This is simply a plot of the 4 normalization parameters in the latest fit result above (Norm_31, Norm_35, Norm_53, Norm_81) as a function of energy. The fit to a 3rd degree polynomial seems good in this example.

The 4 peak position parameters (Peak_31,Peak_35, Peak_53, Peak_81) from the current Ba-133 fit  can be well fit to a 1st order polynomial as well as a power law.