A measurement of w for 150 MeV protons in nitrogen and argon

Ionisation chamber dosimetry for proton radiation therapy requires accurate measurements of w, the average energy necessary to produce an ion pair for protons in the therapeutic energy range of 30-240 MeV. In this investigation, such measurements were made at the Harvard Cyclotron Laboratory for 150 MeV protons in nitrogen and argon with an accuracy of 2%. Ionisation and energy loss were measured simultaneously in a gas cell 1 m in length. A parallel plate ionisation chamber situated inside the gas cell was used to measure ionisation, and the energy loss was determined from time-of-flight and range measurements. The data indicate that w is 26.5 +/- 0.6 eV/(ion pair) for argon and 36.3 +/- 0.8 eV/(ion pair) for nitrogen. These results are consistent with measurements at lower energies.