Isospin effects on the evaporation residue spin distribution

Abstract

Based on a Langevin equation coupled with a statistical decay model, we have studied the evaporation residue spin distribution of the nuclei 194 Pb, 200 Pb, 200 Pb, and 200 Os, and extracted a presaddle nuclear dissipation strength of 5 × 10 21 s −1 by comparing our results with the measured spin distribution of 200 Pb produced in the 16 O + 184 W reaction. We find that with increasing isospin of the system, the sensitivity of the spin distribution to nuclear dissipation decreases substantially. Moreover, for 200 Os, this spin distribution is no longer sensitive to the nuclear dissipation. These results suggest that on the experimental side, to accurately obtain the information of presaddle dissipation strength by measuring the evaporation residue spin distribution, it is best to populate those systems with low isospin.The nature and magnitude of nuclear dissipation is one of the most interesting and challenging problems in nuclear physics. In particular, presaddle nuclear dissipation strength is the focus of current experimental [1-5] and theoretical [6][7][8] research on the fission of highly excited nuclei. Because prescission light particles [9] and giant dipole resonance (GDR) γ rays [10] arise from the contribution of both presaddle and saddle-to-scission emission, it is very difficult to determine the presaddle dissipation strength by merely using the particle multiplicity. Under this circumstance, it is necessary to search for new observables that only depend on the presaddle dissipation effects. Besides the evaporation residue cross section [11,12] and the width of the fissionfragment charge distribution [1,4], the spin distribution of the evaporation residue cross section, namely, the angular momentum distribution leading to evaporation residues, may be a new and an extremely sensitive probe of the presaddle dissipation strength, as suggested in Ref. [13].To extract a precise value of dissipation strength by comparing theoretical predictions with experiment, a Langevin model is certainly preferable to a statistical model. This is because the Langevin model considers the time evolution of the fission decay width and contains a number of dynamical features in the decay of the hot compound nuclei, e.g., the angular momentum dependence of presaddle and saddle-to-scission time, etc. These advantages are not considered in a simple statistical model analysis. In addition, the Langevin model has been employed [14-18] to successfully reproduce a great deal of experimental data on prescission particle multiplicity, evaporation evaporation cross sections, and kinetic distributions of fission fragments over a wide range of excitation energy, angular momentum, and fissility.The present work is devoted to the study of the favorable experimental condition through which presaddle dissipation effects can be better revealed with evaporation residue spin distributions. For this aim, we use the Langevin model to reproduce the measured evaporation residue spin distribution of 200 Pb populated in the 16 O + 184 W reaction...

Disentangling effects of potential shape in the fission rate of heated nuclei

“…The latter method is more accurate but very computer-time-consuming. That is why the combined dynamical-statistical models have been developed [9,10] and are being used widely at present [11][12][13][14][15][16][17][18][19][20]. In these models, after a certain delay time, the dynamical modeling is matched to the statistical description.…”

Significant role of deformation in probing postsaddle nuclear dissipation with light particle emission

“…(2) have been taken from the work of Ignatyuk et al [59] and are consistent with the data [6,14,16,21,60]. About the B s (q), see below.…”

“…An essential conclusion deduced from a comprehensive investigation for this phenomenon is that nuclear dissipation is deformation dependent, namely that presaddle friction is weak and postsaddle friction is strong [6]. Some recent works have reported how to make an accurate determination for the presaddle dissipation strength by analyzing various observables, such as evaporation residue cross sections [17][18][19] and its spin distributions [20,21], fission probabilities [22,23], and the width of the fission-fragment charge distributions [24,25]. However, the magnitude of presaddle friction is still debated [26][27][28][29].…”

“…Since these observables [17][18][19][20][21][22][23][24][25] mentioned above are uniquely connected to presaddle dynamics, to explore postsaddle friction, it is necessary to employ those observables that are sensitive to postsaddle dynamics. As light particles can be evaporated along the whole fission path when a compound nucleus proceeds toward scission, they are considered to be main indicators for the postsaddle dissipation effects.…”

Spallation reaction versus heavy-ion fusion: Fission excitation functions as a fundamental probe of presaddle nuclear dissipation

“…To extract reliable and precise information about β, it is important to explore those experimental signals sensitive to presaddle dissipation effects only. While several observables (e.g., evaporation residue cross section [23,24] and its spin distribution [25,26]) were proposed, these evaporationchannel-related quantities are an indirect indicator of presaddle dissipation as compared to those provided via fission channels. As the most direct consequence of dissipation, fission is hindered; that is, fission probability (or fission cross section) is reduced.…”