Hello,
This is a followup question from the previous topic

I am using USRBDX scoring between Region 3 and Region 2 to score the primary particle. Since I a new to FLUKA, I am rying to understand the impact of varying the parameter of GCR-SPE through SPECSOUR card.

The WHAT(5) card is used to specify the spectral index for sampling. So, If I choose it to be zero, I would expect a flat spectrum from USRBDX scoring(below the transition energy i.e- WHAT(6)). But that is not the case.

The spectral shape does not change for the negative values of spectral index. Please see the attached plot for the same.

Next, for a given spectral index, if the transition energy [WHAT(6)] is changed, it has no impact on the spectral shape. Please see the attached plots

Am I missing something or my choice of parameters for sampling the particle energy is bit vague?
Any help regarding this regard would be much appreciated.

You are missing the fact that the spectral index is not meant to arbitrarily change the shape of the spectrum, which is given in the spc files, rather to alter the energy sampling along the input range, while assigning anyway the proper weight to the sampled energy.
Moreover, SPECSOUR expects the absolute value of the spectral index (gamma, for E^(-gamma) sampling), as in the provided example. Therefore, the negative values you input, disable the E^(-gamma) sampling and imply the 1/E sampling adoption over the full range, making the variation of the spectral index value as well as of the transition energy of no effect at all.