Volume 146B, number 1,2 PHYSICS LETTERS 4 October 1984 HIGH MOMENTUM AND ENERGY TRANSFER INDUCED BY 1760 MeV 40Ar ON 197Au AND 232Th TARGETS ¢r E.C. POLLACCO, M. CONJEAUD, S. HARAR, C. VOLANT, Y. CASSAGNOU, R. DAYRAS, R. LEGRAIN, M.S. NGUYEN, H. OESCHLER 1 and F. SAINT-LAURENT 2 Service de Physique Nucl(aire - Basse Energie, CEN Saclay, 91191 Gif-sur-Yvette Cedex, France Received 30 April 1984 Angular correlations between fission fragments induced by 1760 MeV 4°Ar bombarding 197Au and 232Th targets have been measured. Energies, velocities and masses have been obtained for both fragments. The momentum transfer and energy deposited were deduced from an event-by-event analysis. The recoil velocity and total mass spectra show that mass trans- fer is still a dominating process. Momentum transfer of at least 6.5 GeV/c and 900 MeV excitation energy are deduced. In nucleus--nucleus collisions at intermediate ener- gies, a central issue is momentum and energy transfers. The experimental studies of these quantities allow an understanding of some global features, such as the in- elasticity of the interaction and the maximum energy which may be deposited in a nucleus. In this spirit, a measurement was made for the 4°At + 197Au and 40Ar + 232Th systems at 1760 MeV. The momentum transferred in these reactions was obtained from the angular correlation of the fission products whereas the energy deposited was deduced from the measurement of the mass and velocity of both residual fragments. In this letter we report for the first time an excitation energy of 900 MeV in a nucleus (corresponding to 50% of the total binding energy) with a momentum transfer of 7 GeV/c. Notwithstanding this high energy and mo- mentum transfer the present results show that the fol- lowing fission process behaves similarly to low energy particle induced fission [ 1 ]. The measurement was performed at the GANIL facility with a 44 MeV/u 40Ar beam which bombarded 232Th (250/ag/cm 2 on 25 #g/cm 2 carbon-backing) and 197Au (250/~g/cm 2) targets. Beam spot on the target Experiment performed at the GANIL National Laboratory. I Institut fiir Kernphysik, Technische Hochschule, 6100 Darmstadt, Germany. 2 GANIL, BP 5027, 14021 Caen Cedex, France. 0370-2693/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division) was typically 2 × 2 mm 2 . The experimental set-up de- tected in-plane coincident fission fragments with two movable time-of-flight (TOF) detectors on one side of the beam and a fixed array of seven detectors on the opposite side. A schematic drawing of the experimen- tal set-up is shown in fig. 1. The TOF's consisted of a thin foil secondary electron emitter (CP) as start [2] and a solid state detector as stop (A1 and A2) separated by 40 cm. Detectors in the array (B1-B7) were mounted 40 cm away from the target and they covered an angular range between 40 ° and 100 ° relative to the beam. The high field surface barrier detectors used have an area of 6 cm 2 and provided energy and fast timing signals. Flight times for the two fragments were mea- sured relatively to start fast signals gated by the stop. Energy [3] and time calibrations were obtained by using a thin 252Cf source and by utilizing heavy beams from the FN tandem at Saclay. The velocity resolution on either side of the beam was better than 1.5% while the mass resolution was 3u (FWHM) at the TOF and 4u on the opposite side. Each raw coincident data was analyzed event-by- event. From this analysis the mass, energy and velocity of both fragments in the laboratory and center of mass frame of the fissioning nucleus (fig. 1)were deduced. In doing so, it has been assumed that the fission frag- ment mass ratio before and after evaporation were equal. It is worthwhile to note that the extracted value 29