The specific total and coincidence cosmic-ray-induced neutron production rates in materials

The specific neutron production rate, αT, resulting from the interaction of cosmic rays on targets of C, Al, Fe, Cu, Pb, Ti, Sn and H2O has been measured and found to increase systematically as a function of the molar mass, A, of the target. A power law dependence of the form αT=kAx was used to describe the specific yield function. The best-fit value of x was 0.55±0.13. The same experimental arrangement generated results for the specific coincidence neutron production rate, αR, which is directly proportional to the second factorial moment of the emitted neutron multiplicity distribution. αR was also well represented by a power law with an exponent value of 1.85±0.14. Absolute production rates vary with location (latitude, altitude, overhead shielding) and time (seasonal, solar activity and atmospheric pressure dependence); however, the scaling rules are expected to remain valid. These results are applicable to estimating matrix-dependent background rates in low-level passive neutron assay of plutonium.