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The filename of the Nuclide Library to be used in the spectrum analysis can be the Internal
(working) library or a library on disk. The working library is the library from Library/Select
File..., and will include any modifications made during the interactive analysis mode.
The Background Type can be set to Auto, 1-pt., 3-pt., or 5-pt. These are explained in more detail in Section 6.3.1.
Random Summing is the random summing correction factor discussed in Section 6.11. Entering zero turns this correction off.
The Analysis Range, in channels, can be entered. This is usually used to eliminate analysis of the ends of the spectrum that do not contain useful data. The Analysis Range should be as wide as possible because the automatic energy recalibration feature (see Section 6.3.6) requires separated library peaks to work properly. Also, the correlation of lines from a single nuclide done by the analysis is defeated if the energy range analyzed does not include all the lines.
System Tab
System settings are those settings that are generally the same from sample to sample. However, all of these entries except the Laboratory and Operator names can be different for each sample type. The dialog is shown in Fig. 138.
The Laboratory name, composed of any 64 characters, is printed as the second line on each page of the report. The spectrum name is printed on the next line.
The Operator name is the name of the person operating the system. This name will appear on the analysis reports. This field defaults to the user name entered during Windows installation.
MDA Type — This allows the selection of the type of MDA calculation to be used as the method of calculating the MDA to be on the report. The MDA is a measure of how small an activity could be present and not be detected by the analysis. Many factors affect the MDA, which is reported in units of activity, such as becquerels. The calibration geometry, backgrounds
(system and source-induced), detector resolution, and particular nuclide all seriously affect the
MDA reported. Section 6.9 provides explanations of the different MDA formulas used by
GammaVision.
In the Library section, Match Width sets the maximum amount by which a peak centroid can deviate from the nearest library peak energy and still be associated with that library peak. The value entered is multiplied by the FWHM at the peak energy to get the width used.
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Fig. 138. System Tab.
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If the value is too small, some spectrum peaks will be misidentified due to statistical variation in the centroid; if it is too large, some library peaks will be incorrectly identified.
During efficiency calibration, the net peak area calculation can be affected by the library Match Width setting in effect at the time the calibration is performed. Before starting an efficiency calibration, make sure the Match Width is set to the same value you will be using during analysis. Most users will keep the default setting, 0.5.
If using a different setting, we recommend a value between 0.4 and 0.75.
If the Match Width is set to a value other than 0.5, the value will be printed on the report.
The Fraction Limit is one of the parameters used to determine the presence or absence of a nuclide. The sum of the emission probabilities of the peaks in the spectrum identified with the nuclide is divided by the sum of the emission probabilities of all peaks of the nuclide in the energy range being analyzed. If the result is greater than the fraction limit, the nuclide is marked as being present. To turn off this test, set the limit to zero.
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File for Suspected Nuclides — In the list of unknown peaks, there is a column for suspected nuclides. This is the closest energy in the library file to the unknown peak’s energy. Suspected nuclides are identified in GammaVision using a suspected-nuclide library. This file is a library file (see Section 5.6). Typically, it is a much larger file than the analysis library file. The identification window is also much larger for this list than for the analysis library. This list might contain common lines (such as 511 keV or
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K) that are present in the spectrum, but not desired in the analysis.
If the suspected-nuclide library contains the same lines as the analysis library, a nuclide whose energy is shifted just out of range for the analysis (the default is 0.5 × FWHM) will be marked as suspected on the unknown list.
The File for Suspected Nuclides must not have the same name as the analysis library file. If necessary, make a copy of the analysis library file with a new name.
The Units section allows the selection of either becquerels (Bq) or microcuries (
µCi) as the base units, a Multiplier and Divisor to scale the numbers up or down.
The units label is printed at the top of the activity columns on the report and should reflect the values chosen; that is, if
µCi is chosen with a multiplier of 1000, then “nanocuries” should be entered in the Activity field. If the sample quantity is entered later (see Acquire/Acquisition
Settings... or File/Settings...), the units for quantity (weight or volume) are entered in the Size field. The combined label (activity/quantity) is limited to 14 characters. Optionally enter a
1-sigma sample Size uncertainty (+/-) between 0% and 1000%.
The Multiplier, Divisor, and sample Size values are used to generate the Activity scaling factor listed in the Analysis Parameters section of the report, and used by the NPP32 and ENV32 analysis engines to calculate the report’s Summary of Library Peak Usage table.
The PEAK SEARCH SENSITIVITY sets the sensitivity for the peak search used in the Peak
Search (Section 5.5.2), Interactive in viewed area (Section 5.5.7), and the full-spectrum analysis (Section 5.5.4). Before a suspected peak is accepted, the magnitude of the second difference must be greater than the weighted error of the channel counts. The PEAK SEARCH
SENSITIVITY is a multiplicative factor used in error weighting.
The sensitivity can be set at any integer value from 1 to 5, with 1 the most sensitive (that is, “1" finds the most peaks). A value of 1 will find small peaks, but will also “find” many false peaks.
A value of 5 will locate all the large peaks, but might miss some of the smaller peaks. If too large, some small peaks will be missed. In the interactive mode, many regions will be deconvoluted unnecessarily if the value is too sensitive. The parabolic background method is disabled for energies above 200 keV if the sensitivity is set to 1.
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