Measurements of the neutron background for the Colorado Underground Research Institute (CURIE)
This paper presents the first direct measurements of the radiogenic and muon-induced neutron background at the shallow-underground Colorado Underground Research Institute (CURIE), characterizing the local neutron flux to provide essential quantitative input for planning future low-background experiments.
Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
Deep beneath the Earth's surface, hidden inside solid rock, scientists build laboratories to hunt for the universe's most elusive secrets. These experiments search for rare events, such as particles that might make up dark matter or the mysterious decay of atomic nuclei that could explain why the universe exists. To see these faint signals, the experiments must be shielded from the constant rain of cosmic rays—high-energy particles from space that bombard the Earth's atmosphere and create a noisy background of radiation. While thick layers of rock are excellent at stopping most of this radiation, they cannot stop everything. A specific type of radiation, the neutron, is particularly troublesome because it can slip through heavy shielding and mimic the very signals scientists are trying to find. Understanding exactly how many of these neutrons are present in a specific underground location is therefore critical; without this knowledge, researchers cannot distinguish between a genuine discovery and a false alarm caused by the environment itself.
A team of researchers recently took the first direct measurements of this neutron background at the Colorado Underground Research Institute, a facility located in an old mine in Idaho Springs, Colorado. This site sits under roughly 400 meters of rock, which is deep enough to block most cosmic rays but shallow enough to be accessible for testing new detector technologies. The scientists faced a dual challenge: the underground environment is filled with two very different kinds of neutrons. The first type comes from the natural radioactivity of the rock itself, which constantly emits low-energy neutrons. The second type is created when the few remaining cosmic rays that penetrate the rock smash into the rock's atoms, creating high-energy neutrons. Because these two types behave differently and require different tools to catch, the team used two distinct detectors to map the invisible landscape of the mine.
To measure the low-energy neutrons produced by the rock, the team deployed a custom-built device known as a Bonner Cylinder Spectrometer. This instrument consists of a central tube filled with helium gas, surrounded by six different-sized cylinders made of a dense plastic that acts as a moderator, slowing down fast neutrons so they can be detected. By placing the detector in various orientations and using different plastic thicknesses, the researchers could count how many neutrons arrived at different energy levels. They carefully calibrated their equipment using a known neutron source and simulated the detector's behavior on computers to ensure their counts were accurate. The results revealed a total flow of low-energy neutrons that is dominated by the natural radioactivity of the surrounding rock. In this environment, the neutrons with intermediate energies—those that have been slowed down but not yet stopped—make up the majority of the background, accounting for more than half of the total count. This finding confirms that for experiments at this depth, the local geology is the primary source of low-energy noise.
For the high-energy neutrons created by cosmic rays, the team switched to a different tool: a tank of organic liquid scintillator. This detector works by capturing the flash of light produced when a fast neutron hits an atom inside the liquid. Because these flashes look slightly different depending on whether they were caused by a neutron or by other common radiation like gamma rays, the researchers used a technique called pulse shape discrimination to separate the two. They ran this detector for over 1,400 hours, patiently waiting for the rare high-energy events. After filtering out the background noise and correcting for the detector's efficiency, they identified 55 neutron candidates. This count translates to a specific flow of high-energy neutrons entering the cavern from the rock boundary. When they compared this measured number to previous computer simulations of the same site, they found the real-world measurement was about 12 percent higher than the prediction, though still well within the range of uncertainty. This suggests that while the models are generally accurate, there is still room to refine how we calculate these high-energy backgrounds.
By combining these two measurements, the researchers have created a complete picture of the neutron environment at the Colorado Underground Research Institute. They have shown that the background is a mix of a steady, low-energy hum from the rock and a much quieter, but more penetrating, trickle of high-energy particles from space. These numbers are not just abstract data points; they provide the essential baseline that future experiments will need to design their shielding and interpret their results. Without knowing exactly how many neutrons are present and what energies they carry, scientists cannot be sure if a signal they detect is a new particle or just the familiar chatter of the Earth itself. This work ensures that the next generation of experiments in this mine, and others like it, will be built on a foundation of precise, measured reality rather than just theoretical estimates.
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