When the B-phase order parameter precesses in an external magnetic field, pairwise excitation of spin 1/2 quasiparticles with opposite momentum will occur, in complete analogy to particle physics, where electron-positron pairs are created from the vacuum by a strong electric field. In superfluid 3He the analog of the electric field vector is represented by the time derivative of the orbital momentum of Cooper pairs. It turns out that the effective electric force acting on quasiparticles subject to a time-dependent order parameter is substantially stronger than in particle physics. As a result the pair creation probability at low temperature is predicted to govern the magnetic relaxation in the nonlinear NMR regime. Under certain conditions the distribution of the emitted quasiparticles appears to be thermal, with the temperature proportional to the precession frequency. This is interpreted as an analog to the Unruh effect in particle physics, where accelerated objects in the vacuum emit thermal radiation with temperature proportional to the acceleration.