Abstract
Average transverse momenta have been carefully measured for \ensuremath{\alpha}-particle pairs at laboratory angles of 21\ifmmode^\circ\else\textdegree\fi{}, 31\ifmmode^\circ\else\textdegree\fi{}, 47\ifmmode^\circ\else\textdegree\fi{}, and 67\ifmmode^\circ\else\textdegree\fi{}. They exhibit small but distinct shifts for different relative azimuthal angles \ensuremath{\Delta}cphi. Detection on the same side of the beam (\ensuremath{\Delta}cphi\ensuremath{\approxeq}24\ifmmode^\circ\else\textdegree\fi{}) yields a smaller average tranverse momentum or energy than on the opposite side (\ensuremath{\Delta}cphi\ensuremath{\approxeq}168\ifmmode^\circ\else\textdegree\fi{}). For \ensuremath{\alpha}-\ensuremath{\alpha} pairs detected at \ensuremath{\approxeq}67\ifmmode^\circ\else\textdegree\fi{} to the beam these small shifts are analyzed with a recoil model to extract the average emitter mass for the first \ensuremath{\alpha} particle from the coincident pairs. The results are A=130\ifmmode\pm\else\textpm\fi{}10 to 115\ifmmode\pm\else\textpm\fi{}10 for reactions of $^{107,109}\mathrm{Ag}$ with $^{40}\mathrm{Ar}$ for 7A and 34A MeV, respectively. The particle pairs emitted at 21\ifmmode^\circ\else\textdegree\fi{} are produced from more complex source mixtures. There is evidence from p-Li and \ensuremath{\alpha}-Li pairs that Li fragments are emitted after protons or \ensuremath{\alpha} particles in \ensuremath{\approxeq}1/2 of the events. The overall pattern is consistent with incomplete fusion leading to the formation of a hot, essentially thermalized composite nucleus and its associated statistical decay.
Published Version
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