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

Kapoor, S. S. ; Baba, H. ; Thompson, S. G. (1966) Studies of fragment angular distributions in the fission of Bi209 and U238 induced by alpha particles of energies up to 115 MeV Physical Review, 149 (3). pp. 965-979. ISSN 0031-899X

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Official URL: http://prola.aps.org/abstract/PR/v149/i3/p965_1

Related URL: http://dx.doi.org/10.1103/PhysRev.149.965

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 Exs for each bombarding energy. The transmission coefficients Tl 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 Exs 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 Γnf, 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 Jeff/a½f increases significantly with the bombarding energy. It appears likely that these results suggest a rapid increase in the effective moment of inertia Jeff with the angular momentum, as can be expected on the basis of the observed steep variation in the saddle shapes with Z2/A.

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Source:Copyright of this article belongs to American Physical Society.
ID Code:16910
Deposited On:16 Nov 2010 13:20
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