Neutron spectrum measurement in the Yemi underground laboratory

TL;DR

The study establishes a baseline neutron background for the Yemilab underground laboratory by deploying a high-sensitivity spectrometer based on ten 3He counters with modular HDPE moderators and conducting dedicated internal α-background measurements. Neutron spectra were reconstructed using MAXED unfolding with MCNPX-derived response functions, revealing thermal, epithermal, and fast components across three sites, with Site 2 showing an elevated epithermal flux likely due to local materials and humidity. The results place Yemilab neutron backgrounds in context with other underground labs, providing essential inputs for shielding design and the planning of dark matter and rare-event experiments. Ongoing monitoring and material characterization are highlighted as future steps to refine the background model and ensure optimal detector performance.

Abstract

We report on the measurement of neutron energy spectra at the newly established Yemi Underground Laboratory (Yemilab) in the Republic of Korea, designed to host dark matter and rare-event search experiments. A high-sensitivity neutron spectrometer was employed, consisting of ten cylindrical {}^{3}He proportional counters, eight of which were embedded in cylindrical high-density polyethylene moderators of various sizes. To quantify and mitigate contributions from internal α-backgrounds, each detector underwent a dedicated background measurement using a cadmium-shielded box. These backgrounds, primarily originating from trace amounts of U and Th in the stainless-steel housings, were characterized and subtracted during data analysis. Neutron measurements were carried out at three locations within the Yemilab between March to October 2023. After waveform-based event selection and correction for \alphasym-backgrounds, neutron count rates were estimated and corresponding energy spectra were reconstructed using the unfolding method. The total neutron fluence rates were measured ranged from (3.24 $\pm$ 0.11) to (4.01 $\pm$ 0.10) $\times~10^{-5}~ {cm}^{-2}~{s}^{-1}$, with thermal and fast neutron components (1 - 10 MeV) ranging from (1.32 $\pm$ 0.05) to (1.51 $\pm$ 0.05) $\times 10^{-5}~{cm}^{-2}~{s}^{-1}$ and (0.27 $\pm$ 0.03) to (0.34 $\pm$ 0.10) $\times~10^{-5}~{cm}^{-2}~{s}^{-1}$, respectively.

Figures (13)

• Figure 1: Schematic diagram of the detector and electronics system, illustrating the signal path from each 3He proportional counter through preamplifiers, shaping amplifiers, digitizers, and data acquisition modules.
• Figure 2: Simulated neutron energy response functions for each spectrometer module, calculated using MCNPX. Solid lines represent pure HDPE moderators, while dashed lines denote HDPE/Cu composite moderators with enhanced high-energy sensitivity.
• Figure 3: Energy spectra measured by two 3He detectors, CKB010 (top) and CKB070 (bottom), in underground (blue) and overground (red) locations. The spectra are normalized to daily count rates (counts/day/16keV). Vertical dashed lines indicate the boundaries at 160 keV, 800 keV, and 1.44 MeV, marking analysis regions.
• Figure 4: Summed energy spectrum from all ten 3He detectors during the cadmium-shielded background run. Distinct $\alpha$ peaks from 226Ra , 210Po , 222Rn , and 218Po are visible. The small 764 keV peak indicates residual capture of epithermal neutrons penetrating the shield.
• Figure 5: (Left) Layout of the Yemilab indicating the three measurement sites (yellow dots). (Right) Experimental setup at Site 2 showing the arranged detector modules.