For those needing portability, Rad Pro for Desktop works with Windows 8.1/10 tablets. Will not work with Surface tablets running Windows RT.

This page calculates the dose-rate from point source, gamma emitting isotopes at any distance.
The reverse calculation, determining the activity of a source from a known dose-rate
may also be performed. Radiation shielding may be added as well for
predicting dose-rates from commonly
used shielding materials. The activity of
a source in a pig may be calculated without having to remove the source, given that
the dose-rate outside of the pig is measurable. A third useful calculation
is shield thickness. Input the current dose-rate and the desired dose-rate
and the thickness of the shield required will be calculated for you. Other common, expensive computer codes do not perform that calculation. The shielding
calculations use the latest coefficients from NIST (see references). You may use the linear attenuation
coefficient, the linear energy absorption coefficient or the linear attenuation
coefficient with a buildup factor. In general, the latter with buildup should
be the most accurate, but the choice is the user's. For an explanation of which one to use,
read the shielding and buildup white paper on this site by clicking on the "Documents/Shielding"
button above. Isotopes available for calculation are the most common gamma emitters at nuclear power reactors, in nuclear medicine use and at university, government
and industrial research facilities.