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We have added a new measurement at 100 MeV/nucleon to our previous excitation function for collective flow of light fragments from {sup 40}Ar+{sup 51}V collisions. In the earlier work, flow decreased as the beam energy was raised from 45 to 85 MeV/nucleon. This provided hints to the disappearance of flow, but the lack of measurements at higher beam energies precluded the observation of the reappearance of flow. At a beam energy of 100 MeV/nucleon the flow has reappeared and this allows an experimental determination of the region where attractive scattering balances with repulsive scattering in these collisions. A simultaneous least-squares fit of the flow data for light fragments yields (85{plus minus}10) MeV/nucleon for the balance energy.

Total reaction cross sections, sigma(R), on Si were measured near 40 A MeV for the proton-halo candidate N-12 and the two-proton-halo candidate Ne-17, and were compared with sigma(R) for other light proton-rich nuclei. The A-dependence shows enhanced sigma(R)s for N-12 and Ne-17, relative to their neighbors, but the effect is smaller than for B-8 which has been argued to have a proton halo. In general, nuclei with loosely bound last protons (S-p less than or equal to 1.5 MeV) have significantly larger sigma(R)S than their neighbors. Cross sections for charge-removal from N-12 and Ne-17 also were obtained. (C) 1998 Elsevier Science B.V.

The energy at which the collective transverse flow in the reaction plane disappears, the balance energy ${\mathit{E}}_{\mathrm{bal}}$, is found to increase linearly as a function of impact parameter for $^{40}\mathrm{Ar}$${+}^{45}$Sc reactions. Comparison of our measured values of ${\mathit{E}}_{\mathrm{bal}}$(b) with predictions from quantum molecular dynamics (QMD) model calculations agrees better with an approach incorporating momentum dependence in the nuclear mean field. \textcopyright{} 1996 The American Physical Society.

Total reaction cross sections, sigma R, of 20-60A MeV He-4,He-6,He-8, Li-6-9,Li-11, and Be-10 were measured by injecting magnetically separated, focused, monoenergetic, identified secondary beams of those projectiles into a Si detector telescope and measuring their energy-deposition spectra. These sigma(R)s accurate to about 3%, were compared with predictions of optical, strong absorption, and microscopic models. The latter gave the best overall fit to the data, providing long-tailed matter densities were assumed. The best available optical potentials generally overpredicted the data by about 10%. Strong absorption calculations, in which the isospin-dependant term is quite important, were often unsuccessful, especially for projectiles with large neutron excess. Two-neutron removal cross sections were measured for He-6 and Li-11; the Li-11 data were slightly overpredicted by a microscopic model which includes correlation effects for the Li-11 valence neutrons. Both 2n and 4n removal from He-8 were observed, in about a 2:1 ratio. Subtraction analysis of the data indicates that He-4 is a good core within He-6 and He-8, as is Li-9 within Li-11.

Collective transverse flow of nuclear matter was measured as a function of the ratio of neutrons to protons ( $N/Z$) of the interacting system for the first time. The collisions of three isotopically pure beams of $A\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}58\mathrm{nuclei}$ with two $A\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}58\mathrm{targets}$ were studied at 55 MeV/nucleon. The results for the flow variables demonstrate the sensitivity of transport models to elementary aspects of the nucleon-nucleon collisions.

We have experimentally studied small impact parameter heavy-ion collisions in the (nearly) symmetry entrance channels $^{12}\mathrm{C}$${+}^{12}$C, $^{20}\mathrm{Ne}$${+}^{27}$Al, $^{40}\mathrm{Ar}$${+}^{45}$Sc, $^{84}\mathrm{Kr}$${+}^{93}$Nb, and $^{129}\mathrm{Xe}$${+}^{139}$La, each at many intermediate beam energies. The results from a number of analyses based on a projection of the ``shapes'' of the experimental events called the sphericity are presented. Comparisons of the relative efficiencies of various experimental methods for the selection of central events are made. The importance of autocorrelations between the sphericity and the various impact-parameter--dependent variables is evaluated. Searches for beam energy-dependent transitions from sequential binary disassembly to multifragmentation in the central events are described. Comparisons to dynamic and hybrid model code calculations will be discussed. The average sphericities of the intermediate mass fragments (IMF's, for which 3\ensuremath{\le}Z\ensuremath{\lesssim}20), are presented. The possibility that the IMF emission occurs following the formation of transient toroidal or disk-like geometries in the central events is explored. Increases in the average sphericities of the central events for increasing beam energies are observed which is attributed to transitions from sequential binary disassembly to multifragmentation. The transitional beam energies for the central $^{40}\mathrm{Ar}$${+}^{45}$Sc, $^{84}\mathrm{Kr}$${+}^{93}$Nb, and $^{129}\mathrm{Xe}$${+}^{139}$La reactions are near \ensuremath{\sim}50, \ensuremath{\sim}40, and \ensuremath{\sim}40 MeV/nucleon, respectively.

Fission-like fragments and coincident charged particles have been measured in a 4π geometry over a wide energy range (15–115 A MeV) for the reaction 40 Ar+ 232 Th. The exclusive folding angle distribution data provide direct evidence that fission-like processes following incomplete-fusion are still an appreciable exit channel for beam energies as high as 115 A MeV.

The {ital Z} distributions of fragments emitted from central collisions of {sup 40}Ar+{sup 45}Sc at beam energies from 15 to 115 MeV/nucleon have been fitted to power laws {sigma}({ital Z}){proportional_to}{ital Z}{sup {minus}{lambda}}. The {lambda} parameter reaches a minimum at a beam energy of 23.9{plus_minus}0.7 MeV/nucleon. A percolation model calculation reproduces the observed {ital Z} distributions for all beam energies, using the mean excitation energy as extracted from proton kinetic energy spectra. We extract the critical value of the deposited excitation energy for our system and make predictions for the dependence of this quantity on the size of the fragmenting system.

Multifragment azimuthal correlation functions have been measured as a function of beam energy and impact parameter for the Ar+Sc system ([ital E]/[ital A]=35 to 115 MeV). The observed azimuthal correlation functions---which do not require corrections for dispersion of the reaction plane---exhibit strong asymmetries which are dependent on impact parameter and beam energy. Rotational collective motion and flow seem to dominate the correlation functions at low beam energies. It is proposed that multifragment azimuthal correlation functions can provide a useful probe for intermediate energy heavy ion reaction dynamics.

By studying light charged particles emitted in coincidence with projectile-like fragments, we probe heavy ion reaction mechanisms that result in the production of light particles in peripheral collisions. Triple coincidence {alpha}-{alpha}-{sup 12}C correlations in the reactions of 35 MeV/nucleon {sup 16}O with Ni conclusively identify previously known sources of light particles. The fast, beam velocity component of {alpha} particles in coincidence with projectile-like fragments is associated with the breakup of the projectile. The correlations suggest that the origin of the intermediate velocity component of {alpha} particles is the target nucleus. Both of these components are characterized by {alpha} particles emitted during the early stages of the collision.

Light charged fragments from the reaction {sup 40}Ar+{sup 51}V at 35--85 MeV/nucleon in 10 MeV/nucleon steps have been measured with a 4{pi} detector. The observed transverse collective momentum in central collisions decreases monotonically from 45 to 85 MeV/nucleon. The data are interpreted assuming that the attractive mean-field deflects the fragments to negative scattering angles. As the beam energy is raised, the attractive scattering is opposed by the buildup of compressed nuclear matter. A lower limit of 76 MeV/nucleon is placed on the beam energy where the attractive deflection is balanced by the repulsive side-splash mechanism. The results are compared to theoretical predictions and it is demonstrated the predictions are sensitive to two aspects of nuclear matter physics: the in-medium nucleon-nucleon cross section and the nuclear compressibility.

Azimuthal distributions of fragments with respect to the reaction plane are studied in the Ar+V system as a function of beam energy. Light charged particles are found to exhibit an enhanced emission in the reaction plane which increases with the mass of the detected particle. As the beam energy is increased, the asymmetry decreases. Possible mechanisms behind the asymmetry, such as rotational collective motion and directed transverse momentum, are discussed.