The Role of Surface Entropy in Statistical Emission of Massive Fragments from Equilibrated Nuclear Systems

Statistical fragment emission from excited nuclear systems is studied within the framework of a schematic Fermi-gas model combined with Weisskopf's detailed balance approach. The formalism considers thermal expansion of finite nuclear systems and pays special attention to the role of the diffuse surface region in the decay of hot equilibrated systems. It is found that with increasing excitation energy, effects of surface entropy lead to a systematic and significant reduction of effective emission barriers for fragments and, eventually, to the vanishing of these barriers. The formalism provides a natural explanation for the occurrence of negative nuclear heat capacities reported in the literature. It also accounts for the observed linearity of pseudo-Arrhenius plots of the logarithm of the fragment emission probability {\it versus} the inverse square-root of the excitation energy, but does not predict true Arrhenius behavior of these emission probabilities

Thermally-induced expansion in the 8 GeV/c π−\pi^- + 197^{197}Au reaction

Fragment kinetic energy spectra for reactions induced by 8.0 GeV/c $\rm{\pi^-}$ beams incident on a $\rm{^{197}}$Au target have been analyzed in order to deduce the possible existence and influence of thermal expansion. The average fragment kinetic energies are observed to increase systematically with fragment charge but are nearly independent of excitation energy. Comparison of the data with statistical multifragmentation models indicates the onset of extra collective thermal expansion near an excitation energy of E*/A $\rm{\approx}$ 5 MeV. However, this effect is weak relative to the radial expansion observed in heavy-ion-induced reactions, consistent with the interpretation that the latter expansion may be driven primarily by dynamical effects such as compression/decompression.Comment: 12 pages including 4 postscript figure

Isospin Effects in Nuclear Multifragmentation

We develop an improved Statistical Multifragmentation Model that provides the capability to calculate calorimetric and isotopic observables with precision. With this new model we examine the influence of nuclear isospin on the fragment elemental and isotopic distributions. We show that the proposed improvements on the model are essential for studying isospin effects in nuclear multifragmentation. In particular, these calculations show that accurate comparisons to experimental data require that the nuclear masses, free energies and secondary decay must be handled with higher precision than many current models accord.Comment: 46 pages, 16 figure

Statistical Interpretation of Joint Multiplicity Distributions of Neutrons and Charged Particles

Experimental joint multiplicity distributions of neutrons and charged particles emitted in complex nuclear reactions provide an important test of theoretical models. The method is applied to test three different theoretical models of nuclear multi-fragmentation, two of which fail the test. The measurement of neutrons is decisive in distinguishing between the Berlin and Copenhagen models of nuclear multi-fragmentation and challenges the interpretation of pseudo- Arrhenius plots. Statistical-model evaporation calculations with GEMINI give a good reproduction first and second moments of the experimental multiplicity correlations.Comment: 12 pages, 3 figures Added GEMINI calculations of multiplicity correlations Added brief discussion of how neutron emission is treated in MMM

Fragment Isospin as a Probe of Heavy-Ion Collisions

Isotope ratios of fragments produced at mid-rapidity in peripheral and central collisions of 114Cd ions with 92Mo and 98Mo target nuclei at E/A = 50 MeV are compared. Neutron-rich isotopes are preferentially produced in central collisions as compared to peripheral collisions. The influence of the size (A), density, N/Z, E*/A, and Eflow/A of the emitting source on the measured isotope ratios was explored by comparison with a statistical model (SMM). The mid-rapidity region associated with peripheral collisions does not appear to be neutron-enriched relative to central collisions.Comment: 12 pages including figure

An integrated palaeoenvironmental investigation of a 6200 year old peat sequence from Ile de la Possession, Iles Crozet, sub-Antarctica

International audienceA 6200 year old peat sequence, cored in a volcanic crater on the sub-Antarctic Ile de la Possession (Iles Crozet), has been investigated, based on a multi-proxy approach. The methods applied are macrobotanical (mosses, seeds and fruits) and diatom analyses, complemented by geochemical (Rock-Eval6) and rock magnetic measurements. The chronology of the core is based on 5 radiocarbon dates. When combining all the proxy data the following changes could be inferred. From the onset of the peat formation (6200 cal yr BP) until ca. 5550 cal yr BP, biological production was high and climatic conditions must have been relatively warm. At ca. 5550 cal yr BP a shift to low biological production occurred, lasting until ca. 4600 cal yr BP. During this period the organic matter is well preserved, pointing to a cold and/or wet environment. At ca. 4600 cal yr BP, biological production increased again. From ca. 4600 cal yr BP until ca. 4100 cal yr BP a “hollow and hummock” micro topography developed at the peat surface, resulting in the presence of a mixture of wetter and drier species in the macrobotanical record. After ca. 4100 cal yr BP, the wet species disappear and a generally drier, acidic bog came into existence. A major shift in all the proxy data is observed at ca. 2800 cal yr BP, pointing to wetter and especially windier climatic conditions on the island probably caused by an intensification and/or latitudinal shift of the southern westerly belt. Caused by a stronger wind regime, erosion of the peat surface occurred at that time and a lake was formed in the peat deposits of the crater, which is still present today

Excitation and decay of projectile-like fragments formed in dissipative peripheral collisions at intermediate energies

Projectile-like fragments (PLF:15<=Z<=46) formed in peripheral and mid-peripheral collisions of 114Cd projectiles with 92Mo nuclei at E/A=50 MeV have been detected at very forward angles, 2.1 deg.<=theta_lab<=4.2 deg. Calorimetric analysis of the charged particles observed in coincidence with the PLF reveals that the excitation of the primary PLF is strongly related to its velocity damping. Furthermore, for a given V_PLF*, its excitation is not related to its size, Z_PLF*. For the largest velocity damping, the excitation energy attained is large, approximately commensurate with a system at the limiting temperatureComment: 5 pages, 6 figure

A statistical interpretation of the correlation between intermediate mass fragment multiplicity and transverse energy

Multifragment emission following Xe+Au collisions at 30, 40, 50 and 60 AMeV has been studied with multidetector systems covering nearly 4-pi in solid angle. The correlations of both the intermediate mass fragment and light charged particle multiplicities with the transverse energy are explored. A comparison is made with results from a similar system, Xe+Bi at 28 AMeV. The experimental trends are compared to statistical model predictions.Comment: 7 pages, submitted to Phys. Rev.

Fragment properties of fragmenting heavy nuclei produced in central and semi-peripheral collisions

Fragment properties of hot fragmenting sources of similar sizes produced in central and semi-peripheral collisions are compared in the excitation energy range 5-10 AMeV. For semi-peripheral collisions a method for selecting compact quasi-projectiles sources in velocity space similar to those of fused systems (central collisions) is proposed. The two major results are related to collective energy. The weak radial collective energy observed for quasi-projectile sources is shown to originate from thermal pressure only. The larger fragment multiplicity observed for fused systems and their more symmetric fragmentation are related to the extra radial collective energy due to expansion following a compression phase during central collisions. A first attempt to locate where the different sources break in the phase diagram is proposed.Comment: 23 pages submitted to NP

Multi-Periodic Oscillations in Cepheids and RR Lyrae-Type Stars

Classical Cepheids and RR Lyrae-type stars are usually considered to be textbook examples of purely radial, strictly periodic pulsators. Not all the variables, however, conform to this simple picture. In this review I discuss different forms of multi-periodicity observed in Cepheids and RR Lyrae stars, including Blazhko effect and various types of radial and nonradial multi-mode oscillations.Comment: Proceedings of the 20th Stellar Pulsation Conference Series: "Impact of new instrumentation & new insights in stellar pulsations", 5-9 September 2011, Granada, Spai