Energy loss of the alpha particles is a stochastic process, so a spread in energies is always expected after a monoenergetic beam passes through a medium.  This so-called straggling also causes a skew in the energy distribution, giving a noticeable low-energy tail to the peak. 

Apparatus:

The apparatus is designed to be cheap, relatively high-performance, and flexible.  We are sharing designs and code for reuse.

Source: