Display Full Text | Download Full Text
1.19 MB PDF file

Degree

Doctor of Philosophy, University of Toledo, Physics and Astronomy, 2008.

Abstract

The purpose of this work is to investigate thin film CdTe as a possible large area detector system to be implemented for electronic portal imaging devices (EPID) in radiation therapy application. The material properties, average atomic number of 50, high electron density, remarkable radiation hardness and high band gap, already made them popular as small-area room-temperature crystalline x-ray detectors. The objective of our project is to define the range of the parameters applicable to a thin-film polycrystalline CdTe x-ray detector for mega-voltage energy beam. Due to small thickness of the film, we plan on using photovoltaic CdTe detector in combination with a metal plate converter in order to enhance high-energy photon conversion into secondary electrons, injected from the plate into CdTe layer and resulting in higher output signal. This approach increases efficiency of the device, while minimizing signal spreading due to reduced CdTe sensor thickness. In addition, the presence of the plate helps to reduce input signal-to-noise ratio by shielding the sensor layer from the unavoidable scatter from the patient. The optimal CdTe thickness was found based on both device modeling and experimental verifications. A set of geometrical configurations appropriate for EPID application, was determined along with an approximate compromise between the sensor and metal plate thickness, detector efficiency and spatial resolution, and electrical properties of the material.Our proposed detector system operates on the same basic principle as a regular CdTe based solar cell. When irradiated with x- or gamma-rays, electron hole pairs are produced in response to energy deposition by electrons injected from the metal layer. The device will rely on semiconductor junction as the source of electric field for charge carrier separation, thus eliminating the need for external high-voltage biasing, typically used with x-ray solid state detectors. The generated photocurrent or the developed photo-voltage between the contacts will be used to measure the intensity of incident radiation. Despite the similarities to solar cells, there is a number of important differences discussed in the dissertation that have to be taken into account in the detector design. In this work we 1) present theoretical optimization of novel thin-film based CdTe x-ray detector with a metal plate converter by employing Monte Carlo software package MCNP5, 2) estimate the detector output for the optimized configuration with a thin film device modeling tool SCAPS, and 3) verify the device performance with experiments with a 6MeV clinical x-ray beam. Based on our successful theoretical modeling of an optimized detector and measured device responsivity to radiation, we believe the thin-film CdTe-based detector is well suited for imaging with high energy x-rays used in clinical radiation therapy.

Subject Headings

Physics

Keywords

Photovoltaic detector; therapeutic imaging device; CdTe

Committee / Advisors

Ishmael Parsai, PhD (Committee Chair)
Diana Shvydka, PhD (Committee Member)
Thomas Kvale, PhD (Committee Member)
Song Cheng, PhD (Committee Member)
Jacques Amar, PhD (Committee Member)
Michael Dennis, PhD (Committee Member)

Pages

136p.

Document number: toledo1228060515
Permalink: http://rave.ohiolink.edu/etdc/view?acc_num=toledo1228060515

This ETD has been downloaded 1108 times (through March 2013)