Quantification of spin alignment in fission by simultaneous treatment of gamma and conversion electron angular distributions

The study of the angular momentum properties of fission fragments can shed light about the complex mechanisms that characterize the fission process. One quantity that is of significant interest, and has not yet been studied adequately, is the alignment of the fragments, which is the cause of anisotropy of the {\gamma} rays along the fission axis and has been observed in various past and recent experiments. In this work, we have performed calculations using the FIFRELIN code, in an attempt to quantify the alignment of the nuclear spins after neutron-emission. Under the statistical tensor formalism of angular distributions, the conversion-electron and the {\gamma}-ray angular distributions can be treated simultaneously in an event-by-event calculation. This enables a first prediction of the conversion-electron angular distribution with respect to the fission axis. An average value for the alignment of fission fragments is deduced for 252Cf, with the use of recent experimental data. The method used for the present work can serve as a starting point for future theoretical and experimental studies in terms of {\gamma} and conversion-electron spectroscopy in view of studying the spin alignment of individual fission fragments, which could further improve our understanding on the process of fission

The Quantum Nature of a Nuclear Phase Transition

In their ground states, atomic nuclei are quantum Fermi liquids. At finite temperatures and low densities, these nuclei may undergo a phase change similar to, but substantially different from, a classical liquid gas phase transition. As in the classical case, temperature is the control parameter while density and pressure are the conjugate variables. At variance with the classical case, 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. 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 $^{4}$He-$^{3}$He liquid mixtures. 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.Comment: 5 pages, 4 figure

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, has been experimentally extracted as a function of the fragment atomic number using isoscaling parameters and the variance of the isotope distributions. The extracted values have been 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 but are significantly different from those characterizing the yields of the primary AMD fragments.Comment: 12 pages, 6 figure

Direct Effects of Sulfur on Forests in Europe - A Regional Model of Risk

A simple dynamic model for the sensitivity and risk in forests under long-term exposure to airborne sulfur is presented. The model is an interpretation of results from long-term forest damage and sulfur dioxide measurements in Czechoslovakia, and it focuses on damage caused by direct, foliar impacts. The input to the model is the annual average SO2 concentration, and the accumulation of impact over time is incorporated. In a regional application of the model, sensitivity is defined as a function of the effective temperature sum. Sensitivity and risk maps of Europe in relation to direct impacts of sulfur are presented

Critical behavior of the isotope yield distributions in the Multifragmentation Regime of Heavy Ion Reactions

Isotope yields have been analyzed within the framework of a Modified Fisher Model to study the power law yield distribution of isotopes in the multifragmentation regime. Using the ratio of the mass dependent symmetry energy coefficient relative to the temperature, $a_{sym}/T$, extracted in previous work and that of the pairing term, $a_{p}/T$, extracted from this work, and assuming that both reflect secondary decay processes, the experimentally observed isotope yields have been corrected for these effects. For a given I = N - Z value, the corrected yields of isotopes relative to the yield of $^{12}C$ show a power law distribution, $Y(N,Z)/Y(^{12}C) \sim A^{-\tau}$, in the mass range of $1 \le A \le 30$ and the distributions are almost identical for the different reactions studied. The observed power law distributions change systematically when I of the isotopes changes and the extracted $\tau$ value decreases from 3.9 to 1.0 as I increases from -1 to 3. These observations are well reproduced by a simple de-excitation model, which the power law distribution of the primary isotopes is determined to $\tau^{prim} = 2.4 \pm 0.2$, suggesting that the disassembling system at the time of the fragment formation is indeed at or very near the critical point.Comment: 5 pages, 5 figure

The 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 concentration 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 4He-3He liquid mixtures. We present experimental results which 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.Comment: 14 pages, 18 figure

Isobaric Yield Ratios and The Symmetry Energy In Fermi Energy Heavy Ion Reactions

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, $a_a/T$, as a function of fragment mass A. The extracted values increase from 5 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 ratios determined from fitting the primary fragment distributions from the AMD model calculation are ~ 4 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.Comment: 10 pages, 12 figure