Physics & Astronomy ETDs

Publication Date

2-9-2010

Abstract

Active interrogation systems for highly enriched uranium require improved fieldable neutron sources. The target technology for deuterium-tritium neutron generators is well understood and the most significant improvement can be achieved by improving the deuterium ion source through increased output and, in some cases, lifetime of the ion source. We are developing a new approach to a deuterium ion sources based upon the field desorption/evaporation of deuterium from the surfaces of metal tips. Electrostatic field desorption (EFD) desorbs previously adsorbed deuterium as ions under the influence of high electric fields (several \VpA), without removing tip material. Single etched wire tip experiments have been performed and have shown that this is difficult but can be achieved with molybdenum and tungsten tips. Electrostatic field evaporation (EFE) evaporates ultra thin deuterated titanium films as ions. It has been shown that several $10s$ of atomic layers can be removed within a few nanoseconds from etched tungsten tips. In the course of these studies titanium deposition and deuteration methods were studied and new detection methods developed. Space charge effects resulting from the large ion currents were identified to be the most likely cause of some unusual ion emission characteristics. In addition, on W $<110>$ oriented substrates a surprising body-centered cubic crystal structure of the titanium film was found and studied. The ion currents required for neutron generator applications can be achieved by microfabrication of metal tip arrays. Field desorption studies of microfabricated field emitter tip arrays have been conducted for the first time. Maximum fields of $3$ \VpA have been applied to the array tip surfaces to date, although fields of $\sim2$ \VpA to $\sim2.5$ \VpA are more typical. Desorption of atomic deuterium ions has been observed at fields of roughly $2$ \VpA at room temperature. The desorption of common surface adsorbates, such as hydrogen, carbon, water, and carbon monoxide is observed at fields exceeding $\sim1$ \VpA . In vacuo heating of the arrays to temperatures of the order of $800$ \DegC can be effective in removing many of the surface contaminants observed. For both the field desorption and the field evaporation approaches further improvements to array design and fabrication are required if arrays are to provide sufficient deuterium ion currents to produce $10^9$ to $10^{10}$ n/cm$^2$ of tip array area for the detection systems.

Degree Name

Physics

Level of Degree

Doctoral

Department Name

Physics & Astronomy

First Committee Member (Chair)

Dunlap, David

Second Committee Member

Bassalleck, Bernd

Third Committee Member

Panitz, John

Fourth Committee Member

Brueck, Steven

Project Sponsors

US Department of Energy National Nuclear Security Administration NA\xad22 Office, Robert Mayo, PM Defense Threat Reduction Agency contract HDTRA1\xad07\xad1\xad0036, Major Brad Beatty, PM

Language

English

Keywords

Uranium enrichment, Neutron sources, Deuterium ions, Electron-stimulated desorption.

Document Type

Dissertation

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