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Neutron counting using large arrays of pressurized 3He proportional counters from an aerial system or in a maritime
environment suffers from the background counts from the primary cosmic neutrons and secondary neutrons caused
by cosmic ray‒induced mechanisms like spallation and charge-exchange reaction. This paper reports the work
performed at the Remote Sensing Laboratory–Andrews (RSL-A) and results obtained when using two different
methods to reduce the cosmic neutron background in real time. Both methods used shielding materials with a high
concentration (up to 30% by weight) of neutron-absorbing materials, such as natural boron, to remove the lowenergy
neutron flux from the cosmic background as the first step of the background reduction process. Our first
method was to design, prototype, and test an up-looking plastic scintillator (BC-400, manufactured by Saint Gobain
Corporation) to tag the cosmic neutrons and then create a logic pulse of a fixed time duration (~120 μs) to block the
data taken by the neutron counter (pressurized 3He tubes running in a proportional counter mode). The second
method examined the time correlation between the arrival of two successive neutron signals to the counting array
and calculated the excess of variance (Feynman variance Y2F)1 in the neutron count distribution from Poisson
distribution. The dilution of this variance from cosmic background values ideally would signal the presence of manmade
neutrons.2 The first method has been technically successful in tagging the neutrons in the cosmic-ray flux and
preventing them from being counted in the 3He tube array by electronic veto—field measurement work shows the
efficiency of the electronic veto counter to be about 87%. The second method has successfully derived an empirical
relationship between the percentile non-cosmic component in a neutron flux and the Y2F of the measured neutron
count distribution. By using shielding materials alone, approximately 55% of the neutron flux from man-made
sources like 252Cf or Am-Be was removed.
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