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180mTa, which is the second excited state of 180Ta at Ex = 77 keV, is naturally occurring and is the only known stable isomer. The half-life of 180mTa is considered to be an important parameter for nuclear synthesis models for heavy elements. However, the decay of 180mTa has never been observed even though several groups tried to measure it. We will search for gamma transitions from 180mTa decays in a tantalum sample by using an array of fourteen HPGe detectors recently installed in an underground laboratory at Yangyang, Korea. In preparation for the measurement, Monte-Carlo simulation studies were conducted to optimize the tantalum sample configuration. Based on the simulation study, we decided on a configuration composed of a 2 mm thick disk with diameter of 200 mm and six 2 mm thick rectangular plates with dimension of 158 × 195 mm2. The finalized tantalum sample configuration gives 2.0 and 7.5 coincidence events per year for the EC and the β-decay of 180mTa, respectively. In this study, we used T1/2 =2.0 × 1017 years and T1/2 = 5.8 × 1016 years for the EC and β-decay which are the present best lower-limits as reported by B. Lehnert et al.

Three different ice core samples from Antarctica were analyzed to identify activity concentrations of radioactive isotopes. Tracking migration of radioactive isotopes to Antarctica can provide a key clue to understand global environmental changes caused by radiation exposures because the Antarctic ice cores can preserve unique characteristics of various environmental conditions. We are particularly interested in the 137Cs nucleus, because it is closely related to radiation exposure from nuclear power plant accidents and nuclear bomb tests. With its half life of 30.17±0.03 years, 137Cs can also be used to assess the age of sedimentation occurring after around the year 1945. We selected three ice core samples, called Tarn8, Styx27, and H25, from different time periods; the Tarn8 sample is known to be from earlier than ~ 1000 AD, the Styx27 sample is approximately from the year 1945, and the H25 sample is from the year 2012. Radioactive isotope measurements of the ice core samples were performed using a 100% HPGe detector at Cheongpyeong Underground Radiation Laboratory (CURL). We measured the activity of 137Cs in the H25 sample to be 0.98 ± 0.82 mBq/kg. Although the activity has a large uncertainty mainly due to the limited sample quantity, the 137Cs isotopes in the Antarctic ice core were measured for the first time in Korea.

We constructed an underground laboratory called Cheongpyeong Underground Radiation Laboratory (CURL) for measuring the radioactivity levels of various samples by using HPGe detectors. CURL is located underground at a depth of 1000-m water equivalent in the Cheongpyeong Pumped Storage Power Plant. We developed a shielding system, which consists of 15-cm-thick Pb blocks and 5-cm-thick Cu blocks and completely surrounds a 100% HPGe detector. We measured the background radiations and the gamma peaks from sources with and without the shield. The shielding efficiencies were also estimated using MCNP5 simulations, and they were compared to our measured data. The shielding system blocked more than 99.99% of gamma rays with energies up to 3.0 MeV. The HPGe detector with the shielding system at CURL blocked both high-energy cosmic rays and background radiation from surrounding rocks and materials. Our CURL detector system was optimized for gamma-ray measurements of meterials with ultra-low radioactivity.