Studies of fragment angular distributions in the fission of Bi²⁰⁹ and U²³⁸ induced by alpha particles of energies up to 115 MeV

Abstract

Using semiconductor detectors, the fragment angular distributions have been measured in the cases of fission of Bi²⁰⁹ and U²³⁸ 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 K₀² 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 <l²> of the fissioning nucleus required for deducing K₀² were determined from optical-model calculations. For both the cases of compound nuclei of At²¹³ and Pu²⁴², it is found that the values of K₀² 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 U²³⁸ 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 K₀² were also investigated in detail in the case of the target nucleus U²³⁸. Using the standard expression for Γ_n/Γ_f, the first-chance values of K₀² have been obtained and further corrected for the estimated direct-interaction effects, in the case of the target nucleus U²³⁸. Even after allowance is made for the direct-interaction effects, the energy dependence of K₀² 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 J_eff/a^½_f increases significantly with the bombarding energy. It appears likely that these results suggest a rapid increase in the effective moment of inertia J_eff with the angular momentum, as can be expected on the basis of the observed steep variation in the saddle shapes with Z²/A.