10. Calculate the composite efficiencies D.E.
*
G.E. using one of the following equations:
S and B are defined in steps 7 and 8 under Efficiency Measurement.
A is the activity determined in step 7 under Source Preparation for isotope Xx-123 with units of
microcuries.
Exponent of “2”:
This is a decay correction. For short lived isotopes such as Tc-99m, it is important. For long lived
isotopes such as I-125, it may not be important unless the source is saved for future efficiency
checks.
T
1/2
is the half life of isotope Xx-123, in units of H, days, etc.
T and t are respectively the time of source measurement in the dose calibrator and the same of
counting the source in the well detector. Both should have the same units as T
1/2
.
For example:
If T = 08:31 h and t = 15:49 h, then T - t = -7.300 h.
If T
1/2
= 3.261 days for Ga-67, then T - t should be converted to days = -0.3041 days. In this case,
the decay result would be 0.937.
11. Calculate D.E. from the composite (D.E.
*
G.E.) found in step 10 above and from the calculated value
of G.E. found earlier in the discussion on GEOMETRIC EFFICIENCY. The following equation defines
the calculation.
Note that the two terms (D.E.
*
G.E.) and G.E. are both calculated as percents in the earlier
expressions and the above result preserves D.E. as a percentage.
1. INTRODUCTION
D.E. =
(G.E.
*
D.E.)
*
100%
G.E.
APPENDIX A A-4
(S - B)
counts
D.E.
*
G.E. =
100
sec
*
100%
A
µCi
*
37000
dps/µCi
*
2
(T-t)
T
1/2
(S - B)
counts
D.E.
*
G.E. =
100
sec
*
100%
current activity displayed
OR
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