  Coherence times of electron spins bound to phosphorus donors have been
measured, using a standard Hahn echo technique, to be up to 20 ms in
isotopically pure silicon with [P]$ = 10^{14}$ cm$^{-3}$ and at temperatures
$\leq 4 $K. Although such times are exceptionally long for electron spins in
the solid state, they are nevertheless limited by donor electron spin-spin
interactions. Suppressing such interactions requires even lower donor
concentrations, which lie below the detection limit for typical electron spin
resonance (ESR) spectrometers. Here we describe an alternative method for
phosphorus donor ESR detection, exploiting the spin-to-charge conversion
provided by the optical donor bound exciton transition. We characterise the
method and its dependence on laser power and use it to measure a coherence time
of $T_2 = 130 $ms for one of the purest silicon samples grown to-date ([P]$ =
5\times 10^{11} $cm$^{-3}$). We then benchmark this result using an alternative
application of the donor bound exciton transition: optically polarising the
donor spins before using conventional ESR detection at 1.7~K for a sample with
[P]$ = 4\times10^{12} $cm$^{-3}$, and measuring in this case a $T_2$ of 350 ms.
