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[cdn-nucl-l] Terahertz radiation is gentle but piercing
Posted in US News in the July 28th, 2003 issue and at:
Note that terahertz waves lie between far infrared and millimeter waves, and
carry at least 5 orders of magnitude less energy than x or gamma rays. See:
Terahertz radiation is gentle but piercing
By Kenneth Terrell
Call it Karnak for the age of anthrax. In one envelope is a letter from
Grandma; in the other is a life-threatening dose of a biological weapon. How
can the post office tell the difference before you open your mail? The
answer could lie in a mostly untapped band of the electromagnetic spectrum,
the terahertz range.
Stuck in a no man's land between microwaves and visible light, forms of
radiation harnessed as easily as flicking a switch, terahertz waves for
decades have dodged the yoke. That's changing, as recent
breakthroughs--combined with a range of potential applications stretching
from diagnosing cancer to detecting dangerous flaws in space shuttle
components--are moving T-rays out of the world of academic curiosity toward
the mainstream. "It's matured from a field of engineers just trying to
outgun each other to one where they are trying to find the killer
application that will make this a viable technology," says Daniel van der
Weide, a terahertz researcher at the University of Wisconsin-Madison.
For decades, the region of the spectrum at about a trillion cycles per
second--a terahertz--frustrated scientists, who had trouble producing and
capturing these rays. Technologies that generate microwaves and radio
couldn't reach terahertz frequencies, while lasers work best at frequencies
far above the terahertz range. Detectors were cumbersome and slow. Yet
T-rays beckoned engineers with the promise of X-ray-like penetration without
the harmful side effects but with unique sensitivity to biological materials
and substances like plastics. "Just because it's no man's land doesn't mean
it's not a land of opportunity," says van der Weide.
Medical tool. Now the terahertz realm is opening up, thanks to sensitive,
miniaturized detectors and clever schemes for extracting T-rays from lasers
and other energy sources. Experiments at the Jefferson Lab in Newport News,
Va., for example, used controlled magnetic fields to coax high-speed
electrons into emitting terahertz beams. TeraView, a British company,
assembled semiconductor materials into a filter that converts laser pulses
into more sedate terahertz waves.
Such techniques are letting scientists and companies test out many of the
applications they have long envisioned. Chief among them is medical imaging.
While terahertz waves can penetrate many solids, such as clothes or walls,
they are quickly absorbed by water and similar fluids. That makes them adept
at picking healthy cells from cancerous ones, because blood flows
differently in the two tissue types. At Addenbrooke's Hospital in Cambridge,
England, TeraView is testing a prototype cancer diagnostic machine, which
bounces terahertz pulses off cells and distinguishes skin cancer from
healthy tissue based on the absorption rate.
T-ray technology could save lives in other settings, too. When anthrax-laced
letters began passing through the mail in late 2001, van der Weide tried
adapting the terahertz device he was already developing to see if it could
distinguish between living material--a potential biological agent--and plain
old talcum powder in an envelope. It worked, offering an almost
Van der Weide also says the technology could someday help airport personnel
determine whether passengers or luggage are carrying bombs. X-rays can see
metal components, but a terahertz scan could detect the explosives
themselves. And researchers at Rensselaer Polytechnic Institute found that
T-rays could find weak spots in the foam insulation on a space shuttle's
fuel tank. That might avert another tragedy like the destruction of the
Columbia, triggered when its tank shed a chunk of foam.
Terahertz rays could even make those trips to the dentist's office a little
less painful. Unlike X-rays, which can detect tooth decay only after it has
progressed so far that "drill and fill" is the sole option,
T-rays--sensitive even to the first stages of demineralization--can spot
erosion early enough that it can be remedied with less invasive methods such
as fluoride rinses. Could that be T-rays' killer app? Anthrax may never show
up in your mailbox, but the dentist's chair awaits you every six months.