title: A measurement of w for 150 MeV protons in nitrogen and argon.
abstract: 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.
mesh_terms: D001128:Argon; D003201:Computers; D006801:Humans; D009584:Nitrogen; D011522:Protons; D011878:Radiotherapy / Q000295:instrumentation / Q000379:methods*; D011879:Radiotherapy Dosage
chemical_list: D011522:Protons; D001128:Argon; D009584:Nitrogen