Supporting Analysis for the Development of a Reconfigurable Dipole Chaff Element for the Remote Passive Detection of Neutrons

Open Access
- Author:
- Vresko, Brian Andrew
- Graduate Program:
- Nuclear Engineering
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- None
- Committee Members:
- Jack Brenizer Jr., Thesis Advisor/Co-Advisor
- Keywords:
- neutron
detection
detector
antenna
PCDA
converter
gadolinium
boron
lithium
radiosensitive
radiation - Abstract:
- The objective of thesis research was to evaluate potential candidate materials that were under consideration in the development of a new neutron detecting device. This project was inspired by a detector that consisted of reconfigurable dipole chaff elements for the remote passive detection of chemical agents. It was proposed that a similar detector could be developed to detect neutrons instead of chemicals, by modifying radiosensitive material and adding a neutron converter. The radiosensitive material that was being considered for the neutron detector design was the diacetylene, PCDA. The three different converter materials that were considered for the detector design were gadolinium metal, boron nitride, and lithium silicate. A number of important observations were made from the experiments performed in this study. The results obtained from the gamma irradiation experiments indicated that the incorporation of gadolinium in the PCDA samples increased the materials’ gamma-ray sensitivity. This increased sensitivity was not desirable, since a goal for the detector design was for it to be insensitive to background radiation. The neutron irradiation experiments showed that the radiosensitive material should not be thicker than the range of the ions or charged particles, generated by a neutron-converter interaction. Additionally, there should be a distinct layer of converter material in the detector design, instead of a distribution of converter nanoparticles in the radiosensitive material. Because the amount of neutron converter that can be incorporated in the PCDA is limited, utilizing a discrete layer of converter increases the number of converter atoms. For a given flux, this yields an increased number of neutron absorptions, and therefore increases the energy that can be emitted from the converter into the PCDA. Models of the converter and the radiosensitive material were utilized to determine the neutron interaction distributions in the converter and the reaction product interaction in the radiosensitive material. The Monte Carlo methods based computer programs CASINO and SRIM were used to model the produced electrons and heavy charged particles, respectively. Because the converters were thin, neutron interactions within the converters were modeled assuming simple exponential attenuation. The modeling indicated that different converter materials should be incorporated into the detector design depending on the design limitations and how the device is to be deployed. If boron nitride is used as the converter material in the detector design, the PCDA radiosensitive material thickness should be set to 5 microns and the boron nitride thickness should be equal to or greater than 10 microns. When gadolinium metal foil is used as the converter, the PCDA radiosensitive material should be set to 50 microns and the gadolinium thickness should be equal to or greater than 10 microns. In the case where lithium silicate is used as the converter material, the radiosensitive material thickness should be set to 50 microns and the converter material thickness should be set to 50 microns or greater. For all scenarios mentioned above, the converter material should be placed behind the radiosensitive material. This allows the thickness of the converter material to be increased without any penalty to the energy emitted from the converter material, as long as the alignment of the device is maintained. Enriching the boron and the lithium in their neutron absorbing isotopes is also recommended, since using enriched converters greatly increases the energy emitted from the converter material. It is important to note that the recommended converter thicknesses above would need to be modified if the radiosensitive material in the final detector design does not have the optimum density of 0.92 g/cm3. Based on the experimental and modeling results, enriched boron nitride or enriched lithium silicate should be used as the converter materials. If the detector design does not allow for the recommended thickness mentioned above, then a different converter material should be used based on the design limitations.