D. R. Bowman

Universality of spectator fragmentation at relativistic bombarding energies

Abstract Multi-fragment decays of 129Xe, 197Au and 238U projectiles in collisions with Be, C, Al, Cu, In, Au and U targets at energies between E A = 400 and 1000 MeV have been studied with the ALADIN forward-spectrometer at SIS. By adding an array of 84 SiCsI(Tl) telescopes the solid-angle coverage of the setup was extended to θlab = 16°. This permitted the complete detection of fragments from the projectile-spectator source. The dominant feature of the systematic set of data is the Zbound universality that is obeyed by the fragment multiplicities and correlations. These observables are invariant with respect to the entrance channel if plotted as a function of Zbound, where Zbound is the sum of the atomic numbers Zi of all projectile fragments with Zi ⩾ 2. No significant dependence on the bombarding energy nor on the target mass is observed. The dependence of the fragment multiplicity on the projectile mass follows a linear scaling law. The reasons for and the limits of the observed universality of spectator fragmentation are explored within the realm of the available data and with model studies. It is found that the universal properties should persist up to much higher bombarding energies than explored in this work and that they are consistent with universal features exhibited by the intranuclear cascade and statistical multifragmentation models.

The MSU Miniball 4π fragment detection array

Abstract A compact and highly granular charged particle detection array, covering 89% of 4π in solid angle, is described. In its present configuration, the array consists of 188 fast-plastic CsI(Tl) phoswich detectors arranged in 11 rings coaxial about the beam axis. Each phoswich detector is comprised of a 40 μm thick plastic scintillator foil and a 2 cm thick CsI(Tl) crystal selected for good scintillation uniformity. The detectors are read out by photomultiplier tubes. Elemental identification up to Z ≈ 18 and isotopic identification of H and He nuclei is achieved by exploiting pulse shape discrimination techniques.

Multifragment Emission in the Reaction 36Ar + 197Au at E/A = 35, 50, 80, and 110 MeV

Abstract Multifragment emission in the reaction 36 Ar + 197 Au at E A =35, 80, and 110 MeV has been measured with a low-threshold 4π detector array. Over this broad range of incident energies, the mean values and variances of the intermediate mass fragment (IMF: 3 ⩽ Z ⩽ 20) multiplicity distributions exhibit an approximate scaling with the total charged particle multiplicity. The measured multiplicities of light charged particles and intermediate mass fragments are compared with both a model involving statistical decay of an expanding compound nucleus, and with a model involving microscopic quasi-particle dynamics. The statistical decay model predictions are sensitive to the low-density nuclear equation of state.

Impact Parameter Filters for 36Ar + 197Au Collisions at E/A = 50, 80 and 110 MeV

Abstract Collisions between 36Ar projectiles and 197Au target nuclei at E A = 50, 80 and 110 MeV have been studied with the MSU Miniball, a 4π phoswich array with a low detection threshold. Various impact-parameter filters, based upon total charged-particle multiplicity, transverse energy, midrapidity charge and multiplicity of emitted hydrogen nuclei, are compared and cross-correlated. The relative selectivity of each prescription for small impact-parameter collisions is evaluated by assessing the suppression of fast-particle emission at forward angles.

Expansion effects in intermediate energy heavy-ion reactions

Abstract Transverse kinetic energy spectra of fragments in the reaction 36 Ar + 197 Au are used to probe the multifragmentation mechanism at intermediate energies. The mass dependence of the average transverse kinetic energy is examined in the context of the statistical decay of an expanding, emitting source. The role of source expansion during emission is explored. Results suggest the onset of volume emission at the highest incident energy.

Azimuthal correlations as a test for centrality in heavy-ion collisions

Abstract Azimuthal correlations of particles emitted in collisions between 197Au target nuclei and projectiles 36Ar (at E A = 35, 50, 80 and 110 MeV ) and 129Xe (at E A = 50 MeV ) have been measured with the MSU Miniball, a 4π phoswich array with a low detection threshold. Various impact parameter filters, based on the charge-particle multiplicity, transverse energy and midrapidity charge are tested by their ability to suppress collective motion as measured by the azimuthal correlations. The usefulness of a directivity cut in selecting central collisions for these systems is evaluated.

Multifragment emission in 36Ar+197Au and 129Xe+197Au collisions. Percolation model

Abstract Relative abundances of intermediate mass fragments and light particles measured for 36 Ar+ 197 Au collisions at E A = 50, 80 and 110 MeV lie within the range of percolation model predictions, but for 129 Xe+ 197 Au collisions at E A = 50 MeV , the percolation model predicts too small admixtures of intermediate mass fragments among the emitted charged particles.

Thermalization in nucleus-nucleus collisions☆

Abstract Impact parameter dependent excited state populations of intermmediate mass fragments are investigated for 36 Ar induced reactions on 197 Au at E / A = 35 MeV. Population inversions, indicative of non-thermal excitation mechanisms, are observed in peripheral collisions characterized by low associated charged particle multiplicities. These population inversions disappear for collisions with larger associated charged particle multiplicities, consistent with a more complete thermalization for more complex final states. Discrepancies, observed in central collisions, suggest that the limit of local thermal equilibrium has not yet been observed.

Evidence for the Reducibility of Multifragment Emission to an Elementary Binary Emission in Xe-Induced Reactions

Abstract Multifragmentation for 129Xe-induced reactions on several targets (natCu, 89Y, 165Ho, 197Au) has been studied at E A = 40, 50 and 60 MeV . The probability of emitting n intermediate mass fragments is shown to be binomial at each transversal energy and reducible to an elementary binary probability p. For each target and at each bombarding energy, p shows a thermal behavior as demonstrated by linear Arrhenius plots. A nearly universal linear Arrhenius plot is observed at each bombarding energy, independent of target. The sensitivity of p to the Zthreshold points to its physical significance and rules out auto-correlation effects.

Fluctuations in multifragment decays

Abstract An analysis of charged particle distributions is performed to search for large fluctuations and signals of intermittency in 36 Ar + 197 Au reactions at E/A=35–110 MeV. When the effects of impact-parameter averaging are reduced by appropriate cuts on the total transverse energy, charged particle multiplicity distributions narrower than Poisson distributions are observed. Such distributions are inconsistent with intermittency signals of non-trivial origin.