Betas from the source are assumed to have a flat kinetic energy distribution between 0 and 4 MeV (the distribution is made up of two contributions, from the primary decay of Strontium with a peak at 0.534 MeV and the secondary decay of Yttrium at 2.283 MeV. The half life of Yttrium (64.1 h) is negligible compared to that of Strontium (28.6 y). Thus, if both energy distributions are sufficiently broad, this assumption is not that bad an approximation). Charge deposition is modeled by a Landau distribution with its median energy given by the Bethe Bloch formula. The blue histogram for Betas is the superposition of the contribution from a large number of discrete energies between 0.4 and 4 MeV.
 
These approximations are rather crude. Work is currently going on to compare these results to a full GEANT simulation of the energy deposition.