Operation of a high-Tc DC-SQUID-gradiometer on a non-metallic pulse tube refrigerator

A planar high-Tc DC-SQUID gradiometer has been operated with a specially developed low noise pulse tube refrigerator. The cold finger of the refrigerator consists only of non-metallic and non-magnetic materials. The high sensitive SQUID-gradiometer with a base length of 4 mm has a field gradient resolution of about 3 pT/(cm Hz1/2) at 1 Hz and 0.7 pT/(cm Hz1/2) in the white noise region in magnetically shielded environment. During the operation of both, the sensor and the refrigerator, the noise generated by the cryocooler is below the noise level of the SQUID-gradiometer. We demonstrate the potential of this non-metallic pulse tube refrigerator by measuring the magnetic field originating from a human heart (magnetocardiogram), without additional suppression of the intrinsic refrigerator noise