Determination of the Spin Precession in Realtime

If you place a particle in a magnetic field, which on the one hand has an intrinsic angular momentum (the so-called spin) and on the other hand a magnetic moment, the direction of the spin starts to precess if the magnetic moment and magnetic field lines are not parallel. This is similar to a spinning top, which also precesses if it is not exactly vertical.

For measurements of the electric dipole moment in storage rings, it is important to know and control this precession. For this purpose, we have developed a method that allows us to measure the precession in real time and to manipulate it within certain limits (Phys. Rev. Lett. 115, 094801, Phys. Rev. Lett. 119, 014801). For this purpose, part of the beam is continuously directed onto a carbon target and the spin direction is inferred from the angular distribution of a large number of scattered particles. One difficulty is that the precession frequency of typically 120 kHz is much larger than the rate of the scattering events and the necessary frequency and phase analysis must be carried out in real time.

A direct application of this method was the first measurement of the EDM of deuterons with the so-called RF Wien filter method. Here, the electric and magnetic fields of the Wien filter oscillate at the precession frequency of the deuterons. This is only possible experimentally if both frequencies are measured continuously and tuned to each other so that the phase between the two oscillations is also kept constant.