Z. Phys. A 346, 63-68 (1993) ZEITSCHRIFT FOR PHYSIK A 9 Springer-Verlag 1993 Fission study for the system 84Kr+ Z3ZThat 25, 35 and 45 MeV/u Emanuel C. Pollaeco, Claude Volant, Yves Cassagnou, Monique Conjeaud, Roland Dayras, Samuel Harar*, Robert Legrain, Jean-Etienne Sauvestre DAPNIA/SPhN, CEN Saclay, F-91191 Gif-sur-Yvette Cedex, France Received: 12 October 1991/Revised: 27 October 1992 Abstract. Recoil velocities of heavy residues produced in the bombardment of 84Kr beams on a 232Th target at energies about the Fermi velocity were determined using the method of correlations between fission fragments. The masses of both fragments were also measured. The data show events with high recoil velocity which are produced in incomplete fusion processes. Estimates of the energy transfer are given. Comparisons with incomplete fusion data obtained with other projectiles in the same incident velocity range show an increase of the energy transfer with the mass of the projectile. PACS: 25.70.-z; 25.70.Jj; 25.85.Ge I. Introduction In nucleus-nucleus collisions at bombarding energies up to about 15 MeV/u a large fraction of the cross section goes into fusion making possible the study of nuclei at relatively high excitation energy and angular momentum. With increasing bombarding energy (Ei) this cross section gives way to incomplete fusion [1, 2]. At even higher energies the cross section for incomplete fusion becomes highly suppressed [3, 4]. As for this evolution, interpreta- tions vary from temperature limitations [5] to dynamic processes [6]. In a series of experiments using heavy projectiles on fissile targets, the measurement of the velo- city and mass of fission fragments gave an overview of the reaction and provided an estimate of the transferred en- ergy (Er). In this paper, data are presented for the 84Kr+232Th reaction at 25, 35 and 45 MeV/u incident energies. Unlike the low energy measurements [-7] where fusion is hindered due to barrier penetrabilities, at 25 MeV/u and beyond, incomplete fusion is observed where a large fraction of the projectile (,--70%) fuses with the target. Further, using the same methodology as in [3, 8, 9] the systematics indicate that the energy transfer- Experiment performed at GANIL * Present address: GANIL, BP 5027, F-14021 Caen Cedex, France red to the composite system increases with projectile mass for a given incident velocity. II. Experimental procedure GANIL provided krypton beams at 25, 35 and 45 MeV/u of several tens of nanoamps. The 25 MeV/u beam (35 + charge state) was derived from the primary 35 MeV/u beam (26 § charge state) using a carbon degrader before the analysing magnets. The 23aTh target, 250pg/cm 2 thick, was deposited on a carbon backing (25 pg/cm2). The recoil velocity of the fissioning nucleus was measured by employing the fission fragment (FF) correlation method [8]. Coincidences between the two FF were meas- ured in-plane. One of the FF was detected by one of the two time-of-flight (TOF) detectors located on one side of the beam at 30 ~ and 80 ~ and which consisted of micro- channel plate systems as start and silicon counters as stop (area~900mm2). The flight paths were approximately 45 cm long. To detect the other FF, an array of 8 silicon detectors (area~900 mm 2) were positioned on the oppo- site side of the beam at about 45 cm from the target equally spaced between 20 ~ and 90 ~ . The silicon detectors provided both an energy and timing signal. By measuring the relative timing for particles entering the silicon counters on either side of the beam and correcting for the different flight paths the velocities of both FF were meas- ured. The velocity and mass resolutions for TOF's were typically AV/V~2% and AM/M~4% and better than those for the opposite detectors AV/V~3% and AM/M~6% due to cumulative errors. Energy and time calibrations were obtained by using a 252Cf source on a thin backing and corrections were done for the pulse height defects. The data were analysed event-by-event and gave for each event velocities and energies in the labora- tory system and the deduced masses (M1, ME). By assum- ing binary fission and that evaporation does not change the average velocities and the FF mass ratio, the recoil velocity parallel (V//) and perpendicular to the beam direc- tion as well as the relative velocity (VFF) between the fragments were calculated for each event.