Biomedical Engineering Reference
In-Depth Information
Table 13.4
G
Values (Number per 100 eV) for Various Species at
10
-7
s for Protons of Several Energies and for Alpha Particles of
the Same Velocities
Protons (MeV)
Alpha Particles (MeV)
Species
Type
1
2
5
10
4
8
20
40
OH
1.05
1.44
2.00
2.49
0.35
0.66
1.15
1.54
H
3
O
+
3.53
3.70
3.90
4.11
3.29
3.41
3.55
3.70
e
aq
0.19
0.40
0.83
1.19
0.02
0.08
0.25
0.46
H
1.37
1.53
1.66
1.81
0.79
1.03
1.33
1.57
H
2
1.22
1.13
1.02
0.93
1.41
1.32
1.19
1.10
H
2
O
2
1.48
1.37
1.27
1.18
1.64
1.54
1.41
1.33
Fe
3+
8.69
9.97
12.01
13.86
6.07
7.06
8.72
10.31
overlap and some differences in yields between electron tracks and heavy-ion track
segments. At the highest LET, the reaction efficiency in the heavy-ion track is much
greater than that for electrons of any energy.
Electrons, protons, and alpha particles all produce the same species in local track
regions at 10
-15
s: H
2
O
+
,H
2
O
∗
, and subexcitation electrons. The chemical differ-
ences that result at later times are presumably due to the different spatial patterns
of initial energy deposition that the particles have.
13.6
Biological Effects
It is generally assumed that biological effects on the cell result from both direct
and indirect action of radiation. Direct effects are produced by the initial action of
the radiation itself and indirect effects are caused by the later chemical action of
free radicals and other radiation products. An example of a direct effect is a strand
break in DNA caused by an ionization in the molecule itself. An example of an
indirect effect is a strand break that results when an OH radical attacks a DNA
sugar at a later time (between
∼
10
-9
s). The difference between direct
and indirect effects is illustrated by Fig. 13.4. The dots in the helical configuration
schematically represent the location of sugars and bases on a straight segment of
DNA 200 Å in length in water. The cluster of dots mostly to the right of the helix
gives the positions of the reactants at 10
-11
s and the subsequent times shown after
passage of a 5-keV electron along a line perpendicular to the page 50 Å from the
center of the axis of the helix.
In addition to any transitions produced by the initial passage of the electron or
one of its secondaries (direct effects), the reactants produced in the water can attack
the helix at later times (indirect effects). In these computations, the electron was
made to travel in a straight line. Also, unreacted radicals were assigned a fixed
probability per unit time of simply disappearing, in order to simulate scavenging
10
-12
sand
∼
