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No description available.

This thesis reports on the utilisation of γ-ray coincidences between germanium and cerium doped lanthanum tri-bromide LaBr3(Ce) scintillation detectors for the determination of electromagnetic transition rates in the pico-to-nanosecond regime. The technique uses the high quality full energy peak resolution of the LaBr3(Ce) detectors coupled with their excellent timing responses in order to study discrete energy γ-ray cascades from nuclei populated using the lS0CSO,pn)34p, 130TeC2C,4n)13SCe and 12STeC60,4n)140Nd fusion-evaporation reactions. These studies were carried out at the Tandem van de Graaff accelerator laboratory in Bucharest, Romania. Coincidence measurements were performed using a mixed array of eight HPGe and eight LaBr3(Ce) detectors. The HPGe detectors were used to define the decay path of nucleus of interest, while the LaBr3(Ce) detectors were used to measure the decay times spectra of selected levels. In the first experiment, excited states in 34p were populated using the 18O(18O,pn) fusion-evaporation reaction at beam energy of 36 Me V. In this experiment, a fast timing method was used to mea- sure the half-life of the Iπ = 4- intruder state to be 2.0(1) ns. This structure and decay rate of this level are compared to predictions of restricted basis nuclear shell model cal- culations using on spdf basis which incorporates excitations into the f7/2 negative parity orbital. In the second experiment, N=80 isotones 13SCe and 140Nd were populated using the 130Te(12C,4n) and 12STeC60,4n) fusion-evaporation reactions at incident beam energy of 56 Me V and 66 Me V, respectively. The half-life of the Iπ =6+ yrast state at 2294 ke V in 13SCe was measured to be T1/2=880(19) ps. The B(E2; 6+ -+ 4+) is compared with the predictions of the restricted basis shell model and the systematics of the region and shows an anomalous behaviour compared to the neighbouring isotonic and isotopic chains. Half- lives of the yrast Iπ = 5-, 11+ and 14+ states in 138Ce have also been determined for the first time. Reduced transition probabilities have been calculated for the electromagnetic decays from these states. For 140Nd, the half-life of the Iπ = 8- yrast state at 3240 keY was measured to be Tl/2=100(12) ps. The centroid shift method was used to determine the half-life of this state because the half-life of the nuclear state was significantly shorter than the time resolution (full width at half-maximum) for the LaBr3(Ce) setup. Reduced transition probabilities B(E2) and B(M1) associated with the 178- and 1017 keY transi- tions decaying from the Iπ = 8- state in 140Nd have been calculated for the values of the E2/M1 mixing ratios.

The main aim of this study was to show that scattered radiation in the backward direction can be used as an additional source of material-specific information about a sample measured simultaneously to a primary method in the fOlWard direction, e.g. X-ray diffraction imaging. Many applications for material and structural discrimination employing X-ray scattering and absorption collimate the emitted radiation in a defined direction. Scattered radiation into other directions often remains disregarded or is shielded. The major physical effects characterising the backward-scattered spectra are, besides attenuation, Compton shift and material-specific Doppler broadening of the photon spectrum in the sample. The shape of a Compton-broadened peak can reveal information about the electron momentum of an element or, even to some extent, of a chemical compound. Since most applications utilises polychromatic X-ray sources the focus of this work was the analysis of Compton scattered characteristic anode peaks from sample materials. The multiplet structure of these characteristic peaks superimposed to a broad Bremsstrahlung spectrum is challenging for peak-shape analysis. The initial part of this work was the design and characterisation of an experimental set-up for measuring energy-resolved Compton scattered spectra of sample materials. Subsequently different calculation models were developed for describing the broadening of the characteristic lines of the anode material. These models were adapted to the specific experimental conditions. Essentially, two types of methods were considered. The first calculation procedure evaluated scattered spectra based on tabulated materialspecific Compton profiles. The second model was a phenomenological approach. It fitted a function consisting of Gaussian curves above a linearly approximated background to the curvature of the 2nd derivative of the Compton spectrum. These models were experimentally validated on Compton profiles of a variety of sample materials containing period 2 and period 3 elements. It was proven that in principle comparing measured spectra with calculated spectra provides high material differentiation capabilities, but for most molecules tabulated Compton profiles are not available and the independent atom approximation causes deviations. The phenomenological method employed extracted Gaussian curve fit-parameters to distinguish measured materials quantitatively. Most of the samples could be distinguished from each other based on their profile structure. A principal component analysis of corrected spectra provided an alternative approach and showed capabilities to separate scatter-spectra of different materials. In principle, more sample-specific information valuable for material separation could be extracted as compared to a simple integration method of backscattered radiation.

Experimentally observed x-ray scattering from warm dense samples of iron has been achieved for the first time. This work was carried out at the Vulcan laser facility to investigate the properties of iron under conditions typical of warm dense matter, similar to those found at the boundary between the Earth's inner and outer cores. Suitable samples of iron were generated using the method of direct drive, laser produced shock compression. This was achieved, using intensities of 1033 W/cm2 with frequency doubled (2w at 527 nm) = 1 ns laser pulses, incident upon 7um iron foil targets, coated in 6um of parolee-N plastic. The iron samples were probed using a helium like titanium line source spectrum (ls2-1s2p , 4.75 Kev, 1% D.'A/A ). Spectrally and angularly resolved elastic scattering; collected using 3 HOPG (2d = 6.708 Angstroms) Bragg crystal spectrometers operating In Von Hams geometry, was observed for scattering angles between 30° to 90°, and for a range of plasma conditions (p::::: (2 to 14) g/cc, The (0.2 to 2) eV; and Z=2 to 5). The shock speed has been evaluated by optical streak pyrometer, and comparison with hydrodynamic simulations established that the iron evolves to a molten state at several 100s GA pressure. The experimental design, analysis, and calculation of scattering cross sections have been described in detail. The observed cross sections have been compared with a simple one component plasma model, which highlighted the importance of screening of the inter-ionic potential to the predicted scattering signal. The method of accounting for the long probe duration (- Ins) when processing the results of hydrodynamic simulations, to allow comparison to, and validation of more sophisticated models of scattering, has also been discussed.

This thesis shows how improvements can be made to two dimensional gamma - ray source analysis by using a probabilistic approach to describe the possible source location and likelihood of a candidate gamma-ray event. The mapping algorithm developed assesses the significance of source detection, and background subtraction is implemented both by a modified standard approach and by the development of a multivariate kernel analysis.

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This thesis examines certain aspects of diffraction and scattering of high frequency waves, utilising and extending upon the Geometrical Theory of Diffraction (GTD). The first problem considered is that of scattering of electromagnetic plane waves by a perfectly conducting thin body, of aspect ratio O(k^1/2), where k is the dimensionless wavenumber. The edges of such a body have a radius of curvature which is comparable to the wavelength of the incident field, which lies inbetween the sharp and blunt cases traditionally treated by the GTD. The local problem of scattering by such an edge is that of a parabolic cylinder with the appropriate radius of curvature at the edge. The far field of the integral solution to this problem is examined using the method of steepest descents, extending the recent work of Tew [44]; in particular the behaviour of the field in the vicinity of the shadow boundaries is determined. These are fatter than those in the sharp or blunt cases, with a novel transition function. The second problem considered is that of scattering by thin shells of dielectric material. Under the assumption that the refractive index of the dielectric is large, approximate transition conditions for a layer of half a wavelength in thickness are formulated which account for the effects of curvature of the layer. Using these transition conditions the directivity of the fields scattered by a tightly curved tip region is determined, provided certain conditions are met by the tip curvature. In addition, creeping ray and whispering gallery modes outside such a curved layer are examined in the context of the GTD, and their initiation at a point of tangential incidence upon the layer is studied. The final problem considered concerns the scattering matrix of a closed convex body. A straightforward and explicit discussion of scattering theory is presented. Then the approximations of the GTD are used to find the first two terms in the asymptotic behaviour of the scattering phase, and the connection between the external scattering problem and the internal eigenvalue problem is discussed.

Includes bibliographical references. / ix, 159 p. : ill. ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, Dept. of Physics and Mathematical Physics, 1995

This thesis describes the design and construction of a femtosecond X-ray source based on high harmonic generation in an argon filled hollow capillary waveguide. The system has demonstrated excellent efficiency at generating harmonics in the range 20-50 eV and is tuneable by exploiting the ionization-induced blue-shift that occurs in the waveguide. The X-ray source is designed for X-ray scattering experiments, which will ultimately allow the imaging of single biomolecules. Such experiments will require an intense, focused X-ray beam. Towards this aim, this thesis demonstrates focusing techniques based on tapered capillary optics, including a model that was developed to understand their behaviour.