Sensor Could Detect Concealed Weapons Without X-rays

Unlike X-ray machines or radarinstruments, the sensor doesn't have to generate a signal to detectobjects – it spots them based on how brightly they reflect the naturalradiation that is all around us every day.

There is always acertain amount of radiation – light, heat, and even microwaves – in theenvironment. Every object – the human body, a gun or knife, or anasphalt runway – reflects this ambient radiation differently.

PaulBerger, professor of electrical and computer engineering and physics atOhio State and head of the team that is developing the sensor, likenedthis reflection to the way glossy and satin-finish paints reflect lightdifferently to the eye.

Once the sensor is further developed, itcould be used to scan people or luggage without subjecting them toX-rays or other radiation. And if the sensor were embedded in anairplane nose, it might help pilots see a runway during bad weather.

TheOhio State sensor isn't the only ambient radiation sensor underdevelopment, but it is the only one Berger knows of that is compatiblewith silicon – a feature that makes it relatively inexpensive and easyto work with.

Berger describes the sensor in the current issue ofthe journal IEEE Electron Device Letters. His coauthors include NiuJin, who performed this work for his doctorate at Ohio State and is nowat the University of Illinois at Urbana-Champaign; Ronghua Yu andSung-Yong Chung, both graduate students at Ohio State; Phillip E.Thompson of the Naval Research Laboratory; and Patrick Fay of theUniversity of Notre Dame.

Berger said that the new sensor grewout of his team's recent invention of a device called a tunnel diodethat transmits large amounts of electricity through silicon.

Hewas reading about another team's ambient radiation sensor when herealized that their device worked like one of his diodes -- only inreverse.

“It's basically just a really bad tunnel diode,” heexplained. “I thought, heck, we can make a bad diode! We made lots ofthem back when we were figuring out how to make good ones.”

As it turns out, a really bad tunnel diode can be a really good sensor.

Diodesare one-way conductors that typically power amplifiers for devices suchas stereo speakers. Berger's diode is unique because it is compatiblewith mainstream silicon, so computer chip makers could manufacture itcheaply and integrate it with existing technology easily.

The newsensor is essentially one of these tunnel diodes with a strong shortcircuit running backwards and very little tunneling current runningforwards.

Thompson prepared the films of layered semiconductor material, and the Ohio State team fabricated and tested the sensors.

Theway engineers measure the effectiveness of such sensors is to draw aline graph charting the amount of current passing through them. Thenthey measure the curvature of the line at the point where the currentis zero. A steep curve indicates that a sensor is working well, so thehigher this so-called “curvature coefficient” is, the better.

Inthe laboratory, prototypes of the Ohio State sensor averaged acurvature coefficient of 31. While one other research team has produceda sensor with a coefficient of 39, that sensor is made of antimony – anexotic metal that is hard to work with and not directly compatible withthe silicon circuit that surrounds the sensor element, Berger pointedout.

“So our raw sensor performance isn't quite as good, but ourultimate performance should be superior because you could integrate ourdevice directly with any conventional microchip readout circuitry thatyou wanted to build,” he said.

The team that is making theantimonide sensor has succeeded in combining it with a camera system;the pictures look a lot like X-ray images, with bodies and clothingappearing as dim outlines and metal objects such as guns standing outin sharp relief.

That camera system has performance issues thatBerger thinks could be solved with his silicon-compatible design.Still, the image has inspired him to think big about where his workcould go in the future. Combat pilots, for instance, could potentiallyuse this technology to stealthily identify other aircraft as friend orfoe.

“If you got a fast enough response and a high-resolutionimage, I wonder if you might be able tell one kind of aircraft fromanother without revealing your location to the enemy,” he said.

The National Science Foundation and the Office of Naval Research funded this work.