title: Polarity effects and apparent ion recombination in microionization chambers.
abstract: OBJECTIVE
  Microchambers demonstrate anomalous voltage-dependent polarity effects. Existing polarity and ion recombination correction factors do not account for these effects. As a result, many commercial microchamber models do not meet the specification of a reference-class ionization chamber as defined by the American Association of Physicists in Medicine. The purpose of this investigation is to determine the cause of these voltage-dependent polarity effects.
  
  
  METHODS
  A series of microchamber prototypes were produced to isolate the source of the voltage-dependent polarity effects. Parameters including ionization-chamber collecting-volume size, stem and cable irradiation, chamber assembly, contaminants, high-Z materials, and individual chamber components were investigated. Measurements were performed with electrodes coated with graphite to isolate electrode conductivity. Chamber response was measured as the potential bias of the guard electrode was altered with respect to the collecting electrode, through the integration of additional power supplies. Ionization chamber models were also simulated using comsol Multiphysics software to investigate the effect of a potential difference between electrodes on electric field lines and collecting volume definition.
  
  
  RESULTS
  Investigations with microchamber prototypes demonstrated that the significant source of the voltage-dependent polarity effects was a potential difference between the guard and collecting electrodes of the chambers. The voltage-dependent polarity effects for each prototype were primarily isolated to either the guard or collecting electrode. Polarity effects were reduced by coating the isolated electrode with a conductive layer of graphite. Polarity effects were increased by introducing a potential difference between the electrodes. comsol simulations further demonstrated that for a given potential difference between electrodes, the collecting volume of the chamber changed as the applied voltage was altered, producing voltage-dependent polarity effects in the chamber response. Ionization chamber measurements and comsol simulations demonstrated an inverse relationship between the chamber collecting volume size and the severity of voltage-dependent polarity effects on chamber response. The effect of a given potential difference on chamber polarity effects was roughly ten times greater for microchambers as compared to Farmer-type chambers. Stem and cable irradiations, chamber assembly, contaminants, and high-Z materials were not found to be a significant source of the voltage-dependent polarity effects.
  
  
  CONCLUSIONS
  A potential difference between the guard and collecting electrodes was found to be the primary source of the voltage-dependent polarity effects demonstrated by microchambers. For a given potential difference between electrodes, the relative change in the collecting volume is smaller for larger-volume chambers, illustrating why these polarity effects are not seen in larger-volume chambers with similar guard and collecting electrode designs. Thus, for small-volume chambers, it is necessary to reduce the potential difference between the guard and collecting electrodes in order to reduce polarity effects for reference dosimetry measurements.
authors.affiliation: Department of Medical Physics, University of Wisconsin-Madison, Madison, Wisconsin 53705 and Department of Human Oncology, University of Wisconsin-Madison, Madison, Wisconsin 53792.
authors.forename: Jessica R
authors.initials: JR
authors.lastname: Miller
authors.affiliation: Standard Imaging, Middleton, Wisconsin 53562.
authors.forename: Brian D
authors.initials: BD
authors.lastname: Hooten
authors.affiliation: Department of Medical Physics, University of Wisconsin-Madison, Madison, Wisconsin 53705.
authors.forename: John A
authors.initials: JA
authors.lastname: Micka
authors.affiliation: Department of Medical Physics, University of Wisconsin-Madison, Madison, Wisconsin 53705.
authors.forename: Larry A
authors.initials: LA
authors.lastname: DeWerd
mesh_terms: D003198:Computer Simulation; D004560:Electricity; D004566:Electrodes; D004867:Equipment Design; D006108:Graphite; D007477:Ions; D000072232:Radiation Dosimeters; D011839:Radiation, Ionizing; D012984:Software
chemical_list: D007477:Ions; D006108:Graphite