Elsevier

Nuclear Physics A

Volume 222, Issue 2, 8 April 1974, Pages 405-428
Nuclear Physics A

Isospin conservation and pre-equilibrium decay in (p, ṕ) reactions on neutron rich tin isotopes

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Abstract

Energy spectra for protons emitted in (p, p') reactions on 118Sn, 120Sn and 124Sn have been studied as a function of emission angle at incident energies of 14.0 and 17.8 MeV. These data confirm the importance of isospin selection rules in such reaction systems. While it is not possible to make any definite statements about the mechanism of the reactions populating the higher isospin intermediate states, it is possible to obtain a consistent description of the observed spectra in terms of two pre-equilibrium decay components, one each for the T> and T< states in the composite system. Small T< evaporation peaks for the 118Sn and 120Sn systems may be present. While this explanation is critically dependent on the validity of the extended Griffin model used in the analysis, it is a direct consequence of the inclusion of isospin in the model using techniques adopted from equilibrium calculations. The high energy protons, which have forward-peaked angular distributions, are attributed to pre-equilibrium decay of the T< states and are predicted to be emitted fairly early in the equilibration process. The model also predicts that T> state pre-equilibrium decay should occur primarily toward the end of equilibration and identifies such processes with the low energy, isotropically emitted protons observed in the data. However, the enhanced rate of proton emission from the T> composite states may result in a breakdown of the fundamental statistical assumption of equal population for all n-exciton T> states. On the basis of the statistical model analyses performed it is estimated that 40–80% of the T> states undergo proton emission before isospin mixing occurs and that virtually none of them reach statistical equilibrium.

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    Supported in part by the US Atomic Energy Commission.

    Supported by the National Science Foundation.

    ††

    Present address: D.Ph.N./B.E., CEN Saclay, Gif-sur-Yvette, France.

    ‡‡

    Supported by the New York State Science and Technology Foundation.

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