Li7and Alpha Particles from theLi6+Be9Reaction

Abstract

The kinetic-energy spectra of ${\mathrm{Li}}^7$ nuclei and alpha particles formed as a result of the nuclear reaction between ${\mathrm{Li}}^6$ projectiles of energies 3.0-3.75 MeV and ${\mathrm{Be}}^9$ targets have been studied by a particle-selection system using a gas proportional counter ($\Delta E$) with a very thin window (∼35 μg/${\mathrm{cm}}^2$) and solid-state detectors to measure the energy. No evidence could be found for discrete ${\mathrm{Li}}^7$ energy groups such as would result from a two-product reaction producing ${\mathrm{Be}}^8$ in its ground state or first excited state. The ${\mathrm{Li}}^7$ spectrum is an energy continuum, whereas the alpha-particle spectrum has discrete energies as well as an energy continuum. Absolute values of $\frac{d^2\sigma }{\mathrm{dE}} d\Omega$ have been obtained for these continua in the barycentric system. By extrapolation of the observed results, the total cross section ${\sigma }_t$ for the ${\mathrm{Li}}^7$ continuum is estimated to be 24±8 mb, whereas ${\sigma }_t$ for the alpha continuum, which has contributions from some other reactions also, is estimated to be 103±30 mb. The reaction is interpreted as predominantly a direct three-body breakup ${\mathrm{Be}}^9+{\mathrm{Li}}^6\to {\mathrm{Li}}^7+2\mathrm{}\alpha$ and sequential decay through highly excited states in ${\mathrm{B}}^{11}$. Absolute differential cross sections for the four highest alpha groups from the concomitant reaction ${\mathrm{Be}}^9({\mathrm{Li}}^6,\alpha ){\mathrm{B}}^{11}$ are also given.