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

Abstract

Continuum \ensuremath{\gamma}-ray spectra and angular distributions from decays of highly excited $^{39}\mathrm{K}$, $^{40}\mathrm{K}$, $^{42}\mathrm{Ca}$, and $^{45}\mathrm{Sc}$ compound nuclei produced by bombarding $^{27}\mathrm{Al}$ with $^{12}\mathrm{C}$, $^{13}\mathrm{C}$, $^{15}\mathrm{N}$, and $^{18}\mathrm{O}$ 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\ensuremath{\Elzxh} 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 \ensuremath{\gamma}-ray spectra measured at ${\mathrm{\ensuremath{\theta}}}_{\ensuremath{\gamma}}$=90\ifmmode^\circ\else\textdegree\fi{} 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 $^{39}\mathrm{K}$ and $^{45}\mathrm{Sc}$ nuclei. The width of the giant dipole resonance increases rapidly with spin for all reactions studied, from 11.6\ifmmode\pm\else\textpm\fi{}0.3 MeV at 8\ensuremath{\Elzxh} to 14.7\ifmmode\pm\else\textpm\fi{}0.5 MeV at 18.5\ensuremath{\Elzxh} for $^{45}\mathrm{Sc}^{\mathrm{*}}$ at ${\mathit{T}}_{\mathit{f}}$=1.7--1.8 MeV, for example, presumably due to rotational broadening of the giant dipole resonance. For the reactions $^{12}\mathrm{C}$${+}^{27}$Al at ${\mathit{E}}_{\mathrm{lab}}$=62.7 MeV and $^{18}\mathrm{O}$${+}^{27}$Al at ${\mathit{E}}_{\mathrm{lab}}$=44.9 and 72.5 MeV, angular distributions have been measured. The observed energy dependence of the ${\mathit{a}}_{2}$ coefficient suggests large nuclear deformation consistent with oblate noncollective or prolate collective rotation. The level density in nuclei with A\ensuremath{\simeq}40 has been calculated in the Reisdorf approach with parameters fitted to low-energy level density data for nuclei in this mass region.