  We investigate the low-field relaxation of nuclear hyperpolarization in
undoped and highly doped silicon microparticles at room temperature following
removal from high field. For nominally undoped particles, two relaxation time
scales are identified for ambient fields above 0.2 mT. The slower, T_1s, is
roughly independent of ambient field; the faster, T_1f, decreases with
increasing ambient field. A model in which nuclear spin relaxation occurs at
the particle surface via a two-electron mechanism is shown to be in good
agreement with the experimental data, particularly the field-independence of
T_1s. For boron-doped particles, a single relaxation time scale is observed.
This suggests that for doped particles, mobile carriers and bulk ionized
acceptor sites, rather than paramagnetic surface states, are the dominant
relaxation mechanisms. Relaxation times for the undoped particles are not
affected by tumbling in a liquid solution.
