Sub-barrier Fusion ofH6ewithB

Abstract

The fusion of 6 He with a 209 Bi target has been studied at energies near to and below the Coulomb barrier. Despite the weak binding of the valence neutrons in 6 He, little evidence is found for suppression of fusion due to projectile breakup. Instead, a large enhancement of sub-barrier fusion is observed. It is suggested that this enhancement may arise from coupling to positive Q value neutron transfer channels, resulting in "neutron flow" between the projectile and the target. [S0031-9007 (98)07674-1] PACS numbers: 25.60.Pj, 25.70.JjRecent theoretical studies of near-barrier and subbarrier fusion of the exotic "neutron halo" system 11 Li with 208 Pb (see, e.g., [1][2][3][4][5]) have generated a considerable amount of interest and controversy. The 11 Li nucleus contains two valence neutrons that are only very weakly coupled to a relatively tightly bound 9 Li core. This unusual composition manifests itself in both the structure of the nucleus, as in the existence of the neutron halo and of low-lying E1 modes [6], and also in reactions with other nuclei. Furthermore, neither the n-9 Li nor the n-n subsystems of 11 Li are bound, so that particle stability in this nucleus is achieved via three-body interactions. Systems of this kind, referred to as "Borromean" nuclei [7], provide an unusual opportunity to study three-body interactions in the nucleus.It has been known for some time that sub-barrier fusion of stable nuclei can be enhanced by several orders of magnitude beyond expectations from simple one-dimensional barrier penetration calculations. A qualitative understanding of this phenomenon has been achieved in terms of couplings to internal degrees of freedom of the target and projectile [8], resulting in a lowering of the effective fusion barrier. This dynamical effect is a very sensitive probe of the nuclear structure of the colliding partners. A lowering of the barrier, by 20% or more, is also a general feature in the results for 11 Li 1 208 Pb fusion presented in [1][2][3][4][5], but the leading effect that was calculated in this case is a static one, resulting from the larger radius of the 11 Li "halo" wave function which allows the attractive nuclear force to act at longer distances. However, additional dynamical enhancement was obtained from the coupling to the soft E1 mode [1,2]. The role played by projectile breakup channels, which are possibly important due to the weak binding of the valence neutrons, is considerably more controversial. Several groups [2][3][4] have reported that coupling to these channels reduces the fusion cross section near the barrier, leading to intriguing structure in the excitation function in this region. However, this point of view has been criticized by Dasso and Vitturi [5] who suggest that it results from a misunderstanding of the nature of multidimensional quantum-mechanical tunneling processes. They report only enhancement of the fusion yield, even in the presence of strong breakup channels. It is important to resolve this controversy since the competition between project...

Fusion and reactions of exotic nuclei

“…4, where the two-neutron evaporation channel of the system 6 He + 208 Pb is compared with the corresponding 4 He + 208 Pb system (both producing the same compound nucleus 212 Pb). Despite the differences observed in the data sets, an overall enhancement of the fusion cross sections is observed when compared to the tightly bound 4 He isotope, in agreement with previous results [10][11][12][13][14]. Fig.…”

“…At the experimental side several measurements of the fusion excitation function of 6 He with heavy targets ( 206 Pb, 209 Bi, 238 U) have been performed at different facilities. After the pioneering work of J.J. Kolata and E.F. Aguilera [10][11][12] using the TWINSOL facility at Notre Dame (USA), additional measurements were carried out by R. Raabe [13,14] and R. Wolski [15] using the CRC facility at Louvain-la-Neuve (Belgium), and by Y.E. Penionzhkevich [16] and S.M.…”

Measurements of angular distributions for7Li elastically scattered from58Ni at energies around the Coulomb barrier

“…A clear and specific example of the relevance of elastic scattering experiments is the discovery of the atomic nucleus in 1911, made by Ernest Rutherford. Lately, many experiments and theoretical analyse have been performed using radioactive beams as projectiles to produce a nuclear reaction [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16]. It is well known that in general experiments with halo nuclei are difficult to carry out, mainly because of the low intensities of the respective beams.…”

“…This effect can first be investigated using stable weakly bound beams which have much higher intensities than radioactive beams. The suitable stable nuclei for this kind of study are 9 Be, 6 Li and 7 Li with separation energies of 1.57, 1.47 and 2.47 MeV, respectively.…”

2n-Evaporation Channel In The4,6He+208,206Pb Fusion Reactions

“…Recently a series of experiments have been performed with the aim to study the fusion-fission process using 6 He beams with close-to-barrier energies [3][4][5][9][10][11]. However, the obtained data were rather controversial and showed that higher statistical confidence is necessary as well as more informative experiments giving possibilities of distinguishing different exit reaction channels.…”