title: Zero-shift thimble ionization chamber.
abstract: OBJECTIVE
  Verify experimentally the theoretical prediction of F. Tessier and I. Kawrakow [Med. Phys. 37, 96-107 (2010)] that it is possible to design a thimble ionization chamber with no shift in its effective point of measurement (EPOM), i.e., a chamber that provides a measure of the dose to the medium at the location of its central axis.
  
  
  METHODS
  Measure dose from a 25 MV photon beam incident on water with an Exradin A1SL ionization chamber inside a thin sleeve (as a means of effectively increasing the thimble wall thickness). The depth-dose curve is compared to that obtained using a well-characterized PTW Roos parallel-plate chamber.
  
  
  RESULTS
  With an appropriate increase in thimble wall thickness, the EPOM shift of the Exradin A1SL vanishes. Further increase of the wall thickness yields a chamber with a positive (downstream) shift in its point of measurement.
  
  
  CONCLUSIONS
  It is possible to design a thimble ionization chamber with a zero EPOM shift by adjusting the wall thickness.
authors.forename: Frédéric
authors.initials: F
authors.lastname: Tessier
authors.forename: Brian D
authors.initials: BD
authors.lastname: Hooten
authors.forename: Malcolm R
authors.initials: MR
authors.lastname: McEwen
mesh_terms: D004867:Equipment Design; D019544:Equipment Failure Analysis; D007477:Ions; D011829:Radiation Dosage; D011874:Radiometry / Q000295:instrumentation*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity
chemical_list: D007477:Ions