High-Tc Squid-Based Gradiometer Cooled by a Cryotiger Gas-Mixture Cooler

We investigated the feasibility of a high-Tc SQUID system for fetal magnetocardiography aiming at a system without a magnetically shielded room and cooled by a cryocooler. For demonstration purposes, we have selected the APD-Cryotiger, a gas-mixture Joule-Thomson type cooler. We use an alumina SQUID holder that contains three high-Tc primary measurement SQUIDs and two reference SQUIDs. The three primary SQUIDs can be combined electronically into a second-order gradiometer with 6 cm baseline. Test experiments performed in a magnetically shielded room revealed a significant noise contribution arising from the cooler. Because this noise contribution decreases with increasing distance to the cooler cold head, we expect the source to be located in this cold head, most probably due to remanent magnetization. Because of the large field gradient in this remanent field, a second-order gradiometer configuration is dominated by the cooler noise. As an alternative, we formed a first-order gradiometer of the bottom SQUID and the middle SQUID, and corrected for the cooler noise in it by means of the first-order gradiometer output obtained from the middle SQUID and the top SQUID. In this set up, adult magnetocardiograms were successfully recorded. Outside the magnetically shielded room, however, second-order gradiometer operation is required. Therefore, we attempt to identify the source of the noise contribution in order to be able to remove it from the system.