Recoil Properties of Radionnclides Formed in Photonuclear Reactions on ¹⁹⁷Au at Intermediate Energies

Abstract

The recoil properties of 37 radionuclides with the mass number A = 24-131 produced in the photonuclear reaction on ¹⁹⁷Au by bremsstrahlung of end-point energies (E0) from 300 to 1100 MeV have been investigated using the thick-target thick-catcher method with an aid of intensive chemical separations. The measured mean ranges, FW and BW, in the forward and backward directions, respectively, with respect to the beam axis show E0-independence above 600 MeV, reflecting the resonance character in photonuclear reactions at intermediate energies. The forward-to-backward yield ratios (F/B) are independent of the mass difference (ΔA) between a product (A) and the target (At =197) above ΔA = 66, and the weighted mean value of F/B =1.1 ± 0.1 is clearly distinguished from that of photospallation products with ΔA ≤ 45. The F/B ratios of proton-induced reactions at incident proton energies (Ep) of Ep ? 3 GeV are higher than those of photon-reactions and decrease with an increase of Ep, but become the same values as the photon-reactions at higher Ep. The mean kinetic energies T of the residual nuclei were deduced based on the two-step vector velocity model. The T values are independent of E0 above 600 MeV and increase with an increase of ΔA, reflecting the changes in the reaction mechanism from spallation to fission . The T values of photon-reactions at ΔA > 45 are quite different from those of the proton-reactions, in which the T values decrease with an increase of Ep up to about 3 GeV and become Ep-independent at higher Ep, though the T values of photon-reactions at ΔA ≤ 45 agree well with those of the proton-reactions, irrespectively of Ep. Recoils of ²⁴Na produced from ²⁷Al, ^natV ^natCu, ^natAg, and ¹⁹⁷Au are of special interest from a viewpoint of a change in the production mechanism with respect to At.