Experimental Evidence for Transverse Wobbling in

New rotational bands built on the ν(h_{11/2}) configuration have been identified in ^{105}Pd. Two bands built on this configuration show the characteristics of transverse wobbling: the ΔI=1 transitions between them have a predominant E2 component and the wobbling energy decreases with increasing spin. The properties of the observed wobbling bands are in good agreement with theoretical results obtained using constrained triaxial covariant density functional theory and quantum particle rotor model calculations. This provides the first experimental evidence for transverse wobbling bands based on a one-neutron configuration, and also represents the first observation of wobbling motion in the A∼100 mass region.

Evidence for a spin-aligned neutron-proton paired phase from the level structure of (92)Pd.

Shell structure and magic numbers in atomic nuclei were generally explained by pioneering work that introduced a strong spin-orbit interaction to the nuclear shell model potential. However, knowledge of nuclear forces and the mechanisms governing the structure of nuclei, in particular far from stability, is still incomplete. In nuclei with equal neutron and proton numbers (N = Z), enhanced correlations arise between neutrons and protons (two distinct types of fermions) that occupy orbitals with the same quantum numbers. Such correlations have been predicted to favour an unusual type of nuclear superfluidity, termed isoscalar neutron-proton pairing, in addition to normal isovector pairing. Despite many experimental efforts, these predictions have not been confirmed. Here we report the experimental observation of excited states in the N = Z = 46 nucleus (92)Pd. Gamma rays emitted following the (58)Ni((36)Ar,2n)(92)Pd fusion-evaporation reaction were identified using a combination of state-of-the-art high-resolution γ-ray, charged-particle and neutron detector systems. Our results reveal evidence for a spin-aligned, isoscalar neutron-proton coupling scheme, different from the previous prediction. We suggest that this coupling scheme replaces normal superfluidity (characterized by seniority coupling) in the ground and low-lying excited states of the heaviest N = Z nuclei. Such strong, isoscalar neutron-proton correlations would have a considerable impact on the nuclear level structure and possibly influence the dynamics of rapid proton capture in stellar nucleosynthesis.

High flux of relativistic electrons produced in femtosecond laser-thin foil target interactions: characterization with nuclear techniques.

We present a protocol to characterize the high energy electron beam emitted in the interaction of an ultraintense laser with matter at intensities higher than 10(19) W cm(-2). The electron energies and angular distributions are determined as well as the total number of electrons produced above a 10 MeV threshold. This protocol is based on measurements with an electron spectrometer and nuclear activation techniques, combined with Monte Carlo simulations based on the GEANT3 code. The method is detailed and exemplified with data obtained with polypropylene and copper thin solid targets at a laser intensity of 2x10(19) W cm(-2). Special care is taken of the different sources of uncertainties. In particular, the reproducibility of the laser shots is considered.

Fast electron transport and induced heating in solid targets from rear-side interferometry imaging.

Fast adiabatic plasma heating of a thin solid target irradiated by a high intensity laser has been observed by an optical fast interferometry diagnostic. It is driven by the hot electron current induced by the laser plasma interaction at the front side of the target. Radial and longitudinal temperature profiles are calculated to reproduce the observed rear-side plasma expansion. The main parameters of the suprathermal electrons (number, temperature, and divergence) have been deduced from these observations.

Direct evidence of strongly inhomogeneous energy deposition in target heating with laser-produced ion beams.

We report on strong nonuniformities in target heating with intense, laser-produced proton beams. The observed inhomogeneity in energy deposition can strongly perturb equation of state (EOS) measurements with laser-accelerated ions which are planned in several laboratories. Interferometric measurements of the target expansion show different expansion velocities on the front and rear surfaces, indicating a strong difference in local temperature. The nonuniformity indicates at an additional heating mechanism, which seems to originate from electrons in the keV range.

Electron acceleration by a wake field forced by an intense ultrashort laser pulse.

Plasmas are an attractive medium for the next generation of particle accelerators because they can support electric fields greater than several hundred gigavolts per meter. These accelerating fields are generated by relativistic plasma waves-space-charge oscillations-that can be excited when a high-intensity laser propagates through a plasma. Large currents of background electrons can then be trapped and subsequently accelerated by these relativistic waves. In the forced laser wake field regime, where the laser pulse length is of the order of the plasma wavelength, we show that a gain in maximum electron energy of up to 200 megaelectronvolts can be achieved, along with an improvement in the quality of the ultrashort electron beam.

Relativistic electron generation in interactions of a 30 TW laser pulse with a thin foil target.

Energy and angular distributions of the fast outgoing electron beam induced by the interaction of a 1 J, 30 fs, 2 x 10(19) W/cm(2), 10 Hz laser with a thin foil target are characterized by electron energy spectroscopy and photonuclear reactions. We have investigated the effect of the target thickness and the intensity contrast ratio level on the electron production. Using a 6-microm polyethylene target, up to 4 x 10(8) electrons with energies between 5 and 60 MeV were produced per laser pulse and converted to gamma rays by bremsstrahlung in a Ta secondary target. The rates of photofission of U as well as photonuclear reactions in Cu, Au, and C samples have been measured. In optimal focusing conditions, about 0.06% of the laser energy has been converted to outgoing electrons with energies above 5 MeV. Such electrons leave the target in the laser direction with an opening angle of 2.5 degrees.

Low-energy measurement of the 7Be(p,gamma)8B cross section.

We have measured the cross section of the 7Be(p,gamma)8B reaction for E(c.m.) = 185.8, 134.7, and 111.7 keV using a radioactive 7Be target (132 mCi). Single and coincidence spectra of beta+ and alpha particles from 8B and 8Be* decay, respectively, were measured using a large acceptance spectrometer. The zero energy S factor inferred from these data is 18.5+/-2.4 eV b and a weighted mean value of 18.8+/-1.7 eV b (theoretical uncertainty included) is deduced when combining this value with our previous results at higher energies.