Determination of the air w-value in proton beams using ionization chamberswith gas flow capability

The purpose of this work was to determine the w-value of air for protons us ing the paired gas method. Several plastic- and magnesium-walled chambers w ere used with air, synthetic air, nitrogen, and argon flowing gases. Using argon as a reference gas, the w-value of air was measured and ranged from 3 2.7 to 34.5 J/C for protons with energies encountered in radiotherapy. Usin g nitrogen as a reference gas, the w-value of air ranged from 35.2 to 35.4 J/C over the same range of proton energies. The w-value was found, at a giv en energy, to be independent of the ion chamber used. The uncertainty in th ese measurements was estimated at 5.2% at the 2 sigma level. This uncertain ty was dominated by the 4.4% uncertainty in the w-value of the reference ga s. (C) 2000 American Association of Physicists in Medicine. [S0094-2405(00) 01010-5].