Determination of the air w ‐value in proton beams using ionization chambers with gas flow capability

The purpose of this work was to determine the w ‐value of air for protons using the paired gas method. Several plastic‐ and magnesium‐walled chambers were 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 32.7 to 34.5 J/C for protons with energies encountered in radiotherapy. Using 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 given energy, to be independent of the ion chamber used. The uncertainty in these measurements was estimated at 5.2% at the 2σ level. This uncertainty was dominated by the 4.4% uncertainty in the w ‐value of the reference gas.