Alpha Spectroscopy: Why does Peak Resolution Change?

[says]

I'm trying to understand why peak resolution changed in my alpha spectroscopy experiment.

I've increased the distance between a Am-241 source and a silicon surface barrier (semiconductor) detector and taken spectra along the way, recording information about peak height and FWHM.

As the distance between the source and detector increased the FWHM got lower, but the peak resolution increased. I've calculated the peak resolution = (1/FWHM) * 100 and expressed it as a percentage.

The peak resolution has changed because the FWHM has changed. I understand that much. I don't really understand why the FWHM would decrease though. Is it purely because of the energy degradation of the alpha particles as they travel through air?

 

[ChrisVer]

I don't understand what this plot shows. Couldn't you define the resolution as the uncertainty/standard deviation $\sigma$ or FWHM (they both are related if you fit gaussians) of what you measure over the average value? And why do you have several points at the same distance?
If the above can be explained, why did you stop at that point? probably you would have to remake that measurement because something went wrong at that point?
Your explanation (eg alpha energy degradation) wouldn't explain why your peak gets narrower (decreased FWHM)? What you show (as you define your resolution) says that the further away you get, the best your detector records the particles. How do the actual distributions (the ones you used for the FWHM calculation) look like in the small-distance vs those bad ones in large-distance case?