Evaporation barriers for4He indicate very extended forms for many emitting nuclei

Abstract

We have analyzed a large set of mean energies and angular anisotropies for evaporative 4He emission to obtain barriers to evaporation, B. These exit channel barriers are often substantially smaller than the corresponding empirical s-wave fusion barriers E o. The differences (Eo-B) are interpreted as indicators of the extent of distortion of the emitters. These distortions have in turn been characterized by the deformation parameter for a spheroid %0. For Z=80 the dependence of B or %0 on spin is somewhat suggestive of the superdeformed shapes predicted by the liquid drop model. For Z > 70 significant distortions are indicated for emitters of both large and small spin.How can one get information about the sizes and/or shapes of composite nuclei formed in collisions between heavy ions? This question is of great importance for the understanding of nuclear reactions involving high energies and spins [1]. In several recent papers it has been shown that evaporation-like 4He emission often occurs at a very early stage in the reaction [-2-4], suggesting that these emissions may offer a promising probe of the early evolution of the reaction mechanisms. To explore this possibility we investigate the systematics of some of the experimental properties of these evaporation-like processes, the mean energies ({,)) and anisotropy parameters /32. The data are analyzed with a simple procedure to obtain values for the effective barrier to evaporation, B. These exit channel barriers are then compared to the s-wave fusion barriers Eo for *He [5,6]. The sizable differences (Eo-B) between the two lead us to conclude that in many cases the excited emitter nucleus must be quite deformed (or swollen) compared to its unexcited counterpart in the fusion reaction. As an example we estimate a corresponding quadrupole deformation parameter for evaporation from the tips of a prolate spheroidal emitter as compared to the fusion of spheres. These deformation parameters cannot be taken literally but only as * Permanent address: Institut de Physique Nucleaire, B.P. No. 1, F-91406 Orsay, France semiquantitative indicators of the extent of distortion. They are surprisingly large, and their dependence on Z, energy, and spin shows quite interesting trends. In the first five columns of Table 1 we give the experimental data employed [2,3,[7][8][9][10][11][12][13]. In the others we give quantities derived from this analysis. The basic experimental quantities are the anisotropy parameters /32 and the mean energies {(e)) for 4He, observed in the singles mode at angles greater than 90 ~ c.m. The anisotropy parameters f12 were obtained by fitting the angular distributions to equations from the statistical model [3,10]. If this distribution is isotropic then /32 is zero; increasing forward-backward peaking is characterized by increasing /32-We will employ equations derived for evaporation from a spherical emitting nucleus at a given excitation energy E* and spin J. Therefore we must estimate the effective Z, A and E* of the emitters, and for high Z we must con...

Investigation of the velocity distribution of evaporation residues produced in20Ne+197Au reactions atE/A=5.7?11.4 MeV/u

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