High-Flux, Short-Burst Gamma Ray Source

Bursts of gamma rays, the most energetic form of light, can be made by scattering laser light from an electron beam. Present examples -- including those at the SPring 8 machine in Japan and at Brookhaven National Lab in the United States -- deliver relatively long gamma pulses (greater than 100 picoseconds in duration) with relatively low brightness (yielding about a million gamma per second). A new proposal shows how a gamma source with pulses as short as 100 femtoseconds and fluxes as high as a billion per second could be built.

One of the most important things you can do with a bright stream of gammas is to pass them through a thin target where the gammas can generate electron-positron pairs. From this process, the positrons can be skimmed and, owing to their ability to probe certain processes within materials that cannot be probed by X-rays, be used to study such things as defects in bulk metals.

Basically, the positrons render valuable clues about a material sample (structural and magnetic) by taking up positions throughout the sample, where the positrons meet and annihilate with electrons, creating telltale radiation.

One of the researchers, Yuelin Lin (ylli@aps.anl.gov), says that because the positron bursts are so short -- as short as a trillionth of second -- they can be used to make slow-motion movies of ephemeral and hard-to-watch activities like metals melting at high temperatures.

Li et al., Applied Physics Letters, 10 July 2006
Contact Yuelin Lin
Argonne National Laboratory
ylli@aps.anl.gov