Isospin-projected nuclear level densities by the shell model Monte Carlo method

We have developed an efficient isospin projection method in the shell model Monte Carlo approach for isospin-conserving Hamiltonians. For isoscalar observables this projection method has the advantage of being exact sample by sample. The isospin projection method allows us to take into account the proper isospin dependence of the nuclear interaction, thus avoiding a sign problem that such an interaction introduces in unprojected calculations. We apply our method in the calculation of the isospin dependence of level densities in the complete $pf+g_{9/2}$ shell. We find that isospin-dependent corrections to the total level density are particularly important for $N \sim Z$ nuclei.Comment: 5 pages including 4 figure

The effect of core polarization on longitudinal form factors in 10^{10}B

Electron scattering Coulomb form factors for the single-particle quadrupole transitions in $p$-shell $^{10}$B nucleus have been studied. Core polarization effects are included through a microscopic theory that includes excitations from the core orbits up to higher orbits with 2$\hbar$$\omega$ excitations. The modified surface delta interaction (MSDI) is employed as a residual interaction. The effect of core polarization is found essential in both the transition strengths and momentum transfer dependence of form factors, and gives a remarkably good agreement with the measured data with no adjustable parameters.Comment: 4 pages, 5 figure

Photoconductive switching of a high-voltage spark gap

We have demonstrated photoconductive switching of a gas-filled spark gap. A femtosecond Ti:sapphire laser was focused in a 1 mm spark gap biased at 4.5 kV. There is a clear transition between triggered operation, when only part of the path between the electrodes is ionized, and photoconductive switching, when the entire length of the gap is ionized directly by the laser. The measured standard deviation of the time fluctuations between the rising edge of the transmitted electrical pulse and the laser was less than 15 ps

Polarization-dependent ponderomotive gradient force in a standing wave

The ponderomotive force is derived for a relativistic charged particle entering an electromagnetic standing wave with a general three-dimensional field distribution and a nonrelativistic intensity, using a perturbation expansion method. It is shown that the well-known ponderomotive gradient force expression does not hold for this situation. The modified expression is still of simple gradient form, but contains additional polarization-dependent terms. These terms arise because the relativistic translational velocity induces a quiver motion in the direction of the magnetic force, which is the direction of large field gradients. Oscillation of the Lorentz factor effectively doubles this magnetic contribution. The derived ponderomotive force generalizes the polarization-dependent electron motion in a standing wave obtained earlier [A.E. Kaplan and A.L. Pokrovsky, Phys. Rev. Lett. $\bm{95}$, 053601 (2005)]. Comparison with simulations in the case of a realistic, non-idealized, three-dimensional field configuration confirms the general validity of the analytical results.Comment: 13 pages, 4 figure

On the Solution of the Number-Projected Hartree-Fock-Bogoliubov Equations

The numerical solution of the recently formulated number-projected Hartree-Fock-Bogoliubov equations is studied in an exactly soluble cranked-deformed shell model Hamiltonian. It is found that the solution of these number-projected equations involve similar numerical effort as that of bare HFB. We consider that this is a significant progress in the mean-field studies of the quantum many-body systems. The results of the projected calculations are shown to be in almost complete agreement with the exact solutions of the model Hamiltonian. The phase transition obtained in the HFB theory as a function of the rotational frequency is shown to be smeared out with the projection.Comment: RevTeX, 11 pages, 3 figures. To be published in a special edition of Physics of Atomic Nuclei (former Sov. J. Nucl. Phys.) dedicated to the 90th birthday of A.B. Migda

Experimental investigation of an atmospheric photoconductively switched high-voltage spark gap

We report on the experimental investigation of the photoconductively switched gas-filled spark gap. When the laser intensity of a femtosecond laser is high enough (around 1018 Wm-2), a plasma can be created that spans the complete distance between the electrodes. The gas-filled spark gap is then closed on a femtosecond timescale, similar to photoconductive switching of a semiconductor switch. Stochastic breakdown processes, such as avalanche and streamer formation that cause the breakdown in laser triggered spark gaps, are passed over, which results in faster risetime and less jitter. Measurements of the switched pulses as a function of laser energy were performed in a 1 mm gap at an applied voltage of 4.5 kV. A clear transition from triggering to switching was measured with increased laser energy. Measurements of the output pulses with the gap filled with nitrogen at 1 atm showed results very similar to measurements in air in the same gap. In the switching regime, the amplitude of the switched pulse did not depend strongly on the laser energy. Measurements at lower applied voltages but with the same gap distance showed that it was possible to switch voltages as low as 10% of the self-breakdown voltage. At low applied voltages, a significant difference between the applied voltage and the output voltage is measured. A possible explanation is given based on the dynamic behavior of the laser created plasma. The measured rise time and jitter of the switched pulses were both below the resolution of the measurement equipment, i.e., better than 100 ps and 15 ps, respectively

Electromagnetic field strength prediction in an urban environment: A useful tool for the planning of LMSS

A model for the prediction of the electromagnetic field strength in an urban environment is presented. The ray model, that is based on the Uniform Theory of Diffraction (UTD), includes effects of the non-perfect conductivity of the obstacles and their surface roughness. The urban environment is transformed into a list of standardized obstacles that have various shapes and material properties. The model is capable of accurately predicting the field strength in the urban environment by calculating different types of wave contributions such as reflected, edge and corner diffracted waves, and combinations thereof. Also, antenna weight functions are introduced to simulate the spatial filtering by the mobile antenna. Communication channel parameters such as signal fading, time delay profiles, Doppler shifts and delay-Doppler spectra can be derived from the ray-tracing procedure using post-processing routines. The model has been tested against results from scaled measurements at 50 GHz and proves to be accurate

Thermonuclear Reaction Rate of 23Mg(p,gamma)24$Al

Updated stellar rates for the reaction 23Mg(p,gamma)24Al are calculated by using all available experimental information on 24Al excitation energies. Proton and gamma-ray partial widths for astrophysically important resonances are derived from shell model calculations. Correspondences of experimentally observed 24Al levels with shell model states are based on application of the isobaric multiplet mass equation. Our new rates suggest that the 23Mg(p,gamma)24Al reaction influences the nucleosynthesis in the mass A>20 region during thermonuclear runaways on massive white dwarfs.Comment: 13 pages (uses Revtex) including 3 postscript figures (uses epsfig.sty), accepted for publication in Phys. Rev.

Diapycnal mixing across the photic zone of the NE Atlantic

Variable physical conditions such as vertical turbulent exchange, internal wave, and mesoscale eddy action affect the availability of light and nutrients for phytoplankton (unicellular algae) growth. It is hypothesized that changes in ocean temperature may affect ocean vertical density stratification, which may hamper vertical exchange. In order to quantify variations in physical conditions in the northeast Atlantic Ocean, we sampled a latitudinal transect along 17 ± 5∘ W between 30 and 63∘ N in summer. A shipborne conductivity–temperature–depth (CTD) instrumented package was used with a custom-made modification of the pump inlet to minimize detrimental effects of ship motions on its data. Thorpe-scale analysis was used to establish turbulence values for the upper 500 m from three to six profiles obtained in a short CTD yo-yo, 3 to 5 h after local sunrise. From south to north, average temperature decreased together with stratification while turbulence values weakly increased or remained constant. Vertical turbulent nutrient fluxes did not vary significantly with stratification and latitude. This apparent lack of correspondence between turbulent mixing and temperature is likely due to internal waves breaking (increased stratification can support more internal waves), acting as a potential feedback mechanism. As this feedback mechanism mediates potential physical environment changes in temperature, global surface ocean warming may not affect the vertical nutrient fluxes to a large degree. We urge modellers to test this deduction as it could imply that the future summer phytoplankton productivity in stratified oligotrophic waters would experience little alterations in nutrient input from deeper waters