Long exposure mixed-field imaging of a shielded neutron source

The advent of real-time neutron - gamma-ray discrimination in liquid scintillators has allowed neutron and gamma-ray sources to be imaged simultaneously with a single detector. Exploiting the high penetration of fast-neutrons and their correlation with fissionable materials it may be possible to locate nuclear materials where a significant amount of moderating shielding is present using a passive, stand-off, long exposure imaging method. This scenario frequently presents itself in spent fuel ponds, or in nuclear accident situations such as the flooded BWRs at the Fukushima Daiichi site. Applications may also be relevant in medical imaging, and radiotherapy. The method described allows the independence of neutron imaging, and as a result could be successfully used to effect even in environments with high gamma-ray activity. The imaging system comprises a compact, lightweight probe containing a single, motor positioned, collimated detector; a radiation discrimination unit, data acquisition electronics and a laptop PC. The system is operated remotely and automatically accumulates data over several weeks in the case of this long exposure image. The location of radiation sources is determined from back-projection by an Algebraic Reconstruction Technique (ART) algorithm used in conjunction with the position of the collimator and the observed flux at each position. The imaging system was deployed to image a ²⁵²Cf source (1.0*10⁷ neutrons per second) located in a central position of a 1 m³ water bath. On the exterior, the shielding is sufficient to reduce the dose rate to below 10 μSv/h. Image data was collected for a segment of a full image, the background radiation levels were also monitored during these measurements. Preliminary data suggest that a long exposure image, taken over several weeks, should be sufficient to image the neutron distribution inside the tank and determine the location of the Cf source, all of which will be presented in the full paper. This document is made up of an abstract and the slides of the presentation. (author)