Momentum-resolved study of the saturation intensity in multiple ionization

We present a momentum-resolved study of strong field multiple ionization of ionic targets. Using a deconvolution method we are able to reconstruct the electron momenta from the ion momentum distributions after multiple ionization up to four sequential ionization steps. This technique allows an accurate determination of the saturation intensity as well as of the electron release times during the laser pulse. The measured results are discussed in comparison to typically used models of over-the-barrier ionization and tunnel ionization

A symmetry invariant integral on kappa-deformed spacetime

In this note we present an approach using both constructive and Hopf algebraic methods to contribute to the not yet fully satisfactory definition of an integral on kappa-deformed spacetime. The integral presented here is based on the inner product of differential forms and it is shown that this integral is explicitly invariant under the deformed symmetry structure.Comment: 16 page

Correlation studies of fission fragment neutron multiplicities

We calculate neutron multiplicities from fission fragments with specified mass numbers for events having a specified total fragment kinetic energy. The shape evolution from the initial compound nucleus to the scission configurations is obtained with the Metropolis walk method on the five-dimensional potential-energy landscape, calculated with the macroscopic-microscopic method for the three-quadratic-surface shape family. Shape-dependent microscopic level densities are used to guide the random walk, to partition the intrinsic excitation energy between the two proto-fragments at scission, and to determine the spectrum of the neutrons evaporated from the fragments. The contributions to the total excitation energy of the resulting fragments from statistical excitation and shape distortion at scission is studied. Good agreement is obtained with available experimental data on neutron multiplicities in correlation with fission fragments from $^{235}$U(n$_{\rm th}$,f). At higher neutron energies a superlong fission mode appears which affects the dependence of the observables on the total fragment kinetic energy.Comment: 12 pages, 10 figure

The contrasting fission potential-energy structure of actinides and mercury isotopes

Fission-fragment mass distributions are asymmetric in fission of typical actinide nuclei for nucleon number $A$ in the range $228 \lnsim A \lnsim 258$ and proton number $Z$ in the range $90\lnsim Z \lnsim 100$. For somewhat lighter systems it has been observed that fission mass distributions are usually symmetric. However, a recent experiment showed that fission of $^{180}$Hg following electron capture on $^{180}$Tl is asymmetric. We calculate potential-energy surfaces for a typical actinide nucleus and for 12 even isotopes in the range $^{178}$Hg--$^{200}$Hg, to investigate the similarities and differences of actinide compared to mercury potential surfaces and to what extent fission-fragment properties, in particular shell structure, relate to the structure of the static potential-energy surfaces. Potential-energy surfaces are calculated in the macroscopic-microscopic approach as functions of fiveshape coordinates for more than five million shapes. The structure of the surfaces are investigated by use of an immersion technique. We determine properties of minima, saddle points, valleys, and ridges between valleys in the 5D shape-coordinate space. Along the mercury isotope chain the barrier heights and the ridge heights and persistence with elongation vary significantly and show no obvious connection to possible fragment shell structure, in contrast to the actinide region, where there is a deep asymmetric valley extending from the saddle point to scission. The mechanism of asymmetric fission must be very different in the lighter proton-rich mercury isotopes compared to the actinide region and is apparently unrelated to fragment shell structure. Isotopes lighter than $^{192}$Hg have the saddle point blocked from a deep symmetric valley by a significant ridge. The ridge vanishes for the heavier Hg isotopes, for which we would expect a qualitatively different asymmetry of the fragments.Comment: 8 pages, 9 figure

Dependence of direct neutron capture on nuclear-structure models

The prediction of cross sections for nuclei far off stability is crucial in the field of nuclear astrophysics. We calculate direct neutron capture on the even-even isotopes $^{124-145}$Sn and $^{208-238}$Pb with energy levels, masses, and nuclear density distributions taken from different nuclear-structure models. The utilized structure models are a Hartree-Fock-Bogoliubov model, a relativistic mean field theory, and a macroscopic-microscopic model based on the finite-range droplet model and a folded-Yukawa single-particle potential. Due to the differences in the resulting neutron separation and level energies, the investigated models yield capture cross sections sometimes differing by orders of magnitude. This may also lead to differences in the predicted astrophysical r-process paths. Astrophysical implications are discussed.Comment: 25 pages including 12 figures, RevTeX, to appear in Phys. Rev.

Swiss Democracy on the Web 2010 : Society and Politics in a Connected Age

The rapid adoption of online media like Facebook, Twitter or Wikileaks leaves us with little time to think. Where is information technology taking us, our society and our democratic institutions ? Is the Web replicating social divides that already exist offline or does collaborative technology pave the way for a more equal society ? How do we find the right balance between openness and privacy ? Can social media improve civic participation or do they breed superficial exchange and the promotion of false information ? These and lots of other questions arise when one starts to look at the Internet, society and politics. The first part of this paper gives an overview of the social changes that occur with the rise of the Web. The second part serves as an overview on how the Web is being used for political participation in Switzerland and abroad. Le développement rapide de nouveaux médias comme Facebook, Twitter ou Wikileaks ne laisse que peu de temps à la réflexion. Quels sont les changements que ces technologies de l'information impliquent pour nous, notre société et nos institutions démocratiques ? Internet ne fait-il que reproduire des divisions sociales qui lui préexistent ou constitue-t-il un moyen de lisser et d'égaliser ces mêmes divisions ? Comment trouver le bon équilibre entre transparence et respect de la vie privée ? Les médias sociaux permettent-ils de stimuler la participation politique ou ne sont-ils que le vecteur d'échanges superficiels et de fausses informations ? Ces questions, parmi d'autres, émergent rapidement lorsque l'on s'intéresse à la question des liens entre Internet, la société et la politique. La première partie de ce cahier est consacrée aux changements sociaux générés par l'émergence et le développement d'Internet. La seconde fait l'état des lieux de la manière dont Internet est utilisé pour stimuler la participation politique en Suisse et à l'étranger