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Tuesday, November 6, 2007

Tiny sensor measures tiny magnetic fields

A magnetic sensor smaller than a grain of rice and sensitive enough to detect the brainwave of someone daydreaming has been developed by US scientists who say their research offers the potential for a host of new medical and security uses.
They says the sensor, described online in the journal Nature Photonics, provides a low-cost and portable way to detect changes in a magnetic field.
"What we've done is demonstrate a very good sensitivity with a very small cell," says Dr John Kitching of the US National Institute of Standards and Technology (NIST), who led the project.
The researchers say it's 1000 times more sensitive than NIST's last microchip-sized mini-sensor and can detect magnetic fields down to 70 femtoteslas.
The latest sensor is just a prototype, but Kitching says the device could be used in a range of applications, from foetal heart monitoring to screening for explosives.
Because of its small size, he says it could run for several weeks on a single AA battery.
"Magnetic fields are all over the place," Kitching says. "Anything that has iron in it has a magnetic field."
Electric currents, such as those from power lines, emit a magnetic field, as do the electrical impulses that make the heart contract or brain cells fire.
Larger magnetometers are used by geologists to find iron deposits and by archaeologists to find buried objects. They can also be used by satellites to track the earth's magnetic field.
The most sensitive magnetic sensors - superconducting quantum interference devices or SQUIDS - can detect very weak changes in magnetic fields but must be kept very cold, making them much bigger and far more power-hungry.
While the NIST device sacrifices a bit of that sensitivity, it makes up for it in portability.
"We are able to make something that is almost as sensitive without all the stuff that is needed to make it run," Kitching says.
The prototype consists of a tiny container holding about 100 billion atoms of rubidium.
The researchers shoot an infrared laser beam through the container and measure how much light the atoms absorb. The higher the absorption, the stronger the magnetic field.
Kitching says the device is highly adaptable. It could be used to measure electrical activity in the brain, helping to spot tumours or monitor brain function. Or it could be used in war zones to detect bombs that may not have exploded.
"It really does open up a wide range of possibilities," he says.

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