b 4 c offers a stopping power for neutrons equivalent to that of 2 6 atm of 3 he gas detector 3 fig1 b in addition to high abundance and very low cost of boron on earth the boron coated straw bcs detector offers distinct advantages over conventional 3 he based neutron detectors including sensitivity to both neutrons and gammas rays gamma rays are converted in the straw wall material low weight safety for portable use no pressurization and low cost furthermore in imaging applications the bcs high level of segmentation supports count rate capabilities and parallax free position encoding both difficult to achieve in conventional 3 he pressure vessel designs in addition the straw signal rise time of 45 nsec is about 20 times faster than that of a large diameter 3 he tube affording much higher rate per detection tube and far higher microphonic immunity in homeland security applications there is a need to detect spontaneous fission sources such as plutonium and uranium which may be smuggled into large cargo containers the energy spectrum of neutrons emitted by these sources resembles that of 252 cf a common laboratory source its spectrum peaks between 0 5 and 1 0 mev with a significant tail up to 10 mev the detection of such high energy neutrons is commonly achieved with slow neutron detectors surrounded by a moderator a hydrogen rich material like polyethylene.