R. Tripathi

Measuring the temperature of hot nuclear fragments

Abstract A new thermometer based on fragment momentum fluctuations is presented. This thermometer exhibited residual contamination from the collective motion of the fragments along the beam axis. For this reason, the transverse direction has been explored. Additionally, a mass dependence was observed for this thermometer. This mass dependence may be the result of the Fermi momentum of nucleons or the different properties of the fragments (binding energy, spin, etc.) which might be more sensitive to different densities and temperatures of the exploding fragments. We expect some of these aspects to be smaller for protons (and/or neutrons); consequently, the proton transverse momentum fluctuations were used to investigate the temperature dependence of the source.

Constraining the symmetry term in the nuclear equation of state at subsaturation densities and finite temperatures

Methods of extraction of the symmetry energy (or enthalpy) coefficient to temperature ratio from isobaric and isotopic yields of fragments produced in Fermi-energy heavy-ion collisions are discussed. We show that the methods are consistent when the hot fragmenting source is well characterized and its excitation energy and isotopic composition are properly taken into account. The results are independent of the mass number of the detected fragments, which suggests that their fate is decided very early in the reaction.

Analysis of fragment yield ratios in the nuclear phase transition

The critical phenomenon of the liquid-gas phase transition has been investigated in the reactions {sup 78,86}Kr+{sup 58,64}Ni at beam energy of 35 MeV/nucleon using the Landau free energy approach with isospin asymmetry as an order parameter. Fits to the free energy of fragments showed three minima, suggesting that the system is in the regime of a first-order phase transition. The relation m = -{partial_derivative}F/{partial_derivative}H, which defines the order parameter and its conjugate field H, has been experimentally verified from the linear dependence of the mirror nuclei yield ratio data on the isospin asymmetry of the source. The slope parameter, which is a measure of the distance from a critical temperature, showed a systematic decrease with increasing excitation energy of the source. Within the framework of the Landau free energy approach, isoscaling provided similar results as obtained from the analysis of mirror nuclei yield ratio data. In the present work, it is shown that the external field is primarily related to the minimum of the free energy, which implies a modification of the source concentration {Delta} used in isospin studies.

Quantum suppression of fluctuations and temperatures of reconstructed A ∼ 30 quasi-projectiles

Suppression of multiplicity fluctuations has been observed for three light fermions (protons, tritons and 3He) in the multifragmentation of reconstructed hot quasi-projectiles produced in collisions of 32S (45 MeV/nucleon) with 112Sn. This suppression, predicted by recent calculations, is attributed to Pauli blocking and has also been observed in experiments with trapped Fermi gases. Experimental results on nuclear temperature and density employing a quantal approach based on momentum and multiplicity fluctuations are also presented. The extracted temperatures show a noticeable reduction when compared to a similarly derived classical method. This reduction in temperature is in agreement with previous predictions indicating that classically derived methods overpredict nuclear temperature as they do not take into account the Fermi motion of the nucleons. The present results underline the role of quantum statistics in nuclear disassembly and suggest the need for proper quantum treatment when dealing with the thermodynamic properties of fragmenting heavy ions.

ROLE OF QUASIPROJECTILE ISOSPIN ASYMMETRY IN NUCLEAR FRAGMENTATION

Fragment yield data, selected with different values of the isospin asymmetry of the quasiprojectile from 78, 86Kr+58, 64Ni reactions at Elab = 35 MeV/nucleon, have been analyzed within the framework of Landau free energy approach. Fits to the free energy data indicated the system to be in the regime of a first-order phase transition. The plot of free energy versus isospin asymmetry of the fragments showed a systematic change with increasing isospin asymmetry of the fragmenting source. This observation demonstrated the role of quasiprojectile isospin asymmetry as an external field which governed the minima positions in the free energy plot. The position of the central minimum was found to be in close agreement with the average isospin asymmetry of the fragments calculated excluding neutrons and protons. Significant deviations from the free energy plot were observed for N = Z fragments, particularly for the lighter ones, which could be corrected after inclusion of odd–even effects in the analysis. Analysis of yield data of N = Z nuclei gave a reasonable estimate of temperature. Analysis of free energy data gated with excitation energy of the fragmenting source showed a dependence of fit parameters of Landau free energy equation on the excitation energy as well as isospin asymmetry of the source.

Intermediate Mass Fragment Flow as a Probe to the Nuclear Equation of State

The transverse flow of intermediate mass fragments (IMFs) has been investigated for the 35 MeV/u 70Zn+70Zn, 64Zn+64Zn, and 64Ni+64Ni systems. A transition from the IMF transverse flow strongly depending on the mass of the system, in the most violent collisions, to a dependence on the charge of the system, for the peripheral reactions, is shown. This transition was shown to be sensitive to the density dependence of the symmetry energy using the antisymmetrized molecular dynamics model. The results present a new observable, the IMF transverse flow, that can be used to probe the nuclear Equation of State. Comparison with the simulation demonstrated a preference for a stiff density dependence of the symmetry energy.

Investigation of critical behaviour from nuclear fragment yield ratios

Nuclear fragment yield data has been analyzed using the Landau free energy description to investigate the critical phenomena in the fragmentation of quasiprojectile in the reactions 78,86Kr+58,64Ni at beam energy of 35 MeV/nucleon. The data on mirror nuclei yield ratio for A=3 and 7 showed an exponential dependence on the isospin asymmetry of the quasiprojectile as predicted by the Landau free energy approach. The slope parameter, obtained from linear fit to the plots of the logarithm of mirror nuclei yield ratios as a function of isospin asymmetry of the fragmenting source provided a reasonable estimate of the nuclear symmetry energy, and showed a systematic decrease with increasing excitation energy of the quasiprojectile.

Isoscaling, SMM and the symmetry energy: connecting the dots

The isoscaling and the isobaric yield ratio methods are here analysed with respect to Statistical Multifragmentation Model predictions, to highlight the effects of the secondary decay on the symmetry energy estimation.