Nuno Pinhao

Comparison of calculated and measured optical emission intensities in a direct current argon-copper glow discharge

q . Based on a collisional)radiative model for argon atoms and copper atoms and Cu ions, which was developed as a part of a comprehensive simulation network, optical emission intensities have been calculated for argon and copper lines in a direct current argon glow discharge with copper cathode. Comparison with experimental data has been made, both with respect to the optical emission spectra and to some selected emission lines as a function of axial position. From this study, information can be obtained about the importance of various plasma processes, like electron, fast ion and fast atom impact excitation, and reabsorption of resonant radiation. Q 2000 Elsevier Science B.V. All rights reserved.

Comparison of kinetic calculation techniques for the analysis of electron swarm transport at low to moderate E/N values

We present a comparison between results for the electron velocity distribution function (evdf), and transport and rate coefficients of an electron swarm obtained under different assumptions for the space and angular dependence of the evdf. Several solution techniques for the Boltzmann equation as well as Monte Carlo simulations have been tested. The comparison is made in neon at a constant and homogeneous reduced electric field in the range 10 Td ≤ E/N ≤ 500 Td taking into account the production of electrons in ionizing collisions. The results show that to obtain an accurate description of the electron swarm we need to take into account the variation in space of the electron density in the representation of the evdf. In what regards the angular dependence on velocity we discuss criteria to estimate the importance of the anisotropy of the evdf for any gas. Depending on the solution technique and on the E/N value, we find good to excellent agreement between the Boltzmann results obtained with a half-range method, a multi-term Legendre expansion, an elliptic approximation and the Monte Carlo results. The accuracy of the transport and rate coefficients obtained with each approach is evaluated and it is found that although the two-term velocity expansion is not sufficiently accurate to be used for cross section fitting, the corresponding rate and transport coefficients can generally be used in discharge modelling.

Progress on high-voltage pulse generators, using low voltage semiconductors (<1 kV), designed for plasma immersion ion implantation (PIII)

A modular concept on high-voltage pulse generators, under development for future use in facilities for plasma immersion ion implantation, is presented. The generator proposed uses individual modules, each one consisting of a pulse circuit based on a step-up transformer; the secondary of each step-up transformer is connected in series. Each step-up transformer delivers a fraction of the total voltage with primary voltage supplied via an isolation transformer. With this topology we expect to achieve tens of kiloVolts with low voltage semiconductor switches (<1 kV). A three 5 kV-module initial prototype was assembled with 800 V semiconductor switches and experimentally tested for an output of 11 kV, 5 μs pulse width and 10 kHz-pulse frequency. Different load conditions and results are presented and discussed.

Electron transport parameters in CO2: scanning drift tube measurements and kinetic computations

This work presents the transport coefficients of electrons (bulk drift velocity, longitudinal diffusion coefficient, and effective ionization frequency) in CO2 measured under time-of-flight conditions over a wide range of the reduced electric field, in a scanning drift tube apparatus. The data obtained in the experiments are also applied to determine the effective steady-state Townsend ionization coefficient. These parameters are compared to the results of previous experimental studies, as well as to the results of various kinetic computations: solutions of the electron Boltzmann equation under different approximations (multiterm and density gradient expansions) and Monte Carlo simulations. The experimental data extend the range of E/N compared with previous measurements and are consistent with most of the transport parameters obtained in these earlier studies. The computational results point out the range of applicability of the respective approaches to determine the different measured transport properties of electrons in CO2. They also demonstrate the need for further improvement of the electron collision cross section data for CO2 taking into account the present experimental data.

Oxidation of clofibric acid in aqueous solution using a non-thermal plasma discharge or gamma radiation

In this work, we study degradation of clofibric acid (CFA) in aqueous solution using either ionizing radiation from a60Co source or a non-thermal plasma produced by discharges in the air above the solution. The results obtained with the two technologies are compared in terms of effectiveness of CFA degradation and its by-products. In both cases the CFA degradation follows a quasi-exponential decay in time well modelled by a kinetic scheme which considers the competition between CFA and all reaction intermediates for the reactive species generated in solution as well as the amount of the end product formed. A new degradation law is deduced to explain the results. Although the end-product CO2 was detected and the CFA conversion found to be very high under the studied conditions, HPLC analysis reveals several degradation intermediates still bearing the aromatic ring with the chlorine substituent. The extent of mineralization is rather limited. The energy yield is found to be higher in the gamma radiation experiments.

Time-dependent transport of resonance radiation in an infinite cylinder

A method is described that accounts for the imprisonment of resonance radiation in the calculation of the time-dependent distribution of atoms in resonance states and of the radiation escape in a gas-filled enclosure with cylindrical symmetry. Constant line profile, complete redistribution and pure spectral broadening are assumed. This method is based on the substitution of a matrix product for the radiative terms in the rate equation and the introduction of a symmetrical matrix, related in a simple way to the matrix representation of the radiation transfer probability space. Also, a vector proportional to the vector representation of the radiation escape probability space is introduced allowing the escape radiation to be written as a vector product. This vector and the symmetrical matrix depend only on the type of broadening and number N of discrete intervals considered and can be computed a priori. The concept of escape factor is generalized. The results are applied to standard cases.

125I Measurements for Occupational Exposure Assessment

Whenever there is a risk of occupational exposure to dispersible radioactive material, it is necessary to have a monitoring program to assess the effective dose arising from the intake of radionuclides by workers. In this paper we present our experience in bioassay measurements of 125I in urine samples of workers using high resolution gamma spectrometry. For a 24‐hour excretion period, we found activity values of the order of one Bq and estimated the committed effective doses to be less than one μSv. Although very small, these values led to a re‐evaluation and improvement of the laboratory safety conditions. We discuss the calibration procedure followed for the activity measurements, the estimation of the uncertainty in the excreted activity, the calculation of detection and quantification limits and estimation of performance indicators. Aspects regarding the spectral analysis, true coincidence summing and matrix effects are also considered.

Improving Accuracy and Precision in Gross Alpha and Beta Counting with Proportional Detectors

Following ambiguous results of an intercomparison exercise in 2004, we have reevaluated the sample preparation, measurement procedures and data treatment for gross alpha and beta counting. It was found that the Portuguese standards NP 4332:1996 and NP 4330:1996 do not provide enough background for correct treatment of data. This paper describes the improvements made in gross alpha and beta activities measurement with proportional detectors in simultaneous mode. These improvements include a crosstalk correction both on counting and in the minimum detectable activity; the use of adequate statistical tools for data analysis; the use of QC charts to control the stability of the detectors and the background and, attention to the traceability of results. The calibration curves were computed with an appropriate number of replicates and with a throughout evaluation of the uncertainty budget. Based on this model, we established new criteria to report results.

PLASMAKIN: A chemical kinetics library for plasma physics modeling

Abstract PLASMAKIN is a software library to handle physical and chemical data used in plasma physics modeling and to compute kinetics data from the reactions taking place in the gas or at the surfaces — particle generation and loss rates, photon emission rates and energy exchange rates. PLASMAKIN supports any number of chemical species and reactions, is independent of problem dimensions and can be used both in stationary and time-dependent problems. PLASMAKIN supports a large number of species properties and reaction types. Namely gas or electron temperature dependent collision rate coefficients, vibrational and cascade levels, evaluation of branching ratios, superelastic and other reverse processes, three-body collisions, radiation imprisonment and photoelectric emission. Non-standard rate coefficient functions can be handled by a user supplied routine. The interaction of the user with the library is limited to the preparation of an input file characterizing the interacting species and reactions, accessing the data or the chemical kinetics results through a reduced number of procedure calls with similar interfaces. PLASMAKIN allows the user to concentrate on the physical nature of the problems independently of the gas mixture properties and reactions model. As an example a program has been prepared to evaluate the maintenance field, equilibrium populations, relative contribution of each reaction, photon emission spectra and the energy losses rates on the positive column region of a discharge in electropositive gases. The program is applied to a discharge in neon. The library is written in Fortran 95 and has error support.