Isomeric (γ, n) cross section ratios and the spin dependence of the nuclear level density

Abstract

Isomeric (γ, n) cross section ratios have been measured for 11 nuclei with 59 ≦ A ≦198. The nuclei were excited by gamma rays in the giant resonance region and residual (γ, n) activities were measured with a NaI(Tl) scintillation spectrometer. The isomeric ratios have been analysed using the statistical calculations of Huizenga and Vandenbosch 1) and estimates have been obtained of the parameter σ which characterizes the spin dependence of the nuclear level density. Other estimates of σ are summarized and it is found that for light nuclei ( A≲50), σ≈ σ s the value calculated from statistical theory with corrections for shell effects. For heavy nuclei ( A≳100) σ falls to approximately 0.6 σ s .