Neutron spectrum measurement in the Yemi underground laboratory

This paper reports on the measurement of neutron energy spectra at the Yemi Underground Laboratory using a high-sensitivity spectrometer consisting of $^{3}\text{He}$ proportional counters, providing quantified thermal and fast neutron fluence rates after accounting for internal $\alpha$-backgrounds.

The Gist

The Invisible "Static" in the Deep Dark: A Simple Guide to the Yemilab Neutron Study

Imagine you are trying to listen to a very faint, beautiful whisper from a distant star in the middle of a crowded, noisy football stadium. Even if you wear the best headphones in the world, the roar of the crowd—the shouting, the music, the stomping—will drown out that whisper.

In the world of cutting-edge physics, scientists are looking for "whispers" called Dark Matter and Neutrinoless Double Beta Decay. These are incredibly rare, tiny signals that could explain how our universe works. To hear them, scientists build massive detectors deep underground, hoping the earth acts like a giant soundproof wall to block out the "noise" from space.

But there is a problem: The Earth itself is noisy.

This paper describes how scientists went into a brand-new underground laboratory in South Korea, called Yemilab, to measure a specific kind of "noise" called neutrons.

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1. The Problem: The Earth is "Glowing" with Invisible Particles

Even though we can't see it, the rocks and the concrete walls of the underground tunnels are slightly radioactive. They are constantly spitting out tiny particles called neutrons.

Think of these neutrons like invisible static on a radio. If a scientist is looking for a Dark Matter signal, a neutron might hit their detector and look exactly like Dark Matter. It’s a "fake signal." If we don't know exactly how much "static" is in the room, we might mistake the noise for a discovery.

2. The Tool: The "Neutron Net"

To measure this noise, the researchers built a specialized tool. Imagine a series of different-sized sieves or nets.

• Some nets are small and fine to catch "slow" neutrons (thermal neutrons).
• Some nets are large and heavy to catch "fast, energetic" neutrons.

They used ten high-tech tubes filled with a special gas (Helium-3). When a neutron hits the gas, it creates a tiny electrical spark. By measuring these sparks with different "nets" (called moderators), the scientists can figure out not just how many neutrons are there, but also how fast they are moving.

3. The Cleaning Process: Removing the "Internal Static"

The researchers ran into a tricky issue: even the metal boxes holding their detectors were slightly "noisy" because of tiny impurities in the steel. It was like trying to listen to a radio, but the radio itself was making a buzzing sound.

To fix this, they did a "background check." They put their detectors in a special shielded box to block out all the outside neutrons, leaving only the "internal buzz" of the metal. Once they knew exactly what that internal buzz sounded like, they could mathematically "subtract" it from their final results. It’s like using noise-canceling headphones to silence the hum of an air conditioner so you can hear the music.

4. The Discovery: Not All Tunnels are Equal

The scientists measured three different spots in the Yemilab. They found that one spot (Site 2) was significantly "noisier" than the others. Why? They have a few theories:

• The Concrete Factor: The concrete used to line that specific tunnel might have had more natural radioactive elements in it (like a slightly louder wall).
• The Humidity Factor: It was summer during the test. High humidity can act like a "buffer," slowing down neutrons and changing the type of noise they make.
• The Human Factor: People were moving equipment around in that area, which might have stirred things up.

5. Why does this matter?

The researchers compared Yemilab to other famous underground labs around the world (like those in Italy or China). They found that Yemilab is a relatively "quiet" place, which is great news!

By creating this "map of the noise," they have provided a blueprint for silence. Now, when the next generation of scientists arrives at Yemilab to hunt for the secrets of the universe, they will know exactly what kind of "static" to expect and how to tune their "radios" to hear the whispers of the cosmos.

Technical Summary

Technical Summary: Neutron Spectrum Measurement in the Yemi Underground Laboratory

1. Problem Statement

In rare-event physics experiments—such as dark matter searches, neutrinoless double beta decay studies, and low-energy neutrino detection—neutrons represent a critical background source. Neutrons produced by natural radioactivity in surrounding rock (via $(\alpha, n)$ reactions and spontaneous fission) or by muon-induced spallation can mimic or obscure the extremely rare signals being sought. To design effective shielding and interpret experimental data, it is vital to have an accurate, high-precision characterization of the ambient neutron energy spectra and fluence rates in deep underground environments.

2. Methodology

The researchers developed and deployed a high-sensitivity neutron spectrometer at the newly established Yemi Underground Laboratory (Yemilab) in South Korea (depth $\approx$ 2,500 m.w.e.).

• Instrumentation: The spectrometer utilizes ten high-efficiency $^3\text{He}$ proportional counters. To cover a wide energy range, eight of these are embedded in modular high-density polyethylene (HDPE) moderators of varying sizes. Two "bare" detectors are used for direct thermal neutron measurements. Two specialized composite moderators (HDPE/Cu/HDPE) were designed to enhance sensitivity to high-energy neutrons ($>20$ MeV).
• Background Mitigation: A major challenge in underground spectrometry is the internal $\alpha$-background from trace $^{238}\text{U}$ and $^{232}\text{Th}$ in the detector's stainless-steel housings. The team performed dedicated background measurements using a cadmium-shielded box to isolate and characterize these internal decays, allowing for precise subtraction during data analysis.
• Data Processing:
  • Event Selection: Waveform-based analysis was used to discriminate neutron-induced signals from electronic noise and microphonics, specifically using pulse width ($\Delta T_{DT50-RT50}$) as the primary discriminator.
  • Spectral Unfolding: The neutron energy spectra were reconstructed from the measured pulse-height distributions using the Maximum Entropy method (MAXED), utilizing response functions derived from MCNPX Monte Carlo simulations.
• Measurement Sites: Data were collected at three distinct locations (Site 1, 2, and 3) within Yemilab over a total live time of 196.1 days between March and October 2023.

3. Key Contributions

• Enhanced Sensitivity: The new spectrometer provides approximately ten times higher sensitivity compared to previous Bonner Sphere Spectrometer systems used in Korea, significantly reducing statistical and systematic uncertainties.
• Internal Background Characterization: The study provides a robust methodology for quantifying and correcting internal $\alpha$-backgrounds in $^3\text{He}$ proportional counters, which is essential for long-duration underground measurements.
• Baseline Data for Yemilab: This work establishes the first comprehensive neutron background profile for the Yemilab facility, which is critical for the upcoming generation of rare-event search experiments.

4. Results

• Neutron Fluence Rates: The total neutron fluence rates were measured to be between $(3.24 \pm 0.11)$ and $(4.01 \pm 0.10) \times 10^{-5} \text{ cm}^{-2}\text{s}^{-1}$.
  • Thermal component ($10^{-3}–0.5$ eV): $(1.32 \pm 0.05)$ to $(1.51 \pm 0.05) \times 10^{-5} \text{ cm}^{-2}\text{s}^{-1}$.
  • Fast component (1–10 MeV): $(0.27 \pm 0.03)$ to $(0.34 \pm 0.10) \times 10^{-5} \text{ cm}^{-2}\text{s}^{-1}$.
• Site Variations: Site 2 exhibited a total neutron fluence approximately 25% higher than Sites 1 and 3. This was primarily due to an elevated epithermal neutron flux. The authors attribute this to:
  1. Thicker shotcrete layers at Site 2 containing higher concentrations of U/Th.
  2. Increased seasonal humidity during the measurement period, which enhances neutron moderation.
  3. Higher laboratory activity (personnel and material movement) at Site 2.
• Comparative Analysis: The neutron levels at Yemilab are lower than those previously measured at the YangYang Underground Laboratory but are higher than those at deeper facilities like LNGS or Modane. This confirms that neutron background in these environments is driven more by local radiopurity (rock and lining materials) than by physical overburden depth.

5. Significance

This research provides the essential "baseline" environmental data required for the scientific community to plan and execute high-sensitivity dark matter and neutrino experiments at Yemilab. By accurately mapping the neutron energy spectrum, researchers can optimize radiation shielding, refine detector sensitivity models, and ensure that future discoveries are not obscured by ambient radiogenic backgrounds.