Characterizing the spatial distribution of cosmogenic background neutrons for enhanced localization of special nuclear materials via neutron signatures
The detection of special nuclear materials (SNM) requires the understanding of nuclearsignatures that allow the discrimination against background. In particular, understanding neutron background characteristics such as count rates and energies and their correlations with environmental conditions and surroundings of measurement locations is critical in enhancing SNM detection capabilities. The Mobile Imager of Neutrons for Emergency Responders (MINER) was deployed for 8 weeks in downtown San Francisco (CA) to study such neutron background characteristics in an urban environment. Of specific interest was the investigation of the impact of surrounding buildings on the neutron background count rates and to answer the question whether buildings act as absorber of neutrons or as sources via the so-called ship effect. MINER consists of 16 liquid scintillator detector elements and can be operated as a neutron spectrometer, as a neutron imager, or simply as a counter of fast neutrons. As expected, the neutron background rate was found to be inversely proportional to the atmospheric pressure. In the energy range where MINER is most sensitive, roughly 1-10 MeV, it was found that the shape of the detected background spectrum is similar to that of a detected fission spectrum, indicating the limited discrimination power of the neutron energy. The similarities between the detected background neutron spectrum and fission sources makes it difficult to discriminate SNM from background based solely on the energies observed. The images produced using maximum likelihood expectation maximization revealed that neutrons preferentially were coming from areas in the environment that had open sky. The images produced from the data showed that buildings in the area of deployment act as absorbers of neutrons and not as sources, so the ship effect was not observed. The inherent properties of a neutron scatter camera limit the achievable image quality and the effective deployment to systematically map neutron background signatures due to the low count rate.
Spatial localization of special nuclear materials (SNM) via their neutron signaturesamidst background requires knowledge of the background neutron environment or a means of separating a source from background based on low amounts of information. Neutron scatter cameras have been developed and optimized for rapid detection of high activity sources, but have low imaging efficiency, making it difficult to use them to characterize low rate diffuse sources. The Low Intensity Neutron Imaging System (LINIS) is a collimated neutron imager that has been designed and optimized for imaging diffuse cosmogenic neutron background in the energy range of 0.5-15 MeV. LINIS operates using 16 liquid scintillation detectors shielded by ultra-high molecular weight polyethylene cylindrical collimators in a staggered orientation and rotates to 7 discrete positions, giving it roughly 2 pi sensitivity. LINIS has been characterized using (alpha,n) and fission neutron sources. While designed to operate as a collimated system, LINIS can also function as a neutron scatter camera for spectral and 4 pi spatial detection when the collimators have been removed.
LINIS was deployed at Lawrence Berkeley National Laboratory (LBNL) to observethe location behavior of cosmogenic background neutron radiation for 8 weeks. The observed background neutrons were investigated for the impact of atmospheric pressure on detected rates and surrounding concrete structures on the spatial distribution of detected events. The events projected through the system response matrix revealed that neutrons were preferentially coming from high elevation locations in space. This was confirmed by comparing the MLEM iterated image with the collimated cone backprojection, which shared a similar shape. An overlay of the iterated image with a 4 pi photograph found that the regions of high neutron intensity corresponded with areas of open sky. The inverse behavior of observed neutron rates with increasing atmospheric pressure was similarly confirmed. The ability to derive location information from single event interactions, an inherent feature of a collimated imager, requires a stable calibration and knowledge of the efficiency differences between detectors. These measurements were performed prior to the deployment, but detector shifts occured when LINIS was moved from a lab setting to the deployment location causing need for a new calibration after the fact. Future deployments should take calibration measurements prior to the actual data collection and on a weekly basis.