  A superstrong magnetic field stimulates the spontaneous production of
positrons by naked nuclei by diminishing the value of the critical charge
Z_{cr} . The phenomenon of screening of the Coulomb potential by a superstrong
magnetic field which has been discovered recently acts in the opposite
direction and prevents the nuclei with Z<52 from becoming critical. For Z>52
for a nucleus to become critical stronger B are needed than without taking
screening into account.
