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 LaBr3 nanoparticles and a polyethylene slab containing four 3He
tube detectors. The system was tested via simulation with MCNPX for four Pu samples
of known quality and quantity. These samples had masses ranging from 100-300 g of Pu.
It was found that the designed detector system could be used to determine 240Pu-effective 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
238Pu isotopic content to within 14% accuracy but is completely unable to determine
242Pu 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 (alpha,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 (alpha,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.
| Identifer | oai:union.ndltd.org:tamu.edu/oai:repository.tamu.edu:1969.1/ETD-TAMU-2010-05-7983 |
| Date | 2010 May 1900 |
| Creators | Strohmeyer, Daniel C. |
| Contributors | Charlton, William S. |
| Source Sets | Texas A and M University |
| Language | English |
| Detected Language | English |
| Type | Book, Thesis, Electronic Thesis, text |
| Format | application/pdf |
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