Biophysics with positrons
Positrons are used extensively for clinical diagnostics via Positron Emission Tomography (PET) and have been proposed for the localised treatment of tumours. PET enables body scans with a reasonably high resolution and specificity, locating areas of high metabolic activity. This is useful for imaging cancers and brain function. While detection and imaging systems that are used for PET are state-of-the-art and constantly being improved, the underlying fundamental processes that occur when the positrons enter the tissue and interact with the molecules concerned are not well understood. In PET imaging, the usual practice is to deliver a small amount of a short-lived radioactive isotope (eg. 18F, as part of the molecule fluorodeoxyglucose) to the site to be imaged, either by ingestion or injection. The isotopes used are positron emitters, and the initial energy of the positrons is up to 0.64 MeV. These high-energy positrons quickly thermalise, typically within about a hundred picoseconds, through a process of scattering from the atoms and molecules in the local environment. They lose most of their energy within a few millimetres of the emission site and, at low energies (< 20 eV), can either annihilate directly with a free electron or, though a scattering process with a molecular constituent, form positronium, which is a quasi-stable electron-positron pair. The most common form of positronium, ortho-positronium, also annihilates after a short time (less than 140 nsecs) and in both cases (direct annihilation and positronium decay) two gamma rays result. Coincident detection of these two gamma rays lies at the heart of the PET process.
While PET is an advanced diagnostic technology, little is known about the fundamental interactions that underpin it at the atomic and molecular level. Neither the positron annihilation rates, nor the positronium formation cross sections have been measured for any of the significant bio-molecules. This will be a major thrust of the research in the Centre on bio-systems.
