that already exist in the solid material. 
Neutrons, Fission Fragments 
The nuclear particles in a reactor 
that cause most of the radiation dam- 
age in solids are fast neutrons and fis- 
sion fragments, chiefly because the 
energy of each is tremendous in relation 
to the energy required to create a de- 
fect. The uncharged nature of the 
neutron means that it can interact 
only by direct collision. However, 
once a collision has taken place, the 
knocked-on atom in turn rapidly creates 
subsequent displaced atoms. The inci- 
dent neutron travels many thousands 
of atomic spacings in the soltd before 
it makes another collision. Thus the 
damage resulting from fast neutrons is 
widely spread through a reactor, affect- 
ing all components. 
Fission fragments, possessing ini- 
tially a high charge and mass must dis- 
sipate all their energy within a few 
microns. Therefore the damage is usu- 
ally confined to the fuel volume. The 
total energy dissipated in radiation 
damage is approximately the same for 
both types of particles, but the spatial 
distribution is very different. 
Betas and Gammas 
Other nuclear radiations such as beta 
particles and gamma rays are also capa- 
ble of creating damage. But in general 
the amount is trivial in comparison 
with that of fast neutrons and fission 
fragments. The momentum that can 
be transferred to a lattice atom from 
these radiations is small. 
On the other hand, in studies of the 
basic nature of radiation damage, light 
particles are of interest because of the 
simpler nature of the defects that they 
create. It has been demonstrated in 
the case of electron bombardment that 
one can determine the energy required 
to create a vacancy in a solid (9, 10). 
This is a unique and useful measure- 
ment that is not complicated by the 
presence of additional defects such as 
thermal spikes, induced radioactivity, 
and impurities. These always accom- 
pany fast-neutron- or fission-fragment- 
induced radiation damage. 
HOW DAMAGE OCCURS 
Our present knowledge of defect 
solid state does not allow a detailed 
description of how defects probably 
cause damage. Still we have a reason- 
able qualitative picture of the processes. 
Vacancies 
For example, it is now reasonable to 
assume that the intermingling of atoms 
in the solid state depends on vacancies 
and that at any given temperature 
there is an equilibrium number of 
vacancies present. The larger the 
number of vacancies, the more rapidly 
diffusion takes place. Thus creation 
of excessive vacancies by radiation 
should result in increased reaction 
rates at a given temperature. 
Two experimental observations ap- 
pear to be consistent with this view- 
point. First, a disordered Cu;Au 
specimen can be ordered during expo- 
sure at temperatures well below those 
at which ordering normally occurs (11). 
Second, the precipitation process in 
Cu-Be alloy can be speeded up as a 
result of bombardment (12). This en- 
hancement of diffusion takes place on a 
microscopic scale only. It has not yet 
been shown that appreciable changes in 
mass-transfer rates can be achieved 
under irradiation. 
Interstitials 
The behavior of interstitials is more 
difficult to understand, but the pro- 
nounced increase in yield strength that 
accompanies even moderate neutron 
irradiation of a metal leads one to be- 
lieve that interstitials may be playing 
an important role in the hardening re- 
Many Things to Many Men 
Radiation damage is somewhat unique 
in that it can be considered in so many 
different ways: 
The materials engineer views the 
field simply as an additional environ- 
mental factor to be considered. 
The metallurgist sees it as another 
technique for altering the properties of a 
metal or alloy. It belongs in a category 
82 
with cold working, alloying, and heat 
treating. 
The physicist considers it as a portion 
of the larger field of defect solid state. 
The chemist sees a technique for alter- 
ing the course of chemical reactions. 
The safety engineer sees another 
potential hazard. 
There is truth in all these viewpoints. 
action, though it must be acknowledged 
that a suitable mechanism has not yet 
been devised that leads to detailed 
understanding. 
The problem of understanding the 
behavior of interstitials stems partly 
from our inability to create interstitials 
in close packed good metals by meas- 
ures other than irradiation. 
Impurities 
Impurity atoms can be understood 
in conventional terms. Usually the 
number introduced is small, and the 
effect is small compared to other dam- 
age effects. Thus in materials suitable 
for reactor applications this problem 
usually can be neglected. In the mat- 
ter of fission fragments, however, the 
impurity effect is of much greater mag- 
nitude. The production rate is higher, 
and the size and character of the frag- 
ments is such as to render them nor- 
mally insoluble in the parent matrix. 
This leads to enhanced strain effects, 
and it is readily observed that a residual 
effect of appreciable magnitude re- 
mains after all other types of damage 
have been annealed. On the other 
hand the fuel material does not have 
the permanent residency of other com- 
ponents, so removal for reprocessing 
eventually corrects fission-fragment 
damage. 
Thermal Spikes 
The thermal spike presents the most 
controversial aspect of radiation dam- 
age. It results from that portion of 
the energy that is stored in the form of 
displ: ced atoms or trapped electrons 
and is dissipated as heat during the 
collision process. The amount is com- 
parable to that dissipated in the forma- 
tion of Frenkel defects. It has been 
generally supposed that this energy 
creates a momentary region of high 
temperature that in a good metal may 
involve thousands of atoms and result 
in an increase in temperature of 1000° K 
for probably less than 10-!° sec. With 
fission fragments the number of atoms 
involved and the temperature increase 
would be correspondingly greater. 
A number of low-temperature ex- 
periments have shown that the thermal 
spike may act as a source of vacancies 
and interstitials but does not directly 
cause radiation damage (13). In other 
words the thermal spike may create a 
frozen-in liquid-like region that upon 
subsequent warming may liberate va- 
cancies and interstitials (74). Further- 
