Semicentral Ar+KC1, La+La, and Ar+Pb collisions at 800 MeV/nucleon were studied using a streamer chamber. The results are analyzed in the framework of the transverse momentum analysis and in terms of the average sphericity matrix. A critical examination of the analysis procedures, both experimental and theoretical, is given. New procedures are described to account for overall momentum conservation in the reaction, and to correct for azimuthal variations in the detection efficiency. Average transverse momenta per nucleon in the reaction plane are presented for deuterons emitted in the forward hemisphere, as these provide the most reliable information. A Vlasov-Uehling-Uhlenbeck calculation with a stiff equation of state gives a good At to the momenta in the Ar+Pb reaction. Flow effects parametrized further using the sphericity tensor are found stronger than in the cascade model and consistently weaker than predicted by hydrodynamics. Parameters from the sphericity tensor exhibit a larger variation as a function of multiplicity than do the average momenta per nucleon.to the present measurements, a discussion of measurement losses, and a discussion of particle identification based on ionization measurements supplemented by kinematic considerations.A. Streamer chamber exposureThe experiment was carried out at the Bevalac Streamer Chamber facility. The targets used were 0.60 g/cm and 0.18 g/cm KC1, 1.31 g/cm Pb304, and 0.62 g/cm La. The KC1 and Pb304 targets were exposed to beams of Ar, and the La target to a beam of ' La, all at 800 MeV/nucleon. A plastic scintillator counter covering an angle around the beam axis of +8' in the laboratory, ' with a signal proportional to XZ, , has been used to select central events. In the Ar-KC1 reaction this trigger was 38 120 1988 The American Physical Society 38 COLLECTIVE MOTION IN NUCLEUS-NUCLEUS COLLISIONS. . . 121 adjusted to accept a fraction of 50% and 10% of the total inelastic cross section. The corresponding cross sections of 0.85 and 0.17 barn correspond in the geometrical picture to impact parameter cuts at 5.3 and 2.4 fm. The respective numbers for La+ La and Ar + Pb reactions are 2.2 (b & 8. 5 fm} and 1.0 barn (b & 5. 5 fm}. Reactions of Ar with oxygen in Pb304 were eliminated by the trigger. B. Measurement of events on film An unbiased sample of events in each reaction was selected for a semiexclusive measurement. 'The Ar+ KC1 data were treated as one data sample consisting of 736 events from the more central trigger mode and 470 events from the more peripheral one. The momentum vectors for charged particles in the events were determined by reconstructing trajectories from the track images recorded on film in three views. Table I gives for each reaction the number of measured events together with characteristic features of the three data samples.

The differential cross section for elastic scattering of a particles by He 4 has been measured at center-ofmass angles between 10 and 90° at laboratory bombarding energies of 53. 40±0.15 MeV, 58.49±0.16 MeV, 63.91±0.18 MeV, 69.91±0.20 MeV, 77.55±0.22 MeV, 99.60±0.28 MeV, and 119.86±0.34 MeV. These differential cross sections have been analyzed in terms of complex phase shifts Re bi~\-i Im 5i. The real parts of the phase shifts vary smoothly as a function of energy; the / = 0, 2, and 4 phase shifts continue the trends apparent at lower energies; a broad 1 = 6 resonance is found at a center-of-mass energy of about 26 MeV and evidence is found for an 1 = 8 resonance. The imaginary parts of the phase shifts are appreciable at all the energies of measurement. The real parts of the phase shifts obtained from these and previous measurements have been fitted with an /-dependent potential constructed from a Saxon-Woods repulsive core and a complex Saxon-Woods attractive well of larger radius. The potential is found to be in qualitative agreement with theoretical predictions. Approximate fits have been made to the real parts of the phase shifts using the singlelevel dispersion theory. The energies and reduced widths of the resonances given by these fits are approximately proportional to 7(/+l).

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