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Tiny, Sensitive Magnetic-Field Detectors

Arrays of cheap magnetic sensors could detect improvised explosive devices.
November 16, 2007

Researchers at the National Institute of Standards and Technology (NIST) have developed a new type of magnetometer–or magnetic-field detector–that rivals the sensitivity of its predecessors but is small and cheap, and uses very little power.

Shrinking sensors: At top, metal atoms inside a silicon cube (green) are aligned (arrows) with light from an infrared laser shining onto a detector (blue). At bottom, in the presence of a weak magnetic field–emitted, for example, by a bomb–the atoms are shifted out of alignment and can now absorb light from the laser.

Magnetometers have a wide range of potential applications: where there is an electrical current, there is a magnetic field. Measurements of magnetic fields can reveal information about the electrical activity of the human heart and brain, the chemical identity of a spinning atom, or simply the presence or absence of metal. Because of their small size and sensitivity, the new sensors promise to improve detection of bombs and fetal heartbeats, and could be incorporated into future magnetic resonance imaging (MRI) scanners.

The new sensor, developed by NIST physicist John Kitching, consists of a laser, a cell containing vaporized metal atoms, and a light detector. When the metal atoms are illuminated by the laser, they align such that they don’t absorb any of the light. The presence of even a very weak magnetic field, however, disrupts their alignment, and they absorb some of the light. This change is recorded by the detector.

Other researchers have made similar magnetometers, but Kitching and his team used microfabrication techniques to miniaturize the vapor cell, which in their device consists of a cubic millimeter of silicon. The laser is an infrared diode similar to those in CD drives, so all three components can be mounted on silicon chips, making them easier to work with.

For applications such as the detection of improvised explosive devices or unexploded ordnance in minefields, the small size and low power consumption of the NIST sensors could make a big difference. The sensors could be grouped in arrays, making it possible to gain more data in a given amount of time. Commercially available laser-based magnetic detectors are the size of soda cans, require 20 watts of power, and cost $20,000 each, so grouping them in arrays is impracticable.

Remediation workers use these large sensors to detect unexploded land mines and other weapons in former battlefields, but it’s a “tedious procedure,” says Mark Prouty, president of Geometrics, a San Jose, CA, company that makes magnetic sensors. The heavy sensors must be carried back and forth across a field, then carried back to an office, where magnetic data is synthesized with GPS data to make maps. Then the workers must go back to the field with the maps to dig up the weapons.

With an array of smaller sensors, it would be possible to “gather data in a snapshot and dig [weapons] up in the field,” says Prouty.

The detection of improvised explosive devices is also a big problem for the military, says Prouty. It’s difficult to detect these bombs with individual magnetic sensors because “everything shows up, including the vehicle the sensor is mounted on,” he explains. Single sensors take point measurements; they can detect a metal-containing object like a bomb but can’t give any information about its location or shape. An array of magnetic sensors could “give an answer on the spot,” says Prouty.

Magnetic measurements are also used to study the brain and the heart. Nerve activity in the brain generates very weak magnetic fields–about 10 orders of magnitude smaller than the earth’s. Measuring this weak biomagnetism requires highly sensitive magnetic detectors called SQUIDs, which in turn require superconducting materials. The most sensitive SQUIDs must be cooled to within a few degrees of absolute zero with liquid helium; they cost about $2 million.

Kitching’s magnetometers are nearly as sensitive as SQUIDs and can operate at room temperature. He says that they are currently sensitive enough to measure magnetic fields from the heart but not from the brain. “Fetal heart monitoring is getting a lot of attention in the medical field” but is difficult because it’s not possible to place electrodes directly on a fetus in utero, says Kitching. “Electrical fields don’t get to the surface unaffected [by the mother’s tissues], but magnetic fields do,” he says.

David Cohen, who made some of the first measurements of biomagnetism in the 1960s, says that Kitching’s magnetometers “may get to the point where you can measure the heart,” but he is skeptical that they will be used to study brain activity. He doubts that a device using the NIST sensors to detect biomagnetism would end up being any cheaper than those that rely on SQUID.

Another potential use for the sensors is in future MRI scanners. “For noninvasive biological measures, this could be a really interesting thing,” says Yael Maguire, who, before founding ThingMagic, in Cambridge, MA, worked on miniaturizing nuclear magnetic resonance detectors, a technology similar to MRI. MRI currently requires its own room, specialized technicians, and a large, strong magnet. “The cost of access to the machines” is a problem with MRI, says Maguire. (See “Better Pictures of Proteins.”) Highly sensitive, cheap magnetometers like Kitching’s could be incorporated into future MRI scanners, enabling them to use smaller magnets, bringing their cost down, and potentially making them portable.

But such clinical applications are many years away. Right now, Kitching says that he’s studying the trade-off between the size and sensitivity of the magnetometers and is also designing chips to carry them.

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