A. Kandori

Development of multisample biological immunoassay system using HTSSQUID and magnetic nanoparticles

We developed a prototype magnetic immunoassay system using a high temperature superconductor (HTS) superconducting quantum interference device (SQUID) to investigate the performance and usability of the magnetic immunoassay. The system is designed to measure multiple samples and liquid samples, and it can work in an unshielded environment at a medical facility. To reduce the disturbance from environmental noise, the SQUID and samples are covered with three-layers of permalloy magnetic shield. The SQUID and magnetic shield are set in an aluminum box which acts as an RF shield. A gradiometer with a 5 /spl times/ 10 mm pickup coil, which is cooled by liquid nitrogen through a sapphire/Cu rod, is used as a sensor. We also developed a nonmagnetic sample disk with 12 reaction cells and examined 12 samples in one measurement sequence. The measurement process is controlled by a computer, which perform data averaging. Fe/sub 3/O/sub 4/ nanoparticles with a 25-nm diameter were used as test samples. After applying a magnetic field of about 0.1 T, we measured the remanent magnetic field from the Fe/sub 3/O/sub 4/ nanoparticles. The present system could detect 30 pg of Fe/sub 3/O/sub 4/ nanoparticles. This result was obtained by averaging 100 trials under an unshielded laboratory environment. The measurement time for 100 trials was only 100 s.

Development of a compact DC magnetometer using HTS-SQUID and a rotating sample

We developed a compact DC magnetometer using a high-temperature superconductor (HTS) superconducting quantum interference device (SQUID) to measure very weak magnetic signals from samples such as paramagnetic and diamagnetic materials. The samples were rotated in the DC magnetic field that was detected by a normal conductive pick-up coil. The detected signal was transferred to an input coil that was inductively coupled to the SQUID. To clarify the basic characteristics of this system, the magnetic signal from a magnetic material was measured by varying the sample position and rotation speed. Then, the magnetic signal from pure water was measured under the optimized condition and a very weak magnetic signal from pure water was successfully detected. Therefore, the developed system could be applied to various non-destructive evaluation systems.

Fast Detection of Biological Targets With Magnetic Marker and SQUID

We have been developing a SQUID system for the detection of biological targets. In this system, magnetic markers are bound to the targets, and the magnetic signal from the bound markers is detected with the SQUID. In order to realize fast detection of the targets, we developed a liquid-phase detection method. First, we used large polymer beads as material to capture the targets. Since the polymer beads are uniformly dispersed in liquid, biological targets on the surface of the polymer bead can be easily coupled to the markers, which results in the fast reaction time. Next, we detected the bound markers without using the washing process to separate the bound and unbound markers, which was realized by using the difference in the Brownian relaxation time between them. Using this procedure, we demonstrated the detection of the target called IgE, as well as biotin-coated polymer beads. We obtained a good relationship between the amount of IgE and the magnetic signal. The result was the same as that obtained using the conventional procedure. The reaction time for the coupling between the magnetic marker and the target was 4 min, which was much shorter than the conventional method. These results show the usefulness of the present method.

HTS SQUID Magnetometer Using Resonant Coupling of Cooled Cu Pickup Coil

We have designed and tested an HTS SQUID magnetometer using the resonant coupling of a copper pickup coil cooled at T= 77 K. The coil was made of a twisted multi-filamentary wire so as to reduce the ac resistance. We first showed that the ac resistance of the coil became higher than the dc resistance because of the eddy current loss of the coil. We clarified the dependence of the ac resistance on the parameters of the coil and obtained an empirical expression for the ac resistance. Next, we constructed a magnetometer using a pickup coil with the average diameter D= 50 mm and number of turns N=150 . The pickup coil with an inductance of Lp= 1.05 mH was connected to an input coil through a resonant capacitance of C= 0.29 μF. The HTS SQUID was magnetically coupled to the input coil with a mutual inductance of M= 500 pH. The estimated magnetic field noise of the magnetometer was approximately 2.5 fT/Hz 1/2 at the resonant frequency of f= 9.05 kHz. The Q value of the resonant circuit was 180. The experimental results agreed well with the designed values. The obtained high sensitivity of the magnetometer will be useful for low frequency applications such as low-field NMR.

Applying high-T/sub C/ superconducting quantum interference devices with a room-temperature pickup coil in the measurement of impedance magnetocardiograms

A high-T/sub C/ superconducting quantum interference device (SQUID) magnetometer - in which a room-temperature pickup coil is used to detect an impedance magnetocardiogram (I-MCG) signal - has been developed. The pickup coil (30-mm diameter) is installed outside the cryostat and is connected to the input coil of the high-T/sub C/ SQUID. The magnetometers noise level is 150 fTHz/sup -1/2/ (>10 kHz). The magnetometer was used to measure I-MCG signals (about 12 pT), which were obtained by applying an ac current (15 kHz) of constant amplitude (7 mA, peak-to-peak) to a healthy male subject.

Electronic gradiometer in cylindrical magnetic shield for compact magnetocardiograph

An electronic first-order gradiometer in a cylindrical magnetic shield was investigated in order to determine the best magnetometer configuration for a compact multichannel magnetocardiograph (MCG). It was found that a planar gradiometer is better in terms of noise reduction than an axial gradiometer. This is because the magnetic field inside the cylindrical magnetic shield exhibits an anisotropic distribution, which was confirmed by a numerical calculation using a three-dimensional finite element method. It was also found that MCG measurement in the cylindrical magnetic shield is possible by using a planar electronic gradiometer with high-Tc directly coupled magnetometers. This configuration can successfully detect the P-wave in MCG signal without the need for averaging.

High T/sub c/ dc SQUID utilizing bicrystal junctions with 30 degree misorientation angle

Performances of high T/sub c/ dc SQUID utilizing bicrystal junctions with 30/spl deg/ misorientation angle have been studied. Junction resistance R/sub s/=10 /spl Omega/ and critical current I/sub 0/=10-20 /spl mu/A can be obtained rather easily at T=77 K with this technology. As a result, voltage modulation depth as high as 50 /spl mu/V, and flux noise as low as 5 /spl mu//spl Phi//sub 0//Hz/sup 1/2/ obtained for the 100 pH-SQUID. The measured values of the voltage modulation depth agree reasonably well with the numerical simulation, while the measured values of the flux noise are about 2 or 3 times larger than the simulation. The reason for the high noise level is discussed by taking into account imperfection of junctions.

Direct-Coupled High Tc DC Superconducting Quantum Interference Device Magnetometers on SrTiO3 Substrate: Theoretical Description and Comparison with Experiment

A comprehensive quantitative comparison between the measured performance of direct-coupled high Tc superconducting quantum interference device (SQUID) magnetometers with 30° bicrystal junctions and a numerical simulation is presented. It is shown that the characteristics of the SQUID magnetometer are considerably affected by resonances due to the large dielectric constant of the SrTiO3 substrate. In the realized magnetometer layout, the strip line resonance occurring in the SQUID inductance increases the voltage modulation depth and the output voltage noise of the SQUID, while the flux noise of the SQUID is nearly unchanged. It is also shown that the distortion of the voltage versus flux characteristic is caused by the LC resonance in the pickup loop of the magnetometer in combination with a capacitive feedback. Good agreement between experiment and simulation has been obtained.

Process of fabricating YBCO SQUIDs for 51-channel HTS MCG system

We have constructed a high throughput HTS SQUID fabrication process to develop a 51-channel HTS MCG system. To increase the process throughput of the YBCO deposition, we designed and developed a new deposition system based on a pulsed laser deposition technique. In this deposition system, nine YBCO thin films could be deposited in one deposition sequence by successively depositing the YBCO thin films one by one. In addition to the improvement of the fabrication throughput, the magnetometer pattern was designed to improve the yield of the SQUID fabrication. In our magnetometer design, four SQUIDs were connected with one pickup coil. The yield of good magnetometer chips could be increased by selecting the best SQUID among the four. Using the developed SQUID fabrication process, about one hundred magnetometers were fabricated in a month. The 51-channel HTS MCG system was successfully developed and used for clinical testing.

Noise properties of highly balanced YBa2Cu3Oy directly coupled gradiometers and MCG measurement in magnetically unshielded environment

We have fabricated a first-order, directly coupled gradiometer with a gradiometric balance below 0.1% without balance adjustment by selecting the coupling direction of the pickup coil and the superconducting quantum interference device. The noise of the gradiometers with various balances measured in an unshielded environment decreased in proportion to the improvement in balance. However, the noise of the gradiometer with a balance of 0.03% was limited by the gradient field component of the environmental noise. Magnetocardiogram measurement with this gradiometer in an unshielded environment was also demonstrated.