Direct laser acceleration: A model for the electron injection from the walls of a cylindrical guiding structure.

We use analytical methods and particle-in-cell simulation to investigate the origin of electrons accelerated by the process of direct laser acceleration driven by high-power laser pulses in preformed narrow cylindrical plasma channels. The simulation shows that the majority of accelerated electrons are originally located along the interface between the channel wall and the channel interior. The analytical model based on the electron hydrodynamics illustrates the underlying physical mechanism of the release of electrons from the channel wall when irradiated by an intense laser, the subsequent electron dynamics, and the corresponding evolution of the channel density profile. The quantitative predictions of the total charge of released electrons and the average electron density inside the channel are validated by comparison with the simulation results.

Gamma-ray flash generation in irradiating a thin foil target by a single-cycle tightly focused extreme power laser pulse.

We present a regime where an ultraintense laser pulse interacting with a foil target results in high γ-photon conversion efficiency, obtained via three-dimensional quantum-electrodynamics particle-in-cell simulations. A single-cycle laser pulse is used under the tight-focusing condition for obtaining the λ^{3} regime. The simulations employ a radially polarized laser as it results in higher γ-photon conversion efficiency compared to both azimuthal and linear polarizations. A significant fraction of the laser energy is transferred to positrons, while a part of the electromagnetic wave escapes the target as attosecond single-cycle pulses.

Polarity reversal of wakefields driven by ultrashort pulse laser.

Using an analytical model and computer simulation, we show that the wakefield driven by an ultrashort laser pulse in high-density plasma periodically reverses its polarity due to the carrier-envelope phase shift of the driver. The wakefield polarity reversal occurs on spatial scales shorter than the typical length considered for electron acceleration with the laser-wakefield mechanism. Consequently, the energies of accelerated electrons are significantly affected. The results obtained are important for the laser-wakefield acceleration under the conditions relevant to present-day high-repetition-rate laser systems.

Trends in heavy metal levels in the dissolved and particulate phase in the Dutch Rhine-Meuse (Maas) Delta.

Levels of the heavy metals Cd, Cr, Cu, Pb and Zn, in both the dissolved and particulate phase, were determined in two sampling campaigns in August 1978 and August 1984 in the Dutch Rhine-Meuse (Maas) Delta. Besides the heavy metal concentrations, other important parameters were determined, such as the concentrations of seston, chlorophyll and nutrients. The concentrations of dissolved Cd and of Cd, Pb and Cu in particulate matter were much higher in 1978 than in 1984, especially for Cd. This may be due to the strict regulations implemented in Germany to reduce the pollution of natural waters by heavy metals. The correlation between the high Cd content and the chlorophyll content is explained by the binding of Cd to living and dead organisms. The distribution of the metals between the dissolved and the particulate phase, indicated by the distribution quotient Kd, increases from low values of Kd for Cu and Zn, to higher values for Cd and Cr, and the highest values for Pb. The differences are explained by various binding forms of each of these metals in natural waters.

Application of differential pulse anodic stripping voltammetry to the determination of heavy metals in environmental samples.

A reliable procedure for the determination of the trace metals Cd, Cu, Ni, Pb and Zn in liquid and solid environmental samples by an advanced voltammetric method has been developed. A convenient method of sample pretreatment is wet digestion in a HNO3/HClO4 mixture. The simultaneous voltammetric determination of Cd, Cu, Pb and Zn is made at pH 2 by differential pulse anodic stripping voltammetry (DPASV); the simultaneous determination of Ni and Co at pH 9 after adsorptive accumulation of the respective complex with dimethylglyoxime at the electrode is made by adsorption differential pulse voltammetry (ADPV). The particular advantages of the voltametric approach in food control for heavy metals are high sensitivity, good precision and accuracy, the possibilities for the simultaneous determination of groups of metals and low cost instrumentation.

[Voltammetric determination of heavy metals in meat and the internal organs of slaughter cattle]. / Die voltammetrische Bestimmung von Schwermetallen in Fleisch und inneren Organen von Schlachtrindern.

The most important uptake of heavy metal by man occurs through ingestion. Therefore heavy metal analysis of food becomes of great importance. The whole analytical procedure including sampling, transport, drying, homogenisation, digestion and measurement for the determination of Zn, Cd, Pb, Cu, Ni, and Co in meat is described with due consideration given to the requirements for trace and ultratrace analysis. The particular advantages of the voltammetric approach for heavy metal determinations in meat and meat products are generally featured by this study. A standard deviation of 3-5% RSD in the microgram/g-range and 5-10%-RSD in the ng/g-range was obtained. In order to check the accuracy of the results, mixtures of sample and standard reference material were analysed. While the Cd-content in meat is in the range of 1 ng/g (wet weight) it increases in organs, such as liver (100-500 ng/g wet weight) and kidney (600-2000 ng/g wet weight). Pb shows a similar behaviour. The levels of these elements were found to be below the maximum allowable limits set by the "Bundesgesundheitsamt", with the exception of Cd in the kidneys. There is a statistical significant correlation between the Cd and Pb contents in liver and kidney. In liver the Cu content was especially high, because there are Cu-containing enzymes in this organ. Vitamin B12 is a Co containing complex in the liver. Therefore the Co values in liver are clearly higher than those in muscle meat.

Comparative studies on the distribution of heavy metals in the oceans and coastal waters.

Results of our investigations during the last five years are summarized. The concentrations of Cd, Pb and Cu have been determined in coastal areas of the Western Mediterranean and the North Sea, reflecting the influence of the pollution sources along the shore line. Deep sea water and surface water sample from the Pacific, Atlantic, Mediterranean and Arctic Sea have been analyzed for Cd, Pb, Cu and at some stations also additionally for Ni, Co. It is demonstrated that concentration levels of surface and deep sea waters may differ significantly in diverse oceans. The same is valid to a greater extent for coastal waters affected particularly by anthropogenic input that can be dispersed by tidal currents. Applying reliable sampling methods and the voltammetric determination procedure in particular suitable for heavy metals at the trace level, significant contributions to a consistent data basis for the distribution of heavy metals in the sea could be achieved.

A new electrochemical approach for the in vitro investigation of damage in native DNA by small gamma-radiation doses.

Voltammetry provides a new method to assess gamma-radiation damage to native DNA in vitro after low doses of irradiation. Using single sweep voltammetry at a stationary mercury electrode a substantial labilization of the double helical conformation of gamma-irradiated DNA was found. Up to 40 Gy a linear dose-effect relationship exists reflected for the number of strand breaks and the number of destabilized base pairs. A considerable difference in the frequency of formation of the two kinds of radiation damage is seen in the ratio of their G-values. Base pair labilization occurs depending on aeration conditions 200 to 300 times more frequently than the formation of a single strand break.

A rapid high-performance analytical procedure with simultaneous voltammetric determination of toxic trace metals in urine.

For the monitoring of toxic trace metals in urine a new high-performance trace analytical procedure with simultaneous voltammetric determination is presented. Particular emphasis has been placed on minimizing contamination by reducing the urine sample volume to 1 ml and consequently limiting the needed amount of HNO(3)/HClO(4) and the duration (<20 min) of the wet digestion stage. The procedure consists of three stages. First 20 urine samples (1 ml each) are simultaneously freeze-dried overnight. They then require only a short wet digestion (HClO(4)/HNO(3) 2:1, 20 min, 210 degrees ). They are then adjusted to pH 4.5 with acetate buffer and the trace metals are determined simultaneously by differential pulse anodic-stripping voltammetry at the mercury film electrode, the quartz digestion vessel being used as the voltammetric cell. If precipitates occur in the acidified samples, their trace metal content can be determined in the same manner, avoiding the low results commonly obtained by other methods. The procedure has high sensitivity with fair to good precision, covers a determination range from the sub-mug/l. to the medium mug/l. level and lends itself to automation. It is cheaper and more accurate than atomic absorption. Thus the procedure provides important potentialities for surveillance of occupationally exposed persons as well as for extended ecotoxicological baseline studies in man and cattle.

Toxic trace metals in food. II. A comparative study of the levels of toxic trace metals in wine by differential pulse anodic stripping voltammetry and electrothermal atomic absorption spectrometry.

A new high-performance analytical procedure for the determination of the toxic trace metals cadmium, lead and copper in wines by differential pulse anodic-stripping voltammetry (DPASV) subsequent to UV irradiation of the sample is compared with the hitherto more common application of electrothermal atomic absorption spectrometry (AAS). In this manner also mutually the accuracy attainable with both alternatives has been established. The particularly favourable potentialities of the new voltammetric approach for toxic metal control of wines are demonstrated by the investigation of a typical selection of 36 wines from recent vintages and common vine types of the German and some European wine cultivating regions.

Toxic trace metals in food. I. A new voltammetric procedure for toxic trace metal control of wines.

A new highly sensitive and particularly reliable analytical procedure for the precise determination of the most relevant toxic trace metals cadmium, lead and copper in wine is presented. It consists of a simple and convenient sample pretreatment by UV irradiation (1,5 h, 500-W-Hg-lamp) to release the toxic trace metals bound by organic substances and subsequent voltammetry (DPASV) of cadmium and lead simultaneously at a mercury film electrode (MFE) formed in situ on a vitreous carbon carrier and of copper on a gold electrode. Particular emphasis has been placed on the efficient exclusion of interference due to contamination. Only special wines, with more than 10% of sugar, require substitution of UV irradiation by a wet digestion also described. The procedure may be easily expanded to include the determination of mercury and zinc.