Alternatives
X-rays can also be used for this purpose. It requires a lot of power, and isn't as effective as gamma rays, but no permanent radioactive source is needed.
There is no reason safe methods for handling things like Cobalt-60 (half life of a little over five years) could not be developed, or why one would assume the work would be done by uneducated farm laborers instead of professionals.
Most accidents in such systems have resulted from deliberate bypassing of safety procedures and interlocks.