Capintec CAPTUS 2000, Руководство пользователя

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CAPINTEC, INC CAPTUS
®
2000
(physical and radioactive) in the area around the CAPTUS
®
2000 is unchanging. To stop the
counting process, select ABORT at any time during acquisition. You will return to the main
autocalibration screen. When the process is complete "DONE" appears in the MCA display
area.
After the first count, the software will define a ROI which corresponds to the FWTM (full
width-tenth maximum) of the peak. This same region will be used to define the remaining
counts and appears in red in the spectrum display area.
Detector Resolution
The detector resolution is calculated by determining the FWHM (full width-half maximum)
area of the peak, and dividing by the peak centroid location. The standard deviation, or
sigma, for the peak used to calculate resolution is also listed. For peaks such as the 662
keV peak of Cs137 whose shape is symmetrical or appears to follow a Gaussian distribution,
the FWHM is generally close to 2.35
σ
. Also during the chi-squared test, the software will
include a constancy test for convenience. Upon completion of the test, the chi-squared
value, % FWHM, sigma, and constancy test results will appear in the display area. It may be
necessary to use the scroll bar on the side of the text window to view all of the test results.
The results of each chi-squared test is automatically saved to a history file.
INTERPRETING THE TEST RESULTS
The autocalibrate results require little interpretation. Unless an error message is displayed,
the software will have succeeded in finding a set of adjustments which places the Cs137 32
keV X-ray peak within ± 1 channel and the Cs137 662 keV gamma peak to within ± 2
channels of the theoretical value for a calibration of 2.00 keV per channel. The software has
preset limits which prohibit operation outside of appropriate voltage, gain, and zero adjust
values.
The constancy test compares the Cs137 standard source activity that was entered into the
computer to the activity that was measured. The activity that was entered into the test setup
window (Figure 5-5) is decay corrected by the CAPTUS
®
2000 software. The activity is
actually measured in cpm which must be converted to units of activity by using the efficiency
stored in the isotope library. The deviation between the calculated and measured activity is
the percent error and should be within ± 5%. If the constancy test error is outside 5%, check
the positioning of the rod source, especially with the probe detector. Insure that there are no
other sources in the area, including dosed patients. (NaI detectors are very sensitive. Check
behind walls in adjacent rooms too.) Check the calibration date in the setup so that it
corresponds to your Cesium 137 rod source. Finally, you may have to remeasure the
efficiency of the Cs137 standard so that the measured activity is calculated correctly.
Efficiencies can be measured in the Isotope Library.
The chi-squared test requires more interpretation. Chi-squared values are given in the
following table for 5, 10, 15, and 20 samples. The chi-squared test results should fall
between the 0.95 and 0.05 probability values in the table almost all of the time. The more
stringent 0.90 and 0.10 values are given for reference. A chi-squared test result which is
much higher or much lower than expected should be repeated. Insure that there are no
5-8 QUALITY ASSURANCE July 00