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Capintec CAPTUS 2000 Owner's Manual
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CAPINTEC, INC CAPTUS® 2000(physical and radioactive) in the area around the CAPTUS® 2000 is unchanging. To stop thecounting process, select ABORT at any time during acquisition. You will return to the mainautocalibration screen. When the process is complete "DONE" appears in the MCA displayarea.After the first count, the software will define a ROI which corresponds to the FWTM (fullwidth-tenth maximum) of the peak. This same region will be used to define the remainingcounts and appears in red in the spectrum display area.Detector ResolutionThe 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, orsigma, for the peak used to calculate resolution is also listed. For peaks such as the 662keV 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 willinclude a constancy test for convenience. Upon completion of the test, the chi-squaredvalue, % FWHM, sigma, and constancy test results will appear in the display area. It may benecessary 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 RESULTSThe 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 32keV X-ray peak within ± 1 channel and the Cs137 662 keV gamma peak to within ± 2channels of the theoretical value for a calibration of 2.00 keV per channel. The software haspreset limits which prohibit operation outside of appropriate voltage, gain, and zero adjustvalues.The constancy test compares the Cs137 standard source activity that was entered into thecomputer to the activity that was measured. The activity that was entered into the test setupwindow (Figure 5-5) is decay corrected by the CAPTUS® 2000 software. The activity isactually measured in cpm which must be converted to units of activity by using the efficiencystored in the isotope library. The deviation between the calculated and measured activity isthe percent error and should be within ± 5%. If the constancy test error is outside 5%, checkthe positioning of the rod source, especially with the probe detector. Insure that there are noother sources in the area, including dosed patients. (NaI detectors are very sensitive. Checkbehind walls in adjacent rooms too.) Check the calibration date in the setup so that itcorresponds to your Cesium 137 rod source. Finally, you may have to remeasure theefficiency 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 thefollowing table for 5, 10, 15, and 20 samples. The chi-squared test results should fallbetween the 0.95 and 0.05 probability values in the table almost all of the time. The morestringent 0.90 and 0.10 values are given for reference. A chi-squared test result which ismuch higher or much lower than expected should be repeated. Insure that there are no5-8 QUALITY ASSURANCE July 00 |
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