  We note the existence of a set of magnetic field values where a simple Hahn
echo sequence refocuses the dynamics of the full dipolar interaction, for spin
systems of electron donors in silicon. As the refocussing occurs for both
arbitrary coupling strengths and arbitrary times, these dipolar refocusing
points (DRPs) offer new possibilities for regulating entanglement due to the
always-on spin dipolar interaction. While the experimental effects of DRPs will
be strongly diluted in the measured coherences of thermal (unpolarized) spin
ensembles, we investigate possible signatures in coherence decays arising from
a study of the combined effects of decoherence arising from instantaneous
diffusion and direct flip-flops
