Role of noncollective excitations in low-energy heavy-ion reactions

We investigate the effect of single-particle excitations on heavy-ion reactions at energies near the Coulomb barrier. To this end, we describe single-particle degrees of freedom with the random matrix theory and solve the coupled-channels equations for one-dimensional systems. We find that the single-particle excitations hinder the penetrability at energies above the barrier, leading to a smeared barrier distribution. This indicates that the single-particle excitations provide a promising way to explain the difference in a quasielastic barrier distribution recently observed in ${}^{20}\mathrm{Ne}$$+$${}^{90,92}\mathrm{Zr}$ systems.

Figure 1

The energy spectrum for the model calculation we employ. There is a collective vibrational state at 1 MeV, while single-particle states exist from 2 MeV with an exponentially increasing level density.

Figure 2

The potential penetrability obtained with several methods. The dotted line is obtained without channel coupling, while the dashed line takes into account only the collective vibrational excitation. The solid line shows the result with both the collective and the single-particle excitations.

Figure 3

The barrier distribution defined by the first derivative of the penetrability. The meaning of each line is the same as in Fig. 2.

Figure 4

The $Q$-value distribution for the reflected flux at four energies as indicated in the figure. It is obtained by smearing the discrete distribution with a Lorentzian function with the width of 0.2 MeV. The peaks at $E^{*}=0$ and 1 MeV correspond to the elastic and the collective excitation channels, respectively.

Figure 5

The same as Fig. 2, but for the rotational coupling.

Figure 6

The same as Fig. 3, but for the rotational coupling.

Figure 7

The quasielastic barrier distribution for the ${}^{20}\mathrm{Ne}$$+$${}^{92}\mathrm{Zr}$ system calculated with the weight factors and the eigenbarrier heights for the one-dimensional rotational model discussed in Sec. III B. The meaning of each line is the same as in Fig. 3.