Page 203 - Radiochemistry and nuclear chemistry

Radiation Effects on Matter 187

7.11.1. Radiation sources

The radiation sources normally used can be divided into two groups, those employing
radioactive isotopes (e.g. 6~ and those employing electron beam accelerators (EBA).
Gamma radiation, having a large penetration range (Fig. 7.9), is advantageously used for
initiation of reactions in solids and liquids, food irradiation and sterilization of medical
products. The useful penetration depth of electrons is approximately given by 0.4.E/p (era),
where E is the electron energy (in MeV) and p the density of the irradiated material (in
g/era3). For EBA radiation, dose rate and penetration depth are easily controlled by varying
the beam current and acceleration voltage. Very high radiation intensity can be provided
and hence be applied to high speed processes. Radiation absorption is discussed in detail
in Ch. 6, and particle accelerators in Ch. 13.

7.11.2. Process criteria

The radiation dose required to completely convert 1 kg material to product is

D = (M G) -1 J kg- l (7.15)

where D is the radiation dose (Gy), G is the radiation yield (mol j-l), and M is the molar
mass (kg mol-1).

lOO
I I -
80
60

u~ 40
g

~'f:e/..

~ 2o

1~ 5 10 15 20
DEPTH (cm)
FIG. 7.9. Depth-dose curves for electromagnetic radiation in water. Distance between source
and water surface is 0.8 m; area of beam at water surface 0.01 m 2. (From $pinks and
Woods.)