Morton Kaplan

Elliptic flow: Transition from out-of-plane to in-plane emission in Au + Au collisions

We have measured the proton elliptic flow excitation function for the Au + Au system spanning the beam energy range 2 -- 8 AGeV. The excitation function shows a transition from negative to positive elliptic flow at a beam energy,

Fission and emission of H and He in the reactions of 215 MeV16O with181Ta,208Pb and238U

E_{tr} sim

Mechanisms for emission of4He in the reactions of 334 MeV40Ar with238U

4 AGeV. Detailed comparisons with calculations from a relativistic Boltzmann-equation are presented. The comparisons suggest a softening of the nuclear equation of state (EOS) from a stiff form (K sim 380 MeV) at low beam energies (E_{Beam} le 2 AGeV) to a softer form (K sim 210 MeV) at higher energies (E_{Beam} ge

Studies of4He emission in both fusion-like and inelastic reactions of 340-MeV40Ar+238U

4 AGeV) where the calculated baryon density

Azimuthal dependence of pion interferometry at the AGS

rho sim 4 rho_0

Perturbed angular correlation studies of indium metalloporphyrin complexesa)

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Model-independent source imaging using two-pion correlations in (2 to 8)a GeV Au+Au collisions.

The reactions of 215 MeV16O with12C,181Ta,208Pb and238U have been studied. Inclusive measurements for4He emission are given from each target, and for fission and1,2,3H from Ta, Pb and U. For H/He a high-energy, forward-peaked component is observed with characteristics similar to those reported by others. At backward angles a low-energy, nearly-isotropic component is also observed for4He that cannot be accounted for by emission from fully accelerated fission products. The spectral shapes for this evaporative component are compared with statistical model calculations, and information is obtained concerning the effective barriers to emission. For the reactions of16O with12C, complete fusion seems to be overwhelmed by incomplete fusion. Fission angular distributions and cross sections are also presented and discussed.

New probe of intermediate reaction times: Near-scission emission of 4He for 334 MeV 40Ar + 238U

Emission of4He in the reaction 334 MeV40Ar+238U has been studied by triple coincidence measurements that allow the separate identification of fusion fission and sequential fission. For the4He evaporative spectra from fusion fission the composite system is shown to be the predominant contributor; whereas, for sequential fission the dominant emission is from the fragments. This result demonstrates a correlation between evaporative emission probability and lifetime expectancy of the composite system. To account for the observed4He spectra two other mechanisms are necessary in addition to nuclear evaporation. At forward angles, the4He spectra from both fusion fission and sequential fission exhibit higher intensities and larger energies than those expected from purely evaporative processes. This forward-peaked component must be related to a very rapid or pre-thermalization stage of the reaction. At backward angles yet another component is observed for fusion fission. As it is sensitive to the fragment masses but does not carry the kinematic shift characteristic of their full acceleration, this component must originate near to the time of scission. The average4He energy for this component is approximately 17 MeV (c.m.), and its intensity is correlated with a plane perpendicular to the fission fragment separation axis. These signatures are similar to those for long range alpha particle emission in low energy fission. Alpha particles evaporated from the composite nuclei in fusion-fission reactions are shown to be preferentially associated with fission events which result in the more symmetric masses. This result is consistent with the notion that mass asymmetric fission is a faster process than symmetric fission. Such a correlation between mass asymmetry and lifetime is an essential part of the “fast fission” or “quasifission” idea, which has attracted much current attention.

Mechanisms of light-charged-particle emission in the matched reactions 905 and 1030 MeV 121Sb + 27A1 and 550 and 750 MeV 86Kr + 63Cu

The fission-fragment angular correlation technique has been used in conjunction with a position-sensitive avalanche detector to identify the fusion-like and inelastic collisions of 340-MeV40Ar+238U. For each of these two reaction types, we have characterized the correlated4He emission by measuring three-fold coincidences between two fission fragments and the light charged particle. We find an abundance of both evaporation-like and direct4He emission, each of which is associated with both fusion-fission and sequential-fission processes. Several comparison tests of both the coincidence and singles data strongly indicate that very little4He emission is due to evaporation from accelerated fission fragments in this system. The evaporative4He emission appears instead to have a strong contribution originating from the composite system prior to fission. The forward angle4He emissions, characterized by prominent forward-peaking and relatively high energies, suggest an origin associated with the very early stages of the collision.

Surprising properties of the nuclear stratosphere indicated by energy spectra and large-angle correlations between 4He and 1,2,3H or 4He

Abstract Two-pion correlation functions, measured as a function of azimuthal emission angle with respect to the reaction plane, provide novel information on the anisotropic shape and orientation of the pion-emitting zone formed in heavy ion collisions. We present the first experimental determination of this information, for semi-central Au + Au collisions at 2–6xa0 A GeV. The source extension perpendicular to the reaction plane is greater than the extension in the plane, and tilt of the pion source in coordinate space is found to be opposite its tilt in momentum space.