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
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.
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
(firstname.lastname@example.org), 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