Extended emission sources observed via two-proton correlations

Two-particle correlations between light particles emitted in heavy-ion reactions may be used to extract information on the spatial extent and time development of the emission source. Such measurements have provided evidence for the formation and decay of localized regions of high excitation. An interesting aspect of these measurements is the observation that the less energetic particles are emitted from sources of larger apparent dimensions. This has been interpreted as an indication that the lower energy particles are emitted at a later, more equilibrated stage of the reaction. As originally pointed out by Koonin, the dependence of the two-particle correlation on the direction of the relative momentum, with respect to the direction of emission, may provide information on the source lifetime and shape. Although spatial and temporal effects are not strictly distinguishable, a long-lived spherical emission source will have a characteristically prolate appearance, elongated in the direction of emission. We present an analysis of the directional dependence of the two-proton correlation function and extract the shape of the proton emission source. The source shape is observed to vary from that of the compound nucleus for the emission of energetic protons to an extended spherical shape for the emission of low-energy protons, contrary to expectations for a long-lived evaporative source of compound nucleus dimensions. 2 figs