Recoil properties of radionuclides formed in photospallation reactions on complex nuclei at intermediate energies

The recoil properties of 14, 24, 26, 31, 21 and 20 radionuclides formed in photospallation reactions on ^natV, ^natCu, ⁹³Nb, ^natAg, ^natTa, and ¹⁹⁷Au, respectively, induced by bremsstrahlung of end-point energies (E₀) from 600 to 1100 MeV, have been investigated using the thick-target thick-catcher method based on the two-step vector model. The measured forward-to-backward ratios (F/B) were found to be independent of the mass difference (ΔA) between a product (A_p) and a target (A_t) and also of A_t. The recoil velocities ν from the first step and the mean kinetic energies T of the residual nuclei in the second step were deduced and discussed by comparing with the reported results on proton-induced reactions. A comparison with T values calculated by the PICA (Photon-Induced Intranuclear Cascade Analysis) code at E₀ = 400 MeV was also performed. It was found that although the code well reproduces the experimental results of ^natV and ^natCu, the same calculation for heavier targets gives T values lower than the experimental results, indicating some nuclear-structure effect, such as a medium effect notably at A_t ≥ 100. The parameter values of ε_s = T/(ΔA/A_t), the average kinetic energy of the emitted particles in a random-walk process of both the photon- and proton-reactions, were found to increase along with an increase of A_t, but change slopes to become constant above A_t = 100.