[en] The states in the mass 18 and 19 nuclei which are strongly excited in three- and four-nucleon transfer reactions are considered. A cluster model employing a local effective interaction with the core and approximating the requirements of the Pauli principle with an orthogonality condition is used. A microscopic derivation is presented of the effective cluster-core potentials for alpha cluster states in ¹⁸F and ¹⁹F based on the shell model approximations to the ground states of ¹⁴N and ¹⁵N respectively and an alpha-nucleon scattering potential. (author)

[en] Published in summary form only

[en] Alpha particle cluster structure is known experimentally to persist throughout the mass range 16 ≤ A ≤ 20, and has been very successfully described in this region in terms of the Buck-Dover-Vary local potential cluster model. It is argued that an analogous cluster structure should be present in nuclei at the beginning of the (fp) - shell, and the available experimental data are examined to determine likely alpha particle cluster state candidates in the mass range 40 ≤ A ≤ 44. Calculations of the cluster state spectra and mean square cluster-core separation distances (which may be readily used to evaluate E2 electromagnetic transition rates) for sup(40)Ca, sup(42)Ca, sup(42)Sc, sup(43)Sc, sup(43)Ti and sup(44)Ti using the above mentioned model are presented, and compared with experimental measurements where possible. The agreement between theory and experiment is generally good (although inferior to that obtained in the (sd)-shell) and points to the desirability of an extension and improvement of the measurements of the properties of the excited states in these nuclei. (author)

[en] A code has been written and tested on the CDC Cyber-170 to calculate the prompt fission neutron spectrum, N(E), as a function of both the fissioning nucleus and its excitation energy. In this note a brief description of the underlying physical principles involved and a detailed explanation of the required input data (together with a sample output for the fission of ²³⁵U induced by 14 MeV neutrons) are presented. Weisskopf's standard nuclear evaporation theory provides the basis for the calculation. Two important refinements are that the distribution of fission-fragment residual nuclear temperature and the cooling of the fragments as neutrons are emitted approximately taken into account, and also the energy dependence of the cross section for the inverse process of compound nucleus formation is included. This approach is then used to calculate the average number of prompt neutrons emitted per fission, v-bar_p. At high excitation energies, where fission is still possible after neutron emission, the consequences of the competition between first, second and third chance fission on N(E) and v-bar_p are calculated. Excellent agreement with all the examples given in the original work of Madland and Nix is obtained. (author)

[en] A cluster model is used to calculate the energies of those states in ¹⁷F and ¹⁷O which have a ¹⁴N-trinucleon cluster-core structure. The non-central terms in the cluster-core potential are deduced phenomenologically and also calculated microscopically. They are found to be intimately related to equivalent terms in the potentials for similar cluster-core decompositions of neighbouring nuclei. The results are compared with the spectrum of states excited in a recent experimental study of three-particle transfer onto ¹⁴N. (Author)

[en] A multi-step direct multi-step compound model free of the criticisms suffered by that of Feshbach, Kerman and Koonin is defined in terms of the statistical hypotheses made about the interaction matrix elements. The model is briefly described and discussed. Also described are our unsuccessful attempts to parametrize the level and transition strength densities needed in pre-equilibrium calculations in terms of their moments. (author)

[en] A cluster-model calculation of the properties of triton and alpha-particle cluster states in ⁴³Sc is presented. For the triton cluster states, the ⁴⁰Ca core is assumed to be in its ground state. An effective, local, cluster-core potential containing central and spin-orbit terms, obtained from a simple folding procedure, is used to generate the cluster-state energies and wavefunctions from a single-particle Schroedinger equation. For the alpha-particle cluster states, the ³⁹K core is assumed to be in its ground or first excited state, which is treated as a dsub(3/2) or ssub(1/2) hole in an inert ⁴⁰Ca core. (author)

[en] Some of the bound and unbound states of ⁴⁴ Ti have a pronounced alpha-particle structure, and their energies and widths may be obtained from an alpha ⁴⁰ Ca potential. The differential cross-sections for the elastic scattering of alpha particles by ⁴⁰ a may also be described by such a potential, and some features indicate the presence of unbound states of ⁴⁴ Ti. The attempts to unify these bound and scattering phenomena by the same potential are described, together with some new calculations using a cosh potential. (author)

[en] A cluster model is used to calculate the energies of those states in ¹⁷F and ¹⁷O which have a trinucleon-¹⁴N cluster-core structure. The non-central terms in the cluster-core potential are deduced phenomenologically and also calculated microscopically. They are found to be intimately related to equivalent terms in the potentials for similar cluster-core decompositions of neighbouring nuclei. The results are compared with the spectrum of states excited in a recent experimental study of three-particle transfer onto ¹⁴N. (author)

[en] A strikingly successful phenomenological model which treated a number of low-lying negative-parity states in ¹⁸F and ¹⁸O as having the structure of an (A=3) cluster orbiting an (A=15) core is considered. An attempt is made to justify the assumptions inherent in this model on more microscopic grounds. To do this, an effective, local, cluster-core potential containing central, spin-orbit, spin-spin and tensor-spin components is derived from the shell-model approximation to the ground state of the (A=15) core and a nucleon-trinucleon potential. This latter potential would ideally be obtained from a phase-shift analysis of nucleon-trinucleon scattering data, but in the absence of sufficiently detailed experiments to do this, a family of such potentials, whose non-central terms are taken to have Gaussian geometries of various widths and strengths chosen by considering some of the excited states of the A=4 nuclei, is derived and used. Many of the phemomenological assumptions are then semi-microscopically vindicated, and quite reasonable quantitative agreement with the free parameters of the phenomenological model and the measured energies of the trinucleon cluster states in ¹⁸F and ¹⁸O is obtained. (author)