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The G1 Assessments - CompAct

This page reports an assessment of the results generated by Gordon Gilmore's spectrum analysis program CompAct in anlysing the IAEA G1 test spectra.The assessment is particularly favourable in terms of peak area measurement performance and it's least squares fitting routine. It fails, of cours, in automatic doublet deconvolution, for which it is not designed. If nothing else, this page provides an example of a rigorous assessment.

The IAEA G1 Spectra, and the justification for the indexes, are described in full in Practical Gamma-Ray Spectrometry (Section 15.5.2 in the Second Edition).

The spectra comprise:

  • G1100 - Reference Spectrum containing 20 well defined peaks
  • G1200 - Peak Search Test Spectrum containing 22 very small peaks on high and low backgrounds.
  • G1300 to G1305 - Consistency Test Spectra, each containing 22 easily detectable peaks of varying size. Apart from differing statistical scatter the peaks are identical in all six spectra.
  • G1400 - Doublet Fit Test containing 9 doublets.
The peaks in the various spectra were all produced by shifting the peaks in G1100 by integral numbers of channels and attenuating by known factors. The position and ratio of all the peaks is known, therefore, relative to the G1100 spectrum.

a) All spectra were searched using sensitivity 0.91 and the peak areas estimated using automatic background width selection.

b) G1200 peaks were not all found in the peak search. All peaks were measured using the manual interactive peak fitting process. The G1400 doublets were not resolved by the search process, these were also measured using the fitting procedure.

Full results can be obtained from Nuclear Training Services Ltd. Alternatively use CompAct itself to analyse the G1 Spectra which are provided with the program.

Results and Commentary

G1 Intercomparison Spectra - Summary Sheet
Program Tested:
Sreadsheet Filename:
Analysis Conditions:
Backgd. Width:
Batch search, Sensitivity: 0.91
Spectrum Number of Peaks Search
Peak Position Normalised Area R
Expected Found Spurious Error Std.Dev Error Std.Dev

Targets: 1.00 0.000 0.000 0.00 1.00 1.00

G1200 22 17 0 0.77 0.117 0.274 0.37 0.72 1.17
G1200Fitted 22 22 0 1.00 0.000 0.000 -0.05 0.93 0.72
The peak search routine, in common with most other programs, misses the smallest peaks. 17 peaks detected is good performance, particularly as no spurious peaks were detected. Two more peaks can be detected by increasing sensitivity but then 4 spurious peaks are detected. The R Factor indicates that all the 'errors' are atributable to statistical factors alone.

The large edge around channel 1024 is detected as an 'odd' peak. This edge is unreal and the peak has been ignored for the purpose of these tests.

Fitting the peaks gave a significant area for all peaks in spite of the fact that the smaller peaks are not clearly identifiable by eye. The accuracy of the peak area estimations is an improvement on the simple ROI estimations and is very satisfactory. Since the peak positions are supplied to the fitting process the position error is zero.

Consistency Tests 1-6
G1300 22 21 0 0.95 -0.048 0.139 0.11 1.16 0.58
G1301 22 21 0 0.95 0.005 0.098 0.17 0.84 1.55
G1302 22 21 0 0.95 0.036 0.145 0.47 0.96 2.92
G1303 22 21 0 0.95 0.025 0.114 0.10 1.12 1.42
G1304 22 21 1 0.91 0.021 0.087 0.13 1.30 1.02
G1305 22 21 0 0.95 0.040 0.172 0.33 1.00 1.30
Overall 132 126 1 0.95 0.012 0.127 0.22 1.06 1.52
Peak position accuracy is within 0.05 channels and the precision of the position estimation better than 0.2 channels. The acuracy of the area measurement is good. All peaks were, in fact, detected with only 1 spurious peak in the whole set of spectra.

The problem peak was that at 1010 which lies at the at the top of the curve of the edge feature around channel 1024. The automatically selected ROIs are clearly in error and areas, even with manually set ROIs, are consistently high. Taking into account the shape of the continuum underlying the peak this is not unexpected. This peak has been omitted from the analysis. Taking that into account the overall peak search index would be 0.98.

In all but one spectrum (G1302) the R Factor indicates that area errors can be attributed to statistical scatter alone.

Doublet Resolution Test
G1400Fitted 18 18 0 1.00 -0.101 0.036 0.21 0.80 0.53
The peak search routine does not, and is not designed to, resolve peak doublets as close the peaks encountered here. Three of the doublets have peak only 1 channel apart, another three 3 with peaks 3 channels apart (a little less than 1 FWHM) and the remaining three with components 6 channels apart. In all cases the area accuracy was good; in most cases the area was within one standard deviation of the correct area. This is reflected in the low area error indexes and the low R Factor.

Chi-squared table
Range Found Expected
<-2 3 3
-2<<-1 14 17
-1<<0 34 43
0<<1 45 43
1<<2 25 17
>2 5 3
Total 126 126
Chi-Squared: 7.60
Deg. of Freedom: 5
R Factor: 1.52
The Chi-squared table shows the number of area results found to be within different standard deviation bands. (For example, the first line looks at the number of results more than two standard deviations below the true value.) The actual numbers found is compared to the number statistically expected. The comparison shows that the distribution of errors is symmetrical about zero, i.e. there is no bias on the area estimates, and the numbers within the various bands compares well with expectation. The chi-squared factor of 7.6 for 5 degrees of freedom is well within statistical expectation.

Appendices Data Example Spectra Links Online Course Spreadsheets Test Spectra TOP

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