A=A_{o}e^{( 0.693t/T1/2 )}
Use this equation to find out how much of a radioactive
material is left after a given period of time.
To use the equation:
The information required by the equation is the initial activity
(A_{o}), the time elapsed since the initial activity was
measured (t), and the halflife of the isotope
(T_{1/2}).
 Select the isotope from the list at the top of the page; this
provides the equation with the isotope's halflife. If your
isotope is not on the list, you can still use the equation if you
manually enter the halflife in the Halflife
textbox at the top of the page.
 Next enter the initial activity. It is not necessary to worry
about units here, just keep in mind that the units for the final
activity will be the same as for the initial activity.
 Enter the time elapsed since the initial activity was
measured.
 Select the correct time unit. If the time unit is not
available, you will have to convert the time elapsed to a unit
that is available. For example, if the time elapsed is in minutes
then divide by 60 to convert to hours, and use that value. If you
do not select the time unit, the value NaN (Not a Number)
will be received when the Calculate button is pressed.
 Click the Calculate button and the final
activity is displayed.
An example:
You purchase 9.25 uCi of P32, but it sits for 9 days before you
get a chance to use it. How much of the original 9.25 uCi is
left?
 Select Phosphorus 32 from the isotope list.
14.26 and d show up in the
Halflife and Halflife time
unit textboxes.
 Enter 9.25 in the Initial activity
(A_{o}) textbox.
 Enter 9 in the Decay time
(t) textbox and select the d radio
button.
 Click the Calculate button and the answer
5.972427 is received in the Final
activity (A) textbox.
Advanced:
You can use the equation in reverse to find out the initial
activity if you know the current activity. For example, you have
5.97 uCi of P32 now and you want to know how much was there 9
days ago. Enter 5.97 in the Initial
activity (A_{o}) textbox and 9
in the Decay time (t) textbox.
Calculate shows that you had
9.246240 uCi 9 days ago.
