Biomedical Engineering Reference
In-Depth Information
Fig. 1.2
Initial events
induced by a fast charged
particle that penetrates an
organic or bioorganic solid
composed of molecules R-H
(H
Radiation
e¯
+
[R-H ]
+
[R-H]*
rest of
molecule). Events induced by
secondary electrons are
labeled1to4
D
hydrogen, R
D
-2e¯
[R-H]
R
+ H
or
R
+
+ H¯
or
R¯ + H
+
R
+
+ H
or
R
+H
+
e¯+ [R-H]
*
[R-H]¯
R¯ + H
or
R
+ H¯
energy and momentum of the fast particle practically unchanged, the energy transfer
can be described as an absorption of electromagnetic radiation by the molecules
of the medium [
6
-
8
]. This absorption can lead to the formation of electronically
excited species
, and ionization (i.e.,
C
e
) as shown in Fig.
1.2
,
Œ
R-H
Œ
R-H
C
n
C
ne
/
and multiple ionization
.Œ
R-H
C
[
6
]. The most probable energy loss of
and ionization lies about 22 eV
[
3
,
9
]. Hence, most of the energy of high energy particles is deposited within
irradiated systems by this emission of a succession of low energy quanta. From
the values of the optical oscillator strengths for the dissociative electronic excited
states of hydrocarbons [
9
] and a comparison with the normalized dipole oscillator
strength distribution for DNA and liquid H
2
O[
1
,
3
], one can estimate that about
20% of the energy deposited by fast charged particles in organic matter, including
biological and cellular material, leads to
fast primary charged particles to produce
Œ
R-H
production, whereas the rest leads
to ionization. The ionization energy is shared as the kinetic energy of SE and
potential energy of the cation, with the largest portion of the energy going to SE [
3
].
The products of ionization and electronic excitation that lead to an hydrogen atom
abstraction are shown in Fig.
1.2
, as an example of possible fragmentation produced
by ionizing radiation; for simplicity, products resulting from multiple ionizations are
not shown, but the reaction paths are essentially the same as for single ionization.
A dissociative electronic state
Œ
R-H
can produce two radicals by homologous
bond scission or an ion pair (left vertical arrow in Fig.
1.2
); however, when
ionization occurs, the situation is more complex
due to the emission of at least
one SE
. If the positive ion
Œ
R-H
C
is created in a dissociative state, then a cation
and a radical can be formed as shown by the larger vertical arrow in Fig.
1.2
.The
remaining reactions shown in Fig.
1.2
are due to the SE. By interacting with another
nearby [R-H] molecule, the SE can produce [
10
], depending on its energy, further
ionization (pathway 1) and/or dissociation (pathway 2), or it can temporarily attach
to a nearby molecule to form a temporary transient anion state
Œ
R-H
, which can
Œ
R-H
subsequently dissociate into the products R
H
or R
C
H
, as shown by the
C
