Date of Award

Spring 2006

Document Type

Restricted Thesis

Terms of Use

© 2006 Timothy W. Cronin. All rights reserved. Access to this work is restricted to users within the Swarthmore College network and may only be used for non-commercial, educational, and research purposes. Sharing with users outside of the Swarthmore College network is expressly prohibited. For all other uses, including reproduction and distribution, please contact the copyright holder.

Degree Name

Bachelor of Arts

Department

Physics & Astronomy Department

First Advisor

Amy Lisa Graves

Abstract

Positronium (Ps), the bound state formed by a positron and an electron, serves as a valuable probe of void spaces, particularly in materials with irregular porosity such as glasses, low-k dielectrics, and polymers. One experimental method, Positron Annihilation Lifetime Spectroscopy (PALS), relies on the ability to correlate pore size with Ps lifetime for triplet orthopositronium (0-Ps). A purely theoretical model, developed by Tao and verified by Eldrup (thus called the Tao-Eldrup or TE model) has been shown to fit lifetime vs. pore size curves well for pore radii less than ~ 1nm. In this thesis, we develop a new correspondence between TE models in different geometries; our findings explain in part why the spherical TE model has been robust for other pore shapes. Nonetheless, the TE model and many of its extensions are rather unrealistic, for they do not take into account the two-particle nature of Ps. In this thesis, we undertake a type of computer simulation of fully two-particle Ps called Path Integral Monte Carlo (PIMC). Our simulation results allow us to compare lifetimes of Ps in cylindrical void spaces with PALS experiments and TE models. We find a systematic deviation of our results from both experiment and model, suggesting that we (and the positron community) need to move towards more realistic models of both electronic density and Ps-cavity potential near the pore walls. We also undertake simulation of Ps in cylindrical pores filled with Ar atoms; we find that our annihilation rate vs. Ar density curve has a shape qualitatively similar to that of Ps in bulk Ar. We also observe the tendency of Ps to self-trap in an Ar bubble as the density of Ar in the pore increases. In addition to more realistic modeling of pore walls, future work should also emphasize streamlining the code in order to allow for better comparison of simulation and experiment.

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