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D. Strohmeyer, "Feasibility Study of a Portable Coupled 3He Detector with LaBr3 Gamma Scintillator for Field Identification and Quantification of Nuclear Material," M.S. Thesis, Health Physics, Texas A&M University, College Station, TX (2010).



In recent years, there have been several research endeavors to increase the ability to identify and quantify special nuclear material in field measurements. These have included both gamma spectroscopy and neutron coincidence systems that are portable and work in a variety of environments. In this work, a Monte Carlo Neutral Practicle X (MCNPX) model was used to design an instrument that includes four gamma detection slabs placed within four neutron detection slabs. The combination of gamma spectroscopy and neutron coincidence counting in a single instrument allows for direct measurement of plutonium (Pu) mass without need for assumptions or operator declarations. A combined neutron-gamma instrument was designed for use in characterizing and quantifying Pu in field samples. This detector consists of a plastic scintillator containing LaBr 3 nanoparticles and a polyethylene slab containing four 3He  tube detectors. The system was tested via simulation with MCNPX for four Pu samplesof known quality and quantity. These samples had masses ranging from 100-300 g of Pu.mass to within 3.5% accuracy and to characterize the isotopic content of the Pu to within 2% accuracy for all isotopes except for 238Pu and 242Pu. The system could determine 238 Pu isotopic content to within 14% accuracy but is completely unable to determine 242 Pu content. This system has the ability to Four Plutonium (Pu) samples of known quantity were modeled and tested to determine what data was available from each individual signature. Each model included a separate MCNPX deck for each individual isotope that contributes to the gamma signature in photon mode and a spontaneous fission and ( α,n) deck for the neutron signature. The first three samples were used to create spectrums and efficiency curves for each odd isotope as well as for a Pu effective mass for the neutron signature. The data from these simulations were then used to identify the isotopics in the fourth sample to within acceptable accuracy. From this data, a total Pu mass was obtained as well as an ability to determine the ratio of ( α,n) to  spontaneous fission neutrons without additional simulations. This provides a new method to detect and identify the Pu content within a sample without producing requiring supplemental additional information since isotopics can be determined with the combined use of the gamma and neutron systems.

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Associated Project(s):

  • Safeguards Instrumentation Development for Measuring Pu via Coincident Neutron-Gamma Signals
  • SHIELD (Smuggled HEU Interdiction through Enhanced anaLysis and Detection): A Framework for Developing Novel Detection Systems Focused on Interdicting Shielded HEU

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