Electron-Positron Pair Production by an Electron in a Magnetic Field   Near the Process Threshold
  The electron-positron pair production by an electron in a strong magnetic field near the process threshold is considered. The process is shown to be more probable if the spin of the initial electron is oriented along the field. In this case, the probability of the process is $\sim10^{13} s^{-1}$ when the magnetic field strength is $H=4\cdot 10^{12}$ G. 
O. P. Novak, R. I. Kholodov and P. I. Fomin
hep-ph