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Sci—Sat AM: Brachy — 02: Extracting W air from the 1976 electron beam measurements of domen and lamperti
Author(s) -
Cojocaru CD,
Ross CK
Publication year - 2012
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.4740209
Subject(s) - physics , beam (structure) , electron , cathode ray , nuclear physics , optics
The average energy expended by an energetic electron to create an ion pair in air, W air 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 W air 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 W air 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 W air 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 W air 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.