For those needing portability, Rad Pro for Desktop works with Windows 8.1 tablets (just tested). Will not work with Surface tablets running Windows RT.
uCi cm uR/hr Enter or Select Isotope Select Calculation  
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                       Enter Activity Enter Distance Calculated Dose-Rate   Select Dose-Rate Units   Select Activity Units Select Distance Units Select Activity Calculation Gamma Emitter Point Source Dose-Rate <--to--> Activity and Shielding Calculations (In Air)
       Gamma Emission and Exposure Rate   
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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.