The Radioactive Decay Calculator has the following
Selecting an isotope: to use the calculator's equations
you need to first select an isotope. Simply find the isotope of
interest on the list, highlight it by clicking it once, and click
the select button. The isotope's half-life is displayed. Choosing
another isotope from the list clears the form below.
If your isotope is not on the list: all of the decay
equations require the selected isotope's half-life. Instead of
referring to the isotope databank, they actually use the value
shown in the Half-life textbox. This means that if your isotope
is not in the databank, you can still use the equations. Just
enter the isotope's half-life in the Half-life textbox and the
appropriate time units (h, d or y) in the small textbox to the
right. If the half-life is in minutes you should convert it to
hours; if it is in exponential form you have to enter it in full.
For example, 1.2 * 104 y would be entered as 12000