Cryogenic magnetometer research at Twente University of Technology

In 1982 we started the project ‘Cryogenic Magnetometers’ with the aim to develop SQUID-magnetometers appropriate to a large variety of applications. The first system we developed is a SQUID-magnetometer with an open-ended horizontal access at room temperature. The measuring space inside the pick-up coils is easy accessible and the magnetization (remanent or induced) can be measured of materials in a static position or transported through the access.\ud \ud Further, systems have been developed with the pick-up coil set outside the cryostat, in order to arrive at a flexible system with which the object under investigation can be approached as close as possible. Another part of the project is the optimization of magnetometers with respect to the cryogenic system in order to develop cryogenic magnetometer systems with a very long time between helium refills. The present status of these three research subjects is briefly described

Optimization of a Third-Order Gradiometer for Operation in Unshielded Environments

The optimum geometry of a third-order gradiometer for operation in unshielded environments is discussed. The optimization result depends on the specific signal and noise conditions. The fetal heart is considered as an example of the signal source. We optimized the gradiometer such that the signal-to-noise ratio is maximized in an averaged sense for all relevant environmental noise conditions and distances to the signal source. The resulting design consists of two second-order gradiometers that can be combined to form a third-order gradiometer in noisy environments, whereas a single second-order gradiometer can be used in low-noise environments. The gradiometer can provide the signal-to-noise ratio that allows detection of fetal heart signals in all relevant environmental noise conditions

Partial lattice participation in the spin-lattice relaxation of potassium chromium alum

We developed a SQUID-based frequency sweeping system for a.c. susceptibility measurements. Using this instrument we found that in Potassium Chromium Alum only a part of the lattice system is involved in the spin-lattice relaxation process. This partial lattice participation amounts 60–75% of the total lattice specific heat

Multichannel heart scanner based on high-Tc SQUIDs

A 7-channel magnetometer for magnetocardiography based on high-T c SQUIDs has been realized. This magnetometer is used for test experiments in the development of a multichannel high-Tc SQUID based heart-scanner for clinical applications. The intrinsic noise level of the channels in the 7-channel system is typically 120 fT/¿(Hz) down to 1 Hz. Magnetocardiograms were recorded inside a magnetically shielded room. Introductory experiments were performed on the suppression of noise by combining magnetometers to form planar gradiometers. The noise suppression that can be established appeared to be limited by the imbalance of the gradiometric configuration, which is roughly 2%. This relatively poor balance of the system is caused by inaccuracies in the transfer functions of the individual SQUID magnetometers, and by deviations from the planar geometr