Biomedical Engineering Reference
In-Depth Information
1.4
Damage to biomolecules induced by LEE
1.4.1
Amorphous solid water
Radiation damage to DNA within the nucleous of cells can generally be classified
into two major groups, referred to as “direct” and “indirect”. At least 50% of the
damage induced by high energy radiation within cells is due to the “indirect” effect
[
20
,
57
,
58
]. Whereas “direct” damage results from interaction of primary radiation
and SE with the genome, “indirect” damage is attributed to the immediate species
formed by ionizing radiation in the vicinity, but outside, the volume occupied by
DNA. Owing to the large quantity of water in cells, most of the immediate species
are H and OH radicals and solvated electrons formed by LEE interacting with
water molecules surrounding DNA. These species can further react with water or
diffuse to DNA, where they can trigger other reactions and damage the molecule.
The production of species from LEE-interaction with condensed water molecules
is therefore of considerable relevance to fully understand the indirect effect of
radiationonDNA.
Fragmentation of condensed-phase H
2
O by LEE impact has been investigated
mainly with amorphous ice films. The yield function for desorption of H
[
59
-
61
],
H
2
2
S
3
P
1
D
2
/
[
64
,
65
] were recorded in the range 5-
30 eV. Most of these functions exhibit resonance structures below 15 eV, which are
characteristic of transient anion formation. From anion yields, DEA to condensed
H
2
O was shown to result principally in the formation of H
and the OH
:
radical
from dissociation of the
2
B
1
state of H
2
O
located in the 7-9 eV region. Smaller
contributions arise from the
2
A
1
and
2
B
2
anionic states, which are formed near 9
and 11 eV, respectively [
59
,
60
]. At higher energies, nonresonant processes, such as
dipolar dissociation (e.g.,
[
62
,
63
], D
.
/;
O
.
/
,andO
.
H
C
in Fig.
1.2
) lead to H
2
O
fragmentation with the assistance of a broad resonance that extends from 20 to 30 eV
[
60
]. Kimmel et al. [
64
,
65
] measured the ESD of D
2
R
C
C
H
or R
C
Œ
R-H
!
X
1
†
g
2
S
.
3
P
j
D2;1;0
/
;
D
.
/;
O
,
1
D
2
/
and O
desorption yields from amorphous ice versus incident electron energy.
An apparent threshold was found at
.
6:5
eV with a steadily increasing intensity.
2
S
Above this threshold, the D
.
/
intensity also increases rapidly and exhibits a broad
eV. Above 7 eV, direct electronic excitation of the
3;1
B states
lead to H
:
and OH
:
formation. From 10 eV, ionization progressively takes over and
dominates energy losses. The ensemble of these reactions leads to an abundant
production of OH and H radicals and H
2
molecules.
The integral cross sections per scatterer (i.e. elastic collision, phonon excitations,
vibrational excitations, electronic excitations and ionization) for 1-100 eV electron
scattering in an amorphous film of ice condensed at a temperature of 14 K have
been measured by Michaud et al. [
66
]. The integral cross sections were determined
relative to the total from a two-stream multiple-scattering analysis of the electron
energy distribution backscattered from the film. The magnitude of the electronic
excitation and ionization cross sections and various features found in their energy
resonance for
14
-
21
