CAPINTEC, INC. CAPTUS
®
4000e
8-30 WIPE TESTS November 16
Exiting Summary Reports
Select the Done button on Figure 8-33 Wipe Test Summary Reports Window to
return to Figure 8-23 Wipe Test Search Window.
ISOTOPE EFFICIENCY
Note: A Full Spectrum Efficiency value must be entered in the setup at all times.
Efficiency is a measurement parameter that is defined as the ratio of detected counts (or
counting rate) measured by the system to actual disintegrations (rate of decay). Efficiency
can be defined as follows:
minuteper tionsdisintegra
100minuteper counts
Efficiency %
×
=
The efficiency of an isotope depends upon the geometry, region of interest, energy,
abundance and activity of the source as well as the type of detector system used to measure
the source. For these reasons, efficiency values vary from one isotope to another. The
CAPTUS
®
4000e uses a default value of 65% for full spectrum efficiency. Full spectrum
means the total number of counts detected in all of the MCA channels. This is the nominal
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 tionsdisintegra
100minuteper counts
Efficiency %
×
=
The CAPTUS
®
4000e 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.
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