Experimental determination of cross sections for ionization of DNA constituents by proton impact

This thesis presents a complete set of proton-impact ionization cross section data of DNA constituents determined at the Physikalisch-Technische Bundesanstalt (PTB). The cross section data describe the probabilities for the ionization of gas-phase tetrahydrofuran, pyrimidine and trimethyl phosphate that are structural analogues to the base, sugar, and phosphate residue of the DNA, respectively. Total, single-differential and double-differential ionization cross sections were determined, whereby the differential description refers to the energy or angular dependence of electron emission. Double-differential cross sections were measured for impact energies between 75 keV and 3000 keV over an angular range between 15° and 150° in 15° intervals. Both singledifferential and total ionization cross sections were derived by integration of the measured double-differential cross sections. The measurements of double-differential cross sections were carried out using a crossed-beam arrangement in a high-vacuum chamber. The proton beam, which perpendicularly crosses an effusive gas jet target of DNA constituents inside the chamber, was generated either by the 3.75MV Van de Graaff accelerator of the PTB ion accelerator facility (PIAF) or a newly developed 155 kV ion accelerator. In the measurement a newly constructed electron spectrometer was used to detect the secondary electrons produced by proton collisions with the target molecules. The energy of the secondary electrons was selected by a hemispherical electron energy analyzer, which was mounted on a turntable to allow the spectrometer to rotate around the axis of the gas jet. Hence, the energy spectra could be recorded at defined observation angles with respect to the proton beam. These spectra were converted into absolute cross section values via a normalization procedure based on previously published absolute electron-impact cross section data. The experimentally determined cross section data were compared to various theoretical approaches, such as the classical Rutherford formula and several quantum-mechanical methods, namely the Hansen-Kocbach-Stolterfoht model (HKS), a semi-empirical model based on the dielectric response function (DRF) and an ab initio model based on the first Born approximation with corrected boundary conditions (CB1). The comparison to the CB1 and DRF models shows a good agreement with experimental data in a broad range of emission angles and energies of the secondary electrons. All proton-impact ionization cross sections obtained in this work have been integrated in the GEANT4-DNA and PTra track structure codes to improve the estimation of radiation damage to DNA in ion-beam cancer therapy.