THE CALIBRATION AND IMPLEMENTATION OF CR-39 NUCLEAR TRACK DETECTOR MATERIAL AS A FAST NEUTRON MICRODOSIMETER

A detailed microdosimetric characterization of the M. D. Anderson 42 MeV (p,Be) fast neutron beam was performed using the techniques of microdosimetry and a 1/2 inch diameter Rossi proportional counter. These measurements were performed at 5, 15, and 30 cm depths on the central axis, 3 cm inside, and 3 cm outside the field edge for 10 $\times$ 10 and 20 $\times$ 20 cm field sizes. Spectra were also measured at 5 and 15 cm depth on central axis for a 6 $\times$ 6 cm field size. Continuous slowing down approximation calculations were performed to model the nuclear processes that occur in the fast neutron beam. Irradiation of the CR-39 was performed using a tandem electrostatic accelerator for protons of 10, 6, and 3 MeV and alpha particles of 15, 10, and 7 MeV incident energy on target at angles of incidence from 0 to 85 degrees. The critical angle as well as track etch rate and normal incidence diameter versus linear energy transfer (LET) were obtained from these measurements. The bulk etch rate was also calculated from these measurements. Dose response of the material was studied, and the angular distribution of charged particles created by the fast neutron beam was measured with CR-39. The efficiency of CR-39 was calculated versus that of the Rossi chamber, and an algorithm was devised for derivation of LET spectra from the major and minor axis dimensions of the observed tracks. The CR-39 was irradiated in the same positions as the Rossi chamber, and the derived spectra were compared directly. ^

AN ILLNESS-DEATH PROCESS WITH TIME-DEPENDENT COVARIATES

A general model for the illness-death stochastic process with covariates has been developed for the analysis of survival data. This model incorporates important baseline and time-dependent covariates to make proper adjustment for the transition probabilities and survival probabilities. The follow-up period is subdivided into small intervals and a constant hazard is assumed for each interval. An approximation formula is derived to estimate the transition parameters when the exact transition time is unknown.^ The method developed is illustrated by using data from a study on the prevention of the recurrence of a myocardial infarction and subsequent mortality, the Beta-Blocker Heart Attack Trial (BHAT). This method provides an analytical approach which simultaneously includes provision for both fatal and nonfatal events in the model. According to this analysis, the effectiveness of the treatment can be compared between the Placebo and Propranolol treatment groups with respect to fatal and nonfatal events. ^