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ALADIN collaboration

Publications and source records attributed to ALADIN collaboration.

13 recordsLinked to original sources
arXiv2000

Fragment Kinetic Energies and Modes of Fragment Formation

Kinetic energies of light fragments A <= 10 from the decay of target spectators in 197Au 197Au collisions at 1000 MeV per nucleon have been measured with high-resolution telescopes at backward angles. Except for protons and apart from the observed evaporation components, the kinetic-energy spectra exhibit slope temperatures of about 17 MeV, independent of the particle species, but not corresponding to the thermal or chemical degrees of freedom at breakup. It is suggested that these slope temperatures may reflect the intrinsic Fermi motion and thus the bulk density of the spectator system at the instant of becoming unstable. PACS numbers: 25.70.Pq, 21.65.+f, 25.70.Mn

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arXiv1999

Breakup Density in Spectator Fragmentation

Proton-proton correlations and correlations of protons, deuterons and tritons with alpha particles from spectator decays following 197Au + 197Au collisions at 1000 MeV per nucleon have been measured with two highly efficient detector hodoscopes. The constructed correlation functions, interpreted within the approximation of a simultaneous volume decay, indicate a moderate expansion and low breakup densities, similar to assumptions made in statistical multifragmentation models. PACS numbers: 25.70.Pq, 21.65.+f, 25.70.Mn, 25.75.Gz

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arXiv1998

Thermodynamic Variables from Spectator Decay

New results for the masses, excitation energies, temperatures, and densities of excited spectator systems at breakup are presented. They were obtained in two recent experiments with the ALADIN spectrometer at SIS in which reactions of Au-197 on Au-197 in the regime of relativistic energies up to 1 GeV per nucleon were studied. The methods used to extract these thermodynamic variables are discussed.

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arXiv1998

Breakup Conditions of Projectile Spectators from Dynamical Observables

Momenta and masses of heavy projectile fragments (Z >= 8), produced in collisions of 197Au with C, Al, Cu and Pb targets at E/A = 600 MeV, were determined with the ALADIN magnetic spectrometer at SIS. An analysis of kinematic correlations between the two and three heaviest projectile fragments in their rest frame was performed. The sensitivity of these correlations to the conditions at breakup was verified within the schematic SOS-model. The data were compared to calculations with statistical multifragmentation models and to classical three-body calculations. Classical trajectory calculations reproduce the dynamical observables. The deduced breakup parameters, however, differ considerably from those assumed in the statistical multifragmentation models which describe the charge correlations. If, on the other hand, the analysis of kinematic and charge correlations is performed for events with two and three heavy fragments produced by statistical multifragmentation codes, a good agreement with the data is found with the exception that the fluctuation widths of the intrinsic fragment energies are significantly underestimated. A new version of the multifragmentation code MCFRAG was therefore used to investigate the potential role of angular momentum at the breakup stage. If a mean angular momentum of 0.75$\hbar$/nucleon is added to the system, the energy fluctuations can be reproduced, but at the same time the charge partitions are modified and deviate from the data. PACS numbers: 25.70.Mn, 25.70.Pq, 25.75.Ld, 25.75.-q

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arXiv1998

Temperatures of Exploding Nuclei

Breakup temperatures in central collisions of 197Au + 197Au at bombarding energies E/A = 50 to 200 MeV were determined with two methods. Isotope temperatures, deduced from double ratios of hydrogen, helium, and lithium isotopic yields, increase monotonically with bombarding energy from 5 MeV to 12 MeV, in qualitative agreement with a scenario of chemical freeze-out after adiabatic expansion. Excited-state temperatures, derived from yield ratios of states in 4He, 5Li, 6Li, and 8Be, are about 5 MeV, independent of the projectile energy, and seem to reflect the internal temperature of fragments at their final separation from the system. PACS numbers: 25.70.Mn, 25.70.Pq, 25.75.-q

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arXiv1997

Breakup Temperature of Target Spectators in Au + Au Collisions at E/A = 1000 MeV

Breakup temperatures were deduced from double ratios of isotope yields for target spectators produced in the reaction Au + Au at 1000 MeV per nucleon. Pairs of $^{3,4}$He and $^{6,7}$Li isotopes and pairs of $^{3,4}$He and H isotopes (p, d and d, t) yield consistent temperatures after feeding corrections, based on the quantum statistical model, are applied. The temperatures rise with decreasing impact parameter from 4 MeV for peripheral to about 10 MeV for the most central collisions. The good agreement with the breakup temperatures measured previously for projectile spectators at an incident energy of 600 MeV per nucleon confirms the observed universality of the spectator decay at relativistic bombarding energies. The measured temperatures also agree with the breakup temperatures predicted by the statistical multifragmentation model. For these calculations a relation between the initial excitation energy and mass was derived which gives good simultaneous agreement for the fragment charge correlations. The energy spectra of light charged particles, measured at $θ_{lab}$ = 150$^{\circ}$, exhibit Maxwellian shapes with inverse slope parameters much higher than the breakup temperatures. The statistical multifragmentation model, because Coulomb repulsion and sequential decay processes are included, yields light-particle spectra with inverse slope parameters higher than the breakup temperatures but considerably below the measured values. The systematic behavior of the differences suggests that they are caused by light-charged-particle emission prior to the final breakup stage. PACS numbers: 25.70.Mn, 25.70.Pq, 25.75.-q

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arXiv1997

New Temperature Measurements in 197Au + 197Au Collisions

We report on new measurements of breakup temperatures for target spectators from 197Au + 197Au reactions at 1000 MeV per nucleon. The temperatures rise with decreasing impact parameter from 4 MeV for peripheral to about 10 MeV for the most central collisions, in good agreement with previous results for projectile spectators at 600 MeV per nucleon. The measured temperatures agree quantitatively with the breakup temperatures predicted by the statistical multifragmentation model. For these calculations a relation between the initial excitation energy and mass was derived which gives good simultaneous agreement for the fragment charge correlations. The energy spectra of light charged particles and the behaviour of the mean kinetic energies of neutrons indicate a substantial component of light particle emission prior to the final breakup stage.

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arXiv1997

Source Temperatures and Sizes in Central Collisions

For midrapidity fragments from central 50-200 AMeV Au+Au collisions temperatures from double ratios of isotopic yields were compared with temperatures from particle unbound states. Temperatures from particle unbound states with T = 4-5 MeV show with increasing beam energy an increasing difference to temperatures from double ratios of isotopic yields, which increase from T = 5MeV to T = 12MeV. The lower temperatures extracted from particle unstable states can be explained by increasing cooling of the decaying system due to expansion. This expansion is driven by the radial flow, and freeze out of particle unstable states might depend on the dynamics of the expanding system. Source sizes from pp-correlation functions were found to be 9 to 11 fm.

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arXiv1997

Emission temperatures and freeze out densities from light particle correlation functions in Au+Au collisions at 1 A GeV

A study of emission temperatures extracted from excited state populations and of freeze out radii from light particle intensity interferometry is presented. Three high resolution dE-E-Hodoscopes with a total of 216 detectors are combined with the ALADiN setup in order to study Au+Au collisions at 1 A GeV. In contrast to measurements with the isotope thermometer the extracted apparent temperatures do not vary with impact parameter thus with excitation energy. From the extracted radii a freeze out density was determined which decreases from $0.2ρ_0$ for the most peripheral to less than $0.1ρ_0$ for the most central collisions. A density--dependent feeding correction is applied to the different temperature measurements.

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arXiv1996

Determination of Critical Exponents in Nuclear Systems

Signatures of critical behaviour in nuclear fragmentation are often based on arguments from percolation theory. We demonstrate with general thermodynamic considerations and studies of the Ising model that the reliance on percolation as a reference model bears the risk of missing parts of the essential physics.

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arXiv1996

Last Minute from ALADIN: Temperature measurements in Au+Au reactions at relativistic energies

We report on temperature measurements of nuclear systems formed in the Au+Au collisions at incident energies of 50, 100, 150, 200 and 1000 A MeV. The target spectator matter was studied at the highest energy and the interacting zone (participants) at the lower ones.The temperature deduced from the isotope ratios was compared with the one deduced via the excited states population. An unexpected disagreement was found between the two measurements.

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arXiv1996

The Nuclear Liquid-Gas Phase Transition: Present Status and Future Perspectives

More than two decades ago, the van der Waals behavior of the nucleon - nucleon force inspired the idea of a liquid-gas phase transition in nuclear matter. Heavy-ion reactions at relativistic energies offer the unique possibility for studying this phase transition in a finite, hadronic system. A general overview of this subject is given emphasizing the most recent results on nuclear calorimetry.

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