GammaVision
®
v7 (A66-BW) 783620H / 1013
The energy recalibration affects the multiplets in two ways. For inclusion in the list of deconvolution candidates, the first peak for this nuclide must be present. If the first peak has not been found because it failed the centroid test (see above), then subsequent peaks are not used. The deconvolution will then be performed with fewer candidates. Secondly, the library energies are used to define the location of the multiplet component library peaks. The peak finder energies are derived from the channel number, so although the reported energy might change, the position relative to the actual data does not change. A mismatch of these will result in an inaccurate fit, and a different fit when the peak channels are shifted relative to the spectrum.
6.5.5. Library-Based Peak Stripping
In a few cases, all of the gamma-rays emitted by one isotope are very close in energy to gammarays emitted by other isotopes in the sample. The peaks from the two gamma rays can not be separated correctly using conventional analysis, so the activity for one isotope cannot be correctly determined. This alternative to deconvolution can be used to obtain peak areas of the components of a multiplet. It uses peak areas from other parts of the spectrum to determine the areas of some of the components and calculates the remaining areas. It is also referred to as peakinterference correction.
When enabled on the System tab under Analyze/Settings/Sample Type..., the program can operate automatically using only one library, or manually using three user-defined libraries detailing the peak overlaps.
6.5.5.1. Automatic Peak Stripping
In the automatic or library-based method, GammaVision automatically identifies the isotopes in the library that are too close together and calculates their activities indirectly. This method is simpler to use than the manual method.
The first step is to search the library for isotopes with severely overlapping gamma rays. The criterion for severe is that the peaks are within two channels of one another (regardless of the energy per channel). For peaks that are this close, the peak areas will be more accurate if they are found indirectly. Any such peaks are marked in the library as being too close together.
For example, the only useful gamma ray emitted by
226
Ra is 185.99 keV. The peak overlaps the
185.72 keV peak of
235
U. If these two isotopes are both in the library,
226
Ra and the 185.72 keV
235
U peak are flagged when the library is read. Table 7 shows some other common examples.
These energies are from Erdmann and Soyka. Other references might have different energies, but the overlaps will still occur.
The spectrum is then analyzed as described with one exception: any peaks that are marked as too close are not used in the isotope activity calculations. Instead, the activity of one isotope is based
264
