Measurement of the average energy required to produce an ion pair in air by 7.6 MeV/u12C6+ ions

The average energy required to produce an ion pair (W value) in air by carbon ions must be known with high precision for the ion-chamber dosimetry performed at the GSI tumor treatment unit. As there is a lack of data in the energy range of interest for the therapy we performed a measurement in air with 7.6 MeV/u12C6+-ions. The experimental method is described and the preliminary result of wE=(34.1±0.4) eV is discussed.