EFFECT OF THE DETECTOR RESPONSE ON MEASUREMENTS PERFORMED WITH HEAVY-ION 4-PI DETECTORS

We discuss the effect of geometrical efficiency and detection energy thresholds of a typical heavy-ion 4-pi detector (AMPHORA at the ISN) on measurements at energies near 35 MeV/nucleon. The study is based on simulation data generated by two different models: a microscopic interaction code which describes central as well as peripheral collisions, and a statistical code that describes the emission of pre-equilibrium nucleons followed by evaporation from an equilibrated incomplete-fusion system. Two reactions are studied, S-32 + Al-27 at 37.5 MeV/nucleon and Ca-40 + Ca-40 at 35 MeV/nucleon. We have investigated the effect of the incomplete (and selective) detection of the charged particles on several variables (parallel momentum, multiplicities, charge distribution), momentum distribution variables and moments of the event-by-event charge distribution. The results show that the response of the detector severely distorts the variable distributions and that a selection is required to obtain a set of data that represents the original ensemble. Total detected charge (Z(P) + Z(T)) selection can give, depending on the system, an acceptable representation of some of the variables in central and semi-peripheral reactions but the effect of the detector depends strongly on the kinematics of the studied reaction. The optimum application of this selection appears to be associated to relatively light systems studied under conditions of inverse kinematics.