Spin dependence of the giant-dipole-resonance strength function in highly excited nuclei in the mass region A=39-45

Continuum {gamma}-ray spectra and angular distributions from decays of highly excited {sup 39}K, {sup 40}K, {sup 42}Ca, and {sup 45}Sc compound nuclei produced by bombarding {sup 27}Al with {sup 12}C, {sup 13}C, {sup 15}N, and {sup 18}O ions have been measured and the giant-dipole-resonance strength function has been extracted. The effective temperature associated with the states upon which the giant dipole resonance is built is nearly constant (1.7--1.8 MeV) for these reactions, while the final average spin varies from 8 to 18.5{h bar} as the projectile energy varies from 32 to 72.5 MeV. The parameters of the giant-dipole-resonance strength function extracted from fits of statistical model calculations to the {gamma}-ray spectra measured at {theta}{sub {gamma}}=90{degree} provide direct information on the spin dependence of the hot nuclear shape. The mean energy of the giant dipole resonance shows a weak spin dependence that has a different character for {sup 39}K and {sup 45}Sc nuclei. The width of the giant dipole resonance increases rapidly with spin for all reactions studied, from 11.6{plus minus}0.3 MeV at 8{h bar} to 14.7{plus minus}0.5 MeV at 18.5{h bar} for {sup 45}Sc{sup *} at {ital T}{sub {ital f}}=1.7--1.8 MeV, for example, presumably due to rotational broadening of the giant dipole resonance. For the reactions {sup 12}C+{sup 27}Al at {ital E}{sub lab}=62.7 MeV and {sup 18}O+{sup 27}Al at {ital E}{sub lab}=44.9 and 72.5 MeV, angular distributions have been measured. The observed energy dependence of the {ital a}{sub 2} coefficient suggests large nuclear deformation consistent with oblate noncollective or prolate collective rotation. The level density in nuclei with {ital A}{congruent}40 has been calculated in the Reisdorf approach with parameters fitted to low-energy level density data for nuclei in this mass region.