CAPINTEC, INC. CAPTUS
®
3000
value for whole spectrum efficiency for Tc99m when measured in a 2” × 2” drilled well
detector.
The default value for whole spectrum efficiency can be changed as required. To change the
whole spectrum efficiency, two methods are suggested. Choose the efficiency of the isotope
that is used most frequently at your facility. For example, general nuclear medicine facilities
use Tc99m most often, while nuclear cardiology facilities most frequently use Tl201. This
method will provide an accurate value for most of the wipe tests measured.
Alternatively, calculate the total spectrum efficiency for several isotopes most frequently
used. Input the lowest efficiency. This will provide a conservative estimate for activity by
overestimating the activity of most wipes. If no value is entered in the full spectrum efficiency
box, results will only be displayed as cpm and will be meaningless.
To calculate total spectrum efficiency, first measure the isotope in MCA mode. The total
count rate will appear on the MCA spectral printout in units of cpm. Next convert the current
activity of the source measured in the MCA into units of dpm. Remember that 1µCi = 2.22 x
10
6
dpm. Then calculate the full spectrum efficiency.
minuteper tions
disintegra
100minuteper counts
Efficiency %
×
=
The CAPTUS
®
3000 also measures efficiencies in the Isotope Library, which are used by the
automatic peak finding program to provide both the identity and the activity of a peak
identified in the Wipe Test program.
Note that the Isotope Library measures peak efficiency using only the counts recorded in the
ROI that surrounds the primary photopeak, and does not use total counts. This allows the
peak finding program to better determine the activity of an isotope when multiple peaks or
isotopes are present. For this reason, the efficiency recorded in the Isotope Library will
always be lower than the full spectrum efficiency.
8-30 WIPE TESTS November 13
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