First-order Planar Gradiometers Research Articles

Comparison of Axial Wire-Wound and Planar Thin-Film SQUID-Based Gradiometers for Super-Paramagnetic Relaxometry Measurements

Superparamagnetic relaxometry (SPMR) is a new technology used for detection and localization of cells tagged with super-paramagnetic nanoparticles. SPMR is one of the most promising methods for early cancer diagnosis with the potential for submillimeter size tumor resolution. SQUID-based gradiometers are very suitable for such sensitive measurements. Presently, most commonly used SPMR instruments consist of axial wire-wound gradiometers connected to SQUID-sensors placed inside tight superconducting shields. This configuration avoids flux trapping by the Josephson junctions during magnetization. A thin-film planar gradiometer integrated with a SQUID on a single chip traps flux, if it picks up even small portion of a magnetizing field. Thermal cycling is very slow, therefore it is not practical. Special measures must be applied such as newly developed methods of SQUIDs ac de-fluxing. Planar thin-film gradiometers have advantages over axial wire-wound gradiometers because of their much higher balance level, higher reliability, and reproducibility in production, and significant simplification of multichannel systems design. In this paper, we compare experimental results received using axial wire-wound and planar thin-film gradiometers. This comparison is based on SPMR measurements conducted using the seven-channel axial second-order gradiometer system and two systems with different geometries of the thin-film first-order planar gradiometer arrays. We discuss advantages and drawbacks of the two types of gradiometers.

Characterization of dual high transition temperature superconducting quantum interference device first-order planar gradiometers on a chip

The dual first-order planar gradiometers of high transition temperature (high-Tc) superconducting quantum interference devices (SQUIDs) were designed and fabricated on a 10×10 mm2 SrTiO3 bicrystal substrate. Each gradiometer consists of four bare SQUIDs which are connected to two symmetric pickup loops. Any two of the SQUIDs can be connected in series to obtain better performance. In this study, by selecting the coupling direction of SQUID, a balance resolution of 0.08% was achieved. And, a second-order electronic planar gradiometer was composed of these two first-order gradiometers. Experimental data showed that the noise performance of the second-order electronic planar gradiometer composed of two planar first-order gradiometers can reach 15 μΦ0/Hz1/2 at 1 kHz in an unshielded environment, which is comparable to the noise level of a magnetometer in a shielded environment.

Characterization of a low frequency magnetic noise from a two-stage pulse tube cryocooler

Magnetic noise of a two-stage pulse tube cryocooler (PT) was measured by a fundamental mode orthogonal fluxgate magnetometer and by a LTS Double Relaxation Oscillation SQUID (DROS) first-order planar gradiometer. The magnetometer was installed in a dewar made of aluminum at 12 cm distance from a section containing magnetic regenerative materials of the second pulse tube. The magnetic noise spectrum showed a clear peak at 1.8 Hz, which is the fundamental frequency of the He gas pumping rate. The 1.8 Hz magnetic noise registered a peak, during the cooling down process, when the second cold-stage temperature was around 12 K, which is well correlated with the 1.8 Hz variation of the temperature of the second cold stage. Hence, we attributed the main source of this magnetic noise to the temperature variation of the magnetic moments resulting from magnetic regenerative materials, Er 3Ni and HoCu 2, in the presence of background static magnetic fields. We have also pointed out that the superconducting magnetic shield of lead sheets reduced the low frequency magnetic noise generated from the magnetic regenerative materials. With this arrangement, the magnetic noise amplitude measured with the LTS DROS gradiometer, mounted at 7 cm horizontal distance from the magnetic regenerative materials, in the optimum condition, was lower than 500 pT peak-to-peak, whereas the noise level without lead shielding was higher than the dynamic range of DROS instrumentations which was around ± 10 nT .

Effects of Electromagnetic Radiation From Digital Wireless Telephone Frequencies on a Double Relaxation Oscillation SQUID

In this paper, the effects of electromagnetic fields from the frequency band of digital wireless telephones on a superconducting quantum interference device (SQUID) were experimentally investigated. A first-order planar gradiometer based on the double relaxation oscillation SQUID (DROS) was operated without low-frequency magnetic shielding, and was exposed to radiated electromagnetic fields in the frequency range of 300 MHz-2 GHz. The experiments have been performed in a fully anechoic chamber to minimize reflected waves in the chamber. The flux-to-voltage transfer and the field sensitivity of the gradiometer have been observed under the irradiation of fields. Results show that the performance of the gradiometer was significantly affected at a typical power level of the digital wireless telephones for both horizontally and vertically polarized electromagnetic waves. The noise level of the DROS output voltage also increases as the strength of the external fields becomes higher.

First-order high- Tc SQUID gradiometer

SQUID gradiometers are attractive for magnetic field measurements in noisy environments. A first-order planar gradiometer has been designed and fabricated. The layout of the planar gradiometer used to have two symmetric pickup coils; however, our design adopted one compensation loop with the same size as the SQUID loop at the symmetric position to reduce the imbalance caused by the SQUID. The gradient field resolution is up to 2.2 pT cm −1 Hz −1/2 (white noise) and 30 pT cm −1 Hz −1/2 at 1 Hz.

First-Order Planar Superconducting Quantum Interference Device Gradiometers With Long Baseline

We have developed several types of low-Tc first-order planar gradiometers with long baselines ranging from 3.6 cm to 4 cm. Thin-film planar gradiometers are very attractive for measurements of the off-diagonal components of the magnetic field gradient tensor in noisy environments because of the high intrinsic balance that can be achieved with the precision photolithographic techniques used to fabricate these devices. The gradiometer pickup loops are series configured and connected to the input circuit of a dual-washer gradiometric dc SQUID. Series and parallel SQUID washer configurations have been investigated. For a given chip size, model calculations using a magnetic dipole point source and a distributed current dipole source show that gradiometer performance can be improved by increasing the size of the pickup loops at the expense of a small reduction of the baseline length. Based on these results, an improved planar gradiometer has been developed with 1.175 cm times 1.175 cm square pickup loops integrated with a series-configured dual washer SQUID on a 1.2 cm x 4.8 cm chip. The white flux noise of the improved gradiometer with 3.6 cm baseline is as low as 2.4 muPhi0/Hz1/2 (rms), and the magnetic field sensitivity referred to one pickup loop is 0.63 nT/Phi0. This results in a magnetic field gradient noise of 0.42 fT/cm - Hz1/2. The gradiometers are operable unshielded in typical laboratory environments without losing lock.

Thin-Film Planar Gradiometer with Long Baseline

Gradiometers are attractive for magnetic field measurements in noisy environments. Thin-film planar gradiometers are in particular attractive for measurements of the off-diagonal components of the magnetic field gradient tensor, and they can be fabricated with high intrinsic balance owing to the precision photolithographic techniques used to fabricate these devices. We have developed a low-Tc first-order planar gradiometer with a long baseline of 4 cm. The pickup loops are series configured and connected to the input circuit of a dual washer gradiometric dc SQUID. The white rms flux noise measured in a shielded environment is 1.6 µΦ0/Hz½. The magnetic field sensitivity referred to one pickup loop is 2 nT/Φ0, resulting in an rms magnetic field noise referred to one pickup loop of 3.2 fT/Hz½ and an rms gradient noise of 0.8 fT/cm-Hz½. Based on the lithographic methods used to fabricate the gradiometers, the expected balance level is 1 part in 24, 000 or 0.004%. The gradiometer is operable without shielding in typical laboratory environments without losing lock, with an rms white noise at 10 kHz of 3.5 µΦ0/Hz½.

64-channel magnetocardiogram system based on double relaxation oscillation SQUIDplanar gradiometers

We developed a magnetocardiogram (MCG) system measuring tangential field components usingplanar gradiometers. The SQUID insert has 64 planar gradiometers, of which 32 measure thex-component and theother 32 measure the y-component, where the chest surface is in thex and y plane. The SQUID sensor is a double relaxation oscillation SQUID type with highflux-to-voltage transfers. Thus, a room-temperature dc preamplifier with a modest inputvoltage noise could be used to detect the SQUID output voltage directly. The pickup coilsare first-order planar gradiometers with a baseline of 4 cm, and they were connected to theinput coils using a superconductive bonding of Nb wires. The sensor coverage area of the64 channels is 190 mm in diameter, and the average boil-off rate of liquid He is3.6 L/d in everyday operation of the SQUID system. The SQUID operation of the 64channels was done by digital controls and the average noise of the MCG system was4.6 fT Hz−1/2 at 100 Hz, inside a magnetically shielded room.

High-T c SQUID magnetometer and planar gradiometer and their applications in non-destructive evaluation

We have carried out experimental investigation of eddy current non-destructive evaluation using a high-T c SQUID magnetometer. By mapping out field distributions with the SQUID magnetometer, flaws 10 mm below the surface in conducting aluminum structures can be clearly identified. We have also designed and fabricated a first-order planar gradiometer on a large area LaAlO 3 substrate with a baseline of 16 mm.

A low-noise integrated SQUID gradiometer for biomagnetic sensors

We developed a low-noise and compact SQUID gradiometer for biomagnetic multichannel applications. The SQUID is based on a double relaxation oscillation SQUID and provides very large flux-to-voltage transfer coefficients, allowing simple flux-locked loop electronics to be used for SQUID operation. The pickup coil is a first-order planar gradiometer integrated on the same wafer as the SQUID and has a baseline of 40 mm. The field gradient noise of the SQUID gradiometer in flux-locked loop mode is around 1 fT/cm/spl radic/Hz at 100 Hz, measured inside a magnetically shielded room.

Serial processing of the somesthetic information revealed by different effects of stimulus rate on the somatosensory-evoked potentials and magnetic fields

In order to evaluate information processing in the somatosensory cortex, the effect of two different stimulus rates was investigated by simultaneously recording somatosensory-evoked potentials (SEPs) and magnetic fields (SEFs) in nine healthy adults. During electric stimulation of the median nerve at the wrist, SEFs were recorded with the helmet-shaped whole-head coverage magnetometer array with 122 first-order planar gradiometers while SEPs were simultaneously recorded from seven scalp positions. Interstimulus intervals (ISIs) of 0.9 s and 4 s were compared. In all subjects, N20 as well as its magnetic counterpart, N20m, was clearly demonstrated over the contralateral somatosensory area. Subsequent deflections around 80–200 ms did not make any clear peak and were smaller than those at 20–60 ms (P30m, P40m, N50m and P60m). After 200 ms, SEFs were negligible, whereas SEPs had larger amplitude than those of shorter latencies, constituting a peak around 250 ms (P250). Both SEF and SEP deflections later than 40 ms were decreased in responses at the shorter ISI; this diminution was most prominent for P250. Therefore, it is concluded that the tangential currents in the somatosensory cortex (area 3b) mainly contribute to responses during the first 200 ms after the stimulus, whereas the radially oriented currents (most likely in the crown of the postcentral gyrus) take over for subsequent information processing.

Operation of a hybrid 28-channel neuromagnetometer

A 28-channel system for neuromagnetic measurements operative at CNR-IESS (Instituto di Elettronica dello Stato Solido) is described. The instrument uses 28 Nb/PbAuIn DC-SQUID (superconducting quantum interference device) sensors coupled with a flux transformer configuration. The detection coils are arranged on concentric circumferences. Sixteen axial first-order gradiometers are arranged in the internal circles, while the external circle consists of 12 first-order planar gradiometers. The vertical gradiometers show a white noise level of approximately=5 fT/ square root Hz, while the planar ones have a noise approximately=6 fT/cm square root Hz, with a 1/f corner at a few hertz. The evoked potentials of normal subjects and patients affected by various kinds of vascular disturbances have been studied. The system has succeeded at one-shot localization of some cortical activity. The instrument has also been used to record the spontaneous brain activity in patients affected by focal epilepsy.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

28-channel hybrid system for neuromagnetic measurements

Progress toward the development of a 28-multichannel system for neuromagnetic measurements is described. A novel hybrid design consisting of 16 first-order axial gradiometers and 12 first-order planar gradiometers was chosen, which optimize the use of the available cylindrical volume of the dewar tail. This configuration maintains the symmetry of the detected pattern with respect to rotation of a biomagnetic source located under the center of the array and features a localization power considerably better than an array of all first-order planar gradiometers. The detecting array permits simultaneous magnitude measurements over a circular scalp region of 16-cm diameter. The magnetic sensors used are Nb/PbAuIn DC SQUIDs fabricated at IBM. The devices incorporate resonant damping resistors shunting the inductance, resulting in smooth flux-voltage characteristics and, consequently, very low noise figures in a flux-locked loop configuration. A simple and low-cost electronic system has been designed and fabricated for the DC SQUID sensors, consisting of a compact head mounted on top of the cryogenic probe and a control unit where all the DC remote controls are performed. The dynamic range of 10/sup 7/ square root Hz and the bandwidth of 50 kHz are specifically designed to work in multichannel DC SQUID instrumentation.

Multichannel hybrid system for neuromagnetic measurements

This paper describes progress toward the development of a 28-multichannel system for neuromagnetic measurements. A novel 'hybrid' design consisting of 16 first-order axial gradiometers and 12 first-order planar gradiometers was chosen, which optimises the use of the available cylindrical volume of the dewar tail. This configuration maintains the symmetry of the detected pattern with respect to rotation of a biomagnetic source located under the centre of the array and features a localisation power considerably better than an array of all first-order planar gradiometers. The detecting array permits simultaneous magnetic measurements over a circular scalp region of 16 cm diameter. The magnetic sensors used are Nb/PbAuIn DC SQUIDs fabricated at the IBM. The devices incorporate resonant damping resistors shunting the inductance, resulting in smooth flux-voltage characteristics and, consequently, very low noise figures in a flux-locked loop configuration. A simple and low cost electronic system has been designed and fabricated for the DC SQUID sensors.