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.
 
Days Select Element Select Isotope MBq MBq Days Half LifeSelect Activity Units (start decay today) Date Desired Activity Reached Calculated Time to Decay Select Output Time Units Timed Decay CalculationsEnter Desired Activity Enter Original Activity   
Radioactive Decay   
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Useful for calculating how long it takes for a radioactive isotope to decay to a specified activity. This is a common calculation for radioactive waste. Hospitals and laboratories often have drums of waste containing short-lived isotopes. When the waste in the drums decays to below the release limits, the waste may be disposed of as conventional waste or non-mixed hazardous waste. It is also useful for determining when Eu-152 in activated concrete will decay to below soil release limits or when airborne radionuclides decay below respiratory requirements limits. Nuclear medicine and medical physics people may use the calculator to ascertain when a diagnostic or treatment isotope has lost its effective strength. A feature that sets this calculator apart from the other online timed decay calculators is its ability to give you the actual calendar date that the isotope will reach the desired activity.