Biomedical Engineering Reference
In-Depth Information
sets for both positrons and electrons. The cross section measurements, transport
theory, and implications for the study of charged particle transport in soft matter are
discussed.
14.1
Introduction
The transport of charged particles - electrons, positrons, protons - in gaseous and
liquid media has been a subject of considerable interest in a number of research
and development fields. Charged-particle transport has applications in fields as
diverse as gas discharge physics, atmospheric and astrophysical environments,
environmental science and most recently, the implications for radiation damage in
tissue have been revealed.
We have a program for the measurement and application of both positron
and electron interactions with biologically relevant molecules. Our rationale is
to establish the best and most accurate cross sections for all relevant collision
processes, either through accurate measurement (using both positrons and electrons)
or the application of contemporary, state-of-the-art theoretical models. These cross
sections are compiled into self-consistent sets that can then be used in our Boltz-
mann equation and Monte Carlo modeling codes to establish transport parameters
such as drift and diffusion coefficients and also to model charge penetration or range,
reaction products along particle tracks and energy deposition, amongst other things.
In this paper we consider the interaction of positrons and electrons with water
molecules comprising both gases and liquids, and provide a brief background to
the measurement of e
˙
-
H
2
O cross sections and the way they are applied in the
various approaches to modeling of macroscopic phenomena. Ongoing and future
work involving a series of other bio-molecules is also discussed.
;
14.2
Data Considerations - Measurement and Calculation
of Cross Sections
No single experimental apparatus is used for the measurements that are used
in our modelling studies described here, but rather a range of techniques that
provide absolute scattering cross sections for both positron and electron interactions
have been applied. Where measurements are not available, we have used the best
available theoretical calculations to obtain the relevant cross sections. In some cases,
especially for positrons, neither experimental nor theoretical values are available and
we have had to make educated guesses at the value of the cross sections, usually
based on electron data.
The positron data for water vapour comprises measurements of the grand
total, total elastic and total positronium (Ps) formation cross sections using a
high-resolution, trap-based positron beam [
1
]. Other processes, such as rotational,
