Using a unique two-arm detector system for heavy ions (the BRS, binary reaction spectrometer), coincident fission events have been measured from the decay of ${}^{60}\mathrm{Zn}$ compound nuclei formed at 88 MeV excitation energy in the reactions with ${}^{36}\mathrm{Ar}$ beams on a ${}^{24}\mathrm{Mg}$ target at $E_{\mathrm{lab}}({}^{36}\mathrm{Ar})=195$ MeV. The detectors consisted of two large-area position-sensitive ($x,y$) gas telescopes with Bragg-ionization chambers. From the binary coincidences in the two detectors, inclusive and exclusive cross sections for fission channels with differing losses of charge were obtained. Narrow out-of-plane correlations corresponding to coplanar decay are observed for two fragments emitted in binary events, and in the data for ternary decay with missing charges from 4 up to 8. After subtraction of broad components, these narrow correlations are interpreted as a ternary fission process at high angular momentum through an elongated shape. The lighter mass in the neck region consists dominantly of two or three $\alpha$ particles. Differential cross sections for the different mass splits for binary and ternary fission are presented. The relative yields of the binary and ternary events are explained using the statistical model based on the extended Hauser-Feshbach formalism for compound nucleus decay. The ternary fission process can be described by the decay of hyper-deformed states with angular momenta around $4552\hslash$.

• Received 15 November 2007

DOI:https://doi.org/10.1103/PhysRevC.78.044615