Superconducting quantum interference devices ͑SQUIDs͒ have been a key factor in the development and commercialization of ultrasensitive electric and magnetic measurement systems. In many cases, SQUID instrumentation offers the ability to make measurements where no other methodology is possible. We review the main aspects of designing, fabricating, and operating a number of SQUID measurement systems. While this article is not intended to be an exhaustive review on the principles of SQUID sensors and the underlying concepts behind the Josephson effect, a qualitative description of the operating principles of SQUID sensors and the properties of materials used to fabricate SQUID sensors is presented. The difference between low and high temperature SQUIDs and their suitability for specific applications is discussed. Although SQUID electronics have the capability to operate well above 1 MHz, most applications tend to be at lower frequencies. Specific examples of input circuits and detection coil configuration for different applications and environments, along with expected performance, are described. In particular, anticipated signal strength, magnetic field environment ͑applied field and external noise͒, and cryogenic requirements are discussed. Finally, a variety of applications with specific examples in the areas of electromagnetic, material property, nondestructive test and evaluation, and geophysical and biomedical measurements are reviewed.
We discuss the use of a new SQUID magnetometer for noninvasive measurements of hepatic (liver) iron stores. Placement of the SQUID, detection coil, and magnet in the dewar vacuum region significantly reduced system noise. In addition, the system incorporates multiple magnets and detection coils which may allow thc discrimination of the surface skin layer from the deeper (weaker signal) true liver iron concentration. Measurements indicate an instrumental noise level < 20 pgfg of equivalent iron concentration.
Abdrwt-We have fabricated HTS &SQUID flip-chip the magnetometers could be additionally improved by a sensors with a large area multilayer flux transformers. larger Pickup loop with the flux transformer made on a Different layouts of the flux transformers provide a large largerwafer. variety of magnetometers and planar gradiometers. For the To subtract high magnetic background noise one can use a magnetometers a resolution-6 tT/.\IHz and the planar gradiometric configuration of the pickup coil. Tian et al. [4] gradiometers a resolution of about ,. , 30 fT/cm..\IHz were have achieved a field gradient sensitivity of 73 fT/cm& in routinely obtained at 77 K The noise was nearly white down to the white noise region and 596 fl/cm,,/fi at 1 fi with a frequencies of few Hz. The sensors were vacuum-tight layer gradiometfic flux antenna on a 50 mm si wafer. We have demonstrated [5] a planar HTS flip-chip encapsulated together with a heater and a feedback coil. This makes the handling of the sensors more reproducible and convenient. Production of the magnetometers and gradiometers gradiometer having padometric flux antenna in small series was proven. prepared on a 30 mm wafer. A resolution of-40 ff/cmdHz
More than 50 years ago superconducting quantum interference devices (SQUIDs) were invented. Since then many applications opened up. Already in a 1980 workshop (Weinstock and Overton 1981 SQUID Applications to Geophysics (Society of Exploration Geophysicists)) the application of SQUIDs in geosciences was reviewed. The fabrication and cooling technologies, electronics and other SQUID system components underwent significant improvement within the past years. Thus, SQUIDs are today better suited, more sensitive and effective as well as robust and reliable in operation for geophysical measurements. Many successful application examples, demonstrations and discoveries of mineral resources have been made using them in laboratory devices for investigation of magnetic properties, magnetic exploration, transient electromagnetics and for superconducting gravimeters as well as gravity gradiometers. Therefore, this article intends to review the past, present, and some future aspects of SQUIDs in geo-scientific applications such as e.g. mineral exploration. Since this field is still very active and quite a number of developments are ongoing, this review cannot be comprehensive.
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