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Appendix D: Decay Calculations
Appendix D has three distinct sections. The first section provides two methods of performing
Decay Calculations, the second section contains the Decay Factor Chart, and the third section
explains how the procedures from the first section and the Decay Factor Chart from the second
are generated.
Decay Calculation Methods – Calculator Method
When using a calculator with a Y
x
function:
1. Measure the current activity of the radioisotope A
0
, which can be compared to the decayed
calculated activity.
2. Using the Decay Factor Chart below, look up the half live of T
1/2
for the radioisotope. Note
the time units.
3. T
1/2
= Half life of the isotope
t = Elapsed time
A = Current activity
A
0
= Activity at the time of calibration
4. Calculate
𝐴 = 𝐴
!
× 2
! !
!
! / !
5. Calculate the elapsed time (t) in the same units as the (T
1/2
) that will elapse between the
time the calibrated activity measurement was made and the time at which the decayed
activity of the radioisotope must be known.
Example:
The following is a calculation of the decayed activity of Tc-99m with an initial activity of 43 mCi
after 53 minutes:
A
0
= 43 mCi.
For Tc-99m, T
1/2
= 6.007 h.
Elapsed
𝑡 = 53 𝑚𝑖𝑛 ×
! ! !
!" !"#
= 0 . 8833 ℎ