Effects of weakly coupled channels on quasielastic barrier distributions

Abstract

Heavy-ion collisions often produce fusion barrier distributions with structures displaying a fingerprint of couplings to highly collective excitations. Similar distributions can be obtained from large-angle quasielastic scattering, although in this case, the role of the many weak direct-reaction channels is unclear. For 20Ne+90Zr, we have observed the barrier structures expected for the highly deformed neon projectile; however, for 20Ne+92Zr, we find significant extra absorption into a large number of noncollective inelastic channels. This leads to smearing of the barrier distribution and a consequent reduction in the “resolving power” of the quasielastic method

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

“…Recently, however, a few experimental data which cannot be accounted for by the conventional coupledchannels calculation have been obtained [11][12][13][14][15]. One of these examples is the quasi-elastic scattering experiment for the 20 Ne+ 90,92 Zr systems [15].…”

“…One of these examples is the quasi-elastic scattering experiment for the 20 Ne+ 90,92 Zr systems [15]. The experimental data show that the quasi-elastic barrier distribution for these systems behaves in a significantly different way from each other: the barrier distribution for the 20 Ne+ 92 Zr system is much more smeared than that for the 20 Ne+ 90 Zr system [15]. On the other hand, the coupled-channels calculations that take into account the collective rotational excitations in 20 Ne as well as the vibrational excitations in 90,92 Zr lead to similar barrier distributions for both systems, because the strongly deformed 20 Ne nucleus mainly determines the barrier structure while the difference in the collective excitations in the two Zr targets plays a minor role.…”

Quasi-elastic scattering in the20Ne+90,92Zr reactions: Role of noncollective excitations

“…In the calculations, the rotational excitations of the strongly deformed 20 Ne nucleus dominate the barrier structure, and the collective vibrational excitations in the Zr isotopes are found to play a minor role. Experimental data for the total transfer cross section at an energy near the Coulomb barrier have been found to be essentially the same [21], and the difference in the barrier distributions has been conjectured to originate from differences in the non-collective excitations in the two Zr isotopes. In fact, since the 92 Zr nucleus has two neutrons outside the N = 50 closed shell in 90 Zr, a larger number of non-collective excited states are present in the spectrum (for example, the number of known states up to 5 MeV is only 35 for 90 Zr but 87 for 92 Zr [22]).…”

“…We note again that in the present systems, the noncollective excitations contribute more in the 20 Ne + 92 Zr system, and it would therefore be interesting to compare the experimental Q-value spectra for the two systems at higher energies than studied in Ref. [21]. There the effect of non-collective excitations might be seen more clearly.…”

“…Another example is quasi-elastic scattering in the 20 Ne + 90,92 Zr systems [21]. For these systems, the experimental quasi-elastic barrier distributions exhibit significantly different behavior, that is, the barrier distribution for the 20 Ne + 92 Zr system shows a much more smeared structure than that for the 20 Ne + 90 Zr system.…”