Critical consideration of the statistical model analysis of photonuclear isomeric cross-section ratios

We have measured isomeric cross-section ratios for the photonuclear reactions ¹⁸¹Ta(γ, 3n) ^178m,gTa, ¹⁴²Nd(γ, n)^141m,gNd, ¹¹⁰Pd(γ, n)^{109m₁,m₂,g}Pd, ¹⁰⁸Pd(γ, n)^107m₁,m₂Pd, ¹⁰⁰Mo(γ, n) ^99m,gMo, ⁹²Mo(γ, n)^91m,gMo and ⁸⁹Y(γ, n)^88m₁,m₂Y. The experimental results were compared with statistical model calculations according to the method of Huizenga and Vandenbosch. The applicability and limits of this method, especially the simple decision model for the population of the isomeric state and the ground state of the residual nucleus, were tested and discussed. Additionally, it is shown that for photonuclear reactions on magic nuclei, contributions of a direct reaction mechanism are not negligible.