SNM Detection with a Large Water Cerenkov Detector

摘要

Special Nuclear Material (SNM) can either spontaneously fission, or be induced to do so.Either case results in neutron emission. Since neutrons are highly penetrating anddifficult to shield, they could, potentially, be detected escaping even a well shielded cargocontainer. Obviously, if the shielding is sophisticated, detecting it would require a highlyefficient detector with close to 4π solid angle coverage. Water Cerenkov detectors maybe a cost effective way to achieve that goal if it can be shown that the neutron capturesignature is large enough and if sufficient background rejection can be employed asdetectors get larger. In 2008 the LLNL Advanced Detector Group reported the successfuldetection of neutrons with a ? ton gadolinium doped water Cerenkov prototype. Wehave now built a 4 ton version. This detector is not only bigger, it was designed withphoton detection efficiency in mind from the beginning. We are employing increasedphotocathode coverage and more reflective walls, coated with PTFE. The increasedefficiency should allow better energy resolution. We expect that the better diffusive wallreflectivity will reduce the overall dependence of the detector response on particledirection, again producing a more consistent response. We also believe that as detectorsget larger, both uncorrelated and correlated backgrounds due to gamma-rays and cosmicray interactions near the detector will increase. To prove the effectiveness of thetechnology we must develop new ways to reject these backgrounds while maintaining oursensitivity to SNM neutrons. Better energy resolution will enable us to reject more of thelow energy gamma-ray backgrounds on this basis. Overcoming cosmic ray inducedneutrons is perhaps an even larger concern as detectors get larger. Our detector isdesigned so that we can test various segmentation schemes – effectively dividing thedetector up into smaller ones. In this presentation, we will describe our detector in detail.