comprised of thirteen 48deplCa100MoO4 and five Li2100MoO4 crystals is currently being assembled and installed at Y2L. We have secured 110 kg of 100Mo-isotope-enriched MoO3 powder for the production of crystals for the AMoRE-II phase, which will have ∼200 kg of molybdate crystals and operate at Yemilab, a new underground laboratory located ∼1,100 m deep in the Handeok iron mine that is currently being excavated and with a scheduled completion date of December 2020. AMoRE-II is expected to improve the upper limit on the effective Majorana neutrino mass to cover the entire inverted hierarchy neutrino mass region: 20–50 meV, in the case when no such decays are observed. Results from AMoRE-Pilot and progress of the preparations for AMoRE-I and AMoRE-II are presented.

Measurement of the Background Activities of a 100Mo-enriched Powder Sample for an AMoRE Crystal Material by Using a Single High-Purity Germanium Detector

The Advanced Molybdenum-based Rare process Experiment (AMoRE) searches for neutrino-less double-beta (0{\nu}\b{eta}\b{eta}) decay of 100Mo in enriched molybdate crystals. The AMoRE crystals must have low levels of radioactive contamination to achieve low background signals with energies near the Q-value of the 100Mo 0{\nu}\b{eta}\b{eta} decay. To produce low-activity crystals, radioactive contaminants in the raw materials used to form the crystals must be controlled and quantified. 100EnrMoO3 powder, which is enriched in the 100Mo isotope, is of particular interest as it is the source of 100Mo in the crystals. A high-purity germanium detector having 100% relative efficiency, named CC1, is being operated in the Yangyang underground laboratory. Using CC1, we collected a gamma spectrum from a 1.6-kg 100EnrMoO3 powder sample enriched to 96.4% in 100Mo. Activities were analyzed for the isotopes 228Ac, 228Th, 226Ra, and 40K. They are long-lived naturally occurring isotopes that can produce background signals in the region of interest for AMoRE. Activities of both 228Ac and 228Th were < 1.0 mBq/kg at 90% confidence level (C.L.). The activity of 226Ra was measured to be 5.1 \pm 0.4 (stat) \pm 2.2 (syst) mBq/kg. The 40K activity was found as < 16.4 mBq/kg at 90% C.L.

The Status of AMoRE Double Beta Decay experiment

AMoRE (Advanced Mo-based Rare process Experiment) is searching for neutrinoless double beta decay of 100Mo. A pilot experiment, AMoRE-Pilot, was operated with six 48dep1Ca100MoO4 crystals of total mass ~1.9 kg in a cryostat at the Yangyang underground laboratory (Y2L) with the minimum overburden of 700 m. Since AMoRE-Pilot started in August 2015, two major improvements in its configuration took place to reduce its background level. The first one was replacement of detector components by materials with lower radioactivity level. The second one was the installation polyethylene blocks together with borated rubber sheets to reduce external neutron backgrounds. Using the AMoRE-Pilot data in different configurations, background modelling of the AMoRE-Pilot was performed.

Radioassay and Purification for Experiments at Y2L and Yemilab in Korea

Two major rare-process search experiments, COSINE-100 for WIMP dark matter and AMoRE for neutrinoless double beta decay, have been running at the Yangyang underground laboratory (Y2L) in Korea for about four years. In order to measure radio-activities of materials in both experiments, a number of ultra-low radioactivity measurement detectors have been developed and are now operational. For radioassays of raw materials and detector components, an ICP-MS, an argon gas ionization counter, and a number of HPGe detectors are being used. Three silicon PIN diode-based radon chamber detectors have been either upgraded or constructed for accurate measurements of atmospheric radon in a radon reduction system. A fourteen element HPGe detectors array was installed in 2016 for more sensitive measurements and larger sample capacities than those could be tested in two single crystal HPGe detectors. Since the Y2L space is too small to accommodate the next phases of both experiments, a new underground laboratory, called Y emilab, is being constructed in Jeongseon, Korea with a factor of ten more space (~2,800 m2) and a ~1,100 m overburden compared to ~200 m2 and ~700 m at Y2L. The future experiments require detector materials with substantially lower background levels than those in the current ones. Various types of scintillating crystals such as CaMoO4, Li2MoO4, and NaI(Tl) are being grown with purification methods specifically developed for the raw materials. A summary on radioassay and purification results for experiments at Y2L and Yemilab in Korea will be presented.

Status of COSINE-100

The DAMA/LIBRA collaboration has claimed that an annual modulation signal which is consistent with signal induced by the dark matter interactions is observed by the NaI(Tl) crystal detectors. In order to test the DAMA/LIBRA collaboration’s claim, the COSINE experiment searches for interactions of Weakly Interacting Massive Particles (WIMPs) using the same target material. The COSINE-100 detector has been operating since September of 2016 in the 700-m-deep Yangyang underground laboratory. First annual modulation analysis based on the 2-keV energy threshold has been completed and several other analyses are in progress. In addition, studies for the 1-keV threshold have been on-going and analyses based on this threshold is underway. In this presentation, we report the detector design, performance, and recent results of COSINE-100 as well as plans for the next phase, COSINE-200.

Reduction of radioactive elements in molybdenum trioxide powder by sublimation method and its technical performance

We studied the effectiveness of the purification of molybdenum trioxide (MoO3) powder with the sublimation method. To utilize the method, we have designed sublimation apparatus to purify the powder and annealing apparatus to collect the fine powder, followed by the wet chemistry method. As part of purification R&D, MoO3 powder was purified using a low vacuum sublimation method to remove radioactive elements such as Ra, Th, U, etc. The purification was performed at different temperatures to determine the optimum conditions for high decontamination factors and high recovery efficiencies. After applying the sublimation method, the powder was dissolved in aqueous ammonia; recrystallized to obtain polyammonium molybdate (PAM); and annealed to acquire MoO3 fine powder. The phase of MoO3 powder was studied by using X-ray diffraction (XRD) techniques. The effectiveness of the purification techniques was checked with inductively coupled plasma mass spectrometry (ICP-MS) measurements and the radioactivity from Ra, Th, and U were measured with high purity germanium (HPGe) detectors at Yangyang underground laboratory in Korea. The purified MoO3 powder was used by the AMoRE (Advanced Mo-based Rare process Experiment) collaboration to grow scintillating crystals.

Study of cosmogenic radionuclides in the COSINE-100 NaI(Tl) detectors

COSINE-100 is a direct detection dark matter search experiment that uses a 106 kg array of eight NaI(Tl) crystals that are kept underground at the Yangyang Underground Laboratory to avoid cosmogenic activation of radioisotopes by cosmic rays. Even though the cosmogenic activity is declining with time, there are still significant background rates from the remnant nuclides. In this paper, we report measurements of cosmogenic isotope contaminations with less than one year half-lives that are based on extrapolations of the time dependent activities of their characteristic energy peaks to activity rates at the time the crystals were deployed underground. For longer-lived 109Cd (T1/2=1.27 y) and 22Na (T1/2=2.6 y), we investigate time correlations and coincidence events due to several emissions. The inferred sea-level production rates are compared with calculations based on the ACTIVIA and MENDL-2 model calculations and experimental data. The results from different approaches are in reasonable agreement with each other. For 3H, which has a long, 12.3 year half-life, we evaluated the activity levels and the exposure times that are in reasonable agreement with the time period estimated for each crystal’s exposure.