$1n$ and $2n$ Transfer With the Borromean Nucleus ${}^6\mathrm{He}$ Near the Coulomb Barrier

Angular distributions for $1n$ and $2n$ transfer are reported for the ${}^6\mathrm{He}+{}^{65}\mathrm{Cu}$ system at $E_{\mathrm{lab}}=22.6\mathrm{MeV}$. For the first time, triple coincidences between $\alpha$ particles, neutrons, and characteristic $\gamma$ rays from the targetlike residues were used to separate the contributions arising from $1n$ and $2n$ transfer. The differential cross sections for these channels, elastic scattering, and fusion were analyzed using a coupled reaction channels approach. The large measured ratio of the $2n$-to-$1n$ cross section and the strong influence of $2n$ transfer on other channels indicate that the dineutron configuration of ${}^6\mathrm{He}$ plays a dominant role in the reaction mechanism.

Figure 1

Schematic of (a) the experimental setup. (b) Reaction mechanism for $2n$ and $1n$ transfer

Figure 2

$\gamma$ spectrum in coincidence with $\alpha$ particles and neutrons obtained by selecting events outside the region marked in the inset (see text). The inset shows a 2D plot of the difference in positions of the detected $\alpha$ particle and neutron at the plane of the Si detector.

Figure 3

Angular distributions for ${}^6\mathrm{He}+{}^{65}\mathrm{Cu}$. (a) $1n$ and $2n$ transfer. Calculations for $1n$ and $2n$ transfer are shown. (b) Elastic scattering. Calculations with no coupling (dotted curve), $1n$ transfer couplings only (dashed line), and both $1n$ and $2n$ transfer couplings (solid line) are shown.