Measurement of secondary cosmic-ray neutrons near the geomagnetic North Pole

• Published in: Journal of environmental radioactivity 2019-03, Vol.198, p.189-199
• Main Authors: Woolf, Richard S., Sinclair, Laurel E., Van Brabant, Reid A., Harvey, Bradley J.A., Phlips, Bernard F., Hutcheson, Anthony L., Jackson, Emily G.
• Format: Article
• Language: English
• Subjects: Cosmic rays; Cosmogenic neutrons; Geomagnetic pole; Neutron detectors; Secondary cosmic rays
• ISSN: 0265-931X; 1879-1700
• DOI: 10.1016/j.jenvrad.2019.01.001

The spectrum of cosmogenic neutrons at Earth's surface covers a wide energy range, from thermal to several GeV. The flux of secondary neutrons varies with latitude, elevation, solar activity, and nearby material, including ground moisture. We report the results from a campaign to measure count rates in neutron detectors responding to three different energy ranges conducted near the geomagnetic North Pole at CFS Alert, Nunavut, Canada (82.5°N, 62.5°W; vertical geomagnetic cutoff rigidity, RC = 0 GV) in June of 2016. In November 2016, we performed a follow-on measurement campaign in southern Canada at similar RC (1.5 GV) and elevations. We conducted these measurements, at varying elevation and ground moisture content, with unmoderated and moderated 3He detectors for thermal and epithermal-to-MeV sensitivity, and with EJ-299-33 pulse shape discrimination plastic scintillator detectors for fast neutrons. Background gamma rays were monitored with NaI(Tl) detectors. Using these data sets, we compared the measured count rates to a predictive model. This is the first ever data set taken from this location on Earth. We find that for the thermal and epithermal-to-MeV neutron measurements the predictive model and data are in good agreement, except at one location on rock-covered ground near 1 km elevation. The discrepancy at that location may be attributable to ground moisture variability. Other measurements, during this campaign and prior, support the assertion that ground moisture plays a critical role in determining neutron flux. •We measured the background neutron rate over many decades of energy near the North Pole.•Measurements were compared to the results from a predictive flux model.•Neutron measurements are in agreement with the model, except at one location on rock-covered ground near 1 km elevation.•Local ground moisture variations have an impact on measured neutron flux.