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.
          Select Time UnitcpmDayscpmNew ActivitySelect Activity UnitsCalculated Half LifeOriginal Activity Half Life CalculationsCalculation Date and/or TimeOriginal Date and/or Time          

Radioactive Decay
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< This is the reverse of the radioactive decay calculations.  Inputting two activities at two different dates and/or times, the half life may be calculated.  This is a common calculation in nuclear counting laboratories for short lived isotopes.  It is often used to show that activity on an air sample is from radon daughter products without having to perform gamma spectroscopy.  Dates and/or times may be manually entered or selected with the calendar pickers.