Zap!!!

Here's a report on an unexpected source of x-rays -- Scotch tape!

Just two weeks after a Nobel Prize highlighted theoretical work on subatomic particles, physicists are announcing a startling discovery about a much more familiar form of matter: Scotch tape.

It turns out that if you peel the popular adhesive tape off its roll in a vacuum chamber, it emits X-rays. The researchers even made an X-ray image of one of their fingers.

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In the new work, a machine peeled ordinary Scotch tape off a roll in a vacuum chamber at about 1.2 inches per second.

Rapid pulses of X-rays, each about a billionth of a second long, emerged from very close to where the tape was coming off the roll.

That's where electrons jumped from the roll to the sticky underside of the tape that was being pulled away, a journey of about two-thousandths of an inch, Escobar said.

When those electrons struck the sticky side they slowed down, and that slowing made them emit X-rays.

So is this a health hazard for unsuspecting tape-peelers?

Escobar noted that no X-rays are produced in the presence of air. You need to work in a vacuum — not exactly an everyday situation.

"If you're going to peel tape in a vacuum, you should be extra careful," he said. But "I will continue to use Scotch tape during my daily life, and I think it's safe to do it in your office. No guarantees."

On reflection, it's not too unexpected.  Standard x-ray tubes work by slamming electrons into an anode.  This causes them to slow down very abruptly, and this deceleration converts some of their energy to x-rays.  (Any time you accelerate charged particles, you get electromagnetic radiation, and deceleration is only acceleration in the direction opposite the direction you're going.)

What's unexpected is that the amount of deceleration and the quantity of electrons would be enough to yield a significant amount of x-rays. X-ray tubes use anodes made of tungsten, to provide the maximum decelerating force. Tape, being plastic, is nowhere near as dense as tungsten, and so would not be expected to generate as much x radiation.

 

Why does it matter that the tape's in a vacuum?  Because in a broom, the bristles would get in the way!

 

More to the point, if the roll of tape isn't in a vacuum, it's surrounded by air molecules. I've peeled tape off a roll in the dark, and noticed a glow where the tape is peeling off.  It seems to me the air molecules are acting to slow down any electrons that actually are moving from the roll to the tape. As long as the speeds are kept low, the amount of deceleration possible is reduced, and you don't get x-rays.  You get visible light, and maybe some ultraviolet, but nothing more energetic than that.