|Student Name:||1st Lt Timothy Genda|
|Thesis:||OPTIMIZATION OF PROMPT NEUTRON DETECTOR PLACEMENT FOR STANDOFF PHOTON INTERROGATION OF SPECIAL NUCLEAR MATERIALS|
|Location:||Bldg 640, Room 217|
|Attendance Restrictions:||FOUO, Distribution C|
|Date & Time:||02/04/2014 at 1100|
|Abstract:|| High energy x-ray interrogation of targets of interest can be used to identify the presence of special nuclear material (SNM) at standoff distances by inducing fission and detecting the resulting radiation. This study modeled a potential high energy bremsstrahlung x-ray source and determined optimal detector placement for the detection of prompt neutrons from photo-induced fissions. At low x-ray energies, optimal detector placement is usually at backward angles in order to minimize the active background. However, at x-ray energies above 15 MeV, the x-rays begin to activate materials in the atmosphere and surrounding material, creating a complex noise signal between the beam source and the inspection object that interferes with the ability to detect neutrons at these angles. The results of this study suggest that detector placement slightly forward of shielded SNM over a variety of standoff distances will maximize the prompt neutron signal-to-noise ratio (SNR). The exact detector locations of maximum SNR are sensitive to the amount of shielding and the geometry. Temporal SNR dynamics of a 60 MeV bremsstrahlung pulse are also analyzed and yield increased neutron SNR within the first µs following the end of the interrogating pulse.