6. ANALYSIS METHODS
6.1. General
The GammaVision program analyzes spectrum files and produces a list of the background, net area, counting uncertainty, FWHM, and net count rate for all peaks in the spectrum. If possible, it also gives a list of the average activity of the nuclides in the sample, the activity of each nuclide based on each gamma-ray energy in the library, the MDA for each peak energy of all nuclides in the library, and the reasons the unacceptable peaks were not used for the activity calculation. Peaks that are not the correct shape (according to the calibration) are marked as such; peak areas that are the result of deconvolution of overlapping energies are also marked; gamma-ray energies not in the analysis library are reported as suspected nuclides if a suitable candidate is found in the suspected nuclide library.
The result is a report containing all the descriptions stored with the spectrum file, the analysis parameters, user inputs, and the list of peaks and nuclides found in the spectrum. The standard
GammaVision report is discussed in Chapter 7.
6.2. The Analysis Engines
6.2.1. Analysis Engine Options
GammaVision includes five analysis engines, which are discussed in general terms below. Each one handles the analysis sequence in a slightly different manner. The traditional engines were
WAN32 and GAM32. NPP32 and ENV32 were created in more recent years to meet applications for which WAN32 and GAM32 were not optimized. ROI32 allows you to choose which spectral regions are to be analyzed, after which the unmarked parts of the spectrum are evaluated with a simplified WAN32 analysis. All five routines produce a report file (
.RPT
) and a binary output (
.UFO
) file. The following is a basic summary of processes used by each analysis engine.
Section 6.2.2 contains a decision matrix to assist you in choosing an analysis routine.
6.2.1.1. WAN32
WAN32 does a preliminary library-based peak search of the spectrum. The analysis assumes that all gamma-ray listed in the library exist in the spectrum so it tries to fit a peak at every energy listed in the library. The Peak Cutoff value specified in the Sample Defaults File (SDF) is compared to the 1 sigma counting uncertainty for each peak to determine if that energy will be used for further analysis. With a large Peak Cutoff value (> 200), this engine can “find” false peaks.
A Mariscotti peak search is then implemented on the remainder of the spectrum. For nuclides with no clean (well resolved) gamma rays WAN32 offers Manual-Based peak stripping.
231
