Publications

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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).

We describe an assembly of detectors that quantifies the background radiation present at potential above ground antineutrino detector development and deployment sites. Antineutrino detectors show great promise for safeguard applications in directly detecting the total fission rate as well as the change in fissile content of nuclear power reactors. One major technical challenge that this safeguard application must overcome is the ability to distinguish signals from antineutrinos originating in the reactor core from noise due to background radiation created by terrestrial and cosmogenic sources. To date, existing detectors increase their ability to distinguish antineutrino signals by being surrounded with significant shielding and being placed underground. For the safeguard's agency, this is less than optimal, increasing the overall size and limiting the placement of this system. For antineutrino monitoring to be a widely deployable solution, we must understand the backgrounds found above ground at nuclear power plants that can mimic the antineutrino signal so that these backgrounds can be easily identified, separated, and subtracted rather than shielded. The design, construction, calibration, and results from the deployment of these background detectors at a variety of sites will be presented.

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When searching for SNM simply designing a better detector to optimize the signal S from the source is not enough. It is important to know the background B to maximize S/N, where N is the noise in B. Cosmic rays are a dominant source of neutron background. It is therefore important to know their flux, angular and energy distribution. Over the last 50 years work has been done to study cosmic ray neutrons and their variation. The full hemispherical neutron flux is usually quoted at a certain altitude (e.g. Altitude = 0 meters above sea level, pressure = 1033 g/cm2) and geomagnetic rigidity (e.g. GMR = 1.2GV). Neutron fluxes at other locations are scaled from the sea level data using a well determined prescription. However, there is a lack in knowledge of the angular dependence of the neutron flux at sea level. The angular dependence is important for two reasons; first many detectors have an efficiency that changes with the direction of the incident neutron. Second none of the measurements to date have determined how the flux changes with angle, their data must be modeled to estimate the full hemispherical flux. In this paper we present the cosmic neutron background flux measured by a neutron scatter camera in the energy range 0.2-10MeV. Our measurements are in agreement with the best fit to past data. We present for the 1st time the neutron zenith angle dependence at fission energies which is observed to be a function of the form cos 2.7⊖. ©2007 IEEE.

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