  We analyze the electron spin relaxation rate $1/T_1$ of individual
ion-implanted $^{31}$P donors, in a large set of metal-oxide-semiconductor
(MOS) silicon nanoscale devices, with the aim of identifying spin relaxation
mechanisms peculiar to the environment of the spins. The measurements are
conducted at low temperatures ($T\approx 100$~mK), as a function of external
magnetic field $B_0$ and donor electrochemical potential $\mu_{\rm D}$. We
observe a magnetic field dependence of the form $1/T_1\propto B_0^5$ for
$B_0\gtrsim 3\,$ T, corresponding to the phonon-induced relaxation typical of
donors in the bulk. However, the relaxation rate varies by up to two orders of
magnitude between different devices. We attribute these differences to
variations in lattice strain at the location of the donor. For $B_0\lesssim
3\,$T, the relaxation rate changes to $1/T_1\propto B_0$ for two devices. This
is consistent with relaxation induced by evanescent-wave Johnson noise created
by the metal structures fabricated above the donors. At such low fields, where
$T_1>1\,$s, we also observe and quantify the spurious increase of $1/T_1$ when
the electrochemical potential of the spin excited state $|\uparrow\rangle$
comes in proximity to empty states in the charge reservoir, leading to
spin-dependent tunneling that resets the spin to $|\downarrow\rangle$. These
results give precious insights into the microscopic phenomena that affect spin
relaxation in MOS nanoscale devices, and provide strategies for engineering
spin qubits with improved spin lifetimes.
