Bulletin of the American Physical Society
68th Annual Gaseous Electronics Conference/9th International Conference on Reactive Plasmas/33rd Symposium on Plasma Processing
Volume 60, Number 9
Monday–Friday, October 12–16, 2015; Honolulu, Hawaii
Session ET4: Positron and Electron Collisions |
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Chair: Yuri Ralchenko, NIST Room: 303 AB |
Tuesday, October 13, 2015 10:00AM - 10:30AM |
ET4.00001: New Adventure in Gaseous Positronics - A Cryogenic Beam Invited Speaker: C.M. Surko Buffer-gas-trap based beams have proven a reliable workhorse to study positron scattering and annihilation.\footnote{J. R. Danielson, et al., Rev. Mod. Phys. 87, 247 (2015).} The state of the art beam has a total energy spread $\sim 40$ meV FWHM using 300 K gas. Described here is work to create beams with narrower energy spreads (goal: total spread $\le 5$ meV FWHM using 50 K buffer gas). A Born-approximation model is used to describe cooling on vibrational and rotational excitations. Positron cooling from 1,200 K to 300 K was studied for CF$_4$, N$_2$ and CO to obtain the relevant cross sections (by fits to the model) and then predict cooling to 50 K.\footnote{M. R. Natisin, et al., J. Phys. B 47, 225209 (2014).} Using an additional cryogenic trapping stage, positrons have now been cooled to 50 K on N$_2$ and CO. Since the beam is generated in a magnetic field, the total energy spread is characterized by spreads parallel and perpendicular to the field.\footnote{ M. R. Natisin, et al., Phys. Plasmas 22, 033501 (2014).} While the perpendicular temperature is 4 meV (i.e., kT at 50 K), the parallel energy spread is larger. The currently projected total spread is $\le 10$ meV FWHM - a factor of four better than the 300 K result. Work is in progress to reach the predicted total spread at 50 K of 5 meV FWHM. [Preview Abstract] |
Tuesday, October 13, 2015 10:30AM - 11:00AM |
ET4.00002: Positron scattering measurements for application to medical physics Invited Speaker: James Sullivan While the use of positrons in medical imaging is now well established, there is still much to learn regarding the transport of positrons through the body, and the subsequent damage induced. Current models of dosimetry use only a crude approximation of the collision physics involved, and at low energies misrepresent the thermalisation process to a considerable degree. Recently, collaborative work has commenced to attempt to refine these models, incorporating a better representation of the underlying physics and trying to gain a better understanding of the damage done after the emission of a positron from a medical radioisotope. This problem is being attacked from several different angles, with new models being developed based upon established techniques in plasma and swarm physics. For all these models, a realistic representation of the collision processes of positrons with relevant molecular species is required. At the Australian National University, we have undertaken a program of measurements of positron scattering from a range of molecules that are important in biological systems, with a focus on analogs to DNA. This talk will present measurements of positron scattering from a range of these molecules, as well as describing the experimental techniques employed to make such measurements. Targets have been measured that are both liquid and solid at room temperature, and new approaches have been developed to get absolute cross section data. The application of the data to various models of positron thermalisation will also be described. [Preview Abstract] |
Tuesday, October 13, 2015 11:00AM - 11:15AM |
ET4.00003: Measurement of the Ion Distribution Function in a Dual Frequency Plasma Etch Tool Walter Gekelman, Nathaniel Moore, Patrick Pribyl, Mark Kushner The ion energy distribution function, (IEDF) was measured in detail in an industrial etch tool. The plasma was made with an ICP source (440 kHz, 500 W) and two independently controlled bias sources. The Si wafer was placed on a ceramic electrostatic chuck with an embedded capacitor plate. The first source ran at 2.2 MHz (600 Vpp and 2500 W) with a maximum sheath potential drop of 650 V or 2000V. The second source ran at 19 MHz with Vpp of 600 V. The principal diagnostic was Laser Induced Fluorescence on Argon using 611.49 nm light from a tunable dye laser with ions responding to Doppler shifted light. Using cylindrical lens combinations the laser light was transformed into a sheet 15 cm wide and 0.5 cm thick. The beam could be transverse or parallel to the normal of the wafer. The glowing ions (at 461 nm) were photographed by a CCD camera with 400 micron resolution. The laser was phase locked to the 2.2 MHz rf and the IDDF measured as a function of radial position, height above the wafer and at 8 phases. With Vpp $=$ 600 V the highest energy ions observed were 500 eV, 1.2 mm above the wafer. These observations as well as the angular distribution agreed well with a computer simulation. In the dual frequency case when the potential of the wafer was most negative wrt the bulk plasma the IEDF structure 0.8 mm above the wafer was well fitted by 4 Gaussians. The ion flux to the wafer was far more uniform in the dual frequency case. [Preview Abstract] |
Tuesday, October 13, 2015 11:15AM - 11:30AM |
ET4.00004: Ab-initio calculations of state-to-state rate coefficients for electron- and atom-molecule scattering at high temperatures Vincenzo Laporta, Roberto Celiberto, Fabrizio Esposito, Eswar Josyula In the contribution to conference theoretical calculations of state-by-state rate coefficients for electron-molecule and atom-molecule scattering, by using ab-initio methods, will be presented. In particular nitrogen- and oxygen-involving chemical reactions will be considered. These quantities are of primary importance to study the energy exchange and to implement kinetic models in thermal and chemical non-equilibrium high-temperature aerothermodynamics. [Preview Abstract] |
Tuesday, October 13, 2015 11:30AM - 11:45AM |
ET4.00005: Expression of a momentum-transfer scattering at an inelastic collision on electron transport in a collisional plasma Toshiaki Makabe An expression for the inelastic momentum-transfer scattering on the collision integral of the Boltzmann equation is derived in order to reflect the effect of the inelastic collision of an electron with a molecule on the electron kinetics in gases and collisional plasmas. To our knowledge, this is the first attempt to formulate the effect of the momentum-transfer scattering of an inelastic collision. The present procedure is a traditional one in which the Boltzmann equation of electrons is expanded by the Spherical-harmonics in velocity space. It is shown that the effect of the inelastic momentum-transfer on the electron transport is expressed only when we consider the first anisotropic part of the velocity distribution in the expanded Boltzmann equation. In addition, case studies are performed by considering the dependence of the scattering angle and the magnitude distribution. The influence of the inelastic momentum-transfer scattering on the electron transport should be further investigated, particularly in the case of a Ramsauer gas having the relation $Q_{vib}(v) > Q_{m}(v)$ in the vicinity of the Ramsauer-minimum in SiH$_{4}$, CH$_{4}$, and CF$_{4}$ etc. [Preview Abstract] |
Tuesday, October 13, 2015 11:45AM - 12:00PM |
ET4.00006: Electron Impact Ionization Cross Sections and Rate Coefficients for Single Carbon Freon Molecules Satyendra Pal, Neeraj Kumar Single carbon Freon molecules or chlorofluorocarbons (CFCs) are important industrial material with wide-ranging applications as refrigerant, aerosol propellant and semiconductor etchant, etc. The large-scale industrial consumption is of particular environmental concern because of its potential for ozone destruction in the stratosphere. In the present work, we have extended and generalized the modified Jain-Khare (JK) semi-empirical formalism for the evaluation of the total ionization cross sections corresponding to the formation of the cations in the electron impact ionization of molecules to the electron impact ionization of single carbon freon molecules, viz. CFCl$_{3}$, CF$_{2}$Cl$_{2}$ and CF$_{3}$Cl. The integral partial and the total ionization cross sections as function of incident electron energy are evaluated in the energy range varying from ionization threshold to 1000 eV. In absence of available differential cross sections, the corresponding derived partial and total ionization cross sections revealed a reasonably good agreement with the experimental and theoretical data, wherever available. In addition to the differential and integral ionization cross sections, we have also calculated the ionization rate coefficients using the evaluated partial ionization cross sections and the Maxwell-Boltzmann distribution as a function of electron temperature/energy. [Preview Abstract] |
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