Physics & Astronomy ETDs

Publication Date

7-11-2013

Abstract

Active neutron interrogation is generally accepted as a reliable means of detecting the illicit transportation of special nuclear materials, in particular highly enriched uranium. The development of portable active neutron interrogation systems for field detection applications could be facilitated by the use of a new deuterium ion source which has the potential to advance many of the performance limiting aspects of exiting compact, accelerator-driven neutron generators. The ion source being investigated is a gated array of sharp metal tips that uses high electric fields to generate deuterium ion currents through the physical processes of field ionization and field desorption. The deuterium ions produced by the source are extracted and used to drive a D-D (or D-T) fusion reaction to create neutrons. The basic microstructure for the ion source array is derived from modern semiconductor microfabrication technology for field emitter arrays, though many structural modifications have been made in an attempt to reach the required operating fields of the ion generation processes. Pulsed (field desorption) and d.c. (field ionization) tests conducted with each array design type developed thus far indicate a steady improvement in array tip operating fields. Field ionization studies were conducted with arrays at source temperatures of 77 K and 293 K. Newly developed arrays have demonstrated field ionization currents upwards of ~50 nA, which is roughly 50% of the maximum ion production possible, as presently fabricated. Neutron production by field ionization was demonstrated for the first time from the microfabricated arrays. A maximum neutron yield of 95 n/s (6300 n/s/sqcm of array active area) was observed from a 1.5 sqmm array using a D-D fusion reaction at -90 kV. Field desorption studies at 77 K and 293 K were conducted in parallel with field ionization testing. To date, the arrays have consistently demonstrated the field desorption of deuterium ions from array tip surfaces. The number of deuterium ions desorbed was quantified and found to be significantly less than predicted. The low deuterium ion yields have been attributed to the presence of surface contaminants that inhibit the adsorption of deuterium. As such, thermal and hydrogen plasma cleaning methods are being investigated to condition the array tip surfaces. For both field ionization and field desorption, improved array designs that can achieve higher tip operating fields are required before predicted neutron yields (>10^9 n/s/sqcm) can be demonstrated.

Degree Name

Physics

Level of Degree

Doctoral

Department Name

Physics & Astronomy

First Advisor

Schwoebel, Paul

First Committee Member (Chair)

Dunlap, David

Second Committee Member

Boyd, Stephen

Third Committee Member

Cooper, Gary

Project Sponsors

This project was supported by the U.S. Department of Energy through the National Nuclear Security Administrations O ce of Nonproliferation and Veri cation Research and Development (NA-22).

Language

English

Keywords

Active Neutron Interrogation, Ion Source, Neutron Generator, Deuterium, Tritium, D-D D-T Fusion, Microfabrication, Field Ionization, Field Desorption

Document Type

Dissertation

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