Be9(p, pα)He5Reaction at 57 MeV

The ${\mathrm{Be}}^9(p, p\alpha ){\mathrm{He}}^5$ reaction was studied using 57-MeV protons from the Oak Ridge isochronous cyclotron. The correlated energies of both the emitted protons and $\alpha$ particles were measured at detection angles (${\theta }_p, {\theta }_{\alpha }$) of (94°, -35.3°), (105°, -30°), and (116°, -25.1°), where the detectors were coplanar with the beam. In addition, an angular correlation was measured by fixing ${\theta }_p$ at 105° and varying ${\theta }_{\alpha }$ from -21° to -39° in 3° steps. The data for the reaction leading to the ground state of ${\mathrm{He}}^5$ were found to be consistent with quasifree scattering of the incident protons by $S$-state $\alpha$-particle clusters in ${\mathrm{Be}}^9$. Analysis in terms of a plane-wave impulse approximation yields a momentum density distribution for the struck $\alpha$ particles which has a half-width of 0.27 ${\mathrm{F}}^{-1\mathrm{}}$, and a probability of finding an $\alpha$-particle cluster in ${\mathrm{Be}}^9$ of ${0.25}_{-0.12\mathrm{}}^{+0.25\mathrm{}}$.