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

          Select Time UnitcpmDayscpmNew ActivitySelect Activity Units cpm dpm pCi nCi uCi mCi Ci Bq kBq MBq GBq TBq Calculated Half LifeOriginal Activity Half Life CalculationsCalculation Date and/or TimeOriginal Date and/or Time           Years
Months
Days
Hours
Minutes
Seconds

< 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.