Entropy in central Au+Au reactions between 100 and 400 AMeV

The ratio of the total charge bound in fragments with Z between 2 and 15 to the hydrogen yield has been measured, and the neutron-to-proton ratio n/p has been estimated from the data of central Au+Au reactions between 100 and 400 AMeV, measured with the phase I setup of the detector system FOPI at the GSI, Darmstadt, in the polar angle range between 7^o and 30^o. These two quantities were used to determine the entropy per nucleon S/A by comparing them with the predictions of the FREESCO code. The analysis allows the simultaneous extraction of the values of the baryonic entropy, temperature, and collective flow. The extracted values are in good agreement with the values obtained in earlier FOPI studies and, for the baryonic entropy, with recent hydrodynamic caculations.