Optimization of a compact D-D fast neutron generator for imaging applications

Abstract

In this thesis efforts of optimizing a compact deuterium-deuterium (D-D) fast neutron generator with an emphasis on neutron transmission-based imaging applications are outlined. The main objective was the development and thorough characterization of a high output neutron source with a small neutron emitting spot size. The study is divided into three main sections. In the first part a novel, water-cooled, drive-in rotating beam target was developed, designed, and implemented. The optimized beam target configuration was found using extensive computational fluid dynamics and heat transfer analyses. During experiments, no indication of loss of deuterium due to outgassing was observed with the 40 mm diameter, rotating copper rod that was coated with 5 µm of titanium. At a rotational velocity of 220 rpm the stable neutron output was increased to 2.9 E7 1/s, which was an increase by a factor of more than four compared to the neutron output with a stationary titanium target. In the second main section of this thesis the thorough characterization of the compact D-D neutron generator is presented in terms of overall neutron output and neutron emitting spot size. Both of these variables are extremely important characteristics of a neutron generator tailored to transmission-based imaging methods. Estimation of the overall neutron output was achieved by combining a detailed Monte Carlo model of the neutron source and its surroundings with the reading of an LB6411 neutron probe. The neutron emitting spot size was indirectly measured using an attenuating edge technique where a tungsten block was moved in between the path of direct neutron emission and a detector. The results of the attenuating edge measurements were benchmarked using charged particle tracing simulations with COMSOL Multiphysics. These simulations estimated the distribution of deuterium ions on the surface of the target rod. The third main section describes the design, implementation, and performance of a major design iteration of the compact D-D fast neutron generator. Charged particle tracing simulations were set up to optimize the geometry and dimensions of the neutron generator with respect to ion spot size on the target surface. The goal of further increased neutron output was realized by raising the accelerating potential limit that was achieved by improving the vacuum quality. This included the design of an optimized generator housing and adoption of an electron cyclotron resonance microwave ion source which enabled operation of a deuterium plasma at lower gas pressure levels than the previous radio frequency-based ion source design. The use of an electric field driven electron suppression using an added suppression electrode succeeded in reducing backstreaming secondary electrons from the target, which helped in reduction of high voltage breakdown probability and greatly increased stability of operation of the neutron generator. With all these efforts, the stable neutron output at -120 kV was increased in total by a factor of eight to 6 E7 1/s in comparison to what was achieved before the start of this thesis work. At the highest accelerating potential of -145 kV that was achieved without significant high voltage breakdowns the peak neutron yield was 6.8 E7 1/s. Using the attenuating edge measurement technique, the emitting spot size was determined to be in the range of 2 to 3 mm. This represents a dramatic improvement in imaging performance potential of the neutron generator compared to the starting point of this work.