Superconducting Quantum Interference Device Gradiometer Research Articles

Magnetic field-to-voltage coefficient evaluation of axial SQUID gradiometer in unshielded environment

Due to the existence of earth magnetic field, it is difficult to detect weak magnetic field from some parts of the body like heart, muscle, nerve and so on. In addition to the requirement for high-sensitivity magnetic field sensors, it is also essential to take certain measures to suppress noise. The axial gradiometer based on low-Tc Superconducting QUantum Interference Device (SQUID) can achieve a noise suppression ratio of 30 dB through the spatial gradient difference of the magnetic field, but the calibration of the magnetic field-to-voltage coefficient (B-V coefficient) generally requires a magnetically shielded room (MSR), which is complex and costly. With finite element simulation and practical experiments, a calibration method for B-V coefficient of the axial hardware gradiometer without shielding was presented in this paper. That is, a larger calibration field was generated for coefficient evaluation in an unshielded environment. At the same time, when the test signal was far away, the spatial gradient difference was used to improve the authenticity of the evaluation results. Through simulation and experimental verification, for the same performance and configuration of the gradiometer, the calibration result was 1.6 nT/V with 0.1 nT/V difference from that in a shielding room. The results showed that the unshielded evaluation method proposed in this paper was practical and could be used to evaluate the B-V coefficient of gradiometer.

Magnetotrichography: Measuring the dc magnetic field produced by hair follicles

We here describe the dc magnetic field over the human head produced by healthy hair follicles when the scalp is lightly pressed. This phenomenon was briefly reported decades earlier, where a double-planar SQUID (Superconducting Quantum Interference Devices) gradiometer at a single location was used. We here perform a larger study, using the dcMEG containing 102 double-planar gradiometers covering the whole scalp. The field is displayed as an on-line arrow map over the head, showing the approximate flow of direct current (dc) in the scalp. Standard sets of five arrow maps per subject were measured, where the subject successively pressed parts of their head against the inside of the helmet. These maps were made for 15 normal subjects (5 females), and 2 with alopecia (non-functioning follicles). The directions of “pressed” generating arrows always followed the natural tilt of the follicles, verifying the follicles as generators, with a time constant of about one second. The maximum generator dipole strength was about 24 µA-cm. Scalp electric potentials corresponding to the magnetic signals were masked by much larger electrodermal potentials. Therefore, this magnetic method, called magnetotrichography, is unique in measuring this follicular electrical activity, with possible applications in studying baldness and hair diseases.

Underwater operation of a full tensor SQUID gradiometer system

For the first time a mobile underwater full tensor magnetic gradiometer (FTMG) system based on low-Tc superconducting quantum interference devices (SQUIDs) has been deployed in order to scan the sea floor for magnetized targets. The application is mainly focused on waste deposits and unexploded ordnance (UXO), but could also include shallow geological features as well as archaeological remains. The main methods for detection and localisation of underwater UXO and waste deposits are side sonar scanning and magnetic mapping. While modern sonar scanners can achieve a very high spatial resolution and long detection range, they still have problems detecting targets under cover—for magnetic sensors a layer of non-magnetic sand or ooze does usually not have an effect on the signal apart from an increased distance. In this paper we discuss the setup of the mobile underwater FTMG SQUID system, its challenges and main performance features. It also illustrates its detection and localization capabilities in tests on known magnetic targets.

Investigation of Geomagnetic Orientation Preserved in a Stain Containing Iron Compounds Using a Vector-Type SQUID Magnetometer

We investigate the orientation of the magnetic field preserved in a stain containing iron compounds, such as ink pigment or blood, using a superconducting quantum interference device (SQUID) magnetometer system. The magnetization of a cloth or paper sample is observed by applying a vector-type SQUID gradiometer, which detects three orthogonal components of the magnetic field, and mechanical modulation, which converts dc magnetic signals to time-varying signals detectable by SQUIDs. Numerical experiments indicate that the orientation of the magnetization could be identified by the vector-type SQUID magnetometer when the sample was moved linearly under the probe. This paper describes in detail the instrumentation of the measurement system and a proof-of-principle experiment to confirm the system's effectiveness. The obtained results suggest that the magnetization of iron-compound-containing stains preserved the orientation of the magnetic field in which they were produced, as we hypothesized.

Optimization of pick-up coils for weakly damped SQUID gradiometers**Project supported by the Key Project of Shanghai Zhangjiang National Innovation Demonstration Zone of the Special Development Fund, China (Grant No. 2015-JD-C104-060) and the National Natural Science Foundation of China (Grant No. 61741122).

The performance of a superconducting quantum interference device (SQUID) gradiometer is always determined by its pick-up coil geometry, such as baseline and radius. In this paper, based on the expressions for the coupled flux threading a magnetometer obtained by Wikswo, we studied how the gradiometer performance parameters, including the current dipole sensitivity, spatial resolution and signal-to-noise ratio (SNR), are affected by its pick-up coil via MatLab simulation. Depending on the simulation results, the optimal pick-up coil design region for a certain gradiometer can be obtained. To verify the simulation results, we designed and fabricated several first-order gradiometers based on the weakly damped SQUID with different pick-up coils by applying superconducting connection. The experimental measurements were conducted on a simple current dipole in a magnetically shielding room. The measurement results are well in coincidence with the simulation ones, indicating that the simulation model is useful in specific pick-up coil design.

Gradiometers based on superconducting quantum interference device for nondestructive testing

A prototype of gradiometer for detection and analysis of magnetic signals that are generated by defects in metal structures and materials in the presence of external magnetic bias is based on dc-current superconducting quantum interference device (SQUID). A prototype of single-channel SQUID gradiometer that contains a fiberglass nonmagnetic cryostat, measurement probe with the SQUID sensor and magnetic flux transformer (second-order axial gradiometer), SQUID electronics, and software for control of SQUID gradiometer is studied. The prototype exhibits stable operation under laboratory conditions in the absence of additional magnetic shielding. Upgrade of the SQUID sensors and remaining elements of the prototype of magnetometer is planned for application in nondestructive testing.

Operation of a high-TC SQUID gradiometer with a two-stage MEMS-based Joule–Thomson micro-cooler

Practical applications of high-TC superconducting quantum interference devices (SQUIDs) require cheap, simple in operation, and cryogen-free cooling. Mechanical cryo-coolers are generally not suitable for operation with SQUIDs due to their inherent magnetic and vibrational noise. In this work, we utilized a commercial Joule–Thomson microfluidic two-stage cooling system with base temperature of 75 K. We achieved successful operation of a bicrystal high-TC SQUID gradiometer in shielded magnetic environment. The micro-cooler head contains neither moving nor magnetic parts, and thus does not affect magnetic flux noise of the SQUID even at low frequencies. Our results demonstrate that such a microfluidic cooling system is a promising technology for cooling of high-TC SQUIDs in practical applications such as magnetic bioassays.

Calibration of SQUID vector magnetometers in full tensor gradiometry systems

Measurement of magnetic vector or tensor quantities, namely of field or field gradient, delivers more details of the underlying geological setting in geomagnetic prospection than a scalar measurement of a single component or of the scalar total magnetic intensity. Currently, highest measurement resolutions are achievable with superconducting quantum interference device (SQUID)-based systems.Due to technological limitations, it is necessary to suppress the parasitic magnetic field response from the SQUID gradiometer signals, which are a superposition of one tensor component and all three orthogonal magnetic field components. This in turn requires an accurate estimation of the local magnetic field. Such a measurement can itself be achieved via three additional orthogonal SQUID reference magnetometers. It is the calibration of such a SQUID reference vector magnetometer system that is the subject of this paper.A number of vector magnetometer calibration methods are described in the literature. We present two methods that we have implemented and compared, for their suitability of rapid data processing and integration into a full tensor magnetic gradiometry, SQUID-based, system.We conclude that the calibration routines must necessarily model fabrication misalignments, field offset and scale factors, and include comparison with a reference magnetic field. In order to enable fast processing on site, the software must be able to function as a stand-alone toolbox.

A SQUID gradiometer module with large junction shunt resistors

A dual-washer superconducting quantum interference device (SQUID) with a loop inductance of 350 pH and two on-washer integrated input coils is designed according to conventional niobium technology. In order to obtain a large SQUID flux-to-voltage transfer coefficient, the junction shunt resistance is selected to be 33 Ω. A vertical SQUID gradiometer module with a baseline of 100 mm is constructed by utilizing such a SQUID and a first-order niobium wire-wound antenna. The sensitivity of this module reaches about 0.2 fT/(cm·Hz1/2) in the white noise range using a direct readout scheme, i.e., the SQUID is directly connected to an operational amplifier, in a magnetically shielded room. Some magnetocardiography (MCG) measurements with a sufficiently high signal-to-noise ratio (SNR) are demonstrated.

Development of Metallic Contaminant Detection System Using Eight-Channel High- $T_{c}$ SQUIDs

In this paper, a roll-to-roll eight-channel (8-ch) high- <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Tc</i> superconducting quantum interference device (SQUID) detection system for magnetic contaminants in a lithium-ion (Li-ion) battery anode sheet was developed. Finding ultra-small metallic foreign matter is an important issue for a manufacturer because metallic contaminants carry the risk of an internal short. When contamination occurs, the manufacturer of the product suffers a great loss from recalling the tainted product. Metallic particles with outer dimensions smaller than 100 μm cannot be detected using a conventional X-ray imaging system. Therefore, a highly sensitive detection system for small foreign matter is required. We have already developed a detection system based on a single-channel SQUID gradiometer and horizontal magnetization. For practical use, the detection width of the system should be increased to at least 65 mm by employing multiple sensors. In this paper, we present an 8-ch high- <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Tc</i> SQUID roll-to-roll system for inspecting a Li-ion battery anode with a width of 65 mm. A special microscopic type of a cryostat was developed upon which eight SQUID gradiometers were mounted. As a result, small iron particles of φ35 μm on a real Li-ion battery anode with a width of 70 mm were successfully detected. This system is practical for the detection of contaminants in a Li-ion battery anode sheet.

Visualization of flowing current in braided carbon fiber reinforced plastics using SQUID gradiometer for nondestructive evaluation

In this paper, visualization of flowing current in various braided carbon fiber reinforced plastics (CFRPs) was demonstrated using high-temperature superconductor (HTS) superconducting quantum interference device (SQUID) gradiometer, in order to study electrical properties and integrity of the braided CFRP samples. Step-by-step tensile loading was also applied to the samples, in order to study their mechanical properties and destructive mechanism. Experimental results indicated that the addition of carbon nano fibers and middle-end carbon fiber bundles attributed to modify not only the mechanical properties, but also the electrical properties of the samples. Combining the results by the both methods, a scenario of the destructive mechanism of one sample was estimated.

Full Tensor SQUID Gradiometer for airborne exploration

IPHT Jena and Supracon AG develop and fabricate full tensor magnetic gradiometer (FTMG) systems, based on low-temperature-superconducting (LTS) planar-type SQUID (Superconducting Quantum Interference Device) gradiometers of first order. The systems are operated by Spectrem Air Ltd., jointly owned by Anglo American and De Beers. This paper describes details of the system setup, data processing and achieved performance parameters. The sensors work at 4.2 K in liquid helium. With a base length of only 3.5 cm a real gradient measurement is possible. With six gradiometer the full magnetic gradient tensor can be measured with redundancy, allowing for high system reliability and the application of enhanced noise reduction techniques. The data of the FTMG system will allow for a 3D inversion for the underlying magnetic sources. Examples of 2D maps of tensor components with high spatial resolution are shown.

Simple simulation method for investigating the performance of a SQUID gradiometer corresponding to a baseline length

Since the Superconducting Quantum Interference Device (SQUID) is the most sensitive magnetic field detector, it is widely used for measuring very weak biomagnetic signals, such as a Magnetocardiography (MCG). However, because the SQUID is easily affected by environmental magnetic noises, it is often used as a form of gradiometer suppressing spatially homogeneous field noises. The characteristic of the SQUID gradiometer mainly depends on the distance between the connected coils, that is, a baseline. Accordingly, in order to fabricate a gradiometer, the investigation of a proper baseline for the MCG signal is necessary. For the investigation, however, various gradiometers having different baselines have to be fabricated and tested, which required too much cost and effort. In this study, we suggest a simple simulation method to investigate the performance of a SQUID gradiometer depending on the baseline. In addition, in order to verify the reliability of our simulation, we fabricated wire wound axial first-order gradiometers (50 mm, 70 mm, and 100 mm baseline) and a second-order gradiometer (50 mm baseline) and compared the characteristics of the measured gradiometer signal with the simulated signal.

Analytical Model for the Extraction of Flaw-Induced Current Interactions for SQUID NDE

Incorporating an analytical approach to simulate the interaction of a series of long cracks and the induced current of a double-D excitation coil, we have developed a model-based method to do precise detection of the positions of the cracks in a metallic structure by using eddy-current superconducting quantum interference device (SQUID) nondestructive evaluation (NDE) measurements. Conventionally, the structure of the defects is found by iteratively solving a numerical forward problem, which is usually based on finite-element, boundary-element, or volume-integral method. This, however, incurs a heavy numerical burden, as every time the forward problem is to be solved, a rigorous numerical model should be inevitably employed to extract the complex distribution pattern of the induced current encountering defects of the structure. In this paper, an analytical approach is used for the modeling of the interaction of the induced current and a series of cracks in the sample. It duly considers the distribution of the induced current in the flawed samples, does not call for extremely high computational resource, and thus permits efficient NDE as the forward problem can be solved within a reasonable time. Here, a high- <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Tc</i> first-order radio-frequency SQUID gradiometer is employed as the magnetic sensor of the NDE system to scan the samples with different cracks. The accuracy of the proposed algorithm is verified by having the extracted shape of the defects obtained by applying the proposed algorithm on the SQUID NDE measurements against the actual cracks.

A Room-Temperature Pre-calibration Procedure for Gradiometer Sifting

In order to detect extremely weak magnetic signals, superconducting quantum interference device (SQUID) gradiometers are widely used to suppress environmental noise. A hardware SQUID gradiometer consists of a niobium gradio-antenna and an SQUID, which are coupled via an input coil. Here gradiometer imbalance may greatly reduce its noise suppression performance. The gradiometer balance depends on the geometrical forms of the antenna wound by niobium wire. We describe a simple method based on Faraday's law for the pre-calibration of the gradiometer balance at room temperature, before the gradiometer is set up. The pre-calibrating results are compared with the measured balance of an SQUID gradiometer system. This method may be used for sifting hardware gradiometers for multi-channel systems.

High Tc SQUID Detection System for Metallic Contaminant in Lithium Ion Battery

A highly sensitive detection system for magnetic contaminants using a High-Tc superconducting quantum interference device (SQUID) was developed. Finding ultra-small metallic contaminants is a big issue for manufacturers producing commercial products such as lithium ion batteries. When the contamination does occur, the manufacturer of the product suffers a great loss to recall the tainted products. The outer dimension of metallic particles less than 100 microns can not be detected by X-ray imaging, which is commonly used as the inspection method. In most cases, the matrix of industrial products is magnetized and the magnetic signal from the matrix is large enough to mask the signal from contaminants. We developed a detection system based on a high-Tc SQUID gradiometer. A specially designed low noise planar gradiometer for the system was developed. The flux white noise level of the SQUID gradiometer was 8-16 muphi0/Hz1/2 at 1 kHz. Use of the gradiometer and horizontal magnetization could solve problem above. We tested the performances of the system and found that small iron particles as small as 50 mum times 50 mum on the electrode of the lithium ion battery could be clearly detect. This detection level is difficult to achieve using other methods.

Towards an electrowetting-based digital microfluidic platform for magnetic immunoassays

We demonstrate ElectroWetting-On-Dielectric (EWOD) transport and SQUID gradiometer detection of magnetic nanoparticles (MNPs) suspended in a 2 microl de-ionized water droplet. This proof-of-concept methodology constitutes the first development step towards a highly sensitive magnetic immunoassay platform with SQUID readout and droplet-based sample handling. Magnetic AC-susceptibility measurements were performed on MNPs with a hydrodynamic diameter of 100 nm using a high-Tc dc Superconducting Quantum Interference Device (SQUID) gradiometer as detector. We observed that the signal amplitude per unit volume is 2.5 times higher for a 2 microl sample droplet compared to a 30 microl sample volume.

Detection of proton NMR signal in the Earth’s magnetic field at an urban laboratoryenvironment without shielding

Nuclear magnetic resonance in the Earth’s magnetic field was studiedin an urban laboratory without shielding using a second-orderlow-Tc SQUID (superconducting quantum interference device) gradiometer. Two differentconfigurations were investigated, one with a prepolarization field perpendicularto the measurement field and the other one with the two fields parallel but aπ/2 ACpulse applied perpendicular to them. With a prepolarization field of 10 mT and a measurement fieldof ∼29.1 µT, the proton signal from 15 ml tap water was obtained in a one-shotmeasurement with high signal-to-noise ratio, and the signal was stilldiscernible down to a sample volume of 2–3 ml. Spin–lattice relaxation timeT1∼1.78 s wasmeasured from the variation of signal magnitude on the prepolarization time. Spin–spin relaxationtime T2 was estimated to be around 0.9 s from the calculated FID signal by using a special methoddeveloped based on the principle of a lock-in amplifier.

High-Tc superconducting quantum interference devices: Status and perspectives

In this paper, an overview of the current status of high-Tc superconducting quantum interference devices (SQUIDs), from device engineering to biomagnetic applications, is given. The authors offer a description of the current status of SQUID sensors, challenges encountered, and the solution of fabricating SQUID sensors with low flux noises. The current challenge that we face is to fabricate high-Tc SQUIDs that are not only more reproducible than the current technology but also capable of providing a high IcRn product and fabricating SQUID with high yield. Improvement of flux noises and fabrication yield in the integrated multichoices directly coupled SQUID magnetometer or gradiometer with series SQUID array are presented. High-Tc SQUID magnetometers exhibiting magnetic field sensitivity of ∼30–50fT∕Hz1∕2 or better at 100Hz was demonstrated by incorporating serial SQUID into the pickup loop of the magnetometers. New technologies currently being developed and applications for high-Tc SQUIDs are addressed.

High-Tc SQUID gradiometer system for immunoassays

A high-Tc dc SQUID (superconducting quantum interference device) gradiometer wasdeveloped for magnetic immunoassays where magnetic nanoparticles are usedas markers to detect biological reactions. The gradiometer was fabricated on a5 × 10 mm2 SrTiO3 bicrystal substrate and has a gradiometer resolution of2.1 pT cm−1 Hz−1/2. A magnetic signal was detected from a sample of1 µl ofFe3O4 nanoparticlesin a 40 mg ml−1 solution kept in a microcavity fabricated on Si wafers withSi3N4 membranes using MEMS (micro-electro-mechanical-systems) technology. Itwas found that volumes as small as 0.3 nl in principle would be detectablewith our present device. This corresponds to a total number of particles of2.2 × 107. The estimated average dipole moment per particle is4.8 × 10−22 A m2. We are aiming at reading out immunoassays by detecting the Brownian relaxation ofmagnetic nanoparticles, and we also intend to integrate MEMS technology into our system.