64-channel second-order axial gradiometer system based on DROS for magnetocardiogram in a thin shielded room

Abstract

We fabricated a magnetocardiogram (MCG) system having 64 second-order wire-wound gradiometers for operation inside a thin magnetically shielded room (MSR). In order to provide good quality MCG signals in noisy environments using a thin economic shielded room, we used second-order gradiometers with a baseline of 70 mm. In order to reduce the liquid level necessary to cool the sensor into superconducting state, the total length of the gradiometer was reduced to 160 mm by using a special superconducting connection method between pickup coil and input coil. The SQUID sensor is the low-temperature double relaxation oscillation SQUID (DROS) having large flux-to-voltage transfers to simplify the readout electronics. The coverage area of the 64-channel sensor array is about 256 mm. The MSR is consisted of 1 layer of 1.4-mm thick Permalloy and 1 layer of 12-mm thick Al. The thin MSR has shielding factors of about 17 at 0.1 Hz and 300 at 10 Hz. When the second-order gradiometer system was operated inside the thin MSR, the average noise level is about 7 fT/√Hz at 200 Hz.