Biomedical Engineering Reference
In-Depth Information
Fig. 14.1
The Positronium formation cross section for water vapour. The vertical lines indicate
the Ps threhold (dashed) and the direct ionization threshold (solid)
vibrational and electronic excitation are provided by theory or through comparison
with electron data. In the case of electronic excitation one needs to consider only
all singlet excitation channels as triplet states are not accessible from the singlet
ground state with a positron interaction. The ionization cross section which is
used is that of Campeanu and coworkers [
2
]. Theoretical results were taken as the
existing experimental results were not yet published. An example of the measured
Ps formation cross section for water is shown in Fig.
14.1
.
The electron scattering data set (which is used in both the electron and positron
track models) is based on a range of electron interaction data from the literature
and includes total, elastic (differential and integral), all excitation, ionization and
dissociation channels. It has been discussed previously [
3
].
14.3
Electron transport in water vapour - a test for accuracy
and completeness of cross-section sets
While there has been much recent effort invested in obtaining accurate individual
cross-sections using theoretical, beam and other techniques, for modeling of
macroscopic systems, one must ensure that the cross-section sets are complete and
accurate. One of the tests for completeness and accuracy of cross-section sets is the
comparison of experimental swarm transport properties with those calculated using
the proposed set of cross-sections. A swarm experiment entails measuring physical
properties of an electron swarm (an ensemble of electrons sufficiently dilute so that
electron-electron interactions can be neglected), that is generally in a quasi-steady
