Date of Award

3-22-2012

Document Type

Thesis

Degree Name

Master of Science

Department

Department of Engineering Physics

First Advisor

Larry W. Burggraf, PhD.

Abstract

The purpose of this project is to study the lifetime and annihilation mode of positrons, the antimatter counterpart to electrons, and the formation rate of positronium on a gold surface modified by gold nanoparticles under the influence of radial electric fields measured using two independent systems. The positron annihilation photon pair/triplet spectroscopy system uses a sodium iodide ring detector segmented into six detection regions providing rough position sensitivity to measure the ratio of two to three photon positron annihilation events. An increased rate of three photon annihilation events and a decreased rate of two photon annihilation events are observed when the source is exposed to an externally applied radial electric field. This change is found to be statistically significant suggesting an increased rate of positronium formation. The second system conducts positron lifetime spectroscopy on the same source and measures the effect of radial electric fields on positron lifetime. It uses barium fluoride, a fast inorganic scintillator, to measure the lifetime of a positron directly by comparing the time of arrival of a gamma ray accompanying positron birth in Na-22 with photons resultant from positron annihilation.

Prolongated positron lifetimes, found to be statistically significant, are observed in the source when exposed to electric fields. The effects on positron annihilation and lifetime observed using each system are consistent with each other and suggest that the lifetime of positrons on a gold surface can be modulated by the application of strong radial electric fields.

AFIT Designator

AFIT-NUCL-ENP-12-M03

DTIC Accession Number

ADA558538

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