  We measure the spin-lattice relaxation of donor bound electrons in ultrapure,
isotopically enriched, phosphorus-doped $^{28}$Si:P. The optical pump-probe
experiments reveal at low temperatures extremely long spin relaxation times
which exceed 20 h. The $^{28}$Si:P spin relaxation rate increases linearly with
temperature in the regime below 1 K and shows a distinct transition to a T$^9$
dependence which dominates the spin relaxation between 2 and 4 K at low
magnetic fields. The T$^7$ dependence reported for natural silicon is absent.
At high magnetic fields, the spin relaxation is dominated by the magnetic field
dependent single phonon spin relaxation process. This process is well
documented for natural silicon at finite temperatures but the $^{28}$Si:P
measurements validate additionally that the bosonic phonon distribution leads
at very low temperatures to a deviation from the linear temperature dependence
of $\Gamma$ as predicted by theory.
