The effect of the dipole force and its fluctuation on the motion of Li atoms in an intense, one-dimensional, near-resonant standing light wave has been investigated, both theoretically and experimentally. The duration of the interaction of the atoms with the standing wave was varied from several tens of spontaneous emission lifetimes to several hundred. For a standing wave frequency blue-detuned from resonance, diffusive heating can dominate the time-averaged dissipative dipole force so that there is no steady-state momentum distribution. However, for sufficiently large blue detunings the rate of diffusion is so slow that the resulting distribution approaches a quasi-steady state. For red detunings the diffusion is balanced with the force and a true steady-state is achieved. A Monte Carlo method based on the density matrix equations in the dressed-state representation is applied to simulate the atomic motion. The dynamics of atom channeling is studied. One of the important applications of this investigation is adiabatic laser cooling of atoms. Lithium atoms channeled in the nodes of an intense standing wave radiation field are cooled to near the recoil limit, a single photon's momentum $\hbar{k}$, by adiabatically reducing the radiation intensity. The final momentum distribution has a narrow component with a root mean squared momentum of 2 $\hbar{k}$ in one-dimension. The data are compared with the results obtained by the Monte Carlo method. This process may be useful for cooling and increasing the phase-space density of atoms confined in a magnetic trap.

I have shown that in the fast collision approximation, the scattering operator for resonant x-ray scattering can be expressed in terms of simple spin-orbital moment operators M$\sp{(k)}$(l,s) of the valence shell involved in the resonance. Using these operators, one not only can determine the thermal expectation values of the various spin-orbit multipole moments in the valence shell through the coherent elastic scattering and absorption, but also can obtain the moment-moment correlation functions (Van Hove scattering function S(q,$\omega$)) among the various atoms of the scattering sample from the coherent elastic and inelastic scattering. We have applied the effective scattering operators to the x-ray magnetic scattering in ${La\sb{2-x}Sr\sb{x}CuO\sb4}$ high temperature superconductors, and obtained the relations between the scattering cross-section and the Van Hove scattering function, and the instantaneous correlation function of the spins in these materials. This theory is applicable to the analysis of a broad range of resonant x-ray elastic and inelastic scattering and absorption experiments involving rare earth, actinide and transition elements.

A system to control and stabilize the output of visible laser diodes was developed and used to measure the velocity distribution of lithium atoms in a laser cooling experiment. Circuitry was designed and built for controlling the diode temperature and current, and optical feedback from a grating was used to further tune the laser and to narrow its lineshape. In the experiment, atoms from a thermal lithium beam were slowed to near zero velocity using a multi-frequency relay chirp technique.

The rare gas alkali ionic molecules are established as a new class of ionic excimers which emit in the vacuum ultraviolet (VUV) regime of the electromagnetic spectrum. Temporal and spectral characteristics of these species with (XeRb)$\sp{+}$ at 164 nm and (XeCs)$\sp{+}$ at 160 nm have been investigated by soft x-ray excitation in a laser-produced plasma and by high energy pulsed electron beam pumping in this work. Soft x-ray pumping of XeF(B $\to$ X) and (XeRb)$\sp{+}$ yielded the first observation of excimer molecules formed by reactive kinetics in a laser-produced plasma. The spectrum of (XeRb)$\sp{+}$ was observed. The spectral structure could be assigned to 3 dipole allowed transitions originating from the 0$\sp{+}$(II), 1(II), 1(I) upper states ending in the 0$\sp{+}$(I) ground state. A kinetic study of electron beam pumped mixtures of Ar/Xe/Rb and Ar/Xe/Cs was performed. The observed temporal decays of (XeRb)$\sp{+}$ and (XeCs)$\sp{+}$ were analyzed. The results suggest that electron deactivation is surprisingly not a dominant quenching process for rare gas alkali ions. The radiative lifetimes are 150 $\pm$ 50 ns for (XeCs)$\sp+$ and 250 $\pm$ 50 ns for (XeRb)$\sp+$. In addition several quenching rate constants were extracted from the experimental results. These rate constants and lifetimes were incorporated into a kinetic model for these species. This kinetic model reproduced the experimental observations well providing the electron quenching rate coefficient is kept to a maximum of 5 $\times$ 10$\sp{-9}$ cm$\sp3$s$\sp{-1}$. Estimates for the upper state densities were computed using this model to be $\sim$4 $\times$ 10$\sp{15}$ cm$\sp{-3}$ for (XeCs)$\sp+$ and 2 $\times$ 10$\sp{15}$ cm$\sp{-3}$ for (XeRb)$\sp+$. Including absorption due to the photoionization of the alkali atoms, the net gain coefficients are computed to be on the order of 10$\sp{-2}$ cm$\sp{-1}$. Therefore the rare gas alkali ionic excimers appear to be a promising class of candidates as storage media for VUV lasers.

The quasi-elastic neutron scattering (QNS) method is a useful technique to study biomolecular dynamics. The versatility of the method makes possible motional studies of biomolecules in different forms: powder, crystalline, and solution; and at different temperatures. Thus, it allows investigation of biomolecular dynamics in different states of matter. We have used the QNS method to study the motion of the trypsin chain segments in powder and in D$\sb2$O solution at temperatures of 200K, 280K, and 300K. The scattering spectra S(Q,$\omega$) were measured in constant-Q mode. The S(Q,$\omega$) for trypsin protons in liquid solution exhibits a broadening due to diffusive motion which is absent in the powder and the frozen solution. This diffusive motion has the character of a jump diffusion. The high-frequency thermal motion obtained from the Debye-Waller factor $\langle$u$\sp2\rangle$/3 $\cong$ 0.33A$\sp2$ at T = 300K is consistent with earlier measurements. The DW factor at lower temperatures for trypsin solution shows deviation from theoretical predictions.

Once a day, a flash of gamma-rays erupts somewhere in space and is detected by an international fleet of satellites. Since their first detection over a quarter century ago, these gamma-ray bursts have puzzled researchers who could not determine their distance, emission mechanism, or progenitor. Much of this confusion arose as theorists attempted to create a single model to explain what we now believe are at least two, and probably more, populations of gamma-ray transients. Within the past two years, thanks largely to the Dutch-Italian satellite BeppoSAX, astronomers discovered that bursts have multiwavelength fading afterglows. This helped them determine that most gamma-ray bursts are from distant galaxies. However, it did not answer the questions regarding the emission mechanism or the progenitor. We place constraints on the emission mechanism by closely examining the spectral evolution of gamma-ray bursts observed by the American instrument BATSE. From a sample of 41 distinct pulses in 26 bright gamma-ray bursts, we have determined that the pulses appear to be radiatively cooling. We also studied the evolving spectral shape in 79 bursts. In particular, we found that both the range and evolution of the spectral index below the spectral break conflict with the predictions of a popular synchrotron shock model. They instead suggest inverse Comptonization in a hybrid thermal plus nonthermal plasma as the emission mechanism. With our Monte Carlo codes, we have begun the generation of a library of inverse Compton spectra. Using them, we have made preliminary fits to two bursts with prompt multiwavelength data. The characteristic "terrace-shaped" Compton spectrum is evident in both using BATSE data alone. This shape appears to be confirmed for the January 1, 1997 burst using BeppoSAX X-ray data and for the January 23, 1999 burst using optical data from the ground-based robotic telescope, ROTSE. Both bursts appear to be enshrouded in a material with a high initial Thomson scattering depth tT≳20 . Fitting with a larger, more organized library of Monte Carlo simulations will be required before precise limits can be placed on physical parameters such as the energies, masses, and densities of bursters.

The objective of this work is to use MOSFET dosimeters to accurately measure surface dose delivered during CT examinations in various scanning conditions. To achieve this, the behaviour of MOSFETs under kilovoltage x-ray irradiation first needed to be investigated. A dose-to-dose reproducibility of 4.5%, and a mean change in sensitivity response of 10.4% with accumulated dose were measured. A Monte Carlo model of the x-ray source of a PQ5000 CT simulator was built and validated in order to investigate the MOSFET response characteristics and perform dose calculations. An over-response of 10% was observed when the beam energy was decreased from 140 to 80 kVp, and a slight anisotropy of 8.5% from the mean value over 360º was observed. The dosimeters were calibrated on a solid water phantom using a method involving MC surface dose calculations. Good agreement was found between measurements and simulations of surface dose on a cylindrical PMMA phantom for a stationary tube technique, single axial scan and multiple contiguous axial scans, with generally less than 7.5% discrepancies. Film and MOSFET measurements were then performed for helical adult brain scan parameters using different pitch and collimator settings. The use of five MOSFETs combined in a linear array was found to be suitable to accurately measure surface dose in helical scans for almost all pitch and collimation combinations.

Since portal images are created by megavoltage, forward-directed bremsstrahlung beams, their image quality is inferior to that of images produced by kilovoltage beams. In this study, characteristics of orthogonal bremsstrahlung photons produced by megavoltage electron beams were studied and their suitability for radiotherapy imaging was evaluated. Orthogonal bremsstrahlung beams with kilovoltage effective energies can be obtained from megavoltage electrons striking low atomic number targets. A 10 MeV electron beam emerging out of the research port of a Varian Clinac-18 linac was made to strike carbon, aluminum and copper targets. Percentage depth dose and attenuation measurements of forward and orthogonal beams were performed, and experimental results were compared with Monte Carlo-calculated findings. Images of simple contrast objects taken using the orthogonal bremsstrahlung beams showed superior contrast levels in comparison to those produced by the forward beams.

Dynamic beam delivery (DBD), either in the step-and-shoot or dynamic mode, promises to be useful for developing conformal radiotherapy techniques. The introduction of DBD in the clinic requires stringent evaluation of the DBD system before any dynamic technique can be performed clinically. In this work, a series of quality assurance tests were performed to evaluate a DBD system (DBD Toolbox) on a linear accelerator. The results of the tests have shown that the DBD Toolbox is capable of precisely controlling jaw motion, collimator rotation, and gantry rotation for dynamic beam delivery to within a 1 mm tolerance for jaw motion and 0.5° tolerance for collimator or gantry rotation. In addition, a simple model for dynamic jaw dosimetry was evaluated. The quality assurance tests and model evaluation are presented in this thesis.

Accurate dose calculations and analysis tools are essential to radiotherapy treatment planning. Radiotherapy deliveries utilize the information provided by the treatment planning system and it is generally accepted that clinical outcome can be improved if accuracy in the dose delivery is further improved. Proven Monte Carlo calculations increase the planning accuracy however, most radiotherapy departments do not use Monte Carlo. The McGill Monte Carlo treatment planning system, MMCTP, provides a flexible software environment to integrate Monte Carlo planning with current and new treatment modalities and deliveries. The MMCTP design consists of a graphical user interface, which runs on a simple workstation connected through standard secure-shell protocol to a cluster for lengthy Monte Carlo calculations. The impact of this tool lies in the fact that it allows for systematic, platform independent, large-scale Monte Carlo planning calculations for different treatment sites. Various measurements and patient recalculations were preformed to validate the software and ensure proper functionality.