SAMPLE CALCULATION
6. A
0
= 43 mCi.
7. For Tc-99m, T
1/2
= 6.007 h.
8. Elapsed t = 53 min x
!!!!"
!"!!"#
= 0.8833!hr!
9. Replace equation with following:
A = A
0
/2
n
A = 43 mCi / 2
(0.8833/6.007)
A = 43 mCi / 2
(0.1470451)
A = 43 mCi / 1.1072992
A = 38.83 mCi
Decay Factor Method
The following method is used to perform radioisotope decay calculations using a calculator
without the e
x
function:
1. Measure the current activity A
i
of the radioisotope.
2. Look up half life T
1/2
of the radioisotope in Appendix E. Note the time units.
3. Calculate the time t, in the same units as T
1/2
, that will elapse between when the activity
measurement was made and when you wish to know what the decayed activity of your
radioisotope will be.
4. Calculate t / T
1/2
in terms of N + R, where N is an integer (0, 1, 2, etc.) and R is less than 1.
5. Look up X
n
(N) in Table 1.
6. Look up X
r
(R) in the Decay Factor Chart.
7. Calculate A
f
= A
i
x X
n
(N) x X
r
(R).
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