Monte Carlo simulation of kilovolt electron transport in solids

A Monte Carlo procedure to simulate the penetration and energy loss of low¿energy electron beams through solids is presented. Elastic collisions are described by using the method of partial waves for the screened Coulomb field of the nucleus. The atomic charge density is approximated by an analytical expression with parameters determined from the Dirac¿Hartree¿Fock¿Slater self¿consistent density obtained under Wigner¿Seitz boundary conditions in order to account for solid¿state effects; exchange effects are also accounted for by an energy¿dependent local correction. Elastic differential cross sections are then easily computed by combining the WKB and Born approximations to evaluate the phase shifts. Inelastic collisions are treated on the basis of a generalized oscillator strength model which gives inelastic mean free paths and stopping powers in good agreement with experimental data. This scattering model is accurate in the energy range from a few hundred eV up to about 50 keV. The reliability of the simulation method is analyzed by comparing simulation results and experimental data from backscattering and transmission measurements

Producció de biometà a partir de la co-digestió d'aigües residuals porcines i vitivinícoles

One of the largest winery and cattle industry's environmental impacts is the one derived from the waste waters management. In this study, the problematic setting of residual waste waters produced in winery industry and swine industry is exposed, as well as the process, the microorganisms, the advantages and the limitations of co-digestion. A future application project of anaerobic co-digestion is proposed with an example focused on a catalane vineyard and a swine farm, with the purpose of reducing wastes, the main environmental impacts and the economic costs through both methane and electricity productio

Use of the Bethe equation for inner-shell ionization by electron impact

We analyzed calculated cross sections for K-, L-, and M-shell ionization by electron impact to determine the energy ranges over which these cross sections are consistent with the Bethe equation for inner-shell ionization. Our analysis was performed with K-shell ionization cross sections for 26 elements, with L-shell ionization cross sections for seven elements, L-3-subshell ionization cross sections for Xe, and M-shell ionization cross sections for three elements. The validity (or otherwise) of the Bethe equation could be checked with Fano plots based on a linearized form of the Bethe equation. Our Fano plots, which display theoretical cross sections and available measured cross sections, reveal two linear regions as predicted by de Heer and Inokuti [in Electron Impact Ionization, edited by T. D. Mark and G. H. Dunn, (Springer-Verlag, Vienna, 1985), Chap. 7, pp. 232-276]. For each region, we made linear fits and determined values of the two element-specific Bethe parameters. We found systematic variations of these parameters with atomic number for both the low-and the high-energy linear regions of the Fano plots. We also determined the energy ranges over which the Bethe equation can be used

A novel technique of extracting UCN lifetimes from storage bottle measurements dominated by scattering losses

Neutron lifetime is a critical parameter in the Standard Model. Its measurements using, particularly, the beamline and ultracold neutron storage techniques reveals serious tension. The status of the tension between various measurements have been presented, in light of the insights provided by the $\beta$-decay correlation measurements. When ultracold neutrons are stored in material bottles, they can be lost to various processes, such as $\beta$-decay and up-scattering on material walls. Here, we revisit the lifetime measurement in a material storage bottle, dominated by losses from scattering off the walls of the storage chamber. The neutron energy spectra and its associated uncertainties were, for the first time, well characterized. Such models have been used in the extraction of mean time between wall bounces, which is a key parameter for neutron storage disappearance experiments in search of neutron oscillation. A comparison between the loss model and the number of neutrons stored in a single chamber, used for the neutron electric dipole moment search, allowed us to extract a neutron lifetime of $\tau^*_n=879~({+158}/{-78})_{\text{stat.}}~(+230/-114)_{\text{sys.}}~\text{s~~(68.3\% C.I.)}$. Though the uncertainty on this lifetime is not competent with currently available measurements, the highlight of this work is that, we precisely identify the systematic sources of uncertainty that contribute to the neutron lifetime measurements in material storage bottles, namely from the uncertainty in the energy spectra, as well as the storage chamber parameters of Fermi potential and loss per bounce parameter. In doing so, we finally highlight the underestimation of the uncertainties in the previous Monte Carlo simulations of experiments using ultracold neutron storage in material bottles.Comment: 20 pages, 7 figures, 4 tables. Long unedited version, with extra details. This is an independent analysis, and not a part of nEDM@PSI collaboratio

A simplified model of the source channel of the Leksell Gamma Knife(R)^(R): testing multisource configurations with PENELOPE

A simplification of the source channel geometry of the Leksell Gamma Knife$^{\circledR}$, recently proposed by the authors and checked for a single source configuration (Al-Dweri et al 2004), has been used to calculate the dose distributions along the $x$, $y$ and $z$ axes in a water phantom with a diameter of 160~mm, for different configurations of the Gamma Knife including 201, 150 and 102 unplugged sources. The code PENELOPE (v. 2001) has been used to perform the Monte Carlo simulations. In addition, the output factors for the 14, 8 and 4~mm helmets have been calculated. The results found for the dose profiles show a qualitatively good agreement with previous ones obtained with EGS4 and PENELOPE (v. 2000) codes and with the predictions of GammaPlan$^{\circledR}$. The output factors obtained with our model agree within the statistical uncertainties with those calculated with the same Monte Carlo codes and with those measured with different techniques. Owing to the accuracy of the results obtained and to the reduction in the computational time with respect to full geometry simulations (larger than a factor 15), this simplified model opens the possibility to use Monte Carlo tools for planning purposes in the Gamma Knife$^{\circledR}$.Comment: 13 pages, 8 figures, 5 table

Monte Carlo Simulation of Electron Backscattering in Solids Using a General-Purpose Computer Code

A Monte Carlo study of backscattering of kilovolt electrons in solids, a process of primary importance in electron microscopy and surface analytical techniques, is carried out. Simulations have been performed using the general-purpose simulation code PENELOPE (an acronym for Penetration and ENErgy LOss of Positrons and Electrons ), which generates electron-photon showers in arbitrary materials. A systematic comparison of results from PENELOPE with available experimental data, and with results from simulations with a much more sophisticated code, is given for electron beams with energies between 2.5 and 60 keV and elemental solids with atomic numbers Z = 4 to 92. It is concluded that PENELOPE gives a reliable description of the backscattering process, even for relatively low electron energies and thin targets

Ecosystem function and biodiversity on coral reefs

The article highlights a workshop held in Key West, Florida in November 1993 attended by a group of 35 international scientists where topics of ecosystem function and biodiversity on coral reefs were discussed

Low Energy Electron Point Projection Microscopy of Suspended Graphene, the Ultimate "Microscope Slide"

Point Projection Microscopy (PPM) is used to image suspended graphene using low-energy electrons (100-200eV). Because of the low energies used, the graphene is neither damaged or contaminated by the electron beam. The transparency of graphene is measured to be 74%, equivalent to electron transmission through a sheet as thick as twice the covalent radius of sp^2-bonded carbon. Also observed is rippling in the structure of the suspended graphene, with a wavelength of approximately 26 nm. The interference of the electron beam due to the diffraction off the edge of a graphene knife edge is observed and used to calculate a virtual source size of 4.7 +/- 0.6 Angstroms for the electron emitter. It is demonstrated that graphene can be used as both anode and substrate in PPM in order to avoid distortions due to strong field gradients around nano-scale objects. Graphene can be used to image objects suspended on the sheet using PPM, and in the future, electron holography