  Processes with oppositely charged spinor particles in initial and/or final
states in homogeneous magnetic field B are subject to focusing effects in their
relative motion, which yield the amplifying factors in probabilities growing as
$eB$. In addition the increasing energy of some Landau levels influences the
phase space. As a result some processes in the proper spin states can be
enlarged as $\sim \frac{eB}{\kappa^2}$, where $\kappa^2$ is the characteristic
2d phase space factor available without magnetic field. Several examples,
including neutron $\beta$ decay, positronium decay and $e^+e^-$ pair
production, are quantitatively considered.
