Direct Observation of the Inversion of Flow

In this report, we describe an experiment designed to determine the mean sign of the scattering angles of the emitted particles for the N + Sm at E/A=35, 100 and 155 MeV. A schematic of the experimental set up is shown in Fig. 1. Two ∆E-E telescopes were positioned at φ=0° and φ=180° around the beam axis and subtended approximately 10° ≤ θ ≤ 35°, where θ and φ are the polar and azimuthal angles respectively. These were used to detect and identify charged particles. The ∆E detector consisted of a 5 cm x 5 cm 16-strip Si detector, 300 μm thick, and was positioned 135 mm from the target position. The E detector consisted of nine tapered CsI(Tl) detectors, 7 cm long. These were arranged in a square 3x3 geometry and placed immediately behind the Si detector. A compact cylindrical multiplicity filter, the Minitube, consisting of 58 scintillating fibers, was placed coaxially around the beam axis in the gap between the two polarimeters. The obtained information on the multiplicity of light charged particles was used to derive the reduced impact parameter, b/bmax, during the collision. The measurements were performed at the National Superconducting Cyclotron Laboratory at Michigan State University. The accelerator provided beams of 35A MeV, 100A MeV and 155A MeV N which impinged on an isotropically enriched target of Sm (98.7%) of areal density 3.15 mg/cm. The circular polarization of γ-rays emitted perpendicular to the reaction plane defined by the beam axis and the coincident light charged particles was measured with two forward-scattering polarimeters [1]. These were positioned at θ=90°, φ=90° and θ=90°, φ=270°. The sign convention adopted [2,3] to define the polarizations with respect to the quantization axis n, is given by n = pixpf/|pixpf| where pi and pf are the momentum vectors of the beam and the detected particle, respectively. The measurement relies on the assumption that the angular momentum transferred to the target residue will be preferentially oriented in the direction expected for a friction-like mechanism between the surfaces of the colliding nuclei [3]. Thus, positive circular γ-ray polarizations correspond to a photon spin parallel to n, and a deflection of the emitted particle to negative angles by the nuclear mean field. Negative circular γ-ray polarizations correspond to a photon spin anti-parallel to n and a deflection of the emitted particle to positive angles, caused by repulsive effects of nucleon-nucleon collisions. Zero polarization values may indicate that either the attractive or repulsive interactions balance each other or that the detected lightcharged particles, in the absence of a collective velocity component, are emitted at random azimuthal direction. Experimentally, the count rate asymmetry, Pγ A, is measured. For the doubly symmetric detector system, the count rate asymmetry can be expressed as [1]