Lattice gas model for fragmentation: From Argon on Scandium to Gold on Gold

The recent fragmentation data for central collisions of Gold on Gold are even qualitatively different from those for central collisions of Argon on Scandium. The latter can be fitted with a lattice gas model calculation. Effort is made to understand why the model fails for Gold on Gold. The calculation suggests that the large Coulomb interaction which is operative for the larger system is responsible for this discrepancy. This is demonstrated by mapping the lattice gas model to a molecular dynamics calculation for disassembly. This mapping is quite faithful for Argon on Scandium but deviates strongly for Gold on Gold. The molecular dynamics calculation for disassembly reproduces the characteristics of the fragmentation data for both Gold on Gold and Argon on Scandium.Comment: 13 pages, Revtex, 8 figures in ps files, submitted to Phys. Rev.

Irreversibility and Polymer Adsorption

Physisorption or chemisorption from dilute polymer solutions often entails irreversible polymer-surface bonding. We present a theory of the non-equilibrium layers which result. While the density profile and loop distribution are the same as for equilibrium layers, the final layer comprises a tightly bound inner part plus an outer part whose chains make only fN surface contacts where N is chain length. The contact fractions f follow a broad distribution, P(f) ~ f^{-4/5}, in rather close agreement with strong physisorption experiments [H. M. Schneider et al, Langmuir v.12, p.994 (1996)].Comment: 4 pages, submitted to Phys. Rev. Let

Alphabetisierung - bloß berufliche Notwendigkeit oder mehr?

Anstrengungen zur Verringerung der unzureichend gegebenen Elementarbildung jener Bevölkerungsgruppe, auf die hierzulande die Bezeichnung „AnalphabetInnen“ zutrifft, werden derzeit – so wie Bildungsbemühungen generell – fast durchwegs unter dem Gesichtspunkt des gesellschaftlichen Tausch- bzw. Marktwerts argumentiert. Sie gelten als eine Investition in die „Ressource Mensch“ – auf einen darüber hinausgehenden humanitären oder politischen Wert wird nur selten Bezug genommen. Auch wenn die Bedeutung der Kulturtechniken im Rahmen der ökonomischen Verwertung menschlicher Arbeitskraft selbstverständlich nicht ignoriert werden kann, bedeutet die Befähigung der Menschen zur schriftlichen Kommunikation weit mehr als ihre Zurichtung zu verwertbarem Humankapital. Literalität ist ein grundsätzlicher Aspekt der „Menschwerdung“ des Menschen, sie erst ermöglicht dem einzelnen Individuum eine Teilhabe am kulturellen und gesellschaftlichen Leben, darüber hinaus und darauf aufbauend aber auch die fortschreitende Emanzipation des Menschengeschlechts insgesamt. Die Teilhabemöglichkeit an der menschlichen Gemeinschaft ist nicht bloß ein Vorteil unter vielen anderen, der auf der gleichen Ebene wie die Möglichkeit, seine Arbeitskraft lukrativ verkaufen zu können, angesiedelt ist. Es geht dabei um wesentlich mehr – in letzter Konsequenz um die Überwindung der die Gesellschaft in allen Aspekten bestimmenden Verwertungsprämisse. (DIPF/Orig.

Effects of Compression and Collective Expansion on Particle Emission from Central Heavy-Ion Reactions

Conditions under which compression occurs and collective expansion develops in energetic reactions of heavy nuclei, are analyzed, together with their effects on emitted light baryons and pions. Within transport simulations, it is shown that shock fronts perpendicular to beam axis form in head-on reactions. The fronts separate hot compressed matter from normal. As impact parameter increases, the angle of inclination of the fronts relative to beam axis decreases, and in-between the fronts a weak tangential discontinuity develops. Hot matter exposed to the vacuum in directions perpendicular to shock motion (and parallel to fronts), starts to expand sideways, early within reactions. Expansion in the direction of shock motion follows after the shocks propagate through nuclei, but due to the delay does not acquire same strength. Expansion affects angular distributions, mean-energy components, shapes of spectra and mean energies of different particles emitted into any one direction, and further particle yields. Both the expansion and a collective motion associated with the weak discontinuity, affect the magnitude of sideward flow within reaction plane. Differences in mean particle energy components in and out of the reaction plane in semicentral collisions, depend sensitively on the relative magnitude of shock speed in normal matter and speed of sound in hot matter.Comment: 71 pages, 33 figures (available on request), report MSUCL-94

Statistical nature of cluster emission in nuclear liquid-vapour phase coexistence

The emission of nuclear clusters is investigated within the framework of isospin dependent lattice gas model and classical molecular dynamics model. It is found that the emission of individual cluster which is heavier than proton is almost Poissonian except near the transition temperature at which the system is leaving the liquid-vapor phase coexistence and the thermal scaling is observed by the linear Arrhenius plots which is made from the average multiplicity of each cluster versus the inverse of temperature in the liquid vapor phase coexistence. The slopes of the Arrhenius plots, {\it i.e.} the "emission barriers", are extracted as a function of the mass or charge number and fitted by the formula embodied with the contributions of the surface energy and Coulomb interaction. The good agreements are obtained in comparison with the data for low energy conditional barriers. In addition, the possible influences of the source size, Coulomb interaction and "freeze-out" density and related physical implications are discussed

Liquid-gas phase transition in nuclear multifragmentation

The equation of state of nuclear matter suggests that at suitable beam energies the disassembling hot system formed in heavy ion collisions will pass through a liquid-gas coexistence region. Searching for the signatures of the phase transition has been a very important focal point of experimental endeavours in heavy ion collisions, in the last fifteen years. Simultaneously theoretical models have been developed to provide information about the equation of state and reaction mechanisms consistent with the experimental observables. This article is a review of this endeavour.Comment: 63 pages, 27 figures, submitted to Adv. Nucl. Phys. Some typos corrected, minor text change

Ancient habitat shifts and organismal diversification are decoupled in the African viper genus Bitis (Serpentes: Viperidae)

Aim: The expansion of open habitats during the mid-Miocene has been hypothesized as a driver of allopatric speciation for many African taxa. This habitat-dependent mode of diversification has been implicated in the shift from C3 (e.g. forest/woodland) to C4 dominated systems (i.e. open savanna, grasslands) in a number of African squamates. We examined this hypothesis using a genus of African viperid snakes (Bitis) with both open habitat and forest-dwelling representatives. Location: Africa. Methods: A comprehensive multilocus dataset was used to generate a calibrated species tree using a multispecies coalescent model. Individual gene trees and patterns of nuclear allele sharing were used to assess species monophyly and isolation. To test the habitat-dependent evolution hypothesis, we generated an ancestral character state reconstruction for open and closed habitats using the dated phylogeny. This was related to the timing of open habitat expansion and forest/woodland contraction in Africa. Results: The genus Bitis originated in the Oligocene, with species level diversification in the late Miocene/Pliocene. Four well-supported clades correspond to the recognized subgenera Bitis, Keniabitis, Macrocerastes and Calechidna. Several previously unrecognized lineages potentially represent cryptic species. Main conclusions Habitat-dependent evolution does not appear to have been a main driver for generic level viperine diversification: the ancestral state for Bitis was open habitat and at least one clade moved into forest in the Miocene, long after forest had contracted and fragmented. Forest-dependent species diversified only in the late Miocene, presumably as forest became further reduced in extent, fitting an allopatric model of speciation. Although our results do not favour a general pattern of habitat-dependent diversification in Bitis, cladogenesis within the subgenus Calechidna for “arenicolous” species (Bitis caudalis complex) and “rupicolous” species (B. Atropos-cornuta complex), corresponds to the aridification of southwest Africa. This suggests there are subtleties not captured in the broad open habitat category, which are relevant for understanding the role of habitat-dependent evolution

Searching for the Nuclear Liquid-Gas Phase Transition in Au + Au Collisions at 35 MeV/nucleon

Within the framework of Classical Molecular Dynamics, we study the collision Au + Au at an incident energy of 35 MeV/nucleon. It is found that the system shows a critical behaviour at peripheral impact parameters, revealed through the analysis of conditional moments of charge distributions, Campi Scatter Plot, and the occurrence of large fluctuations in the region of the Campi plot where this critical behaviour is expected. When applying the experimental filters of the MULTICS-MINIBALL apparatus, it is found that criticality signals can be hidden due to the inefficiency of the experimental apparatus. The signals are then recovered by identifying semi-peripheral and peripheral collisions looking to the velocity distribution of the largest fragment, then by selecting the most complete events.Comment: RevTex file, 21 pages + 19 figures available upon request from [email protected]