Biomedical Engineering Reference
In-Depth Information
Table 13.3
G
Values (Number per 100 eV) for Various Species in
Water at 0.28
µ
s for Electrons at Several Energies
Electron Energy (eV)
Species
100
200
500
750
1000
5000
10,000
20,000
OH
1.17
0.72
0.46
0.39
0.39
0.74
1.05
1.10
H
3
O
+
4.97
5.01
4.88
4.97
4.86
5.03
5.19
5.13
e
aq
1.87
1.44
0.82
0.71
0.62
0.89
1.18
1.13
H
2.52
2.12
1.96
1.91
1.96
1.93
1.90
1.99
H
2
0.74
0.86
0.99
0.95
0.93
0.84
0.81
0.80
H
2
O
2
1.84
2.04
2.04
2.00
1.97
1.86
1.81
1.80
Fe
3+
17.9
15.5
12.7
12.3
12.6
12.9
13.9
14.1
the other species, such as H
2
O
2
and H
2
, increase with time. As mentioned earlier,
by about 10
-6
s the reactive species remaining in a track have moved so far apart
that additional reactions are unlikely. As functions of time, therefore, the
G
values
change little after 10
-6
s.
Calculated yields for the principal species produced by electrons of various ini-
tial energies are given in Table 13.3. The
G
values are determined by averaging the
product yields over the entire tracks of a number of electrons at each energy. [The
last line, for Fe
3+
, applies to the Fricke dosimeter (Section 10.6). The measured
G
value for the Fricke dosimeter for tritium beta rays (average energy 5.6 keV),
is 12.9.] The table indicates how subsequent changes induced by radiation can be
partially understood on the basis of track structure—an important objective in ra-
diation chemistry and radiation biology. One sees that the
G
values for the four
reactive species (the first four lines) are smallest for electrons in the energy range
750-1000 eV. In other words, the intratrack chemical reactions go most nearly to
completion for electrons at these initial energies. At lower energies, the number of
initial reactants at 10
-12
s is smaller and diffusion is more favorable compared with
reaction. At higher energies, the LET is less and the reactants at 10
-12
s are more
spread out than at 750-1000 eV, and thus have a smaller probability of subsequently
reacting.
Similar calculations have been carried out for the track segments of protons and
alpha particles. The results are shown in Table 13.4. As in Fig. 7.1, pairs of ions
have the same speed, and so the alpha particles have four times the LET of the
protons in each case. Several findings can be pointed out. First, for either type of
particle, the LET is smaller at the higher energies and hence the initial density of
reactants at 10
-12
s is smaller. Therefore, the efficiency of the chemical development
of the track should get progressively smaller at the higher energies. This decreased
efficiency is reflected in the increasing
G
values for the reactant species in the first
four lines (more are left at 10
-7
s) and in the decreasing
G
values for the reaction
products in the fifth and sixth lines (fewer are produced). Second, at a given velocity,
the reaction efficiency is considerably greater in the track of an alpha particle than
in the track of a proton. Third, comparison of Tables 13.3 and 13.4 shows some
