title: Polarity and ion recombination corrections in continuous and pulsed beams for ionization chambers with high Z chamber walls.
abstract: In this work, the response of Farmer-type ionization chambers fitted with high atomic number (Z) walls is studied, and results of the effects of such walls on polarity and ion recombination correction factors in both continuous and pulsed beams are presented. Measurements were made in a continuous Co-60 beam and a pulsed 6MV linac beam using an Exradin-A12 ionization chamber fitted with the manufacturer's C-552 plastic wall, as well as geometrically identical walls made from aluminum, copper and molybdenum. The bias voltage was changed between 10values (range: +50 to +560V). Ion recombination was determined from Jaffé plots and by using the "two-voltage technique". The saturation charge measured with each chamber wall was extrapolated from Jaffé plots. Additionally, the effect of different wall materials on chamber response was studied using MCNP simulations. Results showed that the polarity correction factor is not significantly affected by changes in chamber wall material (within 0.1%). Furthermore, although the saturation charges greatly vary with each chamber wall material, and charge multiplication increases for higher atomic number wall materials, the standard methods of calculating ion recombination yielded results that differed by only 0.2%. Therefore, polarity and ion recombination correction factors are not greatly affected by the chamber wall material. The experimental saturation charges for all the different wall materials agreed well within the uncertainty with MCNP simulations. The breakdown of the linear relationship in Jaffé plots that was previously reported to exist for conventional chamber walls was also observed with the different wall materials.
authors.affiliation: Department of Medical Physics, Cedars Cancer Centre, McGill University Health Centre (Glen Site), DS1 7141, 1001 boul. Decarie, Montreal, Quebec H4A 3J1, Canada. Electronic address: Ghada.Aldosary@McGill.ca.
authors.forename: Ghada
authors.initials: G
authors.lastname: Aldosary
authors.affiliation: Department of Medical Physics, Odette Cancer Centre, Sunnybrook Health Sciences Centre, Room TG276, T wing, 2075 Bayview Avenue, Toronto, Ontario M4N 3M5, Canada.
authors.forename: Habib
authors.initials: H
authors.lastname: Safigholi
authors.affiliation: Department of Medical Physics, Odette Cancer Centre, Sunnybrook Health Sciences Centre, Room TG276, T wing, 2075 Bayview Avenue, Toronto, Ontario M4N 3M5, Canada.
authors.forename: William
authors.initials: W
authors.lastname: Song
authors.affiliation: Department of Medical Physics, Cedars Cancer Centre, McGill University Health Centre (Glen Site), DS1 7141, 1001 boul. Decarie, Montreal, Quebec H4A 3J1, Canada.
authors.forename: Jan
authors.initials: J
authors.lastname: Seuntjens
authors.affiliation: Department of Medical Physics, Odette Cancer Centre, Sunnybrook Health Sciences Centre, Room TG276, T wing, 2075 Bayview Avenue, Toronto, Ontario M4N 3M5, Canada.
authors.forename: Arman
authors.initials: A
authors.lastname: Sarfehnia
mesh_terms: D000535:Aluminum; D003037:Cobalt Radioisotopes; D003198:Computer Simulation; D003300:Copper; D004867:Equipment Design; D018499:Linear Energy Transfer; D008982:Molybdenum; D009010:Monte Carlo Method; D010969:Plastics; D055618:Radiation Equipment and Supplies
chemical_list: D003037:Cobalt Radioisotopes; D010969:Plastics; D003300:Copper; D008982:Molybdenum; D000535:Aluminum
keywords: Energy dependence; Ionization chamber walls; Jaffé plots; Polarity and ion recombination corrections