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The helium-ion-induced fission excitation functions of rhenium, lutetium, and thulium have been measured in the energy interval of 30 to 80 MeV using a "fission-track" technique with plastic films as detectors. By analyzing the ratio $\frac{{\ensuremath{\Gamma}}_{f}}{{\ensuremath{\Gamma}}_{n}}$, which measures competition between fission and neutron emission, fission thresholds of 22.4, 23.6, 27.5, and 28.7 MeV have been determined for ${\mathrm{Ir}}^{189}$, ${\mathrm{Ir}}^{191}$, ${\mathrm{Ta}}^{179}$ and ${\mathrm{Lu}}^{173}$, respectively. These values are obtained using a level-density parameter ${a}_{f}=\frac{A}{10}$, with the uncertainty of this choice introducing a possible error of \ifmmode\pm\else\textpm\fi{}2.5 MeV in the calculated barriers. The thresholds, which are of particular interest because the nuclides involved are removed from the closed-shell region, are in agreement with predictions of a recently proposed semi-empirical mass formulation based on a charged liquid drop. Correlations are given showing the relationship of $\frac{{\ensuremath{\Gamma}}_{f}}{{\ensuremath{\Gamma}}_{n}}$ with $\frac{{Z}^{2}}{A}$ and ${{E}_{f}}^{\ensuremath{'}}\ensuremath{-}{{B}_{n}}^{\ensuremath{'}}$, where ${{E}_{f}}^{\ensuremath{'}}$ and ${{B}_{n}}^{\ensuremath{'}}$ are the effective fission barrier and the neutron binding energy. The results suggest that shell effects tend to persist even at excitation energies of 30 MeV. Comparisons of the correlations with heavy-element data point out the marked discontinuity near $Z=90$, and support the general two-mode fission hypothesis.

Cross sections have been determined for 25 representative fission products in the fission of ${\mathrm{Np}}^{237}$ by 20.0-, 25.0-, 28.4-, 32.5-, 37.5-, and 40.5-MeV helium ions and for ${\mathrm{Pu}}^{239}$ by 21.5- and 32.0-MeV helium ions. Mass distribution curves have been constructed from the data and the fission and total reaction cross sections determined. Various nuclear parameters are calculated and compared with other fission systems, and with theory.

The absolute fission yields of approximately twenty-five nuclides from the helium-ion-induced fission of ${\mathrm{U}}^{235}$ were determined with an accuracy of \ifmmode\pm\else\textpm\fi{}5-15% for several energies ranging from 20 to 40 Mev. Such features of fission as the symmetric-asymmetric modes of fission, the relation of total fission cross section to compound nucleus theory, fine structure in fission product distribution, valley to peak ratios, and neutron emission are discussed as well as some of the experimental detail involved.