Source Shape Determination with Directional Correlation Functions

Authors: A. Le Fèvre for the INDRA-ALADIN collaboration
Published: Proceedings of the 41th International Winter Meeting On Nuclear Physics (Bormio 2003), 26 Jan -2 Feb 2003, Bormio, Italy
    edited by I. Iori and A. Moroni (Ricerca Scientifica ed Educazione Permanente, Supplemento no 120, Milano, 2003), p. 178
Abstract: Non-isotropic effects have been observed in central collisions of 129Xe+natSn at 50 A.MeV and 197Au+197Au at 60, 80 and 100 A.MeV incident energies, measured with INDRA at GSI. In all reactions, anisotropy of the fragment distributions in size and mean kinetic energy indicate an incomplete shape relaxation of the multifragmenting source. Using the Berlin microcanonical multifragmentation model (MMMC) extended to a non-spherical source, we have shown that the standard assumption of a spherical source, expanding with either isotropic or anisotropic radial flow, cannot simultaneously explain the angular dependence of both the charge distributions and the fragment kinetic energies. This anisotropy can be understood as the consequence of the statistical multifragmentation of a prolate expanding source aligned along the beam direction. In order to improve the shape characterization of the source, we have constructed correlation functions between fragments from the projected relative velocities. Their sensitivity to various properties of the source like its geometry (volume and shape) has been tested in model studies using results from MMMC calculations. As compared to the correlation functions with angular cuts, the new method employing projections shows a higher sensitivity. The application to the experimental data confirms the spatial elongations of the sources that have previously been determined.
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