▼ The American Association of Physicists in Medicine Task Group Report43 (AAPM-TG43) and its updated version TG-43U1 rely on the LiF TLD detector to determine the experimental absolute dose rate for brachytherapy. The recommended uncertainty estimates associated with TLD experimental dosimetry include 5% for statistical errors (Type A) and 7% for systematic errors (Type B). TG-43U1 protocol does not include recommendation for other experimental dosimetric techniques to calculate the absolute dose for brachytherapy. This research used two independent experimental methods and Monte Carlo simulations to investigate and analyze uncertainties and errors associated with absolute dosimetry of HDR brachytherapy for a Tandem applicator. An A16 MicroChamber and one dose MOSFET detectors were selected to meet the TG-43U1 recommendations for experimental dosimetry. Statistical and systematic uncertainty analyses associated with each experimental technique were analyzed quantitatively using MCNPX 2.6 to evaluate source positional error, Tandem positional error, the source spectrum, phantom size effect, reproducibility, temperature and pressure effects, volume averaging, stem and wall effects, and Tandem effect. Absolute dose calculations for clinical use are based on Treatment Planning System (TPS) with no corrections for the above uncertainties. Absolute dose and uncertainties along the transverse plane were predicted for the A16 microchamber. The generated overall uncertainties are 22%, 17%, 15%, 15%, 16%, 17%, and 19% at 1cm, 2cm, 3cm, 4cm, and 5cm, respectively. Predicting the dose beyond 5cm is complicated due to low signal-to-noise ratio, cable effect, and stem effect for the A16 microchamber. Since dose beyond 5cm adds no clinical information, it has been ignored in this study. The absolute dose was predicted for the MOSFET detector from 1cm to 7cm along the transverse plane. The generated overall uncertainties are 23%, 11%, 8%, 7%, 7%, 9%, and 8% at 1cm, 2cm, 3cm, and 4cm, 5cm, 6cm, and 7cm, respectively. The Nucletron Freiburg flap applicator is used with the Nucletron remote afterloader HDR machine to deliver dose to surface cancers. Dosimetric data for the Nucletron 192Ir source were generated using Monte Carlo simulation and compared with the published data. Two dimensional dosimetric data were calculated at two source positions; at the center of the sphere of the applicator and between two adjacent spheres. Unlike the TPS dose algorithm, The Monte Carlo code developed for this research accounts for the applicator material, secondary electrons and delta particles, and the air gap between the skin and the applicator. Advisors/Committee Members: Spitz, Henry.

▼ This thesis sets out to analyze the use of shielding to protect the lens of the eye from secondary particles produced by Galactic Cosmic Rays interacting with the structure of spacecraft. The specific aim is to develop a model that will utilize a shield to reduce frontal dosage to an astronaut's lens by 20%. In the MCNPX model, a proton beam will pass through an aluminum structure to create the secondary particles. Dosage to the lens will then be determined after the various shielding materials are placed between the eye and the source. Advisors/Committee Members: Spitz, Henry.

▼ The gamma irradiation facility at the High Flux Isotope Reactor (HFIR)is used to deliver a pure gamma dose to any target of interest. in addition to the gamma rays, there are neutron impurities due to spontaneous fissions, gamma neutron reactions, and alpha neutron reactions. Steps have been taken to eliminate the neutrons in the area of interest, but the actual level of neutrons in the facility is unknown. Measurements and MCNPX modeling have verified that neutrons exist in the gamma irradiation facility in quantities as high as 4.43X105 n/cm2s Advisors/Committee Members: Spitz, Henry.

▼ Problems often arise that require the use of MCNP to simulate a small response change caused by a variation in system parameters. The direct computation approach for such problems often leads to a too large statistical uncertainty for the predicted response change to be acceptable when the response change is small, unless extremely large number of particle histories is used in the MCNP simulations. Consequently, correlated sampling and the differential operator perturbation technique have been developed for MCNP to simulate small response changes with an acceptable uncertainty at reasonable number of particle histories. A thorough investigation is conducted on the theory, performance, and applicability of the two methods in this dissertation research and it consists of three components. First, the performance of correlated sampling is investigated. It is found that under certain conditions using the default output format of MCNP5 and the original practice of batch statistics developed for correlated sampling may overestimate the uncertainty of the change in system response. The cause of the overestimation is analyzed; correspondingly two new improved procedures are proposed for correlated sampling to ensure correct estimation of the uncertainty. The performances of the improved procedures of correlated sampling are also compared with that of direct, uncorrelated computation. Results show that the improved correlated sampling method may yield a standard deviation that is up to one magnitude smaller than that predicted by the direct simulation at the same number of particle histories. Secondly, the performances of improved correlated sampling method and the differential operator perturbation technique of MCNP are compared for different types of fixed source problems. In terms of precision of response changes, it is found that the MCNP perturbation technique significantly outperforms correlated sampling for one type of problems but performs comparably with or even underperforms correlated sampling for the other two types of problems. In terms of accuracy of changes in system response, it is found that the MCNP normal perturbation calculations may produce biased results for two types of problems. The magnitude of the bias is problem-dependent and can be quite significant even when response changes are very small. However, accurate results can be obtained for all the test problems if the MCNP perturbation calculations are done by the midpoint correction technique that takes into account the effect of second order cross-differential terms. Thirdly, the two methods are applied to the pin diversion analysis of PWR spent fuel assemblies, for which the small change in neutron flux needs to be estimated when some of the spent fuel pins are missing or replaced with dummy pins. Since the pin diversion analysis involves variation in particle source, the current MCNP perturbation technique cannot be directly applied to such problems. The correlate sampling method in theory can be applied to the pin diversion problem, but it cannot reduce the uncertainty of response change significantly. Some kind of source treatment must be introduced in order for the two methods to work effectively. Two different source treatment strategies are proposed for the pin diversion analysis in this work. It is demonstrated that when coupled with an appropriate source treatment strategy, the MCNP perturbation technique may still produce inaccurate results thus it is not recommended to be used in the pin diversion analysis; however, the MCNP correlated sampling method works well for all the pin diversion problems. Advisors/Committee Members: Su, Bingjing.

▼ Research has shown that particulate matter of 2.5 micrometers in diameter or smaller is harmful to humans. Air quality sampling for particles of this size is a way to characterize the air by the elements that are present. This research compares two methods of air quality sampling, one which is a long term sample and one which is a short term sample. The long term sample is a new method which collects 5 short term samples on one filter. The objective of utilizing a long term sampling method was to accumulate a greater amount of mass on the filters but still evaluate a limited period of exposure when vehicular traffic is expected to be greatest, i.e., during the morning rush hour. The samples were analyzed using X-ray fluorescence (XRF) and neutron activation analysis (NAA). NAA results confirm that for most elements analyzed, the long term sampling method results in accumulation of greater elemental mass than collected on the short term air sample filter. NAA proved to be a superior for measuring many of the important vehicle-related particulate matter than XRF. However, XRF was useful for detecting several important elements, such as calcium, that cannot be detected with NAA. Advisors/Committee Members: Spitz, Henry.