DEVELOPMENT OF THE SOLID-STATE NUCLEAR TRACK DETECTOR CR-39 AS A MICRODOSIMETER FOR HIGH-ENERGY NEUTRON CANCER THERAPY

The Solid State Nuclear Track Detector (SSNTD) CR-39 plastic is developed for use as a microdosimeter for the high-energy neutron beams used in neutron cancer therapy. This was accomplished through a combination of experimental investigations including equipment development and numerical particle transport calculations.;The experimental portion of the research involved the irradiation of several SSNTDs by a clinical neutron cancer therapy beam at the Cleveland Clinic / NASA therapy facility. Ten CR-39 SSNTDs were irradiated in Tissue Equivalent (TE) liquid on the beam centerline and perpendicular to the beam. Further analysis which was performed at the University of Illinois consisted of etching and track major axis measurement. The etching was performed using a High-Temperature Etch Bath. The analysis of the SSNTDs was performed using a microcomputer-based SSNTD Automatic Readout and Analysis (SARA) system which was developed at the University of Illinois.;The calculational portion of the development of CR-39 as a microdosimeter involved the computer simulation of the Cleveland Clinic / NASA neutron beam using the Monte Carlo method. Neutrons were transported through TE liquid and water using the Monte Carlo code HETC which was obtained from the Radiation Shielding Information Center (RSIC) at Oak Ridge National Laboratory. The results from this code were analyzed by user-written analysis codes in order to calculate the energy-dependent neutron angular flux spectra at various depths in TE liquid and water. The neutron angular flux spectrum was used to obtain energy-dependent charged-particle angular flux spectra at various depths in TE liquid and water through the use of charged-particle production codes which were also written by the user. The energy-dependent charged-particle angular flux spectra were then used to calculate an LET-dependent correlation function which would relate the measured track major axis length distribution to the microdose rate (defined as the derivative of the dose rate with respect to LET) as a function of LET. Using the calculated correlation function, it is possible to calculate an LET spectrum from a measured track major axis length distribution on the surface of a CR-39 SSNTD.