Direct measurements of cosmic-ray neutron intensity were recorded with a neutron scatter camera developed at SNL. The instrument used in this work is a prototype originally designed for nuclear non-proliferation work, but in this project it was used to characterize the response of ambient neutrons in the 0.5-10 MeV range to water located on or above the land surface. Ambient neutron intensity near the land surface responds strongly to the presence of water, suggesting the possibility of an indirect method for monitoring soil water content, snow water equivalent depth, or canopy intercepted water. For environmental measurements the major advantage of measuring neutrons with the scatter camera is the limited (60{sup o}) field of view that can be obtained, which allows observations to be conducted at a previously unattainable spatial scales. This work is intended to provide new measurements of directional fluxes which can be used in the design of new instruments for passively and noninvasively observing land-surface water. Through measurements and neutron transport modeling we have demonstrated that such a technique is feasible.
We describe the design, calibration, and measurements made with the neutron scatter camera. Neutron scatter camera design allows for the determination of the direction and energy of incident neutrons by measuring the position, recoil energy, and time-of-flight (TOF) between elastic scatters in two liquid scintillator cells. The detector response and sensitive energy range (0.5-10 MeV) has been determined by detailed calibrations using a {sup 252}Cf neutron source over its field of view (FOV). We present results from several recent deployments. In a laboratory study we detected a {sup 252}Cf neutron source at a stand off distance of 30 m. A hidden neutron source was detected inside a large ocean tanker. We measured the integral flux density, differential energy distribution and angular distribution of cosmic neutron background in the fission energy range 0.5-10 MeV at Alameda, CA (sea level), Livermore, CA (174 m), Albuquerque, NM (1615 m) and Fenton Hill, NM (2630 m). The neutron backgrounds are relatively low, and non-isotropic. The camera has been ruggedized, deployed to various locations and has performed various measurements successfully. Our results show fast neutron imaging could be a useful tool for the detection of special nuclear material (SNM).