Calibration of cavity ionization chambers

Abstract

d commercial thimble ionization chamber, intended for use as a Bragg-Gray cavity chamber a t high photon energies, must be calibrated in such a way that the reading of the instrument can be converted into the " ion dose " in the air volume of the chamber. i.e. the quotient of the ion charge and the mass of air in that volume.The factor for this conversion is independent of the photon energy. A method for this calibration is proposed and measurements of a correction factor are described. The theoretical expression for the energy dependent conversion factor, in calculating the absorbed dose in water from the reading of the dosemeter when the chamber is used in a water phantom, is compared with calculations of other authors.

The photon calibration of a tissue-equivalent ionisation chamber for neutron dosimetry

“…(4) The wall correction factor has been discussed by Franz (1971). For 250 kV x-rays with a 1.5 mm copper filter he has shown that kwall = 1.000 for wall thicknesses up to 300mgcm-*.…”

“…The internal radius of the secondary standard chamber is 3.6 mm and, if a displacement correction were applied to its reading, a similar correction would have to be made to the reading from the TE chamber which has an internal radius of 4.8 mm, and this would tend to cancel out. Franz (1971) suggested that for measurements in a phantom the centre of measurement should be considered to be displaced by three quarters of the radius of the cavity in front of the centre of the chamber. For the two chambers used, which have a difference of 1.2 mm in their internal radii, the net correction, calculated from depth dose tables, would be 0.997 for a 10 cm x 10 cm field in a 6oCo beam.…”