The enhancement in the production of $\mathrm{even}-Z$ nuclei observed in nuclear fission has also been observed in fragments produced from heavy ion collsions. Beams of ${}^{40}\mathrm{Ar},$ ${}^{40}\mathrm{Cl},$ and ${}^{40}\mathrm{Ca}$ at 25 MeV/nucleon were impinged on ${}^{58}\mathrm{Fe}$ and ${}^{58}\mathrm{Ni}$ targets. The resulting fragments were detected using the MSU $4\pi$ detector array, which had additional silicon detectors for better isotopic resolution. Comparison of the ratios of yields for each element showed enhancement of even-$Z$ fragment production. The enhancement was more pronounced for reactions with a greater difference in the $N/Z$ of the compound system. However, this effect was less for systems that were more neutron rich. The average $N/Z$ for fragments also displayed an odd-even effect with a lower average $N/Z$ for the $\mathrm{e}\mathrm{v}\mathrm{e}\mathrm{n}-Z$ fragments. This is related to the greater availability of neutron-poor isotopes for $\mathrm{e}\mathrm{v}\mathrm{e}\mathrm{n}-Z$ nuclei.

• Received 30 September 1999

DOI:https://doi.org/10.1103/PhysRevC.63.014601