By far the best shielding against radioactivity
is Gauss' law: the intensity of a point source
falls off as the square of its distance from
the observer. All localized sources are labelled with their
activity at a given distance, for instance
``10 mr/h at 1 m.''
If one keeps at least 10 m
away from such a source, one will receive less than
0.1 mR per hour, which is not worrisome.
Needless to say, one should never touch
a radioactive source, because 1/r2 can be
very large as r goes to zero. Other safety measures
include lead aprons, which are effective only
for X-rays and -rays, and thick concrete
shielding for neutrons and high-energy charged particles
(these are much in evidence at TRIUMF).