Positron and Electron Collisions with Bio-Molecules
- Chief Supervisor
- Prof. S.J. Buckman (Australian National University)
- Supervisors
- Suzanne Smith (ANSTO)
- Dr James Sullivan (Australian National University)
- Dr. Julian Lower (Australian National University)
- Dr. Michael Brunger (Flinders University)
- Centre Nodes
- Australian National University
- ANSTO
- Flinders University
- Collaborators
- University of California, San Diego
- University of Nebraska, Lincoln
Recent experiments have demonstrated the critical role that low energy electrons play in the damage to human cells induced by ionizing radiations. Positrons are also a tool which is used in the diagnosis of human tumours, through the use of Positron Emission Tomography (PET). They are also mooted as a possible therapeutic tool (positherapy) in the treatment of tumours. This project will investigate and quantify, at the fundamental level, the interactions of both positrons and electrons with a number of important biomolecules.
Until relatively recently it was the commonly held belief that ballistic, high-energy impacts were responsible for the bulk of the cell and tissue damage when ionizing radiation enters the body. This has now been shown to be only partially correct. It now appears that the high energy ionizing radiation entering the body liberates many low (sub-ionization) energy electrons which in turn attach to the numerous electro-negative components of DNA (bases, sugars, water) and, through the process of dissociative attachment, either directly lead to single or double DNA strand breaks, or form free radicals, which then chemically react with the DNA molecules to lead to strand breaking. There have been tremendous breakthroughs in this area in recent years and our goal is to add to this experimental endeavour by providing the first, accurate absolute cross sections for a range of these important building blocks of DNA.
Positrons are now widely used in PET diagnostics of human tumours. PET scans involve detecting the annihilation gamma rays that are produced when positrons are selectively introduced to the site of a tumour. Most of the radioactive positron emitters which are used for these purposes (eg 18F) emit high energy (~600 keV) positrons, which must thermalise, via scattering, before entering the energy regime which is favoured for annihilation (<~20 eV). Annihilation takes place either by direct means, whereby the positron picks up an electron from the surrounding medium and annihilates, or through the formation of positronium, an electron positron pair, which subsequently annihilates. Direct annihilation is only strongly favoured at very low positron energies (<1 eV) while positronium formation is typically strongest in the 5-20 eV region and is generally a process with a much larger probability.
This project will focus on measurements of fundamental scattering processes (total and elastic scattering, vibrational excitation, electronic excitation, positronium formation) using both electrons and positrons. The goal is to provide benchmark cross sections for important biomolecules, ranging from H2O and simple acids, such as formic and acetic acid, to the sugars and bases that make up DNA and RNA strands.
Further Reading
B. Boudaiffa, P. Cloutier, D. Hunting, M. Huels and L. Sanche Science 287 1658 (2000)
Positron Emission Tomography: Principles and Practice, P. E. Valik, D. G. Bailey, D. W. Townsend, M. N. Maisey, Springer-Verlag, 2003
F. Martin, P.D. Burrow, Z. Cai, P. Cloutier, D. Hunting and L. Sanche Phys. Rev. Lett. 93 068101 (2004)
