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Appendix D:
Decay Calculations
This appendix provides methods of performing radioisotope decay calculations.
The first section provides two methods of performing Decay Calculations, the second section
provides the DECAY FACTOR CHART, and the third section provides an explanation of how the
procedures used in the first section, and the DECAY FACTOR CHART in the second section, are
generated.
Decay Calculation Methods - Calculator Method
If You Are Using A Calculator With The Y
x
Function, Perform The Following:
1. Measure the current activity of the radioisotope (A
0
), which can be compared to the decayed
calculated activity.
2. Look up the half life T
1/2
of the radioisotope in Appendix E. 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
5. Calculate the elapsed time (t) in the same units as the (T
1/2
) that will elapse between when the
calibrated activity measurement was made and when you wish to know what the decayed
activity of your radioisotope will be.
Sample Calculation
The following is a calculation of the decayed activity of Tc-99m, with an initial activity of
43 mCi, after 53 minutes:
6. A
0
= 43 mCi.
7. For Tc-99m, T
1/2
= 6.007 h.
8. Elapsed t = 53 min x 1 hr = 0.8833 hr
60 min
9.
A = 43 mCi x 2
(-0.8833/6.007)
A = 43 mCi x 2
(-0.147045)
A = 43 mCi x 0.903
A = 38.83 mCi
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