Biomedical Engineering Reference
In-Depth Information
Table 16.2
Binding energies (in eV) for the different sub-shells of the
water molecule in gaseous and liquid phases [
49
]
Water phase
1b
1
3a
1
1b
2
2a
1
1a
1
Vapor
12:61
14:73
18:55
32:20
539:70
Liquid
10:79
13:39
16:05
32:30
539:00
16.3.2
From vapor to liquid water
As highlighted above, numerical track-structure MC simulations were successfully
developed for modeling the charged particle transport in biological medium and
then providing a detailed description of the three-dimensional energetic deposit
cartography. To that end, the modeling of the ion-induced ionizing processes in
water and more particularly of ionization is of prime importance. Water ionization
by charged particle impact (electrons as well as heavy charged particles) has been
a matter of active research since the 70's particularly in the field of radiobiology
for modeling the radio-induced damages (see for example [
44
]and[
45
]), the
biological matter being commonly simulated by water. However, due to the scarcity
of experimental measurements in its liquid phase, water was essentially studied in
its vapor phase by assuming that describing the particle track-structure in liquid
matter could be done, in a first step, either by applying the well-known “gas-phase
approximation”
i.e
. via a simple linear extrapolation to unit density environment of
the liquid or by converting the highly excited Rydberg states occurring in gaseous
water into ionization (see for example [
46
]).
Another approach has consisted in implementing into the cross section calcula-
tions the binding energies of the liquid water phase whose values differ from gaseous
water by about 2-4 eV essentially for the three outermost subshells see Table
16.2
.
Thus, we find an abundant literature dedicated to heavy charged particles-transport
numerical simulations in gaseous and liquid water: see for example Nikjoo
et al
.
[
45
] and Gonzalez-Mu noz
et al
.[
47
] and more particularly the interesting work
of Emfietzoglou and co-workers (see for example [
48
]) where the influence of the
water phase on the singly differential and total ionization cross sections for protons
was analyzed.
16.3.3
The DNA target
As stated above, ion-induced collisions on DNA bases have been rarely experi-
mentally investigated. On the theoretical side, only few attempts were proposed
for predicting total ionization cross sections. Among these, we essentially find two
approaches: a first (semi)-classical one generally based on a classical-trajectory
Monte Carlo (CTMC) type description and a second one developed in the quantum-
mechanical framework and limited - for the major part of the existing studies -
to the use of the first Born approximation. The “semi-classical group” may be
