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Studies of Fragment Angular Distributions in the Fission of Bi 209 and U 238 </…

1966; American Institute of Physics; Volume: 149; Issue: 3 Linguagem: Inglês

10.1103/physrev.149.965

1536-6065

S. S. Kapoor, H. Baba, S.G. Thompson,

Nuclear reactor physics and engineering

Using semiconductor detectors, the fragment angular distributions have been measured in the cases of fission of ${\mathrm{Bi}}^{209}$ and ${\mathrm{U}}^{238}$ induced by alpha particles of various energies ranging from 23 to 115 MeV obtained from the Berkeley 88-in. variable-energy cyclotron. The center-of-mass angular distributions were analyzed by a least-squares fitting code to obtain the value of ${{K}_{0}}^{2}$ corresponding to the saddle-point excitation energy ${{E}_{x}}^{s}$ for each bombarding energy. The transmission coefficients ${T}_{l}$ and the mean square of the orbital angular momentum $〈{l}^{2}〉$ of the fissioning nucleus required for deducing ${{K}_{0}}^{2}$ were determined from optical-model calculations. For both the cases of compound nuclei of ${\mathrm{At}}^{213}$ and ${\mathrm{Pu}}^{242}$, it is found that the values of ${{K}_{0}}^{2}$ increase more rapidly with ${{E}_{x}}^{s}$ than expected on the basis of the Fermi-gas model, irrespective of the assumptions made about the multiple-chance fissions. The fission cross sections of ${\mathrm{U}}^{238}$ for alpha-particle energies up to 110 MeV, also measured in this work, enabled us to check the accuracy of the optical-model calculations. The presence of direct interactions and their effects on the deduced values of ${{K}_{0}}^{2}$ were also investigated in detail in the case of the target nucleus ${\mathrm{U}}^{238}$. Using the standard expression for $\frac{{\ensuremath{\Gamma}}_{n}}{{\ensuremath{\Gamma}}_{f}}$, the first-chance values of ${{K}_{0}}^{2}$ have been obtained and further corrected for the estimated direct-interaction effects, in the case of the target nucleus ${\mathrm{U}}^{238}$. Even after allowance is made for the direct-interaction effects, the energy dependence of ${{K}_{0}}^{2}$ appears to be significantly different from that expected on the basis of the simple Fermi-gas model. These results can be explained within the framework of the Fermi-gas model if it is assumed that $\frac{{J}_{\mathrm{eff}}}{{{a}_{f}}^{\frac{1}{2}}}$ increases significantly with the bombarding energy. It appears likely that these results suggest a rapid increase in the effective moment of inertia ${J}_{\mathrm{eff}}$ with the angular momentum, as can be expected on the basis of the observed steep variation in the saddle shapes with $\frac{{Z}^{2}}{A}$.