Angular momentum dependence of 200Pb fission studied by comparison of 19F on 181Ta with 16O on 184W

The time development of fission in highly excited Pb nuclei has been studied by the crystal blocking technique. Thin Ta crystals were bombarded with ¹⁹F ions in the energy range 90–120 MeV and the yield of fission fragments was measured for emission directions close to a strong axis. The experimental blocking dips are compared with calculated dips containing a superposition of two components, corresponding to short- and long-lived compound nuclei. The information extracted is the energy dependence of the relative amount of fission which comes from compound nuclei with lifetimes of τ≳3 × 10⁻¹⁷s. The total fission cross section and angular distribution of fission fragments were also measured for ¹⁹F bombardment of ¹⁸¹Ta in the energy range 84.3–114.7 MeV and for ¹⁶O bombardment of ¹⁸⁴W in the energy range 83.4–107.9 MeV. The results of the three types of measurements have been interpreted through comparison with statistical model calculations that follow the spin and energy distribution of compound nuclei through the neutron evaporation cascade. The ¹⁹F + ¹⁸¹Ta measurements, when compared with the present ¹⁶O + ¹⁸⁴W cross section and angular distribution measurements and earlier lifetime measurements for ¹⁶O + W, yield information on the spin distribution for the compound nucleus and its influence on the fission process.