Physics Question #2919

Daniel, a 15 year old male from SIngapore asks on September 10, 2005,

Is it possible to focus gamma rays into one beam and then reflect it off an orbiting mirror onto a target?

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The answer

Donald J. Barry answered on January 2, 2006

No one has yet made an effective gamma ray reflector. It is much easier to "stop" gamma rays than to bounce them backwards. Since several methods of detecting gamma rays don't entirely stop them, but merely cause them to lose some energy, it's possible to chart the course of a gamma ray with multiple observations, thus giving (somewhat crudely) a direction from which it came.

It would be highly useful to actually be able to image gamma rays directly, as when using a lens or mirror with light. Since reflection seems to be out of the question, refraction (bending) remains. There are some technologies under development that could be applied to future imaging gamma ray satellites. The main problem is that gamma rays are hard to bend -- most techniques only manage to slightly alter their direction. As a result, focal lengths from a "gamma ray lens" to a detector must be huge with currently proposed techniques -- hundreds of meters to thousands of kilometers! This is too large for an individual spacecraft, but could be achieved by spacecraft flying in formation, one with a detector, one with a lens.

You can read about one effort at the web pages of the European Space Agency.

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