In polarization experiments at storage rings, one of the challenges is to
maintain the spin-resonance condition of a radio-frequency spin rotator with
the spin-precessions of the orbiting particles. Time-dependent variations of
the magnetic fields of ring elements lead to unwanted variations of the spin
precession frequency. We report here on a solution to this problem by shielding
(or masking) one of the bunches stored in the ring from the high-frequency
fields of the spin rotator, so that the masked pilot bunch acts as a
co-magnetometer for the other signal bunch, tracking fluctuations in the ring
on a time scale of about one second. While the new method was developed
primarily for searches of electric dipole moments of charged particles, it may
have far-reaching implications for future spin physics facilities, such as the
EIC and NICA.Comment: 5 pages, 3 figures + references + supplemental material (6 pages, 2
figures, 6 tables + references