Experimental investigations on the first Townsend coefficient in pure isobutane

In this work we present results of the first Townsend coefficient (alpha) in pure isobutane by measuring the current growth as a function of the electric field strength in a pulsed irradiation regime. A Resistive Plate Chamber (RPC)-like configuration was used. To validate this method, as well as to crosscheck the experimental apparatus, measurements of the alpha parameter were firstly carried out with pure nitrogen and the results compared to the accurate data available in the literature. The data obtained with isobutane in a field range from 145 Td up to 200 Td were well-matched to those calculated with Magboltz versions 2.7.1 and 2.8.6. The experimental consistency of these results with other published data in the range of 550-1300 Td was very good, as demonstrated by the use of the Korff parameterization. (C) 2012 Elsevier B.V. All rights reserved.

Electronic stopping cross sections for protons in Al2O3: an experimental and theoretical study

The electronic stopping cross section (SCS) of Al2O3 for proton beams is studied both experimentally and theoretically. The measurements are made for proton energies from 40 keV up to 1 MeV, which cover the maximum stopping region, using two experimental methods, the transmission technique at low energies (similar to 40-175 keV) and the Rutherford backscattering at high energies (approximate to 190-1000 keV). These new data reveal an increment of 16% in the SCS around the maximum stopping with respect to older measurements. The theoretical study includes electronic stopping power calculations based on the dielectric formalism and on the transport cross section (TCS) model to describe the electron excitations of Al2O3. The non-linear TCS calculations of the SCS for valence electrons together with the generalized oscillator strengths (GOS) model for the core electrons compare well with the experimental data in the whole range of energies considered.

Interfacial concentrations of chloride and bromide in zwitterionic micelles with opposite dipoles: Experimental determination by chemical trapping and a theoretical description

Interfacial concentrations of chloride and bromide ions, with Li+, Na+, K+, Rb+, Cs+, trimethylammonium (TMA(+)), Ca2+, and Mg2+ as counterions, were determined by chemical trapping in micelles formed by two zwitterionic surfactants, namely N-hexadecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (HPS) and hexadecylphosphorylcholine (HDPC) micelles. Appropriate standard curves for the chemical trapping method were obtained by measuring the product yields of chloride and bromide salts with 2,4,6-trimethyl-benzenediazonium (BF4) in the presence of low molecular analogs (N,N,N-trimethyl-propane sulfonate and methyl-phosphorylcholine) of the employed surfactants. The experimentally determined values for the local Br- (Cl-) concentrations were modeled by fully integrated non-linear Poisson Boltzmann equations. The best fits to all experimental data were obtained by considering that ions at the interface are not fixed at an adsorption site but are free to move in the interfacial plane. In addition, the calculation of ion distribution allowed the estimation of the degree of ion coverage by using standard chemical potential differences accounting for ion specificity. (C) 2012 Elsevier Inc. All rights reserved.