Capintec CAPTUS 2000, Owner'S Manual

Page: 97 / 206

CAPINTEC, INC CAPTUS
®
2000
You may exit the add wipe test screen at any time by selecting the SAVE button. Any
information which has been entered will be saved to the appropriate wipe test data file. If
you do not wish to save the information, choose the
CANCEL button. The software will
return to the original wipe test location screen without saving any information.
ISOTOPE EFFICIENCY
Efficiency is a measurement parameter which defines the counting losses which occur
between the radioactive rate of decay and the counting rate measured by the system.
Efficiency can be defined as follows:
minute per tions disintegra
100 minute per 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
®
2000 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 select an
appropriate whole spectrum efficiency, two methods are suggested. Choose the efficiency of
the isotope which is used most frequently at your facility. For example, general nuclear
medicine facilities use Tc99m most of the time, while nuclear cardiology facilities use Tl201
most of the time. 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. Enter 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.
To calculate total spectrum efficiency, first measure the isotope in the 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 1
µ
Ci = 2.22 x
10
6
dpm. Then calculate the full spectrum efficiency.
minute per tions disintegra
100 minute per counts
Efficiency %
×
=
The CAPTUS
®
2000 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 which surrounds the primary photopeak, and does not use total counts. This allows the
July 00 WIPE TESTS 7-3