SQUIDs as detectors in a new experiment to measure the neutron electric dipole moment

Abstract

A new experiment has been proposed Los Alamos National Laboratory; to measure the neutron electric dipole moment (EDM) to 4/spl times/10/sup -28/ ecm, a factor of 250 times better than the current experimental limit. Such a measure of the neutron EDM would challenge the theories of supersymmetry and time reversal violation as the origin of the observed cosmological asymmetry in the ratio of baryons to antibaryons. One possible design for this new experiment includes the use of low temperature superconducting (LTS) SQUIDS coupled to large (/spl sim/100 cm/sup 2/) pick-up coils to measure the precession frequency of the spin-polarized /sup 3/He atoms that act as polarizer, spin analyzer, and detector for the ultra-cold neutrons used in the experiment. The method of directly measuring the /sup 3/He precession signal eliminates the need for very uniform magnetic fields (a major source of systematic error in these types of experiments). It is estimated that a flux of /spl sim/2/spl times/10/sup -16/ Tm/sup 2/ (0.1 /spl Phi//sub 0/) will be coupled into the pick-up coils. To achieve the required signal-to-noise ratio one must have a flux resolution of d/spl Phi//sub SQ/=5/spl times/10/sup -6/ /spl Phi//sub 0//Hz/sup 1/2/ at 10 Hz. While this is close to the sensitivity available in commercial devices, the effects of coupling to such a large pick-up coil and flux noise from other sources in the experiment still need to be understood. To determine the feasibility of using SQUIDs in such an application we designed and built a superconducting test cell, which simulates major features of the proposed EDM experiment, and we developed a two-SQUID readout system that will reduce SQUID noise in the experiment. We present an overview of the EDM experiment with SQUIDs, estimations of required SQUID parameters and experimental considerations. We also present the measured performance of a single magnetometer in the test cell as well as the performance of the two SQUID readout technique.