Page 199 - Radiochemistry and nuclear chemistry

Radiation Effects on Matter 183

Direct energy transfer from excited solvent molecules (A*) to solute molecules (B) to form
excited solute molecules may also take place:

A* + B ~ B*

The reprocessing of used reactor fuel elements involves solvent extraction processes with
organic solvents. In these processes the solvents are subjected to high radiation fields with
subsequent decomposition of the organic solvent. The design of chemical reprocessing
systems must take into account any interference by the radiolytic products (Ch. 20).
Labeled compounds experience self-radiolysis induced by the radioactive decay. The
extent of such radiation effects depends on the half-life, the decay energy, the specific
activity of the sample, and the G-value for decomposition. The presence of other substances
can considerably affect the amount of damage. Aromatic compounds such as benzene (as
a solvent) can serve as a protective medium to minimize radiation self-decomposition,
whereas water or oxygen enhance it.
Radiation doses of 105 Gy can induce decomposition effects of the order of 1%. Samples
whose specific activity exceed 40 GBq (1 Ci)/mol for 14C or about 400 GBq (10 Ci) per
tool for 3H will receive a dose of this magnitude in a period of a year. Samples may be
stored in benzene solution in vacuo or in deep freeze to minimize self-radiation effects and
should be re-purified before use if the decomposition products are likely to affect the
experiment.

7.9. Experimental methods

Radiation chemistry is characterized by the very fast generation of reactive species
followed by extensive competition between recombination reactions and reactions with
solutes. A complete description of a radiation chemical process requires information about
the f'mal products and the transient species.
The final products can be analyzed with standard chemical methods and much information
has been gained through the use of selectively reacting scavengers.
Information about structures and identities of primary ionic species has been obtained
from mass spectrometry, photoelectron and vacuum ultraviolet spectroscopy. Electron spin
resonance techniques have been used extensively in the study of free radicals. Low
temperature, matrixes and "spin traps" have been employed to stabilize the short lived
radicals. Much insight into radiation processes and radiation induced radical reactions has
been gained by means of the pulse radiolysis method which is based on irradiation of
samples with a short pulse of ionizing radiation. The radiation source is generally an elect-
ron accelerator or, to a much lesser extent, a heavy particle accelerator. Techniques used
to follow the transient behavior of the radiolytically generated short-lived species are optical
absorption spectroscopy, esr, conductivity and polarography. The time resolution is
generally in the femtosecond to microsecond range.