Development, characterization, and application of a charged particle microbeam for radiobiological research

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 2005."September 2005."Includes bibliographical references (leaves 194-197).The goal of this work is to develop a charged-particle microbeam for use in radiobiological research at the MIT Laboratory for Accelerator Beam Applications (LABA). The purpose of this device is to precisely explore the radiation response of biological systems on a cellular and subcellular level, particularly in the area of temporal and spatial effects of radiation on in vitro systems. An accelerator-based 750 keV proton source was characterized and integrated into a laboratory-scale device that includes a deflection/gating system, single-particle detection system, imaging and positioning system, and a collimation system with two designed modes: a "charged-particle microslit" for delivering a -3 micron by 1 mm dose profile; and a pinhole aperture for delivering a -3 micron diameter pattern of radiation. The entire device measures less than 4 m, requires minimal radiation shielding, and utilizes a dedicated ion source. The charged particle microslit has been fully characterized and used to deliver a radiation pattern to a series of mammalian fibroblast cell monolayers that have subsequently been assayed for direct and indirect chemical effects of radiation, double-stranded DNA damage, and DNA repair protein localization. These studies will contribute to the understanding of the radiation-induced bystander effect, which is generally defined as the induction of biological effects in cells that are not directly traversed by ionizing radiation.(cont.) Analysis of the range of assays performed on the microbeam-irradiated cells demonstrates that even though the physical radiation dose is confined to a subnuclear width ( 40 microns) and show dependence on the initial radiation dose delivered to the directly irradiated cells. As an experimental system, the LABA Microbeam was designed to be practically turn-key, and most applications require only one operator to perform. The LABA Microbeam represents a significant step towards a cost-effective and easily operated charged-particle microbeam appropriate for use as a standard laboratory research tool. Further work remains in automation of the microbeam subsystems and optimization/characterization of the pinhole-aperture collimator, as well as expanding the scope of the radiobiological assays performed using the charged-particle microslit.by Michael R. Folkert.Ph.D

Monte Carlo simulation of neutron shielding for proton therapy facilities

Thesis (S.B. and S.M.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 1998.Includes bibliographical references (leaves 60-63).A study was performed to develop a Monte Carlo method of modeling neutron shielding of proton therapy facilities in a complex, realistic environment. The bulk neutron shielding of the Northeast Proton Therapy Center (Massachusetts General Hospital, Boston, MA) was used as the basis of the design work. A geometrical model of the facility was simulated using the LAHET Code System, a set of Monte Carlo codes developed at Los Alamos National Laboratory. Additional software tools for reading and analyzing the simulation data that the model provides have been developed and tested. In order to verify the computer simulations, neutron detection and data acquisition systems have been assembled, modified, and thoroughly tested in order to monitor the neutron dose equivalent during proton beam operation at several locations on a continuous basis. Preliminary tests show that the geometry and physics models proposed in this work are valid.by Michael R. Folkert.S.B.and S.M

Spin-orbit interaction in a dual gated InAs/GaSb quantum well

Spin-orbit interaction is investigated in a dual gated InAs/GaSb quantum well. Using an electric field the quantum well can be tuned between a single carrier regime with exclusively electrons as carriers and a two-carriers regime where electrons and holes coexist. Spin-orbit interaction in both regimes manifests itself as a beating in the Shubnikov-de Haas oscillations. In the single carrier regime the linear Dresselhaus strength is characterized by $\beta =$ 28.5 meV$\AA$ and the Rashba coefficient $\alpha$ is tuned from 75 to 53 meV$\AA$ by changing the electric field. In the two-carriers regime the spin splitting shows a nonmonotonic behavior with gate voltage, which is consistent with our band structure calculations