Electrons tell a
tale of the tape
Adhesive produces X-rays by
mimicking particle accelerator
By Marissa Cevallos
Forget fancy particle accelerators—a
cheaper tool for emitting X-rays is right
there in the supply cabinet. Peeling back
clear sticky tape emits X-rays, the same
high-energy light emanating from airport
scanners or galaxy clusters, and scientists
now have a better understanding why.
Physicists were dumbfounded two
years ago when UCLA researchers produced quick flashes of X-rays by pulling
back Scotch tape in a vacuum (SN Online:
10/22/08). The X-rays pulse at the rate of
a billion times a second, radiating from a
region only about 100 micrometers wide,
says UCLA physicist Seth Putterman.
Spotting the X-rays was perplexing
because they are 100,000 times more
energetic than the chemical bonds holding tape’s sticky side down.
Now, two teams of scientists have an
explanation: Peeling tape separates positive and negative charges, creating an
electric field. The field jump-starts free
electrons in the neighborhood, accelerating them enough to emit X-ray photons
known as bremsstrahlung radiation. It’s
just like radiation created in accelerators
as they whip charged particles around.
No need to worry about radiation
exposure at the office, though. At atmospheric pressure, the electrons run into
other particles before having a chance to
accelerate and radiate X-rays.
To track the X-rays, Josip Horvat
of the Institute for Superconducting
and Electronic Materials near Wollongong, Australia, and colleagues rigged
Year that clear
Scotch tape was
up Scotch tape on a spool driven by a
motor. The X-rays mostly sprayed at a
right angle to the direction the tape was
pulled, the researchers report online
September 29 in Applied Physics Letters.
That’s convenient, because herding light
into a straight line normally absorbs
the light’s energy, but the tape naturally
emits X-rays in a straight line to within
“Tape is an even better use as an X-ray
source than we thought,” says Putterman,
who in May in Applied Physics B: Lasers
and Optics identified bremsstrahlung
radiation as the X-ray source. He imagines that medical workers in the field
could use a hand crank to peel off adhesives and X-ray a finger.
But it’s still a mystery, Putterman says,
how tape separates enough charge to
create a strong electric field, just as physicists don’t know how charge separates
in clouds to create lightning bolts.
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