Measurement of Neutron Background at the Pyhasalmi mine for CUPP Project, Finland

TL;DR

This study measures the natural neutron background at various depths in the Pyhasalmi mine to support underground experiments, revealing flux levels and energy spectra crucial for background understanding.

Contribution

It provides the first detailed measurement of neutron background at multiple depths in the Pyhasalmi mine using a specialized spectrometer, advancing background characterization for underground physics.

Findings

Neutron fluxes range from 10^{-7} to 10^{-6} cm^{-2}s^{-1} above 1 MeV at different depths.

Energy spectra of neutrons extend up to 25 MeV, varying with depth.

Background levels are characterized for future underground experiments.

Abstract

A natural neutron flux is one of significant kind of background in high-sensitive underground experiments. Therefore, when scheduling a delicate underground measurements one needs to measure neutron background. Deep underground the most significant source of neutrons are the U-Th natural radioactive chains giving a fission spectrum with the temperature of 2-3 MeV. Another source is the U-Th alpha-reactions on light nuclei of mine rock giving neutrons with different spectra in the 1-15 MeV energy region. Normal basalt mine rocks contain 1 ppm g/g of U-238 and less. Deep underground those rocks produce natural neutron fluxes of 10^{-7} - 10^{-6} cm^{-2}s^{-1} above 1 MeV. To measure such a background one needs a special techniques. In the Institute for Nuclear Research, Moscow, the neutron spectrometer was developed and built which is sensitive to such a low neutron fluxes. At the end of 2001 the collection of neutron data at the Pyhasalmi mine was started for the CUPP project. During 2002 the background and rough energy spectra of neutron at underground levels 410, 660, 990 and 1410 m were measured. The result of the measurement of the neutron background at different levels of the Pyhasalmi mine is presented and discussed. Data analysis is performed in different energy ranges from thermal neutrons up to 25 MeV and above.