  The integral relaxation time describing the thermoactivated escape of a
uniaxial quantum spin system interacting with a boson bath is calculated
analytically in the whole temperature range. For temperatures T much less than
the barrier height \Delta U, the level quantization near the top of the barrier
and the strong frequency dependence of the one-boson transition probability can
lead to the regularly spaced deep minima of the thermoactivation rate as a
function of the magnetic field applied along the z axis.
