Extracting W(air) from the electron beam measurements of Domen and Lamperti.

The average energy expended by an energetic electron to create an ion pair in dry air, Wair , is a key quantity in radiation dosimetry. Although Wair is well established for electron energies up to about 3 MeV, there is limited data for higher energies. The measurements by Domen and Lamperti [Med. Phys. 3, 294-301 (1976)] using electron beams in the energy range from 15 to 50 MeV can, in principle, be used to deduce values for Wair , if the electron stopping power of graphite and air are known. A previous analysis of these data revealed an anomalous variation of 2% in Wair as a function of the electron energy. We use Monte Carlo simulation techniques to reanalyze the original data and obtain new estimates for Wair , and to investigate the source of the reported anomaly. Domen and Lamperti (DL) reported the ratio of the response of a graphite calorimeter to that of a graphite ionization chamber for broad beams of electrons with energies between 15 and 50 MeV and at different depths in graphite (including depths well beyond the range of the primary electrons, i.e., in the bremsstrahlung photon regime). Using a detailed EGSnrc model of the DL apparatus, as well as up-to-date stopping powers, we compute the dose ratio between the ionization chamber cavity and the calorimeter core, for plane-parallel electron beams. This dose ratio, multiplied by the DL measured ratio, provides a direct estimate for Wair . Despite an improved analysis of the original work, the extracted values of Wair still exhibit an increase as the mean electron energy at the point of measurement decreases below about 15 MeV. This anomalous trend is dubious physically, and inconsistent with extensive data for Wair obtained at lower energies. A thorough sensitivity analysis indicates that this trend is unlikely to stem from errors in extrapolation and correction procedures, uncertainties in electron stopping powers, or bias in calorimetry or ionization chamber measurements. However, we find that results are quite sensitive to the intrinsic graphite mass thickness of the detectors and to the incident beam energy. The DL experiment provides data in an energy regime where the electron stopping power is insensitive to the mean excitation energy of graphite - an issue plaguing Wair experiments at lower energies. Unfortunately, state-of-the-art scrutiny of the original data cannot explain the anomalous trend in terms of perturbation effects or extrapolation bias. It can only be understood in terms of speculative offsets in graphite mass thickness or beam energy. Therefore higher accuracy measurements for electron energies above 15 MeV are recommended to further resolve the value of Wair .

Tessier F ，Cojocaru CD ，Ross CK 《-》

Sci-Sat AM: Brachy - 02: Extracting W(air) from the 1976 electron beam measurements of domen and lamperti.

The average energy expended by an energetic electron to create an ion pair in air, Wair is an important quantity in radiation dosimetry. The data obtained by Domen and Lamperti using electron beams in the energy range from 15 to 50 MeV can be used to extract a value for Wair if the electron stopping powers of graphite and air are assumed to be known. We use Monte Carlo techniques to reanalyze these data and obtain a new estimate for the value of Wair Using the EGSnrc Monte Carlo and its associated user codes, as well as the best availabl-e stopping power data for graphite, we calculate the perturbation effects due to the calorimeter and ionization chamber and the effect of extrapolating from scattered to plane-parallel beams. Without further adjustments, the extracted values of Wair show a significant trend as the mean electron energy decreases. We show that part of this trend can be attributed to an incorrect value of the density assigned to the graphite absorbers and part to the likelihood that the nominal energy assigned to the low-energy electron beams is not correct. Using all the data, we obtain a value for Wair of 33.84 eV per ion pair with a relative standard uncertainty of 0.4 %. This result serves to complement values obtained using 60 Co γ-rays, for which the value of the mean excitation of graphite contributes significantly to the uncertainty.

Cojocaru CD ，Ross CK 《-》

Determination of W(air) in high-energy electron beams using graphite detectors.

ICRU Report 90 on Key Data for Ionizing-Radiation Dosimetry: Measurement Standards and Applications (2014) has reaffirmed the recommended value of the mean energy required to create an ion pair in air, Wair , to be 33.97(12) eV. The report also indicates that this "constant" of radiation dosimetry is energy independent above 10 keV, since there is no theoretical or experimental evidence to the contrary. The goal of this investigation is to obtain additional experimental determinations of Wair in high energy beams and thus to verify the suggested energy independence. Wair can be evaluated by combining ionometric and calorimetric measurements with a calculated ratio of the absorbed dose in the ion chamber air cavity and that of the calorimeter absorbing element. In this investigation, a graphite parallel plate chamber and a graphite calorimeter were used and the dose ratio was calculated using the EGSnrc Monte Carlo code. Measurements were made in electron beams from the NRC Vickers linear accelerator at two incident energies, 20 and 35 MeV. A range of average energies at the measurement point were obtained by inserting graphite plates in the primary beam. The average value of Wair obtained in this investigation is 33.85(18) eV which is consistent with the recommended value of 33.97(12) eV where the number in brackets represents the combined standard uncertainty of the value, referring to the corresponding last digits. The individual values of Wair do not show any statistically significant energy dependence. The overall combined uncertainty of 0.5% meets the original target of the investigation. A larger-scale investigation, involving more individual energy points and a wider range of electron energies is required to go further and, for example, comment on the Wair energy dependency question raised by Tessier et al. [Med. Phys. 2018;45:370-381].

Bourgouin A ，Cojocaru C ，Ross C ，McEwen M ... -  《-》

Perturbation correction factors for the NACP-02 plane-parallel ionization chamber in water in high-energy electron beams.

Verhaegen F ，Zakikhani R ，Dusautoy A ，Palmans H ，Bostock G ，Shipley D ，Seuntjens J ... -  《PHYSICS IN MEDICINE AND BIOLOGY》

Use of the BIPM calorimetric and ionometric standards in megavoltage photon beams to determine Wair and Ic.

Burns DT ，Picard S ，Kessler C ，Roger P ... -  《-》