Page 192 - Radiochemistry and nuclear chemistry

176 Radiochemistry and Nuclear Chemistry

Table 7.2. Spur reactions in water
eaq + eaq -* H 2 + 2OH-
eaq + "OH -- OH
eaq + H3 O+ -* H-+ H20
eaq + H'-- H 2 + OH
H" + H" --. H 2
9 OH + -OH ~ H202
9 OH + H"-~ H20
H3 O+ + OH- -- H20

7.6. Water

The consequences of ionization and excitation depend on the physical state and the
molecular composition of the irradiated material. In this section we introduce chemical
phenomena into the description using the radiolysis of water as an example.
The time scale for the sequence of events on the radiolysis of water is shown in Figure
7.5. The ionization event occurs on the time scale of an electronic transition ( < 10-16 s).
The positive ion H2 O+ formed reacts with water within 10-14 s, forming an "OH radical
and H30 +. The electron, if liberated with sufficient kinetic energy, can ionize further water
molecules before its energy falls below the ionization threshold of water (12.61 eV). The
electron thereafter loses the rest of its energy by causing vibrational and rotational
excitation of the water molecules and, finally, becomes solvated.
The solvation process has been shown to occur within 10 -12 s. The excited states
dissociate within 10-14 _ 10-13 S, i.e on the same time scale as a molecular vibration to
form O.,H.,.OH and H 2. The physical and physicochemical (pre-thermal) processes are
thus completed within 10-12 s leaving the species in thermal equilibrium with the water.
The radiolysis products are clustered in spurs (Fig. 7.1); i.e., they are inhomogeneously
distributed in the water and proceed to diffuse out of the spur volume. During this "spur
diffusion" process, recombination reactions take place leading to the formation of molecular
or secondary radical products. The spur expansion is complete within 10-7 s, at which time
the radiolysis products are those shown in Figure 7.5. Spur reactions are listed in Table
7.2. G-values for the radiolysis products in water irradiated with different types of radiation
are given in Table 7.3. It is seen that, as the LET of the radiation increases, the G-values

Table 7.3. Product yields (lanol/J) in irradiated neutral water
,,,,
Radiation G(- H20) G(H 2) G(H202) G(e~q) G(H') G(-OH) G(.HO2)

Liquid water:
7 and fast 0.43 0.047 0.073 0.28 0.062 0.28 0.0027
electrons
12 MeV He :+ 0.294 0.115 0.112 0.0044 0.028 0.056 0.007

Water vapor:
7, electrons 0.85 0.05 0 G(e-) =0.31 0.75 0.85
,