Characterisation of the Bedretto Underground Site for Fundamental Physics Experiments

TL;DR

This study performs a comprehensive environmental characterization of the Bedretto underground site to assess its suitability for fundamental-physics experiments. By measuring atmospheric muon, gamma, and neutron fluxes along with radon levels, magnetic background, and seismic activity, the work demonstrates a muon flux suppression of about $10^6$ relative to the surface, an effective depth near $4000$ m w.e., rock-dependent gamma and neutron backgrounds, and exceptionally low magnetic and seismic noise. The results, together with shielding and ventilation considerations, position Bedretto as one of Europe’s deepest and quietest, horizontally accessible underground laboratories, capable of supporting R&D and large-scale experiments in rare-event searches, quantum sensing, and gravitational-wave science. The findings provide concrete inputs for facility design, shielding strategies (e.g., shotcrete, borated materials), and long-term monitoring necessary to realize a world-class, expandable underground physics facility at TM3500.

Abstract

Underground laboratories provide the ultra-low background and low-vibration environments essential for rare-event searches, gravitational-wave detection, and quantum-sensing technologies. We report a comprehensive environmental characterisation of the Bedretto tunnel in Ticino, Switzerland, a site offering horizontal access, excellent infrastructure, and the potential to be be Europe's second-deepest and quietest underground laboratory. At the prospective physics site, located beneath an overburden exceeding 1400 m, we measure the cosmic-muon, gamma-ray, and neutron fluxes, as well as the radon concentration, magnetic-field spectrum, and seismic backgrounds. The muon flux is suppressed by six orders of magnitude relative to the surface, consistent with an effective depth of about 4000 metre water equivalent, gamma-ray and neutron measurements reflect the local geology and guide shielding requirements for future particle and nuclear physics experiments. Magnetic and seismic noise levels are found to be exceptionally low, meeting or exceeding the criteria for next-generation atom-interferometric gravitational-wave detectors. These results establish the site as a highly competitive, accessible deep-underground location for fundamental-physics experiments.

Figures (10)

• Figure 1: The Bedretto tunnel and the profile of the mountain with the different rock formations overlaid rast2022geologynasa2019aster. The black markers indicate the gamma-ray rate as measured using a 3 in NaI(Tl) detector. The red line indicates the site of the planned fundamental physics lab. Serendipitously, the laboratory's intended location, the gamma-ray flux is close to minimal while the overburden is maximal. The locations of rock niches are indicated in grey in the bottom panel.
• Figure 2: Measured muon flux rates and the equivalent water depth for different underground laboratories. The muon flux in Bedretto is suppressed by about a factor of $10^{6}$, better than almost all existing underground laboratories in Europe designated in red. Muon fluxes taken from Refs. Woodley:2024elnSoudan_MuonYemilab_MuonBoulby_MuonLSC_MuonLNGS_MuonLSM_MuonSURF_MuonCJPL_MuonSNO_Muon.
• Figure 3: Gamma-ray and muon energy spectra measured via a single muon panel in surface calibration run. The blue and red curves represent the fitted gamma-ray and muon contributions, respectively, with the combined fit shown in cyan. The orange distribution corresponds to muon events when applying a four-panel coincidence requirement. The dashed green line is the energy threshold applied to select muons.
• Figure 4: Calibrated energy spectrum from the TM3500 Bedretto granite measured in Gator. The spectra are dominated by gamma lines of the $^{238}$U and $^{232}$Th decay chains; the $^{40}$K line at 1460 keV is also seen.
• Figure 5: $^{3}$He neutron spectrum of the ambient neutron flux as measured on the surface and at TM3500. The primary neutron capture peak is visible at 765 keV. The low energy structure is from gamma and X-ray interactions with the detector.