biodex ATOMLAB 500 - Page 205

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Biodex Medical Systems, Inc. © 2016 205
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. 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).
Sample Calculation
The following is a calculation of the decayed activity of Tc-99m, with an initial activity of
43 mCi, after 32.5 hours:
1. A
i
= 43 mCi.
2. For Tc-99m, T
1/2
= 6.007 h.
3. t = 32.5 h.
4. t / T
1/2
= 32.5 h / 6.007 = 5.410. N = 5, R = 0.410.
5. From Table 1, X
n
(5) = 0.03125.
6. From the Decay Factor Chart, the .41 row and the .000 column,
X
r
(.410) = .75262.
7. A
f
= A
i
x X
n
(N) x X
r
(R) = 43 mCi x (0.03125) x (.75262) = 1.01 mCi.