Directionally Sensitive Neutron Detector For Homeland Security Applications



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With an increase in the capabilities and sophistication of terrorist networks worldwide comes a corresponding increase in the probability of a radiological or nuclear device being detonated within the borders of the United States. One method to decrease the risk associated with this threat is to interdict the material during transport into the US. Current RPMS have limitations in their ability to detect shielded nuclear materials. It was proposed that directionally sensitive neutron detectors might be able to overcome many of these limitations.

This thesis presents a method to create a directionally sensitive neutron detector using a unique characteristic of 10B. This characteristic is the Doppler broadening of the de-excitation gamma-ray from the 10B(n, alpha) reaction. Using conservation principles and the method of cone superposition, the mathematics for determining the incoming neutron direction vector from counts in a boron loaded cloud chamber and boron loaded semiconductor were derived.

An external routine for MCNPX was developed to calculate the Doppler broaden de-excitation gamma-rays. The calculated spectrum of Doppler broadened de-excitation gamma-rays was then compared to measured and analytical spectrums and matched with a high degree of accuracy.

MCNPX simulations were performed for both a prototype 10B loaded cloud chamber and prototype 10B loaded semiconductor detector. These simulations assessed the detectors' abilities to determine incoming neutron direction vectors using simulated particle reactant data. A sensitivity analysis was also performed by modifying the energy and direction vector of the simulated output data for 7Li* particles. Deviation coefficients showed a respective angular uncertainty of 1.86 degrees and 6.07 degrees for the boron loaded cloud chamber and a boron loaded semiconductor detectors. These capabilities were used to propose a possible RPM design that could be implemented.