The first and major part of this thesis deals with electron capture collisions, while the second part is concerned with excitation of atomic hydrogen by electron and proton impact. In Chapter 1 the formal theory of scattering is used to obtain an exact expression for the cross section for electron capture by a structure-less particle in atomic hydrogen, and the Born series of approximations is introduced via a formal expansion of the Green's operator for the system. Chapter 2 begins with a discussion of the OBK approximation; relevant experimental investigations are then briefly described, a number of Born approximation calculations are discussed and a detailed description is given of some impact parameter methods which have proved useful in this field. The chapter ends with remarks on some very recent work. The impulse approximation is derived in Chanter 3 and the evaluation of cross sections for the processes and is described in detail. Results for these processes are presented and are compared with the values obtained by other authors. The behaviour of electron capture cross sections in the high energy limit forms the subject of Chapter. Most of the standard approximations are discussed but the main emphasis is placed on the Born and impulse approximations, for which the analysis is given in detail. The application of the impulse approximation to the excitation of atomic hydrogen by electron and proton impact is discussed in Chapter 5. It is shown that the results obtained by Akerib and Borowitz for electron impact are unreliable. New results are given which are in poor agreement with experiment and reasons for the discrepancy are discussed.

In this thesis we develop and investigate the use of a semi-classical impact parameter method for treating the excitation of atoms by electron impact. Our particular interest is forbidden, non-exchange transitions. Chapter 1 contains a brief description of the method along with reasons for considering such a method. Previous work employing the same general ideas is reviewed, as are other theoretical calculations and experimental results of direct relevance to this thesis. The paper of Seaton (1962) which applies the impact parameter method to optically allowed transitions is reviewed in some detail as our treatment of the subject is based largely on this work. In Chapter 2 we derive expressions for the transition probabilities and introduce the idea of the averaged probabilities. Cross section formulae are given which are applicable to dipole and quadrupole transitions and which include a strong coupling form. The formulae are generalized to apply to an incident point particle of arbitrary mass and charge. Chapter 3 contains formulae for the averaged probabilities for arbitrary interaction potentials and these are applied to the case where the potential is a sum of spherical tensor operators. Expressions for cross sections, applicable to arbitrary transitions induced by electron or proton impact, are given and the explicit form evaluated for the case of hydrogen. In Chapter 4 are presented results of calculations of cross sections for certain forbidden transitions in hydrogen and helium, both for electron and proton impact. We examine the validity of the approximations made and discuss the choice of the cut-off. Chapter 5 contains an evaluation of the method as well as suggestions for improving it and for further applications. The derivation of certain results used in the thesis is presented in the appendices.

Several calculations have been carried out using the transition operator technique. The radiative decay in a two-level quantum system with excited state coupled by an external perturbation has been investigated and an expression obtained for P(t), the probability of finding the atom in its excited state at time t. This is seen to possess a steady value for yt >> 1 and a third term which decays at nearly half the rate of its second. Next radiative decay of an atom with two close-lying excited states is considered and |b_q|², the spectral density for spontaneous emission from the uppermost level, calculated. This shows that the proximity of the upper two levels makes the line-shape non-Lorentxian. Heisenberg equations of motion were then derived for the transitions operators of an ℓ-level atom undergoing radiative decay, the possibility of overlapping pairs of levels being ignored. These master equations were then used to obtain the spectral profiles of certain atomic lines, employing the so-far avoided Markoff approximation. First the spectral density of the scattered radiation was found for transitions between levels 5P_3/2 and 4S (ground state), and 6S and 5P_3/2, in the potassium atom when driving fields of arbitrary strength coupled levels 6S and 4P_3/2, and 4P^3/2 and 4S. Triple-peaked profiles were obtained. Spectral profiles for emission were then derived for transitions between levels 3P and (ground state), and 3P and in the hydrogen atom when a driving field of arbitrary strength coupled levels The former profile was double the latter quadruple-peaked. Lastly master equations were derived for an atom, with two excited levels undergoing radiative decay, when allowance was made for thepossibility of their overlapping. A driving field of arbitrary magnitude coupled the uppermost and ground states. The spectrum for the scattered light resulting from transitions between the uppermost and ground state was found and graphs computed for various field strengths and separations of the excited states.

The plasma produced by focusing the output of a high-powered, neodymium glass laser onto plane, metallic targets has been used as a source for studying the grazing incidence spectra of the elements in the periodic table from chromium to nickel. Over 400 new identifications are presented, the vast majority with transitions belonging to the n k n-1 k+12s 2p - 2s 2p transition arrays in the 0 I to Be I isoelectronic sequences. Of particular interest are identifications of inter combination lines in the N I to Be I sequences, which are observed for the first time in laboratory spectra of fourth period elements. The new data, taken together with other published results, allow a complete n k n-1 k+1 listing of the 2s 2p - 2s 2p transitions in the elements chromium to nickel, from which the appropriate term schemes have been determined. In addition to the n = 2 - 2 transitions, the wavelengths of some sodium-like transitions of the elements chromium to nickel are presented. The laser system used in this study has an output power of, typically, 50 GW in pulses of a few nanoseconds duration. Special features of this system are its high repetition rate and the reproducibility of its output and these features have been exploited in experiments to distinguish between the emission from different ionisation stages. A full description of the laser system is given. Consideration is, also, given to the various atomic structure calculations which have been used to give wavelength predictions of n = 2 - 2 transitions. The emphasis is placed on Hartree-Fock methods. The importance of the n = 2 - 2 transitions is evidenced by the variety of plasmas in which they occur and their uses both as a diagnostic of certain plasma parameters and for comparison with atomic structure calculations. Their occurrence in different plasmas is illustrated by the application of the term scheme analysis to tokamak and solar flare plasmas, both of which are of topical interest. Predictions of the allowed and forbidden, n = 2 - 2 transitions of metallic impurity elements which are expected to have sufficient intensity to be observed in tokamak discharges are listed and some new identifications in solar flare spectra are presented. Of particular note amongst these predictions and identifications are those of then forbidden transitions within the 2s 2p configurations, some of which fall in the near-UV and visible spectral regions, where high resolution spectroscopic techniques can be used. An example of a diagnostic use is the measurement of electron densities from line intensity ratios and this method is used to estimate the electron densities of some solar flare plasmas. In addition, the application of this method to high density plasmas, which is being investigated by other authors, is discussed. Finally, some of the observed wavelengths are compared with the results of atomic structure calculations.

The eigenfunction expansion method is used to describe the scattering of low energy electrons by atoms or ions having any number of incomplete subshells. Hartree Fock wave functions are used to describe the target system and allowance is made for the inclusion of any number of excited electronic configurations in the expansion. The continuum electron wave functions are given as the solutions of coupled integro differential equations with prescribed boundary conditions. Expressions for the photoionisation cross sections are also derived within this approximation. The formalism presented in this thesis uses the technique of angular momentum recoupling which greatly simplifies the evaluation of the potential terms which appear in the equations and the expressions for the photoionisation cross sections. This has enabled us to write, for the first time, a computer program for the calculation of the various electron scattering and photoionisation cross sections for a general atomic system, which requires as input only the Hartree Fock functions for the bound orbitals and the parameters needed to specify the terms to be included in the eigenfunction expansion. A program which calculates the necessary recoupling coefficients for any given expansion is described.

The close-coupling approximation has been used to compute cross sections for a wide range of processes (electron impact, photodetachment and photoionization). Errors in previous formulations have been pointed out and corrected. The results are compared with previous calculations and experiments and are correlated to recent work on the effects of configuration inter-action. A description of the computer code used is also given.

Two low energy positron beams have been designed, built and their performance characteristics evaluated. Both are magnetically guided systems using a combination of a solenoid and Helmholtz coils. Slow positrons are produced by the moderation of fast positrons, from a 22Na source, in annealed polycrystalline tungsten mesh with efficiencies greater than 2 x 10^-4. The original beam, built to high vacuum specifications, has been incorporated into a fully automated microcomputer controlled Doppler-broadening spectrometer system. The newer beam line is built, to UHV specifications, into a liquid helium cryostat. It also has a vertical geometry making the whole system far more versatile and allowing the study of liquid surfaces. Doppler-broadening analysis has been applied to measurements taken using both beams applied to pure metals (Mo and Ga). A two-state model incorporating diffusion of thermal positrons back to the surface has been found to be inadequate at low incident positron energies. In this regime epithermal positron and positronium emission is significant. A model of simple back-scattering of epithermal positrons was found to be successful in fitting the experimental lineshape parameters. Both fast and slow positron techniques have been applied to the study of inert gas precipitates in metals. Using conventional methods a detailed Doppler-broadening study has been carried out on the anealing of bulk Cu samples containing 3 atomic % Kr in the form of a high concentration of solid precipitates at 300K. Melting of the Kr and bubble growth are clearly seen. Deconvolution of the annihilation lineshape indicates that positrons are trapped at the Cu-Kr interface. Slow positrons have been used to profile the defect distribution of Mo implanted with a high dose of Kr ions. Using the simplest model of a step-function distribution there is reasonable agreement with the expected Kr profile.

A study of the ion behaviour in the HBTX1A/B Reversed Field Pinches (RFP's) is presented. A comparison of measured and calculated energy spectra of the neutral deuterium atoms emitted from the plasma enabled the deuterium ion temperature to be measured and its radial profile estimated. The measured ion temperatures of 80.0eV to 600.OeV cannot be explained by collisional heating of the ions by the electrons even indirectly through typical (2.0% CV, 2.0% OVI, 0.1% FeXII) or extreme (9.0% FeXII) concentrations of impurity ions. Measurements of the Doppler broadening of carbon ion line emission, CV(2271.oA), enabled estimates to be made of the CV ion temperature of 150.OeV to 450.OeV and fluid velocity of 3.0x103ms1 to 1.4x10 ms1 toroidally. The measured ion temperatures are shown to be consistent with the heating power per particle being proportional to the particle mass. An ion power balance model is described in which it is assumed thatthe excess power input into the plasma above the Ohmic heating power into the electrons, typically 6.OMWnf3 on average, heats the ions locally through the damping of fluctuations. The resistivity estimated from a helicity balance model can be made to agree with the Spitzer value only if the excess toroidal loop voltage due to helicity loss at the plasma boundary is included. Ion energy losses are calculated from the relative neutral deuterium density profile assuming equal ion and electron energy and particle diffusion coefficients of typically 50.0m s on axis. Using this model the calculated ion temperature profile and ratio of ion to electron temperature, Ti/Te, are consistent with those measured over a wide range of conditions. Ti/Te is predicted to increase with increased helicity loss at the plasma edge. This was demonstrated by insertion of a tile into the plasma edge which increased Ti/Te from 1.0 with limiters to 2.0 and by removal of the limiters which decreased Ti/Te to 0.5 to 0.3 as predicted. A review of ion heating mechanisms indicated that ion heating by viscous damping of fluctuations is the most likely candidate. Estimates of the order of the viscous heating powerusing measured fluctuation levels are of the required magnitude. It isconcluded that the ion temperatures observed can be explained if the power input to the fluctuations associated with the RFP 'dynamo' heats the ions through the viscous damping of these fluctuations.

A brief introduction on the scope of gamma ray spectroscopy is given in Chapter I, Chapter II deals with, the codification of the detecting system is of the prolate spheroidal field b-ray spectrometer (large spectrometer) and the medium size magnetic lens spectrometer (small spectrometer). The "Venetian blind" E.M.I photomultipliers previously used in both the spectrometers have been replaced by 36 a'VP photomultipliers and fast electronics using avalanche discriminator circuits. The NE 102 phosphors were replaced by the fast NE 104 scintillators. further improvement in the large spectrometer was made by replacing the conical phosphor and a straight light guide by an equiangular spiral light guide. Chapter IV describes the preparation of p-sources. An improved technique using the electrospraying method has been applied in the preparation of Th B and Ce sources. The coincidence measurements of 144Ce using the two spectrometers have been described in Chapter VI. But the most important and original contribution of this thesis is the e y coincidence measurements of 144Ce using the large spectrometer and a Go(Li) K-ray detector in con-junction. The e-y coincidence measurements have been described in Chapter VI. Very possibly a magnetic spectrometer and a Ge(Li) x-ray detector have been used for the first time time in these e - y coincidence measurements on 144Ce. The single y-ray spectra of 144Ce neasured with the 5 cc Ge(Li) detector and with X-ray detector are also described in this chapter. The use of a G-e(Li) X-ray detector in the e - y coincidence neasurenents of Ce has nade it possible to solve the ambiguities in the literature concerning sone low energy transitions and in the placing of the upper excited level in 144Pr The results can be talcen as the nost reliable ones obtained so far. Our results agree best with Geiger et al (1960,61). Chapter VII describes some (Tl)/Ge(Li) y - y coincidence measurenents of 207Bi and it is possible that these are the first measurenents on 207Bi with this type of equipment.

Studies on (d,d);(3He,3He); (d,3He);(3He,d) and (d,n) reactions are reported in this work. The (d,d) angular distributions were measured at an incident energy of 12.0 MeV on 45Sc, 40Ca and at 13.0 MeV on 37Cl, 35Cl, 27A1 and 23Na. The (3He,3He) angular distributions were measured at an incident energy of 14.0 MeV on 29Si and 25Mg. The results of elastic scattering studies were analysed to extract optical potential parameters for the analysis of the transfer reaction experiments. The (d,3He) angular distributions were measured on targets 45Sc, 40Ca at an incident energy of 12.0 MeV and on 37Ck, 35Cl, 27Al and 23Na at 13.0 MeV. The (3He,d) angular distributions were measured on targets of 29Si and 25Mg at an incident energy of 14.0 MeV. The reaction 29Si(d,n)30P was studied at 3.0 MeV. The results of transfer reaction studies were analysed using DWBA and Hauser-Feshbach theories and discussed in the light of the structure of the nuclear states involved. It has been possible in some cases to assign spins and parities to nuclear levels from these studies. Implications of reaction mechanism in some cases have also been considered. The problem of absolute normalization of DWBA calculations has been investigated using the results from some (d, 3He) studies.