Key data for ionizing-radiation dosimetry: measurement standards and applications, ICRU Report 90

Ionizing radiation dosimetry is used to describe those measurements or calculations that provide information on the energy deposited by the interaction of ionizing radiation with matter. Radiation dosimetry is of importance in several areas, including radiation therapy, radiation protection, and the industrial use of radiation. Several techniques have been developed for measuring energy deposition, with ionization chambers and calorimeters being the most important. These techniques form the basis of primary measurement standards for ionizing radiation, and these primary standards require values and uncertainty estimates for certain key parameters used to relate the result of measurement to the desired quantity. The calculation of the interaction of radiation with matter using Monte Carlo techniques is now well developed and can be used to study energy deposition for problems where measurements are difficult or impossible. Both measurements and calculations require knowledge of basic quantities related to the interaction of radiation with matter. These include data on photon cross sections, electron stopping powers, and the average energy to create an ion pair, to name a few. These data are often referred to as “key data.” This Report examines key data for stopping powers for charged particles ranging from electrons to carbon ions. Values and uncertainties are assigned to the mean excitation energies for air, graphite, and liquid water, and tables of stopping powers covering the energy range from 1 keV to 1 GeV, or higher, are provided. Photon cross sections for air, water, and graphite are reviewed, examined, and compared with relevant measurements to estimate their uncertainties. Values are recommended for the average energy to create an ion pair in air, Wair. The available data for the chemical yield for Fricke dosimetry and for the heat defects for graphite and liquid water are summarized, as is the humidity correction factor for air-filled ionization chambers and the correction to the measured charge due to the initial ion pairs created by an incident photon. Data for the correction in photon and electron beams at low energies for the deviation of Wair from the recommended asymptotic value are also summarized. The impact of the recommended changes is discussed. Important changes are an increase in the uncertainty for air-kerma measurements with free-air chambers and a decrease of about 0.7 % in 60Co air-kerma measurements. The recommended stopping powers for graphite and liquid water differ by up to 1 % from those previously recommended. For radiation dosimetry based on ionization chambers calibrated against absorbed-dose-to-water calorimeters, changes in the measured absorbed dose to water will not exceed 0.5%.