Common problems during nonlinearity determination

Although nonlinearity determination is almost a trivial task with HyperLab, there may be common problems for the users. One such problem may arise with peak matching.

Decays not set

If you have selected a measurement, where the sample is not set to a radioactive source, HyperLab is not able to automatically assign decays and their radiations to the measurement. In this case the user should manually assign decays to the measurement.

To add decays, select Input data tab, then click on the Searched decay lines node under the measurement in question.

If you open this node and no decay has been set, click Add decay on the right.

Select proper decay and click OK. HyperLab now loads the decay and its radiations, and performs a fit with the currently identified matching radiation-peak pairs.

Radiations not found

When only a simple linear energy calibration is applied to a spectrum, large discrepancies may exist between the expected and observed peak positions. If this difference is greater than the identification threshold (3 channels by default), radiations will not be identified. Therefore the precise energy calibration is of great importance in the Peak evaluator. Please double-check your calibration, use as many calibration points for the contained isotope as possible.

It is also advisable to drop the original – rather imprecise – energy calibration points for the sample spectra.

In case of proper linear energy calibration, the distribution of well‑selected energy calibration points of multi-line sources should resemble the common expected nonlinearity, as depicted in this figure.

Another option is to increase the identification limit under Options / Fitting options / Peak position threshold.

Note: after you are using a specific peak evaluation for nonlinearity analysis, you had better not modify the peak evaluation afterwards – e.g. by adding or removing peaks from it. When you open such a peak evaluation with the spectrum evaluator module, it will warn you about this, suggesting opening the evaluation only in read-only mode.

If you want to use a measurement for multiple purposes, e.g. for nonlinearity as well as efficiency analysis or quantitative evaluation, it is advisable to create a separate peak evaluation for each purpose.

Another reason for missing peak matching is the missing radiation usage flag. HyperLab uses a gamma line for nonlinearity purposes only if that radiation is equipped with “Use for nonlinearity” flag. See further details in section Nuclear data library management.