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Low-altitude measurements of the neutron background

Low-altitude neutron measurements were made to better define the neutron background in airborne radiation detection systems. An early publication shows a little-known effect that occurs in the thermal and epithermal component of the neutron flux at approximately 150-300 ft (~90m). Both components of the neutron flux decrease as they approach 150 ft in altitude and increase with further increases in altitude. This work better defines the altitude dependence of the neutron flux near the ground. Unmoderated He-3 tubes were used to measure the thermal neutron background component, and a EJ-299 plastic pulse shape discrimination (PSD) detector was used to measure the fast neutron background component. The direct energy measurements were also some of the first altitude measurements made with this type of neutron detector. Measurements were made using a 6 ft tethered helium-filled balloon at altitudes of 50 to 500 ft (~150 m) in 50 ft increments. Measurements above 500 ft were made on board the Navy's MZ-3A (Figure 1) airship. The results show reasonable agreement to simulations performed in SoftWare for Optimization of Radiation Detectors (SWORD), which interfaces with MCNP6. The simulated results for the thermal neutron rates agreed with the experimental measurements showing an increase in the count rate near the air/ground interface.