title: Experimental analysis of general ion recombination in a liquid-filled ionization chamber in high-energy photon beams.
abstract: OBJECTIVE
  To study experimentally the general ion recombination effect in a liquid-filled ionization chamber (LIC) in high-energy photon beams.
  
  
  METHODS
  The general ion recombination effect on the response of a micro liquid ion chamber (microLion) was investigated with a 6 MV photon beam in normal and SRS modes produced from a Varian(®) Novalis Tx(TM) linear accelerator. Dose rates of the linear accelerator were set to 100, 400, and 1000 MU∕min, which correspond to pulse repetition frequencies of 60, 240, and 600 Hz, respectively. Polarization voltages applied to the microLion were +800 and +400 V. The relative collection efficiency of the microLion response as a function of dose per pulse was experimentally measured with changing polarization voltage and pulse repetition frequencies and was compared with the theoretically calculated value.
  
  
  RESULTS
  For the 60 Hz pulse repetition frequency, the experimental relative collection efficiency was not different from the theoretical one for a pulsed beam more than 0.3% for both polarization voltages. For a pulsed radiation beam with a higher pulse repetition frequency, the experimental relative collection efficiency converged to the theoretically calculated efficiency for continuous beams. This result indicates that the response of the microLion tends toward the response to a continuous beam with increasing pulse repetition frequency of a pulsed beam because of low ion mobility in the liquid.
  
  
  CONCLUSIONS
  This work suggests an empirical method to correct for differences in general ion recombination of a LIC between different radiation fields. More work is needed to quantitatively explain the LIC general ion recombination behavior in pulsed beams generated from linear accelerators.
authors.affiliation: Medical Physics Unit, McGill University, Montreal General Hospital (L5-113), 1650 Cedar Avenue, Montreal, Quebec H3G 1A4, Canada. eachung@medphys.mcgill.ca
authors.forename: Eunah
authors.initials: E
authors.lastname: Chung
authors.forename: Stephen
authors.initials: S
authors.lastname: Davis
authors.forename: Jan
authors.initials: J
authors.lastname: Seuntjens
mesh_terms: D004867:Equipment Design; D019544:Equipment Failure Analysis; D007477:Ions / Q000737:chemistry* / Q000528:radiation effects*; D010315:Particle Accelerators; D017785:Photons; D011874:Radiometry / Q000295:instrumentation* / Q000379:methods; D011879:Radiotherapy Dosage; D011882:Radiotherapy, High-Energy / Q000295:instrumentation*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D012996:Solutions
chemical_list: D007477:Ions; D012996:Solutions