W. C. Hsi

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.

Statistical Multifragmentation in Central Au+Au Collisions at 35 MeV/u

Abstract Multifragment disintegrations, measured for central Au + Au collisions at E A = 35 MeV , are analyzed with the Statistical Multifragmentation Model. Charge distributions, mean fragment energies, and two-fragment correlation functions are well reproduced by the statistical breakup of a large, diluted and thermalized system slightly above the multifragmentation threshold.

Thermal source parameters in Au+Au central collisions at 35 A MeV

Central Au+Au collisions at 35 A MeV are analyzed to find the characteristics of a thermal source, in the framework of the statistical multifragmentation model SMM. A recently introduced backtracing protocol provides an effective comparison of theory and experiment. For the first time the distributions of the central source parameters (density, mass number, excitation energy) are found. The collective energy of primary fragments is also deduced. It is shown that the backtracing procedure allows an estimation of the pre-equilibrium emission. c 1998 Published by Elsevier Science B.V.

Multifragment production in Au+Au at 35 MeV/u

Abstract Multifragment disintegration has been measured with a high efficiency detection system for the reaction Au + Au at E / A = 35 MeV. From the event shape analysis and the comparison with the predictions of a many-body trajectories calculation the data, for central collisions, are compatible with a fast emission from a unique fragment source.

Circumstantial evidence for critical behavior in peripheral Au+Au collisions at 35 MeV/nucleon.

The fragmentation resulting from peripheral Au + Au collisions at an incident energy of {ital E}=35 MeV/nucleon is investigated. A power-law charge distribution, {ital A}{sup {minus}{tau}} with {tau}{approx_equal}2.2, and an intermittency signal are observed for events selected in the region of the Campi scatter plot where {open_quote}{open_quote}critical{close_quote}{close_quote} behavior is expected. {copyright} {ital 1996 The American Physical Society.}

Circumstantial evidence for critical behavior in peripheral {ital A}{ital u}{bold +}{ital A}{ital u} collisions at 35 MeV/nucleon

The fragmentation resulting from peripheral Au + Au collisions at an incident energy of {ital E}=35 MeV/nucleon is investigated. A power-law charge distribution, {ital A}{sup {minus}{tau}} with {tau}{approx_equal}2.2, and an intermittency signal are observed for events selected in the region of the Campi scatter plot where {open_quote}{open_quote}critical{close_quote}{close_quote} behavior is expected. {copyright} {ital 1996 The American Physical Society.}

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.

Caloric curve of8GeV/cπ−,p¯+197Aureactions

The relationship between nuclear temperature and excitation energy of hot nuclei formed by 8 GeV/c negative pion and antiproton beams incident on 197Au has been investigated with the ISiS 4-pidetector array at the BNL AGS accelerator. The double-isotope-ratio technique was used to calculate the temperature of the hot system. The two thermometers used (p/d-3He/4He) and (d/t-3He/4He) are in agreement below E*/A ~ 7 MeV when corrected for secondary decay. Comparison of these caloric curves to those from other experiments shows some differences that may be attributable to instrumentation and analysis procedures. The caloric curves from this experiment are also compared with the predictions from the SMM multifragmentation model.

Time scale for proton emission from highly excited projectiles

Abstract Highly fragmented exit channels produced by the decay of excited 24Mg projectiles were detected. The longitudinal velocity spectra of protons and alpha particles from such channels are offset. This offset is consistent with post-breakup Coulomb accelerations from the 197Au target nuclei implying that the protons were emitted within 3 X 10−22 s after the target-projectile separation.

Fragment emission from modestly excited nuclear systems

Abstract Fragment emission patterns occurring in nuclear systems of modest excitation are studied. Exclusive measurement of fragment emission in 14 N+ 197 Au reactions at E A = 100, 130 and 156 MeV allows selection of central collisions where a single source dominates the decay. Low threshold measurement of IMF emission for these events allows investigation of the influence of detector threshold effects. The time scale of fragment emission is deduced using fragment-fragment velocity correlations. Comparisons are made to the predictions of a statistical decay model.