Positron and Electron Impact Ionization of Biomolecules
- Chief Supervisor
- Professor B. Lohmann (Adelaide University)
- Supervisors
- Professor Stephen Buckman (Australian National University)
- Dr. James Sullivan (Australian National University)
- Centre Nodes
- Adelaide University
- Australian National University
There is considerable interest in the role that low energy electrons play in the damage caused by ionizing radiation, such as DNA strand-breaking in biological systems. These low energy electrons are produced, in copious amounts, by ionization events in which incident high-energy radiation (photons, charged particles, etc.) ionize biomolecules, and produce cascades of low energy electrons. These electrons have energies that are typically below 20 eV, and they are produced at a rate of about 2 x 104 electrons per Mev of incident particle energy. They subsequently cause extensive molecular and cell damage through a range of possible interaction processes, the most important of which appears to be dissociative attachment. In this process, even very low energy electrons (< 1 eV) can cause molecular fragmentation and lead to the formation of reactive molecular radicals, which can lead in turn to further bond damage.
Similarly, the way in which positrons interact with biomolecules is an area of considerable interest, driven largely by the potential application to medical diagnostic technologies such as Positron Emission Tomography (PET). In this process high energy positrons (>600 keV) are introduced into the body by the decay of a radionuclide which is introduced on a carrier molecule, such as a sugar. These positrons must essentially thermalise before they can annihilate, and they do so through a series of scattering processes with molecules and cells. As ionization is one of the most efficient energy loss processes, it is an important interaction in the PET diagnostic process, and essentially no information is available for the way in which positrons ionize common biomolecules. Positrons can also annihilate with molecular electrons OR form positronium, an electron-positron pair. These processes both involve the removal of an electron from the molecule and, as such, also lead to ionization, even (and most typically) at energies below the ionization potential of the molecule.
This project will explore the fundamental processes of ionization of biomolecules by both positron and electron impact. Detailed differential (in energy and angle) studies of electron impact ionization of biomolecules will be performed at the University of Adelaide. Complementary experiments, for both low energy (near-threshold) positrons and electrons, which lead to the measurement of the total ionization cross section, will be carried out on the Australian Positron Beamline Facility at the ANU.
