APPENDIX C:
DECAY CALCULATIONS
OVERVIEW
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 calculated decayed
activity.
2) Look up the half life T
1/2
of the radioisotope in Appendix
D. 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
0
x 2
(-t/T
1/2
)
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.
The following is a calculation of the decayed activity of Tc-
99m, with an initial activity of 43 mCi, after 53 minutes:
1) A
0
= 43 mCi.
2) For Tc-99m, T
1/2
= 6.007 h.
ATOMLAB Dose Calibrator Operation Manual Appendix C
Biodex Medical Systems C-1 Release 2.0
Sample
Calculation
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