No enhancement of fusion probability by the neutron halo of 6He

Abstract

Quantum tunnelling through a potential barrier (such as occurs in nuclear fusion) is very sensitive to the detailed structure of the system and its intrinsic degrees of freedom. A strong increase of the fusion probability has been observed for heavy deformed nuclei. In light exotic nuclei such as 6He, 11Li and 11Be (termed 'halo' nuclei), the neutron matter extends much further than the usual nuclear interaction scale. However, understanding the effect of the neutron halo on fusion has been controversial--it could induce a large enhancement of fusion, but alternatively the weak binding energy of the nuclei could inhibit the process. Other reaction channels known as direct processes (usually negligible for ordinary nuclei) are also important: for example, a fragment of the halo nucleus could transfer to the target nucleus through a diminished potential barrier. Here we study the reactions of the halo nucleus 6He with a 238U target, at energies near the fusion barrier. Most of these reactions lead to fission of the system, which we use as an experimental signature to identify the contribution of the fusion and transfer channels to the total cross-section. At energies below the fusion barrier, we find no evidence for a substantial enhancement of fusion. Rather, the (large) fission yield is due to a two-neutron transfer reaction, with other direct processes possibly also involved.

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.…”

Radioactive ion beam physics at the cyclotron research centre Louvain-la-Neuve

“…however, the strong presence of direct processes, reducing the fusion cross section to levels comparable with those expected for a well-bound, non-halo projectile such as the stable 4 He [92].…”

“…(from Ref. [92]). Schematic view of the set-up used in the measurement of the 6 He + 238 U reaction.…”

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.…”