Measurement of the ambient neutron flux and spectral distribution at the Canfranc Underground Laboratory

Abstract

We report on the measurement of the ambient neutron flux and its energy distribution over a broad range of neutron energies, conducted in Hall A of the Canfranc Underground Laboratory using the High Efficiency Neutron Spectrometry Array (HENSA). In particular, we obtained a thermal neutron flux of $3.4(2) \times 10^{-6}$ cm$^{-2}$ s$^{-1}$ and a total energy-integrated flux of $14.8(2) \times 10^{-6}$ cm$^{-2}$ s$^{-1}$. This work represents the first long-term measurement of the total neutron flux, with full spectral characterization across a wide energy interval, performed in an underground laboratory to date. The results are relevant for rare-event search experiments located underground, ranging from nuclear astrophysics to astroparticle physics.

Figures (3)

• Figure 1: HENSA response to neutrons simulated with GEANT4. Moderators and covering materials are indicated.
• Figure 2: Energy spectrum of the ambient neutron flux in Hall A of LSC, obtained by deconvoluting the measured neutron rate Orr_2022 with the detector response shown in Fig. \ref{['Fig1']}, using two different algorithms Tain_2007: BAYES (red dashed line) and CHIMEM (blue solid line). The black dotted line represents the prior flux distribution Mon_2021, used as the initial solution in both iterative algorithms, with the value in the first energy bin fixed to our determined thermal flux value (see the text).
• Figure 3: Reconstructed neutron counting rate using BAYES (red dashed line) and CHIMEM (blue solid line), compared with the data from Orr_2022 (black points).