Re: WHAT ARE THE CAUSES OF ENERGY SPECTRUM BROADENING IN ALPHA SOURCES

My guess based on the limited information is that the alpha particles, 
because they are so large (atomically) and have a +2 charge love to 
interact with whatever is in their path.  Your source contains a parent 
nuclide which must eject the alpha out of the nucleus of the atom and from 
their everything it interacts with will degrade it's original energy level.

Thus the spectrum will be a distribution of energies from zero to the 
maximum energy of the alpha particle, in theory.

Hope this helps.
[Moderator note:  Ideally, an alpha particle source that provides a sharp 
energy peak should be very thin, so that  the emitted alpha doesn't lose much 
energy exiting the source. ]