The w-value in air for proton therapy beams

An important quantity in ionization chamber dosimetry is the energy required to produce an ion pair in the gas filling. In proton therapy beams, the appropriate number is the differential value (w) which is not reliably known. A thorough re-evaluation of all available proton w-values in air has been undertaken. The most reliable value for clinical beams should be based on comparisons between calorimetric and ionometric measurements. These data yield a w-value for proton therapy beams in dry air of 34.2 J C−1±0.4%.