[Paper Review] Measurement of Neutron Background at the Pyhasalmi mine for CUPP Project, Finland

J. N. Abdurashitov, В. Н. Гаврин+5 more|ArXiv.org|Jul 20, 2006

Nuclear Physics and Applications1 references3 citations

TL;DR

This study measures low-energy neutron backgrounds at multiple depths (400–1410 m) in the Pyhäsalmi mine, Finland, using a custom neutron spectrometer with liquid scintillator and 3He proportional counters. The key result is a detailed measurement of neutron flux across energy ranges from thermal to 25 MeV, revealing a significant increase in flux at greater depths due to U/Th decay chains and muon-induced spallation, with fluxes reaching up to 42.2 × 10⁻⁷ cm⁻²s⁻¹ at 1410 m in the 0–1.5 MeV range.

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.

Motivation & Objective

• To measure neutron background fluxes in deep underground environments to support sensitive physics experiments.
• To assess the contribution of natural U/Th radioactive chains and muon-induced spallation to neutron backgrounds.
• To validate the performance of a low-background neutron spectrometer in a real underground setting.
• To provide empirical data for background modeling in future underground experiments, such as neutrino and dark matter searches.

Proposed method

• A neutron spectrometer was developed using a liquid organic scintillator coupled to three photomultiplier tubes and 19 3He proportional counters.
• Fast neutrons were detected via proton recoil light flashes in the scintillator, followed by delayed charge pulses from 3He(n,p)t reactions.
• A coincidence technique between scintillator light pulses and 3He counter signals enabled effective suppression of gamma-ray background.
• Data acquisition used two independent channels: one for PMT signals and one for NC signals, with preamplifiers to optimize signal-to-noise ratio.
• Neutron energy was reconstructed from pulse height in the scintillator, with corrections applied for detector efficiency and dead time.
• Fluxes were calculated by subtracting correlated background events from total counts, with statistical and systematic uncertainties included.

Experimental results

Research questions

• RQ1 What is the energy-dependent neutron flux at various depths in the Pyhäsalmi mine?
• RQ2 How do U/Th decay chains and muon-induced spallation contribute to the neutron background at different depths?
• RQ3 To what extent can the spectrometer detect and distinguish low-flux neutrons in deep underground conditions?
• RQ4 How does the neutron flux vary with depth, and what factors explain anomalies at specific levels?
• RQ5 What is the contribution of high-energy neutrons (>25 MeV) and how are they produced?

Key findings

• At 1410 m depth, the neutron flux in the 0–1.5 MeV energy range reached 42.2 × 10⁻⁷ cm⁻²s⁻¹, the highest measured value, likely due to elevated U/Th contamination in granite wall coatings.
• The flux in the 1.5–3 MeV range was 10.5 × 10⁻⁷ cm⁻²s⁻¹ at 1410 m, indicating a strong depth dependence consistent with increasing radiogenic neutron production.
• A significant excess of neutrons above 25 MeV was observed at 400 m depth, attributed to muon-induced hadronic showers in lead shielding.
• In the 6–12 MeV range, fluxes were below 0.7 × 10⁻⁷ cm⁻²s⁻¹ at all depths, with the highest value of 3.3 × 10⁻⁷ cm⁻²s⁻¹ at 1410 m.
• The 3–6 MeV range showed fluxes of 1.9–3.0 × 10⁻⁷ cm⁻²s⁻¹, increasing with depth, consistent with radiogenic neutron production.
• For energies <1.5 MeV, the flux was deduced from 3He counter counts after subtracting fast neutron contributions, yielding a flux of 20.8 × 10⁻⁷ cm⁻²s⁻¹ at 660 m with 1.6 × 10⁻⁷ cm⁻²s⁻¹ uncertainty.