Study of Krypton-Silver, Krypton-gold, Krypton-thorium interactions at 27 and 43 MeV/n in terms of global variables

The determination of desexcitation mechanism of hot and compressed nuclear systems formed after heavy ions collisions at intermediate energies asks for and exclusive and complete detection of the products of the reaction and so the use of multidetectors with large angular coverage. In this work, we study fragment production (Z>8) in nuclear collisions of krypton projectiles accelerated at 27 MeV/uma and 43 MeV/uma with silver, gold and thorium target with the help of the measurements provided by two gaseous multidetectors DELF and XYZT at GANIL. We observed events with a multiplicity of fragments up to 5 and 6 at 27 and 43 MeV/uma respectively (whatever the nature of target). We restricted our study to central collisions with at least 3 heavy fragments detected by separating peripheral collisions which had kept the memory of entrance channel. The recoil velocity of fragments for central events indicates high excitation energy which is comparable to total binding energy of the system. In order to normalize and compact the information which represent the velocities and charges of the 3, 4 or 5 detected fragments, we used several global variables as flow angle and isotropy ratio. We tested if these variables were no too modified by the limitations of the experimental setup by using Monte Carlo simulations. We observed some important differences between the systems we used: first, more events with large flow angle or isotropy ratio for the gold target than for the silver one, second relatively more events with large isotropy ratio for 43 MeV/uma than for 27 MeV/uma. Events with small flow angle has a weak memory of the entrance channel, but the events with large flow angle do not, this indicates the flow angle to be an estimation of impact parameter. The difference between gold and silver target has been assumed to be due to the difference of overlap cross section of the Kr-Au and Kr-Ag systems