Nuclear Engineering ETDs

Publication Date

Summer 7-1-2017

Abstract

Neutron irradiation damages material by atomic displacements. The majority of these damage regions are microscopic and difficult to study, though they can cause a change in density and thus a change in refractive index in transparent materials. This work utilized CaF2 crystals to track refractive index change based on neutron radiation dose. High precision refractive index measurements were performed utilizing a nested-cavity mode-locked laser where the CaF2 crystal acted as a Fabry-Pérot Etalon (FPE). By comparing the repetition rate of the cavity and the repetition rate of the FPE, refractive index change was determined. Following several irradiation experiments, the change in refractive index was measured to examine correlation between dose and the change in refractive index.

An examination of the causal effects behind refractive index change was also performed by molecular dynamics simulations, leading to a statistical determination of the threshold displacement energy (TDE) in a CaF2 crystal lattice. Additionally, damage cascade analysis suggests that the number of atomic displacements and vacancies caused by neutron irradiation increases linearly as a function of incident neutron energy. Simulations were performed for a Primary Knock-on Atom (PKA) energy range of 100 eV to 5 keV, which was the upper limit of the computing constraints for this work. This finding is in line with the irradiation induced refractive index change theory for crystalline solids.

Light irradiation of crystalline solids was performed demonstrating noticeable trends in the change of refractive index when correlated to absorbed dose. Unfortunately, due to uncertainties in the data caused by several unknown factors, higher dose irradiations must be performed to confirm the trend. This type of experimental measurement of microscopic damage in the bulk of the material through the refractive index will supports several potential applications. This technique may be adopted and modified for dosimetry applications as well as being utilized as a nondestructive method for understanding microscopic neutron damage in bulk materials.

Keywords

CaF2, Radiation Damage, Refractive Index, Interferometry, Fabry-Perot Etalon, LAMMPS

Document Type

Dissertation

Language

English

Degree Name

Nuclear Engineering

Level of Degree

Doctoral

Department Name

Nuclear Engineering

First Committee Member (Chair)

Adam Hecht

Second Committee Member

Gary Cooper

Third Committee Member

Cassiano de Oliveira

Fourth Committee Member

Jean-Claude Diels

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