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TitleMeasurement of secondary cosmic-ray neutrons near the geomagnetic North Pole
AuthorWoolf, R S; Sinclair, L EORCID logo; Van Brabant, R A; Harvey, B J A; Phlips, B F; Hutcheson, A L; Jackson, E G
SourceJournal of Environmental Radioactivity vol. 198, 2019 p. 189-199, Open Access logo Open Access
Alt SeriesNatural Resources Canada, Contribution Series 20170337
PublisherElsevier BV
Mediapaper; on-line; digital
File formatpdf
AreaCanadian Forces Station Alert
Lat/Long WENS -62.3500 -62.3333 82.5000 82.4833
Subjectsgeophysics; Science and Technology; Nature and Environment; geomagnetic fields; isotopes; helium; modelling; soil moisture; solar variations; elevations
Illustrationsplots; photographs; bar graphs; tables; time series
ProgramCanadian Hazard Information Service
Released2019 01 11
AbstractThe 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.
Summary(Plain Language Summary, not published)
The motion of charged particles is affected by magnetic fields. Therefore one might expect the rate of cosmic rays at the earth's surface to be different at the poles, where the magnetic field lines are nearly vertical, from the cosmic ray rate at lower latitudes. We measured cosmic neutron count rates near the geomagnetic north pole and compared this to expectations extrapolated from lower latitudes. This is the first measurement of cosmic neutron count rates from near the north pole. There is one observation at high altitude which is different from the model prediction. This difference may or may not be explainable by a dependence of the cosmic neutron rate on soil moisture. The effect of soil moisture should be included in future versions of the model.

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