Page 189 - Radiochemistry and nuclear chemistry

Radiation Effects on Matter 173

the higher Err (where it may be about one third of the calculated value). Atomic
displacements cause many changes in the properties of metals. Usually electrical resistance,
volume, hardness and tensile strength increase, while density and ductility decrease.
The microcrystalline properties of metals are particularly influenced by irradiation.
Although low-alloy steel in modem reactor tanks are rather radiation resistant (provided
they are free of Cu, P and S impurities), stainless steel (e.g. of the 18 % Cr, 8 % Ni type)
has been found to become brittle upon irradiation due to the formation of microscopic
helium bubbles, probably due to n,t~ reactions in 54Fe and impurities of light elements (N,
B, etc.). This behavior is accentuated for metallic uranium in reactors because of the
formation of fission products, some of which are gases. As a result of this radiation effect
it is not possible to use uranium metal in modem power reactors, where high radiation
doses are accumulated in a very short time. The fuel elements for power reactors are
therefore made of nonmetallic uranium compounds.
The displaced atoms may return to their original lattice positions through diffusion if they
are not trapped in energy wells requiting some activation energy for release. Such energy
can be provided by heating or by irradiation with electrons or 3'-rays (these do not cause
new displacements). This "healing" of particle radiation damage is commonly referred to
as annealing. The thermal annealing rate increases with temperature as does radiation
annealing with radiation dose. Doses in the 10 kGy range are usually required for
appreciable effect.

7.5. Inorganic nonmetallic compounds

The time for a high energy particle to pass by an atom is _< 10 -16 s. Ill this time the atom
may become excited and/or ionized, but it does not change position (the Franck-Condon
principle) provided there is no direct collision. The excited atoms are de-excited through
the emission of fluorescence radiation, usually within 10 -8 s. The ionization can result in
simple trapping of the electrons and production of "electron holes" in the lattice, especially
at impurity sites. The local excess (or deficiency) of charge produced in this way leads to
electronic states with absorption bands in the visible and ultraviolet regions of the spectrum.
For example, irradiation of LiCI results in a change of the color of the compound from
white to yellow. Similarly, LiF becomes black, KCI blue, etc. The irradiation of ionic
crystals also leads to changes in other physical properties much as conductivity, hardness,
strength, etc. Frequently, heating returns the properties and color to the normal state (or
close to it) accompanied by the emission of light; this forms the basis for a radiation dose
measurement technique named "thermoluminescence dosimetry" (w
Following a collision between a heavy particle (n, p, etc.) and an absorber atom in a
crystalline material the recoiling ion produces lattice vacancies and, upon stopping, may
occupy a non-equilibrium interstitial position (Fig. 7.3). The localized dissipation of energy
can result in lattice oscillations, terminating in some reorientation of the local regions in the
crystal lattice. These crystal defects increase the energy content of the crystal.
Semiconductors, where the concentration of charge carriers is very small, have their con-
ductivity reduced by introduction of lattice defects during irradiation. The production of
interstitial atoms makes the graphite moderator in nuclear reactors stronger, harder, and
more brittle. Since these dislocated atoms are more energetic than the atoms in the lattice,