Thomas Keutgen

Experimental determination of the symmetry energy of a low density nuclear gas

Experimental analyses of moderate-temperature nuclear gases produced in the violent collisions of 35 MeV/nucleon Zn-64 projectiles with Mo-92 and Au-197 target nuclei reveal a large degree of alpha particle clustering at low densities. For these gases, temperature- and density-dependent symmetry energy coefficients have been derived from isoscaling analyses of the yields of nuclei with A <= 4. At densities of 0.01 to 0.05 times the ground-state density of symmetric nuclear matter, the temperature- and density-dependent symmetry energies range from 9.03 to 13.6 MeV. This is much larger than those obtained in mean-field calculations and reflects the clusterization of low-density nuclear matter. The results are in quite reasonable agreement with calculated values obtained with a recently proposed virial equation of state calculation.

Quantum Nature of a Nuclear Phase Transition

At finite temperatures and low densities, nuclei may undergo a phase change similar to a classical liquid-gas phase transition. Temperature is the control parameter while density and pressure are the conjugate variables. In the nucleus the difference between the proton and neutron concentrations acts as an additional order parameter, for which the symmetry potential is the conjugate variable. We present experimental results which reveal the N/Z dependence of the phase transition and discuss possible implications of these observations in terms of the Landau free energy description of critical phenomena.

Stand-alone Cosmic Muon Reconstruction Before Installation of the CMS Silicon Strip Tracker

The subsystems of the CMS silicon strip tracker were integrated and commissioned at the Tracker Integration Facility (TIF) in the period from November 2006 to July 2007. As part of the commissioning, large samples of cosmic ray data were recorded under various running conditions in the absence of a magnetic field. Cosmic rays detected by scintillation counters were used to trigger the readout of up to 15% of the final silicon strip detector, and over 4.7 million events were recorded. This document describes the cosmic track reconstruction and presents results on the performance of track and hit reconstruction as from dedicated analyses. Comment: Added missing LaTex command / no change in contents w.r.t. v1

Isospin dependence of the nuclear equation of state near the critical point

We discuss experimental evidence for a nuclear phase transition driven by the different concentrations of neutrons to protons. Different ratios of the neutron to proton concentrations lead to different critical points for the phase transition. This is analogous to the phase transitions occurring in He-4-He-3 liquid mixtures. We present experimental results that reveal the N/A (or Z/A) dependence of the phase transition and discuss possible implications of these observations in terms of the Landau free energy description of critical phenomena.

Isocaling and the symmetry energy in the multifragmentation regime of heavy-ion collisions

The ratio of the symmetry energy coefficient to temperature, a(sym)/T, in Fermi energy heavy-ion collisions, was experimentally extracted as a function of the fragment atomic number using isoscaling parameters and the variance of the isotope distributions. The extracted values were compared to the results of calculations made with an antisymmetrized molecular dynamics (AMD) model employing a statistical decay code to account for deexcitation of excited primary fragments. The experimental values are in good agreement with the values calculated from the final ground-state products but are significantly different from those characterizing the yields of the primary AMD fragments.

Alignment of the CMS silicon strip tracker during stand-alone commissioning

The results of the CMS tracker alignment analysis are presented using the data from cosmic tracks, optical survey information, and the laser alignment system at the Tracker Integration Facility at CERN. During several months of operation in the spring and summer of 2007, about five million cosmic track events were collected with a partially active CMS Tracker. This allowed us to perform first alignment of the active silicon modules with the cosmic tracks using three different statistical approaches; validate the survey and laser alignment system performance; and test the stability of Tracker structures under various stresses and temperatures ranging from +15C to -15C. Comparison with simulation shows that the achieved alignment precision in the barrel part of the tracker leads to residual distributions similar to those obtained with a random misalignment of 50 (80) microns in the outer (inner) part of the barrel. Comment: 41 pages, 63 postscript figures, submitted to JINST

Novel determination of density, temperature, and symmetry energy for nuclear multifragmentation through primary fragment-yield reconstruction

For thefirst time primary hot isotope distributions are experimentally reconstructed in intermediate heavy-ion collisions and used with antisymmetrized molecular dynamics (AMD) calculations to determine density, temperature, and symmetry energy coefficient in a self-consistent manner. A kinematical focusing method is employed to reconstruct the primary hot fragment-yield distributions for multifragmentation events observed in the reaction system 64Zn+112Sn at 40 MeV/nucleon. The reconstructed yield distributions are in good agreement with the primary isotope distributions of AMD simulations. The experimentally extracted values of the symmetry energy coefficient relative to the temperature, asym/T, are compared with those of the AMD simulations with different density dependence of the symmetry energy term. The calculated asym/T values change according to the different interactions. By comparison of the experimental values of asym/T with those of calculations, the density of the source at fragment formation was determined to be ρ/ρ0=(0.63±0.03). Using this density, the symmetry energy coefficient and the temperature are determined in a self-consistent manner as asym=(24.7±1.9)MeV and T=(4.9±0.2) MeV.

Experimental reconstruction of primary hot isotopes and characteristic properties of the fragmenting source in heavy-ion reactions near the Fermi energy

The characteristic properties of the hot nuclear matter existing at the time of fragment formation in multifragmentation events produced in the reaction 64Zn+Sn112 at 40 MeV/nucleon are studied. A kinematical focusing method is employed to determine the multiplicities of evaporated light particles, associated with isotopically identified intermediate-mass fragments. From these data the primary isotopic yield distributions are reconstructed using a Monte Carlo method. The reconstructed yield distributions are in good agreement with the primary isotope distributions obtained from antisymmetrized molecular dynamics transport model simulations. Utilizing the reconstructed yields and power distribution, characteristic properties of the emitting source are examined. The primary mass distribution exhibits a power-law distribution with the critical exponent A-2.3 for A≥15 isotopes but significantly deviate from that for lighter isotopes. Based on the modified Fisher model, the ratios of the Coulomb and symmetry energy coefficients relative to the temperature, ac/T and asym/T, are extracted as a function of A. The extracted asym/T values are compared with results of the antisymmetrized molecular dynamics simulations using Gogny interactions with different density dependencies of the symmetry energy term. The calculated asym/T values show a close relation to the symmetry energy at the density at the time of fragment formation. From this relation the density of the fragmenting source is determined to be ρ/ρ0=0.63±0.03. Using this density, the symmetry energy coefficient and the temperature of fragmenting source are determined in a self-consistent manner as asym=24.7±3.4 MeV and T=4.9±0.2 MeV.

Isobaric yield ratios and the symmetry energy in heavy-ion reactions near the Fermi energy

The relative isobaric yields of fragments produced in a series of heavy-ion-induced multifragmentation reactions have been analyzed in the framework of a modified Fisher model, primarily to determine the ratio of the symmetry energy coefficient to the temperature, a(sym)/T, as a function of fragment mass A. The extracted values increase from 5 to similar to 16 as A increases from 9 to 37. These values have been compared to the results of calculations using the antisymmetrized molecular dynamics (AMD) model together with the statistical decay code GEMINI. The calculated ratios are in good agreement with those extracted from the experiment. In contrast, the values extracted from the ratios of the primary isobars from the AMD model calculation are similar to 4 to 5 and show little variation with A. This observation indicates that the value of the symmetry energy coefficient derived from final fragment observables may be significantly different than the actual value at the time of fragment formation. The experimentally observed pairing effect is also studied within the same simulations. The Coulomb coefficient is also discussed.

Highest temperatures sustainable in heavy nuclei produced in Ar+Au collisions at 60 AMeV

The specific decay mode of hot nuclei surviving fission or fragmentation and leading in the final state to the production of a large residue has been studied in Ar+Au collisions at 60 A MeV. The temperature of these residues has been obtained by analysing the kinematical characteristics (angular and kinetic energy distributions) of the coincident neutrons detected by the DEMON neutron detector, A total cross section of 50 mb has been measured for the production of residues with an initial temperature of 6 MeV or more. This result is compared with the predictions of a microscopic transport model