Sensitivity of Small-Angle Correlations of Light Charged Particles to Reaction Mechanisms in the 16O+27A1 Reaction at 40MeV/Nucleon

Small-angle $p$-$p$, $p$-$d$, $d$-$\ensuremath{\alpha}$, and $\ensuremath{\alpha}$-$\ensuremath{\alpha}$ correlation functions were measured following the reaction ${}^{16}$O+${}^{27}$Al at 40 MeV/nucleon ${}^{16}$O. These light charged particles (LCP's) were measured with a closely packed hexagonal array of CsI detectors, located at $35\ifmmode^\circ\else\textdegree\fi{}$, with a center to center opening angle of 2.35$\ifmmode^\circ\else\textdegree\fi{}$ for adjacent detectors. Coincident particles were simultaneously detected in the NSCL $4\ensuremath{\pi}$ detector. This measurement was intended to be a complement to earlier results from the same system. Based on studies of this system at lower energies and other published correlation measurements, it was expected that at 40 MeV/nucleon there would be significant positive correlations from the nuclear force and deep anticorrelations from Coulomb repulsion. However, correlation functions from this higher energy are remarkably similar to those previously measured at $\ensuremath{\approx}$15 MeV/nucleon. Correlation functions formed from events with a high multiplicity or high total detected energy (central collisions) are not significantly different from the inclusive measurements. As a possible explanation we suggest that significant correlations are most readily seen in experiments sensitive to LCP's from fast preequilibrium processes and that measurements at more backward angles are primarily sensitive to LCP's from a longer-lived source formed after preequilibrium processes are done. This idea is supported by trends of $p$-$p$ correlation functions from a wide range of systems. A schematic calculation based on a Boltzmann-Ueling-Uhlenbeck (BUU) model and statistical emission qualitatively reproduces the results from this work.