title: W values and radial dose distributions for protons in TE-gas and air at energies up to 500 MeV.
abstract: The dosimetry of charged-particle beams is frequently performed by measuring the ionization yield produced in gas-filled ionization chambers and converting it into absorbed dose using the mean energy W expended in a gas per ion pair formed, or its differential value omega for the filling gas. Because of the increasing importance of proton therapy and the lack of accurate values of W and omega at high proton energies in particular, we investigated the ionization yield formation by protons in air and methane-based tissue-equivalent gas in the energy range up to 500 MeV. For this purpose, we analysed the available experimental values of W and omega and, in a second step, investigated the ionization-yield formation by protons as a function of their initial energy using an analytical method based on the Spencer-Fano equation in the framework of the continuous-slowing-down approximation. To estimate possible ionization losses which can be expected if the ionometric method is applied in the case of an incomplete secondary-electron equilibrium, a new analytical model has been developed which can be used to calculate the radial distribution of the ionization yield produced by secondary electrons around high-energy proton tracks.
authors.affiliation: Physikalisch-Technische Bundesanstalt, Braunschweig, Germany.
authors.forename: G
authors.initials: G
authors.lastname: Grosswendt
authors.forename: W Y
authors.initials: WY
authors.lastname: Baek
mesh_terms: D000388:Air; D004583:Electrons; D005740:Gases; D006801:Humans; D008962:Models, Theoretical; D019047:Phantoms, Imaging; D061766:Proton Therapy; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted; D011882:Radiotherapy, High-Energy; D015203:Reproducibility of Results
chemical_list: D005740:Gases