Breakup of ${}^6\mathrm{He}$ incident on ${}^{209}\mathrm{Bi}$ near the Coulomb barrier

A neutron-α-particle coincidence experiment was performed to study breakup of ${}^6\mathrm{He}$ on a ${}^{209}\mathrm{Bi}$ target at an incident energy of 22.5 MeV. Direct breakup, as well as breakup through the $2^{+}$ excited state of ${}^6\mathrm{He}$ and also the ${}^5\mathrm{He}$$+n$ channel, could be distinguished. A total breakup cross section of $205\pm 65$ mb was measured. The data compare well with a coupled-channels calculation that assumes a 50% reduction in the B(E1) computed from a pure ${}^4\mathrm{He}$$+2n$ cluster configuration, in agreement with results obtained at much higher energies. A value of $\mathrm{B}(\mathrm{E}2\Uparrow )=5.4\pm 0.7e^2{\mathrm{fm}}^4$ is deduced for the $2^{+}$ state in ${}^6\mathrm{He}$.

Figure 1

Upper panel: The reaction distance deduced from the neutron energy spectrum. The two horizontal bars give the fusion radius (deduced from the fusion cross section, Ref. [3]), and the distance of closest approach deduced from the reconstructed ${}^6\mathrm{He}$ scattering angle. Lower panel: The observed laboratory neutron energy spectrum in coincidence with alpha particles at $60°$. The background has been subtracted and the spectrum has been corrected for the neutron detection efficiency.

Figure 2

The relative energy between two neutrons coincident with an α particle at $60°$ to the beam.

Figure 3

The angular distribution for ${}^6\mathrm{He}$ breakup on a ${}^{209}\mathrm{Bi}$ target at an incident energy of 22.5 MeV. The circles are the measured total breakup cross section. The triangles are the “resonant breakup” cross section through the $2^{+}$ state in ${}^6\mathrm{He}$. The diamonds represent the direct breakup computed from the $\alpha \mathit{nn}$ triple-coincidence spectrum. The squares are the cross sections for breakup through the ${}^5\mathrm{He}$$+n$ channel. The solid and dot-dashed curves are calculations of total and $2^{+}$ “resonant” breakup, respectively, from Ref. [10]. The “total” breakup calculation does not include the ${}^5\mathrm{He}$$+n$ channel.