Quasielastic barrier distributions for the ${}^{20}\mathrm{Ne}+{}^{58,60,61}\mathrm{Ni}$ systems: Influence of weak channels

Background: Near-barrier fusion can be strongly affected by the coupling between relative motion and internal degrees of freedom of the collision partners. The coupled channels (CC) method is a usual way of describing the reaction dynamics in this energy region. In the standard approach in the CC method only strong reaction channels (collective excitations) are taken into account. However, in some cases this approach fails to describe experimentally obtained barrier height distributions.

Purpose: The influence of weak (noncollective) reaction channels on barrier height distributions was studied.

Method: The barrier height distributions were determined from quasielastic scattering of ${}^{20}\mathrm{Ne}$ on ${}^{58,60,61}\mathrm{Ni}$ targets. The scattered ions were registered at backward angles (130–150 degrees).

Results: In the ${}^{58}\mathrm{Ni}$ and ${}^{60}\mathrm{Ni}$ cases one observes a “structure” (two peaks) in the barrier height distribution which is completely smoothed out for ${}^{61}\mathrm{Ni}$.

Conclusions: The results support the hypothesis that noncollective excitations of the target nuclei, much more numerous in ${}^{61}\mathrm{Ni}$ than in ${}^{58}\mathrm{Ni}$ and ${}^{60}\mathrm{Ni}$, influence the barrier height distribution and are responsible for smoothing out the structure.

Figure 1

Comparison of level densities in ${}^{58,60,61}\mathrm{Ni}$: (a) experimental levels adopted from [57] and (b) single-particle level densities calculated with the HFB method based on the BSk14 Skyrme interaction [58].

Figure 2

$Q$-value spectra for the ${}^{20}\mathrm{Ni}+{}^{61}\mathrm{Ni}$ system (for beam energy 51 MeV) obtained from data registered at 35 degrees [panels (a) and (c)] and 140 degrees [panels (b) and (d)] presented in linear and logarithmic scales.

Figure 3

Excitation functions for quasielastic backscattering of ${}^{20}\mathrm{Ne}$ on ${}^{58,60,61}\mathrm{Ni}$ targets registered at ${130}^{\circ }$, ${140}^{\circ }$, and ${150}^{\circ }$ (in the laboratory).

Figure 4

Barrier height distributions for the ${}^{20}\mathrm{Ne}+{}^{58,60,61}\mathrm{Ni}$ systems derived using the Eq. (2) from the excitation functions presented in Fig. 3. Theoretical predictions obtained by means of the ccqel code, convoluted with experimental resolution, are marked with solid lines.

Figure 5

Comparison of barrier height distributions shown in Fig. 4 after renormalization of barrier positions for ${}^{60}\mathrm{Ni}$ and ${}^{61}\mathrm{Ni}$, taking into account in an approximate way sizes of the target nuclei (see the text). The lines, being spline approximations of experimental distributions, are to guide the eye.