Understanding Neutron Radiography Reading V-Kodak Part 2 of 3

Therefore, protection requirements for an iridium 192 source should be based
on the radiation output, in terms of roentgens per hour at a known distance.
The values of source strength, in curies, are merely a rough guide, and
should be used only if the radiation output of the source is unknown. The
table above can be extended to sources having higher or lower radiation
outputs than those listed. The half-value layer of iridium 192 radiation in lead
is about 1/6 inch. Therefore, if the radiation output is doubled or halved, the
lead thicknesses should be respectively increased or decreased by 1/6 inch.
Tables VII and VII are based on the storage of a particular amount of
radioactive material in a single protective lead container. The problem of
protecting film from gamma radiation becomes more complicated when the
film is exposed to radiation from several sources, each in its own housing.
Charlie Chong/ Fion Zhang
Radiography in Modern Industry. Rochester, NY: Eastman Kodak Co. 1980