Shape of things to comeNIST shows tiny sensor with biomedical, homeland security applications

Published 9 November 2007

Tiny sensor can detect magnetic field changes as small as 70 femtoteslas — equivalent to the brain waves of a person daydreaming; in addition to medical uses, sensor may be deployed in airport screening for explosives based on detection of nuclear quadrupole resonance in nitrogen compounds

Magnetic field changes are measured with femtoteslas. A femtotesla is one quadrillionth (or a millionth of a billionth) of a tesla, the unit which defines the strength of a magnetic field. For comparison, the Earth’s magnetic field is measured in microteslas, and a magnetic resonance imaging (MRI) system operates at several teslas. The brain waves of a person daydreaming clock at 70 femtoteslas. Now, researchers at the National Institute of Standards and Technology (NIST) have demonstrated a tiny sensor that can detect magnetic field changes as small as 70 femtoteslas. The sensor could be battery-operated and could reduce the costs of noninvasive biomagnetic measurements such as fetal heart monitoring. The device also may have applications such as homeland security screening for explosives.* The prototype device is described in an article in the November issue of Nature Photonics. It is almost 1,000 times more sensitive than NIST’s original chip-scale magnetometer demonstrated in 2004 and is based on a different operating principle. Its performance puts it within reach of matching the current gold standard for magnetic sensors, so-called superconducting quantum interference devices (SQUIDs). These devices can sense changes in the 3- to 40-femtotesla range but must be cooled to very low (cryogenic) temperatures, making them much larger, power hungry, and more expensive. The NIST prototype consists of a single low-power (milliwatt) infrared laser and a rice-grain-sized container with dimensions of 3 by 2 by 1 millimeters. The container holds about 100 billion rubidium atoms in gas form. As the laser beam passes through the atomic vapor, scientists measure the transmitted optical power while varying the strength of a magnetic field applied perpendicular to the beam. The amount of laser light absorbed by the atoms varies predictably with the magnetic field, providing a reference scale for measuring the field. The stronger the magnetic field, the more light is absorbed. “The small size and high performance of this sensor will open doors to applications that we could previously only dream about,” project leader John Kitching says.

The new NIST mini-sensor could reduce the equipment size and costs associated with some non-invasive biomedical tests. To make a complete portable magnetometer, the laser and vapor cell would need to be packaged with miniature optics and a light detector. The vapor cell can be fabricated and assembled on semiconductor wafers using existing techniques for making microelectronics and microelectromechanical systems (MEMS). This design, adapted from a previously developed NIST chip-scale atomic clock, offers the potential for low-cost mass production. Magnetometers need to be designed with applications in mind; smaller vapor cells require less power but are also less sensitive. Thus, an application for which low power is critical would benefit from a very small magnetometer, whereas a larger magnetometer would be more suitable for a different application requiring high sensitivity. The NIST work evaluates the tradeoffs between size, power, and performance in a quantifiable way. “This result suggests that millimeter-scale, low-power, inexpensive, femtotesla magnetometers are feasible … Such an instrument would greatly expand the range of applications in which atomic magnetometers could be used,” the Nature Photonics article states.

The NIST device could be used in a heart monitoring technique known as magnetocardiography (MCG), which is sensitive enough to measure fields of few picoteslas emitted by the fetal heart from small currents in heart muscle cells, providing complementary and perhaps better information than an electrocardiogram. With further improvements, the NIST sensor also might be used in magnetoencephalography (MEG), which measures the magnetic fields produced by electrical activity in the brain, helping to pinpoint tumors or determine function of various parts of the brain. Kitching said the NIST sensor also may have applications in MRI or in airport screening for explosives based on detection of nuclear quadrupole resonance in nitrogen compounds.

-read more in Vishal Shah et al., “Subpicotesla Atomic Magnetometry with a Microfabricated Vapour Cell,” Nature Photonics 1 (1 November 2007): 649-52 (doi:10.1038/nphoton.2007.201) (sub. req.)

* on the use of highly sensitive magnetometers in the detection of unexploded ordinance, see Bill Delaney and Delores Etter, “Report of the Defense Science Board Task Force on Unexploded Ordinance”