Studies of Alpha Particles from theLi6-Li6Nuclear Reaction

The reaction mechanism and the states in ${\mathrm{Be}}^8$ in a ${\mathrm{Li}}^6$-induced reaction ${\mathrm{Li}}^6({\mathrm{Li}}^6, \alpha ){\mathrm{Be}}^8\to 3\alpha$, were studied by a double-coincidence experiment in which angles and energies of the two simultaneous alphas were recorded. The bombarding energy was 1.9 MeV. When the data are transformed to the center-of-mass system, it is shown that a mechanism in which the entire available energy is divided equally between two alphas emitted 180° apart is some 23 times more probable than one in which one of the alphas recoils from ${\mathrm{Be}}^8$ in its ground state. There are also satellite coincidence maxima at 169° and 192°. The pattern can be interpreted as due to a cluster reaction mechanism in which ${\mathrm{Li}}^6$ combines with a deuteron cluster to produce a state of ${\mathrm{Be}}^8$ at 20.95±0.3 MeV which is (${\mathrm{Li}}^6$+$d$)-like. The pattern is fitted best, if a Breit-Wigner probability distribution for the excited ${\mathrm{Be}}^8$ state is assumed, by a total width of 3.4 MeV. It is estimated that the $\alpha$-decay width of this state is 260±90 keV. Such a mechanism has previously been suggested by Coste and Marquez, following ideas of Temmer. The angular distribution of the alpha group which leaves ${\mathrm{Be}}^8$ in its ground state is nearly isotropic in the barycentric system. It seems that the reaction leading to the ground state of ${\mathrm{Be}}^8$ proceeds through an intermediate compound nucleus, ${\mathrm{C}}^{12*}$. The absolute total cross section for an intermediate stage involving the ground state of ${\mathrm{Be}}^8$ is 5.8×${10}^{-29\mathrm{}}$ ${\mathrm{cm}}^2$; for the 20.85 MeV state it is 13×1$0^{-28\mathrm{}}$ ${\mathrm{cm}}^2$.