Microscopic study of Ca++Ca fusion

We investigate the fusion barriers for reactions involving Ca isotopes $\mathrm{^{40}Ca}+\mathrm{^{40}Ca}$, $\mathrm{^{40}Ca}+\mathrm{^{48}Ca}$, and $\mathrm{^{48}Ca}+\mathrm{^{48}Ca}$ using the microscopic time-dependent Hartree-Fock theory coupled with a density constraint. In this formalism the fusion barriers are directly obtained from TDHF dynamics. We also study the excitation of the pre-equilibrium GDR for the $\mathrm{^{40}Ca}+\mathrm{^{48}Ca}$ system and the associated $\gamma$-ray emission spectrum. Fusion cross-sections are calculated using the incoming-wave boundary condition approach. We examine the dependence of fusion barriers on collision energy as well as on the different parametrizations of the Skyrme interaction.Comment: 11 pages, 13 figure

Le sans-abrisme: au croisement des représentationset des construits sociaux

Il existe une représentation sociale du clochard affranchi et heureux. Celui qui a troqué ses contraintes contre la Liberté et qui, pour manifester son refus d’un mode de vie, aurait choisi de vivre en marge faisant ainsi coïncider Précarité avec Liberté. Existe-il un lieu de recevabilité pour ses besoins? Que comprendre du vécu subjectif des plus démunis quand, ils se détournent des dispositifs d’aide, voire qu’ils y préfèrent la rue

Angular Dependence in Proton-Proton Correlation Functions in Central 40Ca+40Ca^{40}Ca+^{40}Ca and 48Ca+48Ca^{48}Ca+^{48}Ca Reactions

The angular dependence of proton-proton correlation functions is studied in central $^{40}Ca+^{40}Ca$ and $^{48}Ca+^{48}Ca$ nuclear reactions at E=80 MeV/A. Measurements were performed with the HiRA detector complemented by the 4$\pi$ Array at NSCL. A striking angular dependence in the laboratory frame is found within p-p correlation functions for both systems that greatly exceeds the measured and expected isospin dependent difference between the neutron-rich and neutron-deficient systems. Sources measured at backward angles reflect the participant zone of the reaction, while much larger sources observed at forward angles reflect the expanding, fragmenting and evaporating projectile remnants. The decrease of the size of the source with increasing momentum is observed at backward angles while a weaker trend in the opposite direction is observed at forward angles. The results are compared to the theoretical calculations using the BUU transport model.Comment: 8 pages, 3 figures, submitted to PR

The RCK1 domain of the human BK_(Ca) channel transduces Ca^(2+) binding into structural rearrangements

Large-conductance voltage- and Ca^(2+)-activated K^+ (BK_(Ca)) channels play a fundamental role in cellular function by integrating information from their voltage and Ca2+ sensors to control membrane potential and Ca^(2+) homeostasis. The molecular mechanism of Ca^(2+)-dependent regulation of BKCa channels is unknown, but likely relies on the operation of two cytosolic domains, regulator of K^+ conductance (RCK)1 and RCK2. Using solution-based investigations, we demonstrate that the purified BK_(Ca) RCK1 domain adopts an α/β fold, binds Ca^(2+), and assembles into an octameric superstructure similar to prokaryotic RCK domains. Results from steady-state and time-resolved spectroscopy reveal Ca^(2+)-induced conformational changes in physiologically relevant [Ca^(2+)]. The neutralization of residues known to be involved in high-affinity Ca^(2+) sensing (D362 and D367) prevented Ca^(2+)-induced structural transitions in RCK1 but did not abolish Ca^(2+) binding. We provide evidence that the RCK1 domain is a high-affinity Ca^(2+) sensor that transduces Ca^(2+) binding into structural rearrangements, likely representing elementary steps in the Ca^(2+)-dependent activation of human BK_(Ca) channels

Modeling effects of L-type ca(2+) current and na(+)-ca(2+) exchanger on ca(2+) trigger flux in rabbit myocytes with realistic T-tubule geometries.

The transverse tubular system of rabbit ventricular myocytes consists of cell membrane invaginations (t-tubules) that are essential for efficient cardiac excitation-contraction coupling. In this study, we investigate how t-tubule micro-anatomy, L-type Ca(2+) channel (LCC) clustering, and allosteric activation of Na(+)/Ca(2+) exchanger by L-type Ca(2+) current affects intracellular Ca(2+) dynamics. Our model includes a realistic 3D geometry of a single t-tubule and its surrounding half-sarcomeres for rabbit ventricular myocytes. The effects of spatially distributed membrane ion-transporters (LCC, Na(+)/Ca(2+) exchanger, sarcolemmal Ca(2+) pump, and sarcolemmal Ca(2+) leak), and stationary and mobile Ca(2+) buffers (troponin C, ATP, calmodulin, and Fluo-3) are also considered. We used a coupled reaction-diffusion system to describe the spatio-temporal concentration profiles of free and buffered intracellular Ca(2+). We obtained parameters from voltage-clamp protocols of L-type Ca(2+) current and line-scan recordings of Ca(2+) concentration profiles in rabbit cells, in which the sarcoplasmic reticulum is disabled. Our model results agree with experimental measurements of global Ca(2+) transient in myocytes loaded with 50 μM Fluo-3. We found that local Ca(2+) concentrations within the cytosol and sub-sarcolemma, as well as the local trigger fluxes of Ca(2+) crossing the cell membrane, are sensitive to details of t-tubule micro-structure and membrane Ca(2+) flux distribution. The model additionally predicts that local Ca(2+) trigger fluxes are at least threefold to eightfold higher than the whole-cell Ca(2+) trigger flux. We found also that the activation of allosteric Ca(2+)-binding sites on the Na(+)/Ca(2+) exchanger could provide a mechanism for regulating global and local Ca(2+) trigger fluxes in vivo. Our studies indicate that improved structural and functional models could improve our understanding of the contributions of L-type and Na(+)/Ca(2+) exchanger fluxes to intracellular Ca(2+) dynamics

Optimization of 2-d lattice cellular automata for pseudorandom number generation

This paper proposes a generalized approach to 2-d CA PRNGs – the 2-d lattice CA PRNG – by introducing vertical connections to arrays of 1-d CA. The structure of a 2-d lattice CA PRNG lies in between that of 1-d CA and 2-d CA grid PRNGs. With the generalized approach, 2-d lattice CA PRNG offers more 2-d CA PRNG variations. It is found that they can do better than the conventional 2-d CA grid PRNGs. In this paper, the structure and properties of 2-d lattice CA are explored by varying the number and location of vertical connections, and by searching for different 2-d array settings that can give good randomness based on Diehard test. To get the most out of 2-d lattice CA PRNGs, genetic algorithm is employed in searching for good neighborhood characteristics. By adopting an evolutionary approach, the randomness quality of 2-d lattice CA PRNGs is optimized. In this paper, a new metric, #rn is introduced as a way of finding a 2-d lattice CA PRNG with the least number of cells required to pass Diehard test. Following the introduction of the new metric #rn, a cropping technique is presented to further boost the CA PRNG performance. The cost and efficiency of 2-d lattice CA PRNG is compared with past works on CA PRNGs