An analysis of existing data forW_air,I_cand the productW_airs_c,air

Abstract

An analysis is made of the published results of over forty experiments that have determined either the mean energy Wair required to create an ion pair in dry air, the mean excitation energy Ic for graphite, or the product of Wair and the electron stopping-power ratio sc,air for graphite and air for 60Co gamma radiation. A minimization process is used in which the best estimates for Wair and sc,air are those that minimize the sum of the deviations, taking uncertainties into account. This approach yields self-consistent values for Wair, sc,air and the product Wairsc,air. The results of this process are the values Wair = 33.97 eV with standard uncertainty 0.11 eV, sc,air = 0.9926 for 60Co, with relative standard uncertainty 0.32%, and the product Wairsc,air = 33.72 eV for 60Co with standard uncertainty 0.08%. The resulting value for Ic depends on the treatment of the density effect in graphite, the best estimate being the value Ic = 81.1 eV with standard uncertainty 2.0 eV obtained when using the grain density in the evaluation of the graphite stopping power. The robustness of the analysis and the consequences for air-kerma primary standards for radiation dosimetry are discussed.

Aerrow: A probe‐format graphite calorimeter for absolute dosimetry of high‐energy photon beams in the clinical environment

“…Even among primary standards, calorimetry is considered the most direct and absolute method of measuring absorbed radiation dose since device calibration can be achieved in terms of quantities with traceable standards (i.e., electrical and temperature), entirely independent of radiation . This avoids the need to rely on dosimetric quantities such as (the average energy required to produce an ion pair in dry air) and () (the product of the molar extinction coefficient and the radiation chemical yield of ferric ions), the knowledge of which are relatively more uncertain than current electrical and temperature‐based standards . To date, calorimeter designs have primarily been driven by national metrology institutes, whose principal motivation is to achieve the lowest possible measurement uncertainty .…”

“…[1][2][3] This avoids the need to rely on dosimetric quantities such as ðW=eÞ air (the average energy required to produce an ion pair in dry air) and (ðGÞ 3þ Fe ) (the product of the molar extinction coefficient and the radiation chemical yield of ferric ions), the knowledge of which are relatively more uncertain than current electrical and temperature-based standards. 1,4 To date, calorimeter designs have primarily been driven by national metrology institutes, whose principal motivation is to achieve the lowest possible measurement uncertainty. 5 Utility and usability of the devices are secondary considerations, and as a result, most calorimeters today are generally both bulky and fragile, and are operated by only handful of individuals possessing the required specialized equipment and tacit knowledge.…”

Extracting Wair from the electron beam measurements of Domen and Lamperti

“… is incorrect. Burns has reanalyzed the available data on , W air , and the product (where is the graphite to air stopping power ratio), to conclude that the best consistency overall is obtained when eV. A subsequent reanalysis of key data for dosimetry in ICRU Report 90 resulted in the recommendation eV.…”

Direct measurement of electron beam quality conversion factors using water calorimetry

“…4,5 Generally, the former two techniques rely on a well-characterized radiation field as a reference. Ionometry relies on the knowledge of the average energy required to produce an ion pair in dry air, (W/e) air , 6 and a restricted graphite-to-air stopping power ratio. Fricke dosimetry relies on an accurate transfer of absorbed dose from a calibration quantity and on the knowledge of the product of the molar extinction coefficient and the radiation chemical yield of ferric ions, (εG) Fe 3+.…”

“…5,7 This phenomenon is less of a problem for other types of dosimetry systems, but nonetheless can affect the result of measurement if neglected. 6 Current electron standards include a total absorption-calibrated Fricke-based system operated by The Swiss Federal Office of Metrology and Accreditation (METAS) 9 and a graphite calorimeter employed by the National Physical Laboratory (NPL) in the UK. 10 McEwen and DuSautoy 7 have reviewed these standards and compared them to the NRC and McGill University water calorimeters.…”