An Underground Particle Physics Lab in Idaho Springs, Colorado

Honestly, I probably wouldn't be blogging this article if it weren't discussing a new major physics experiment facility less than an hour away from my home. It is even closer to the University of Colorado at Boulder, and the Colorado School of Mines, both of which have high quality advanced physics programs staffed by elite physics PhD professors. CU-Boulder has been the home institution of quite a few Nobel prize winners.

The problem is that cosmic rays that include muons, and other surface bound sources of background noise, prevent scientists from conducting precision studies of low-energy muon production in ordinary university laboratories on the surface.

The basic strategy at this laboratory is to remove that background noise by putting the detection instruments deep in a mine originally dug for gold mining (the high school in Idaho Spring's somewhat titillating mascot as a result of this legacy of gold mining is the "gold diggers"). Initial calibrating tests show that the cosmic muon flux deep in the mine that was detected matches what the designers of the experiment expected to a reasonable degree. 

This laboratory setting provides a relatively inexpensive space to do research without background noise from cosmic and Earth-bound sources of contaminating charged particles, which is important for high precision measurements that could be done in a university laboratory but for their background interference. It reduces background noise 700-fold despite providing shielding only equivalent to a depth of about 415 meters.

There are only five underground laboratories of this kind in North America and only about a dozen in the entire world, not including CURIE, and they are in high demand.

We present the characterization of cosmogenic muon backgrounds for the Colorado Underground Research Institute (CURIE), located in the Edgar Experimental Mine (EEM) in Idaho Springs, Colorado. 

The CURIE facility at the EEM offers a versatile shallow underground environment, with accessible horizontal tunnel access and stable rock formations ideal for low-background physics experiments. We have measured the total underground muon flux in two locations, Site 0 and Site 1, yielding values of ϕ = 0.246 ± 0.020 sys. ± 0.012 stat. and 0.239 ± 0.025 sys. ± 0.010 stat. μ/m2/s, respectively. 

We have utilized GEANT4 and PROPOSAL Monte Carlo simulations with Daemonflux and MUTE to model the muon flux at both sites, as well as an additional future location. We find good agreement between measurement and simulations, demonstrating the first instance of this computational framework being successfully used for depths < 1 km.w.e. The measured underground flux corresponds to a factor of 700 reduction compared to the sea level flux. 

Additionally, we present a new depth-intensity relationship to normalize the mountain overburden to an equivalent flat depth, enabling direct comparison with other underground facilities. We report an average equivalent vertical depth of 0.415 ± 0.027 km.w.e. Based on our measurements, this work highlights the facility's capability for hosting low-background experiments, addressing the demand for shallow underground research spaces.

Dakota K. Keblbeck, Eric Mayotte, Uwe Greife, Kyle G. Leach, Wouter Van De Pontseele, "Cosmogenic Muon Background Characterization for the Colorado Underground Research Institute (CURIE)" arXiv:2411.01626 (November 3, 2024).