Læknablaðið - 01.10.1968, Síða 58

216
LÆKNABL AÐIÐ
GLASS
LEAD
I E
HVL
80Kv 0.2mm
'mOOKv
LEAD
HVL
8mm
1
LEAD
Co-60
TYPES OF RADIATION.
1
i
HVL
I 2mm
HVL in CONCRETE
2.5in.
SHOWING STOPPING POWER OR HALF-VALUE LAYER OF VARIOUS SUBSTANCES
Fig. 2.
Comparison of important physical properties of various types of radia-
tion.
or to re])lace ihose cells killed. Hence, assuming a sublethal dose
of radiation, there is usually an appreciable degree of restoration
of form and function. Some of the damage done is not reparable,
however, and, consequently, 'with eacli exposure to radiation there
is a residual portion of permanent injury.
A latent period without obvious evidence of injury often
occurs follo'wing the initial acute effects of radiation due to the
repair that has taken place. However, as time goes on tliis repair
tends to lose its effectiveness, and the descendants of the irradiated
cells may become increasingly divergent from the normal. As
these cellular changes occur, delaved necrosis of the tissue, often
with ulceration and other abnormalities, may appear.
After a number of generations the damaged cells may even
proliferate autonomously and become independent of the
organism — cancerous — in their growth pattern. The germ
cells are highly sensitive to radialion, although they have some
slight power of repairing the damage done. Thus, a dose of a few
hundred R may lead to sterility lasting for months or even a few
years, ultimately followed by recovery. In general, germinal
epithelium is much more sensitive than are somatic cells. Hence,
prevalence of damage to tlie former has ljecome tlie controlling
factor in setting safe levels for radiation protection. For all