Advanced Physics for Brachytherapy 2018

Chapter 6: Calibration Using Free In-air Measurements

6.1. General The free in-air measurement technique cannot be used for low-energy sources due to air-kerma calibration factor uncertainties, low air-kerma rates, and uncertainties due to air humidity. For long-lived radionuclides, e.g., 137 Cs, a source can be a working standard.

6.2. Formalism for reference air kerma rate K R reference air kerma rate N K

chamber air kerma calibration factor at desired photon energy M U measured charge, corrected for T, P, recombination, transit error t time for collecting charge k scat correction for room scatter k n correction factor for non-uniform electron fluence within the cavity d measurement distance from source center to chamber center d ref reference measurement distance (i.e., 1 meter)