  We present three Monte Carlo models for the propagation of athermal phonons
in the diamond absorber of a composite semiconducting bolometer `Bolo 184'.
Previous measurements of the response of this bolometer to impacts by $\alpha$
particles show a strong dependence on the location of particle incidence, and
the shape of the response function is determined by the propagation and
thermalisation of athermal phonons. The specific mechanisms of athermal phonon
propagation at this time were undetermined, and hence we have developed three
models for probing this behaviour by attempting to reproduce the statistical
features seen in the experimental data. The first two models assume a phonon
thermalisation length determined by a mean free path $\lambda$, where the first
model assumes that phonons thermalise at the borders of the disc (with a small
$\lambda$) and the second assumes that they reflect (with a $\lambda$ larger
than the size of the disc). The third model allows athermal photons to
propagate along their geometrical line of sight (similar to ray optics),
gradually losing energy. We find that both the reflective model and the
geometrical model reproduce the features seen in experimental data, whilst the
model assuming phonon thermalisation at the disc border produces unrealistic
results. There is no significant dependence on directionality of energy
absorption in the geometrical model, and in the schema of this thin crystalline
diamond, a reflective absorber law and a geometrical law both produce
consistent results.
