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THE FIVE PRINCIPAL MECHANISMS of radiation damage are ionization, vacancies, interstitials, impurity atoms, and thermal spikes. 
Here is how a neutron might give rise 
damage is concerned with what hap- 
pens to the solid matter during the 
attenuation process. The same equa- 
tions must govern many of the calcula- 
tions of both workers. Practically, the 
attenuation process may in time render 
the material unsuitable for shielding 
purposes. 
DAMAGE MECHANISMS 
Radiation damage originates from 
the interaction of energetic radiation 
and matter (2). This interaction dis- 
places atoms from their equilibrium 
positions and places them in nonequi- 
librium positions. It creates transient, 
local high-temperature regions. It in- 
troduces impurity atoms either through 
radioactive capture and decay or by 
inclusion of fission fragments. And it 
breaks chemical bonds and forms free 
radicals. 
The results of these effects we list by 
the five names: vacancies, interstitials, 
thermal spikes, impurity atoms, and 
ionization effects. All will cause 
changes in many of the properties of 
to each of them in coppe-. 
matter, whether it be solid or liquid. 
The first four items are of most con- 
cern to those who deal with solid re- 
actor components. The last is of con- 
cern in liquids and gases. It is the 
principal topic in radiation chemistry. 
Estimates of Magnitudes 
Numerous theoretical estimates have 
been made of the number of vacancies 
and interstitials that are created per 
incident neutron and fission fragment. 
Sizes and durations of thermal spikes 
accompanying the collision process 
have been estimated (3, 4). These 
calculations are difficult to check ex- 
perimentally. The effect per defect on 
properties such as electrical resistivity 
is not known with sufficient accuracy. 
Where experimental verifications have 
been attempted, it appears that theory 
leads to an overestimate of the number 
of defects by a factor of 2-5. 
On the other hand, where the effect of 
radiation is to enhance solid-state re- 
actions that are diffusion controlled, it 
is found that the number of defects 
Grid-line intersect‘ons are equilibrium positions for atoms 
required has been underestimated. 
Defect Behavior 
At the present time experiment and 
theory apparently agree within an 
order of magnitude, but closer agree- 
ment will depend in large measure on 
gaining additional information on the 
behavior of defects in general. In the 
case of radiation damage, it is clear 
that we must be dealing with a collec- 
tion of defects that react with the mat- 
ter and with one another in a fashion 
not clearly understood. 
It has recently been shown that when 
radiation experiments are carried out 
at a temperature low enough to inhibit 
all defect motion, it is useful to con- 
sider the damaged region as resembling 
a frozen-in liquid (4, 6). Surprisingly, 
defect mobility has been observed at 
—243° C (7, 8). Type and extent of 
defect motion in a solid is a function of 
the type and structure of the solid con- 
taining the defects. The effect of radi- 
ation on the behavior will depend also 
on the latent opportunities for change 
81 
