Gamma-ray study of pion-induced reactions on the nickel isotopes

The spectra of residual nuclides following 100-, 160-, and 220-MeV ${\pi }^{+}$ and ${\pi }^{-}$ bombardment of ${}_{}{}^{58,60,62,64}{}_{}{}^{}\mathrm{Ni}$ have been measured by detecting prompt and, at 220 MeV, $\beta$-delayed gamma rays. A wide spectrum of residual nuclides extending along the valley of stability down to Ca, is seen. Where radioactivities were measured, the total (prompt + delayed) observed cross section amounted to ≈ 900 mb. The mean number of nucleons removed increased from about 5 for the ${}_{}{}^{58}{}_{}{}^{}\mathrm{Ni}$ target to about 8 for ${}_{}{}^{64}{}_{}{}^{}\mathrm{Ni}$. The residual nuclide spectrum depends sensitively on the target neutron excess, but is essentially independent of pion charge or pion energy in the range observed. A Monte-Carlo cascade-evaporation calculation involving an intermediate $\Delta$ resonance reproduces the yield of residual nuclides far (more than about eight nucleons) removed from the target, but it fails to reproduce the yield of nearer nuclides which is a more sensitive measure of the early stages of the reaction.