3. Realization of Quantities by Primary Standards Laboratories

Abstract

In principle, both kerma and absorbed dose can be determined for any material at any energy. Given the considerations noted above regarding secondaryphoton contributions and charged-particle equilibrium (CPE), it is not surprising that the relationship between kerma and absorbed dose changes significantly with energy and material. Because of the very limited beam penetration and the relatively low absorbed-dose rates involved, absorbed dose is very difficult to realize directly for kilovoltage x rays, although a few water calorimeter standards have been developed (see, e.g., de Prez and de Pooter, 2008; Krauss et al., 2012; Rapp et al., 2013). Most primary standards for these radiations are based on kerma, in particular on the determination of the air kerma using a free-air ionization chamber. At Co g-ray energies, the need for CPE would require a prohibitively large free-air chamber, and cavity-ionization chambers are used as primary standards. In more recent years, the direct determination of absorbed dose by graphite and water calorimetry has produced standards with an overall uncertainty that matches, and in some cases reduces, that derived from a determination of air kerma. Nevertheless, air kerma for Co g rays remains a very important reference quantity, particularly for standards laboratories. At the high energies produced by particle accelerators, the determination of air kerma free in air in these beams is no longer used to determine absorbed dose. In more recent years, absorbed-dose standards at high energies have been used, either directly for instrument calibrations or more commonly to determine values for correction factors that convert an ionization-chamber absorbed-dose calibration for Co g rays for use at higher energies (McEwen, 2010; Seuntjens et al., 2000). Several review articles describing the standards used for radiation dosimetry have been published in a special issue of Metrologia (Sharpe, 2009).