Biomedical Engineering Reference
In-Depth Information
Table 13.1
Time Frame for Effects of Ionizing Radiation
Times
Events
Formation of H
2
O
+
,H
2
O
∗
, and subexcitation electrons,
Physical stage
10
-15
s
e
-
, in local track regions (
0.1
µ
m)
Three initial species replaced by H
3
O
+
,OH,e
aq
,H,
Prechemical stage
10
-15
sto
10
-12
s
∼
∼
and H
2
The four species H
3
O
+
, OH, e
aq
, and H diffuse and
Chemical stage
10
-12
sto
10
-6
s
∼
∼
either react with one another or become widely
separated. Intratrack reactions essentially complete by
∼
10
-6
s
Biological stages
10
-3
s
Radical reactions with biological molecules complete
1 s
Biochemical changes
Minutes
Cell division affected
Days
Gastrointestinal and central nervous system changes
Weeks
Lung fibrosis develops
Years
Cataracts and cancer may appear; genetic effects in
offspring
by the radiation induce changes as follows. First, in about 10
-14
s, an ionized wa-
ter molecule reacts with a neighboring molecule, forming a hydronium ion and a
hydroxyl radical:
H
2
O
+
+H
2
O → H
3
O
+
+OH.
(13.1)
Second, an excited water molecule gets rid of its energy either by losing an elec-
tron, thus becoming an ion and proceeding according to the reaction (13.1), or by
molecular dissociation:
H
2
O
+
+e
-
H+OH
.
H
2
O
∗
→
(13.2)
The vibrational periods of the water molecule are ∼
10
-14
s, which is the time that
characterizes the dissociation process. Third, the subexcitation electrons migrate,
losing energy by vibrational and rotational excitation of water molecules, and be-
come thermalized by times
∼
10
-12
s. Moreover, the thermalized electrons orient
the permanent dipole moments of neighboring water molecules, forming a clus-
ter, called a hydrated electron. We denote the thermalization-hydration process
symbolically by writing
e
-
e
aq
,
→
(13.3)
where the subscript aq refers to the fact that the electron is hydrated (aqueous
solution). These changes are summarized for the prechemical stage in Table 13.1.
Of the five species formed, H
2
does not react further.
