Page 202 - Radiochemistry and nuclear chemistry

186 Radiochemistry atut Nuclear Chemistry

radiation during the ages, imperfections are produced in the crystal lattice. When heated
these crystals produce light in proportion to the dose received. If the dose rate is known the
age of the exposed material can be calculated. There are numerous examples of the use of
this technique e.g. for authentication of old porcelain or ceramics. Recently British Museum
was forced to remove a large number of Mexican, Greek and Roman sculptures and vases
because TLD-dating showed that they had been manufactured during the 19th century.
A modification is the thermocurrent dosimeter. In this case the detector may be a thin
crystal of synthetic sapphire (~ 10 mm, thickness _< 1 mm) between thin metal electrodes.
When the crystal is heated after having received a certain dose, the trapped electrons are
released and cause a current to pass between the electrodes. The peak of the integrated
current is a measure of absorbed dose. The effect is referred to as RITAC (radiation
induced thermally activated current) and the technique is named TC (thermocurrent)
dosimetry. Because it is instrumentally easier to measure an electric current than light, the
TC dosimeter may replace the TLD in time.
Doses can also be calculated from the product of the dose rate and the time of exposure.
The most common dose rate meter is the ionization chamber. Because of the close
connection between this instrument and pulse type ionization counters, which measure
individual nuclear particles entering the detector, the discussion of ionization chambers is
deferred to Ch. 8, which deals in more detail with radiation measurement techniques.
If the number, energy, and type of nuclear particles being absorbed in a material can be
measured or estimated, the absorbed dose can be calculated as described previously. Such
calculations are very important, particularly in the field of radiation protection.

7.11. Large-scale non-biological applications

In this section we deal only with the more important non-biological applications, as the
latter are treated in Ch. 18, which specifically deals with biological effects of radiation.
Ionizing radiation produces ionized and excited atoms and molecules in all materials.
Excited molecules formed directly or by recombination reactions between electrons and
cations decompose in the vast majority of systems to highly reactive free radicals. The
reactive species formed on radiolysis are precursors of further reactions, such as reduction,
oxidation, polymerization, cross linking and so on. It should therefore be possible to apply
radiation chemical methods to industrial processes and, consequently, extensive applied
research and development of radiation chemistry has been carried out during the past three
decades.
A large proportion of the radiation induced reactions can be brought about by thermal,
photochemical or chemical initiation, but the advantages of radiation initiation are generally
claimed to be:

o No contamination by catalyst and catalyst residue.
o Temperature independence.
o Easy control of radiation intensity and hence rate of induced reactions.
o High spe~ treatment capability.
o Ionizing radiation offers the advantage of greater penetrating power compared
to initiation by UV-light.