Biomedical Engineering Reference
In-Depth Information
N
+
projectile:
CN
_
/dR
NH
_
/dR
CN
_
/R
NH
_
/R
CN
_
/T
CN
_
/dT
+
N
2
projectile
CN
_
/dR
CN
_
/R
0
0 0 0 0 0 0 0
Incident Ion Energy (Lab. energy in eV)
Fig. 11.2
Left: Kinetic energies for atomic fragment ions from 4 keV/amu (C
C
:2keV/amu)C
qC
induced fragmentation of thymine [
28
]. Right: Desorption energy thresholds of NH
and CN
anions during N
C
and N2
2C
ion irradiation of deoxyribose dR, ribose R, thymine T and thymidine
dT films on Pt substrate [
29
]
only a few eV of kinetic energy can efficiently damage nucleobases collisionally
(see Fig.
11.2
b, [
20
,
22
]). Nucleobase damage due to reactive scattering even seems
to occur at almost zero kinetic energy of the impinging ion (see Fig.
11.2
b, for
ribose and deoxyribose, NH
can only be formed in a reactive hydrogen abstraction
process involving the projectile N atom, [
29
]). KeV ion-induced biomolecular
fragmentation in a biological environment is thus likely to trigger a whole avalanche
of damage, which is interesting because biological effects of ionizing radiation are
closely related to the complexity of the induced DNA strand breaks. The relative
biological effectiveness (RBE) for double strand break production is typically higher
for densely ionizing radiation (e.g. heavy ions) than for sparsely ionizing (low
linear energy transfer (LET)) radiation, e.g. photons. A possible explanation for this
observation could be that densely ionizing radiation leads to formation of clustered
double strand breaks [
33
]. Psonka-Antonczyk
et al.
found evidence for formation
of such clustered lesions in atomic force microscopy studies of heavy ion irradiated
plasmid DNA [
34
].
A motivation for ion-biomolecule collision studies is the need for absolute
ionization and fragmentation cross sections which can serve as input for track
structure calculations. By determination of the layer thickness deposited on a cooled
aluminum plate opposite the sublimation oven, Tabet
et al.
[
35
] recently were able to
determine densities for various nucleobase targets and report absolute cross sections
for direct ionization and for electron capture following 80 keV proton collisions.
Typical cross sections were
17:5 10
15
cm
2
for direct ionization of the nucleobases uracil, thymine and adenine. For
cytosine, surprisingly 3 times smaller cross sections were obtained. First quantum
mechanical predictions of these ionization cross sections (performed within the
6 10
15
cm
2
for capture from and
