In a particle accelerator, electrons race around a ring at nearly the speed of light. When such an electron collides with a photon, a particle of light, the photon bounces off like a ball hitting a moving truck and flies away with much greater energy. This phenomenon is called Compton scattering and has long been used to produce gamma radiation — the most energetic form of light.
Usually, a laser beam is directed into the accelerator to collide with the electrons. But laser photons carry relatively little energy, so the resulting gamma rays are modest. In the new experiment, physicists took a different approach. As electrons turn in the accelerator ring, they emit ultraviolet radiation, like headlights on a curve. This radiation was captured and sent to a multilayer mirror, which reflected it back into the ring, precisely into the electron beam. The ultraviolet photons carry 92 electron-volts of energy — hundreds of times more than laser photons. The collision boosted them to an energy of 0.543 gigaelectron-volts.
The experiment was conducted at Japan's NewSUBARU ring with an electron energy of 1 gigaelectron-volt. The gamma-ray count rate was 1400 per second. The theoretical calculation matched the measured spectrum exactly — meaning the method works as intended. Now such a gamma-ray source can be used to study photoproduction of hadrons — processes where a gamma quantum gives birth to particles that make up atomic nuclei.
The ultraviolet mirror is made of alternating layers of molybdenum and silicon: each layer only a few atoms thick, together acting as a single reflective surface.
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