Tuesday, May 8, 2012

Gamma-Ray Bending Opens New Door for Optics

Gamma-Ray Bending Opens New Door for Optics: Theory says that gamma rays, being even more energetic than x-rays, ought to bypass orbiting electrons altogether; materials should not bend them at all and the refractive index for gamma rays should be almost equal to one...

ILL is a research reactor that produces intense beams of neutrons. Habs, Jentschel, and colleagues used one of its beams to bombard samples of radioactive chlorine and gadolinium to produce gamma rays. They directed these down a 20-meter-long tube to a device known as a crystal spectrometer, which funneled the gamma rays into a specific direction. They then passed half of the gamma rays through a silicon prism and into another spectrometer to measure their final direction, while they directed the other half straight to the spectrometer unimpeded. To the researchers' surprise, as they report in a paper due to be published this month in Physical Review Letters, gamma rays with an energy above 700 kiloelectronvolts are slightly bent by the silicon prism...

So what drives this new bending effect? Although he can't be sure, Habs believes it resides in the nuclei at the heart of the silicon atoms. Although electrons don't normally reside in nuclei because of the very strong electric fields there, quantum mechanics allows pairs of "virtual" electrons and antielectrons, or positrons, to blink briefly into existence and then recombine and disappear again. Habs thinks the sheer number of these virtual electron-positron pairs amplifies the gamma-ray scattering, which is normally negligible, to a detectable amount.

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