We have developed a 12-channel SQUID magnetometer to measure the magnetic field in the cryoEDM experiment. This will allow us to track changes in the field during neutron EDM measurements and correct shifts in the spin precession frequency.

Both the pick-up loops and the SQUID sensors are inside superfluid helium. The pick-up loops measure the magnetic field inside the Ramsey Cell, where the neutrons are stored. The SQUID sensors are located outside of the magnetic shields, as the cables carrying the bias current to the SQUID sensors generate a significant magnetic field. The pick-up loops are connected to the sensors by twisted wire pairs, which run inside superconducting capillaries made from solder wire with the flux removed. We have also installed heaters, to heat the loops into their conducting state if necessary; and a set of calibration coils. We are currently working on the electronics and software required to control this.

As this will be the first time a SQUID magnetometer is used in an EDM experiment, we anticipate some further challenges. It will be necessary to determine the best way to measure the different components of the magnetic field; extrapolate this away from the pick-up loops; and integrate these measurements over an experimental cycle.

As measurements taken in our Oxford laboratory are limited by the noisy environment, we tested a smaller version of our system in the very low noise environment at the Laboratoire Souterrain à Bas Bruit (LSBB) in Rustrel, Provence, France. This system worked very well, limited by the magnetic field of the Earth.

References

A SQUID magnetometry system for a cryogenic neutron electric dipole moment experiment S. Henry, C. Clarke, A. Cottle, M. Pipe, Nuclear Instruments and Methods in Physics Research A, 763 (2014) 483-494.

Characterisation of superconducting capillaries for magnetic shielding of twisted-wire pairs in a neutron electric dipole moment experiment S. Henry, M. Pipe, A. Cottle, C. Clarke, U. Divakar and A. Lynch, Nuclear Instruments and Methods in Physics Research A, 763 (2014) 155-162.

SQUID magnetometry for the cryoEDM experiment -- Test at LSBB, S Henry et al., Journal of Instrumentation 3 (2008) P11003.