Woolf Richard S, Sinclair Laurel E, Van Brabant Reid A, Harvey Bradley J A, Phlips Bernard F, Hutcheson Anthony L, Jackson Emily G
Space Science Division, U. S. Naval Research Laboratory, 4555 Overlook Ave., SW, Washington, DC, 20375, USA.
Natural Resources Canada, Government of Canada, 930 Carling Ave (CEF, Bldg 7, Observatory Cr.), Ottawa, Ontario, K1A 0Y3, Canada.
J Environ Radioact. 2019 Mar;198:189-199. doi: 10.1016/j.jenvrad.2019.01.001. Epub 2019 Jan 11.
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, R = 0 GV) in June of 2016. In November 2016, we performed a follow-on measurement campaign in southern Canada at similar R (1.5 GV) and elevations. We conducted these measurements, at varying elevation and ground moisture content, with unmoderated and moderated He 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.
地球表面宇宙成因中子的能谱覆盖了很宽的能量范围，从热中子到几个GeV。次级中子的通量随纬度、海拔、太阳活动以及包括地面湿度在内的附近物质而变化。我们报告了2016年6月在加拿大努纳武特地区弗罗比舍湾站（北纬82.5°，西经62.5°；垂直地磁截止刚度，R = 0 GV）地磁北极附近进行的一项测量活动的结果，该活动测量了响应三种不同能量范围的中子探测器的计数率。2016年11月，我们在加拿大南部以相似的R值（1.5 GV）和海拔进行了后续测量活动。我们使用未慢化和慢化的氦探测器来测量热中子和超热中子至MeV的灵敏度，以及使用EJ - 299 - 33脉冲形状甄别塑料闪烁体探测器来测量快中子，在不同海拔和地面湿度条件下进行了这些测量。用碘化钠（铊）探测器监测背景伽马射线。利用这些数据集，我们将测量的计数率与一个预测模型进行了比较。这是首次从地球上这个地点获取的数据集。我们发现，对于热中子和超热中子至MeV的测量，预测模型与数据吻合良好，只有在海拔近1 km的岩石覆盖地面的一个位置除外。该位置的差异可能归因于地面湿度的变化。在此次活动及之前的其他测量结果支持了地面湿度在确定中子通量方面起关键作用这一论断。
