Radiation Trapping in a Cold and Dense Atomic Ensemble in a Magnetic Field

We report a theoretical investigation of cooperative spontaneous decay of atomic excitation in cold ensemble under condition that average interatomic separation is comparable with resonant, light wavelength. We show that magnetostatic field causes some acceleration of decay at shot time interval upon turn off the excitation pulse arid essential decreasing of decay rate of long-lived sub-radiant states. The dependence of this effect on duration of excitation pulse as well as on its carrier frequency is considered.