Biomedical Engineering Reference
In-Depth Information
Chapter 8
Positronium Formation and Scattering
from Biologically Relevant Molecules
G. Laricchia, D.A. Cooke, and S.J. Brawley
Abstract
Recent progress in our experimental studies of positronium formation
and scattering from simple atomic and molecular systems are reviewed. The former
are used to highlight key features of ionizing collision by positrons before consider-
ing recent phenomena observed in the case of molecular targets, including positron
impact excitation-ionization and the electron-like scattering of positronium. The
guiding theme of this review arises from the role that repeated cycles of formation
and dissociation of positronium are expected to play in the accurate description of
positron interaction with matter.
8.1
Introduction
The study of the interaction of positrons and positronium (Ps, the bound-state of
an electron and a positron) with atoms and molecules is variously motivated, for
example, by the need to understand basic matter-antimatter interactions, to assist
the development of accurate scattering theories, to aid the analysis of astrophysical
events and to support tests of QED bound-state problems (e.g. [
1
]). Recently, the
outstanding success of positron-emission-tomography (PET) in imaging human
pathologies and physiological functions through the visualization of metabolic
pathways, has resulted in its proliferation across major hospitals around the world
[
2
]. In turn, this has highlighted the lack of accurate
ˇ
C
dosimetry, most protocols
presently being based on macroscopic doses which are computed by dividing the
total energy deposited in the body by its total mass (sometimes allowing for the non-
uniform dose patterns arising from different permanence times in various organs)
[
3
]. Thus, as discussed elsewhere in this topic (Chaps. 13,14,16), event-by-event
