Fission-Excitation Functions in Interactions ofB11,C12,N14, andF19with Various Targets

Fission cross sections for the systems ${}_{}{}^{169}{}_{}{}^{}\mathrm{Tm}$ + ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$, ${}_{}{}^{175}{}_{}{}^{}\mathrm{Lu}$ + ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$, and ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$, ${}_{}{}^{174}{}_{}{}^{}\mathrm{Y}$ + ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$, ${}_{}{}^{182}{}_{}{}^{}\mathrm{W}$ + ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$, ${}_{}{}^{165}{}_{}{}^{}\mathrm{Ho}$ + ${}_{}{}^{14}{}_{}{}^{}\mathrm{N}$, and ${}_{}{}^{159}{}_{}{}^{}\mathrm{Tb}$ + ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ have been measured for heavy-ion bombarding energies up to 10.4 MeV per nucleon. The experimental technique consisted of counting coincident fission-fragment pairs with two gold-surface-barrier silicon-diode detectors. For the above systems, fission takes place only in reactions in which a compound nucleus is formed between the incident projectile and the target nucleus. Values of the compound-nucleus cross sections for these reactions are estimated from other data in order to account for surface reactions which occur in heavy-ion bombardment. The difference between the cross section for compoundnucleus formation and that for fission is assumed to be equal to the cross section for neutron-evaporation products. The ratio of the fission cross section to that for neutron evaporation is then taken to be equal to $〈\frac{{\Gamma }_f}{{\Gamma }_n}〉$, the ratio of the level widths for the two competing processes averaged over the various reaction channels. A theoretical fit to the $〈\frac{{\Gamma }_f}{{\Gamma }_n}〉$ values is obtained for the low-energy region of the excitation function, where first-chance fission is highly probable. We find the ratio of the level-density parameter for fission to that for neutron emission to be 1.2 ± 0.1, and values for the fission barrier to be in agreement with those predicted by Myers and Swiatecki.