The electron-nuclear double resonance (ENDOR) spectrum of 57Fe 3+ in the two alums RbA1(SO4)2·12H 20 and RbGa(SO4)2·12H20 is examined at ∼9.4 GHz and 4.2 K. With the magnetic field in a general direction of the crystals, the ENDOR spectra of the four sites in the unit cell of the crystals were observed. A spin Hamiltonian expressed in trigonal form adequately described the observed frequencies of the four sites for the magnetic field in two directions. The hyper-fine parameters were found to be slightly anisotropic. The nuclear spectroscopic splitting factor gI was found to be isotropic within experimental error, with a value of gI = 0.180 +/- 0.004. An additional term of the type S3I was required for a successful fit of the experimental results to the spin Hamiltonian. The parameters are compared with those obtained by others in EPR or ENDOR studies of 57Fe3+ in different crystals systems.

At the present time, the meaning of the cluster model and what is meant by clustering in actual nuclei has not been fully clarified. It is thought that some progress is made here in an understanding of clustering in nuclei. Along with defining the cluster model, the amount of ad hoc involved in the choice of a certain cluster model of a nucleus is given. Also presented here, is a quantitative measure of the amount of clustering occurring in the nucleus. A calculation of this measure of the degree of clustering in the Be 8 nucleus is performed with success. It is demonstrated that Be 8 is slightly ellipsoidal in shape and much less dense than a free alpha particle, The radius of the alpha particles within Be8 is calculated and compared with the radius of a free alpha particle. The separation of the alpha particles is found to be much less than the magnitude of the alpha cluster radius. This investigation also revealed certain new mathematical and physical properties of the cluster model.

A study has been made of the low-energy charged particles emitted from the interactions of 6 BeV and 27 BeV protons with silver and bromine nuclei in nuclear emulsions. The energy spectra of the protons and alpha particles are compared with the Weisskopf evaporation theory and with experimental results at other bombarding energies. The evaporation temperatures of the excited nuclei were determined for the emission of protons (including deuterons and tritons) and alpha particles, respectively, for the two primary proton energies of 6 BeV and 27 BeV. The two proton energy spectra are consistent with an evaporation temperature of 4 to 5 MeV and a potential barrier V° ∼ 3-4 MeV. For the alpha particle spectra, the parameters for the evaporation curves which give the best fit to the experimental distributions are a temperature, T, of the order of 10 to 12 MeV and a potential barrier V° very nearly equal to zero. These abnormally high temperatures (greater than the binding energy of a nucleon inside the nucleus) indicate the emission process cannot be entirely explained in terms of a normal mechanism of evaporation from a thermodynamic equilibrium state. Applying the value of T obtained from the proton energy spectra, with a value of V° - 7-8 MeV, gives a satisfactory fit of the theoretical curve to the experimental distribution for alpha energies less than 30 MeV. A study of the angular distributions of the emitted particles alpha shows that the emission is anisotropic, the distribution being peaked in the forward direction; this peaking is more pronounced at 6 BeV than at 27 BeV. In both cases, the forward peaking appears to be due exclusively to alpha particles of energy greater than 30 MeV. We may conclude, therefore, that these higher-energy particles are probably associated with secondary interactions provoked by the cascade, rather than with the pure evaporation process. In an attempt to understand the processes of interaction of 6 BeV protons with heavy nuclei, the angular distributions of the black tracks have been studied for stars previously analyzed in terms of the charged pi-meson distribution. The results are discussed for the cases when the incident proton collides: (a) with a single nucleon in the target nucleus, and (b) with 3 to 5 target nucleons. The results indicate that the interactions with a single target nucleon occur at the periphery of the nucleus, the two nucleons then leaving the nucleus without initiating a cascade. The star size and the average excitation energy are found to be smaller for the single nucleon interactions than in the case of the (3-4-5) target nucleon interactions and the probability of fragment emission appears to be greater.

Abstract not available.

Abstract not available.

The question of the self-consistency of spin wave theory as applied to different spin arrangements in magnetically ordered crystals has been reinvestigated. A set of equations, involving the probabilities of finding a given number of spin deviations at a given site first proposed by Van Kranendonk and Van Vleck (1958) in connection with a simple cubic antiferromagnet at a temperature of 0°K, is generalised and solved exactly for an arbitrary temperature. Two sets of equations are solved both for the case of a simple cubic antiferromagnet and for more general spin arrangements, collectively referred to as spiral spin arrangements. In solving for the probabilities a method is developed for easy calculation of the thermal average of certain functions of number operators. Finally, numerical results are given for some probabilities connected with: (i) the simple cubic antiferromagnet and (ii) a model of the rare earth metal, dysprosium. The latter is of some interest in view of the investigations of spiral spin arrangements in recent years. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

This thesis is concerned with two nuclear reactions which have astrophysical significance. First, for the reaction O¹⁶(p, γ)F¹⁷ a theoretical analysis of the currently available data has been made. Second, for the reaction D(p, γ)He³, experimental results have been obtained at lower energies than previously reported and a crude theoretical analysis of the data is presented. The analysis of the reaction O¹⁶(p, γ)F¹⁷ is based on a relatively simple nuclear model. The system, O¹⁶+p, is described in terms of a single particle wave function for the odd proton, moving in a potential provided by the O¹⁶ core. The proton is assumed to be captured directly from the continuum to one of the final bound states of F¹⁷ with the emission of gamma rays. A square well plus, coulomb potential is assumed to represent the interaction between the O¹⁶ core and the odd proton. A check on the bound state wave functions produced by the model was made by calculating the lifetime for the quadrupole transition between the 2s[subscript ½] first excited state and 1d[subscript 5/2] ground state. A similar check on the validity of the continuum wave functions given by the present model was made by comparing the O¹⁶(p,p)O¹⁶ scattering cross section predicted by the model with the experimental data of Eppling. For direct capture, the radial integrals for electric dipole transitions from continuum P-states to final 2s[subscript ½] and 1d[subscript 5/2] states were evaluated numerically for radii of 4.8 and 3.65 fermis. For 4.8 fermi radius, the absolute cross section, energy dependence and ratio of the transitions to the first excited and to the ground states are in good agreement with the experimental data. The cross sections for the 3.65 fermi radius are a factor of two low. For energies lower than 0.5 Mev, there is a considerable increase in the astrophysical S factor contributed by S[subscript s], corresponding to the first excited state transition, while S[subscript d], corresponding to the ground state transition remains practically constant over the whole energy range. [ ... ] / Science, Faculty of / Physics and Astronomy, Department of / Graduate

A search has been made for the radiative capture reaction d(α,ɤ)Li⁶ at the energy corresponding to the 2.184 Mev level in Li⁶. The search was carried out using heavy ice targets varying from 100 Kev to 700 Kev thick. The radiative capture reaction was not observed. An upper limit on the resonant capture process was set at (1.4 ± 0.6) x 10⁻³¹ cm². A gamma radiation peak was observed at 1.64 Mev for runs on D₂O, H₂O, and tungsten oxide targets. The cross-section was roughly estimated at 4 x 10⁻²⁹ cm². This radiation was attributed to the reaction O¹⁷(α,n)Ne²⁰. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Through the technique of adiabatic demagnetization of paramagnetic salts substances may be cooled to temperatures of the order of 0.1° K and 0.01° K. At these low temperatures the hyperfine structure interaction can produce an appreciable degree of nuclear orientation. The orientation of an assembly of radioactive nuclei may be detected by measuring the anisotropy in the emission of gamma radiation. By this method we have investigated nuclear orientation in paramagnetic, antiferromagnetic and ferromagnetic substances. We have studied the nuclear orientation of Pr¹⁴² and Yb¹⁷⁵ introduced as impurities in paramagnetic single crystals of cerium magnesium nitrate. The anisotropy of the 1.57 Mev. γ-ray of Pr¹⁴² was measured as a function of temperature in the range of 0.003° K to 1.0° K both in zero external magnetic field and in a field of 320 gauss parallel to the trigonal crystal axis. Values for the magnetic moment of Pr¹⁴² were assigned from our results for two assumed decay schemes, these are 0.11 nuclear magnetons for the spin assignments 2→[formula omitted] and 0.15 nuclear magnetons for the spin assignments [formula omitted]. Similar experiments were carried out on Yb¹⁷⁵. Measurements were made in a variety of external magnetic fields up to 700 gauss, and at temperatures as low as 0.003⁰K. No anisotropy was observed for the 396 kev γ-ray, nor for the 282 kev γ-ray. The most likely explanation for this result is that the lifetime of the γ - emitting state is about 10⁻¹⁰ seconds. This conclusion has since received independent confirmation. We have established that nuclear orientation can be produced in antiferromagnetic single crystals. We have explored some of the possibilities and features of nuclear orientation in this class of substances by investigating salts of manganese and cobalt with transition temperatures differing by an order of magnitude. Single crystals of these antiferromagnetic salts were cooled in thermal contact with potassium chrome alum and the anisotropy of the gamma radiation emitted by Co⁶⁰ and Mn⁵⁴ introduced in the lattice was observed. These measurements show that the hyperfine structure splittings in the antiferromagnetic state are comparable to those found in the paramagnetic state and give indications that nuclear spin relaxation times may be of the order of minutes and hours in antiferromagnetic materials at temperatures below 1°K. We have attempted to detect nuclear orientation arising from a possible hyperfine structure interaction at the anion in antiferromagnetic MnBr₂4 H₂O and MnCl₂4 H₂O. To detect the nuclear orientation we observed the anisotropy of the gamma radiation emitted by Br⁸² introduced into MnBr₂4 H₂O and I¹⁸¹ in both McCl₂4 H₂O and MnBr₂4 H₂O. This attempt yielded negative results. The anisotropy of the gamma radiation of Co⁶⁰ in a ferromagnetic single crystal of cobalt metal was measured before and after heat treatment of the crystal. The results before heat treatment show a significant discrepancy with the data reported by other workers and differ from those found for the heat treated crystal. A qualitative explanation of these results in terms of crystalline stacking faults in cobalt metal is presented. Work was initiated on nuclear orientation in binary ferromagnetic alloys. The nuclear orientation may arise from the hyperfine structure interactions which may exist in one or both components of a ferromagnetic binary alloy. The inter-metallic compound chosen for special study was MnBi. Although only preliminary and inconclusive results on the orientation of Mn⁵⁴ nuclei in this substance were obtained the technique adopted is briefly described. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

The differential cross section for the capture of tritium by alpha particles to form Li⁷ has been measured using a tritium-zirconium target and 1.64 mev. alpha particles. The differential cross section for the gamma-ray transition to the ground state of Li⁷ was found to be 2.54± .46 x 10¯³¹ cm.² per steradian at 90° to the incident alpha beam direction. At the same energy, the ratio of the differential cross section at 90° for transitions to the first excited state to that for transitions to the ground state of Li⁷ was found to be approximately 0.40. The T(α,ɣ )Li⁷ 90° differential cross section has been measured relative to the differential cross section at 1.64 mev. using alpha particles of energies 0.515, 0.72, 0.98, 1.23, and 1.94 mev. From the smooth change of the reaction cross section with energy it can be concluded that the reaction proceeds by direct radiative capture. At all energies, the ratio of the differential cross section at 90° for transitions to the first excited state to that for transition to the ground state was approximately 0.4. Preliminary angular distribution measurements at 0° and at 90° to the incident alpha beam direction indicate that the angular distribution is not isotropic. The ratio of the yield at 0° to the yield at 90° was found to be 1.40 ± 0.37 at an alpha particle energy of 1.64 mev. Differential cross section measurements for direct radiative capture of protons by 0¹⁶ have been made relative to the cross section at 800 kev., using a solid tungsten-dioxide target and protons of energies 0.618, 09823, 1.13, 1.536, and 2.04 mev. Absolute 90° differential cross section values were based on the measurement of the 90° differential cross section for transitions to the first excited state in F¹⁷ at 800 kev. of 10.4 ± 1.3 x 10¯³² cm.² per steradian made by Robertson. The differential cross section for the gamma ray transitions to the first excited state of F¹⁷, at 90° to the incident proton beam direction, was found to vary smoothly from 0.41 x 10¯³¹ cm.² per steradian at 0.618 mev. to 11.9 x 10¯³¹ cm.² per steradian at 2.04 mev. At all energies the ratio of the differential cross section at 90° for transitions to the ground state to that for transitions to the first excited state was approximately 0.20. / Science, Faculty of / Physics and Astronomy, Department of / Graduate