Understanding Neutron Radiography Reading V-Kodak Part 2 of 3

These show the necessary emitter-film distances and thicknesses of lead that
should surround the various gamma-ray emitters to provide protection of
stored film. These recommendations allow for a slight but harmless degree of
fog on films when stored for the recommended periods. The lead thicknesses
and distances in tables above are considered the minimum tolerable. To
apply the cobalt 60 table to radium, values for source strength should be
multiplied by 1.6 to give the grams of radium in a source that will have the
same gamma-ray output and hence will require the same lead protection.
This table can be extended to larger or smaller source sizes very easily. The
half-value layer, in lead, for the gamma rays of radium or cobalt 60 is about ½
inch. Therefore, if the source strength is doubled or halved, the lead
protection should be increased or decreased by 1/2 inch. The table can also
be adapted to storage times longer than those given in the tabulation. If, for
example, film is to be stored in the vicinity of cobalt 60 for twice the
recommended time, the protection recommendations for a source twice as
large as the actual source should be followed. Iridium 192 has a high
absorption for its own gamma radiation. This means that the external
radiation from a large source is lower, per curie of activity, than that from a
small source.
Charlie Chong/ Fion Zhang
Radiography in Modern Industry. Rochester, NY: Eastman Kodak Co. 1980