laser to make powerful bursts of X-rays or gamma rays

‘Lasetron’ could probe the nucleus

8 February 2002

Pulses of radiation short enough to probe the nucleus could be
generated with existing technology, according to physicists in
the US. Alexander Kaplan and Peter Shkolnikov have calculated
that a high-power laser could make electrons emit powerful
bursts of X-rays or gamma rays just zeptoseconds – 10-21
seconds – long. This ‘lasetron’ could also produce magnetic
fields as intense as those found near white dwarf stars (A
Kaplan and P Shkolnikov 2002 Phys. Rev. Lett. 88 074801).

The lasetron proposed by Kaplan, of Johns Hopkins University in
the US, and Shkolnikov, of the State University of New York at
Stony Brook, is based on the principle of synchrotron radiation.
Electrons accelerating in a magnetic field continuously emit
radiation, and if the electrons are moving in a circle, a stationary
observer will see a burst of radiation each time the electrons
complete a circuit.

Electronic effects in atoms, such as ionization, typically occur in
less than a femtosecond – 10-15 seconds – and these have
already been probed by pulses of radiation lasting attoseconds,
or 10-18 seconds. But nuclear processes take place about a
thousand times faster and existing synchrotrons cannot generate
bursts of radiation short enough to study them.

Kaplan and Shkolnikov believe that this problem could be solved
by using a super-powerful laser to make electrons circulate
extremely quickly. It is well known that electrons circulate in the
electromagnetic field of a circularly polarized laser. But according
to the pair, existing petawatt lasers – with outputs of 1015 watts
– could force electrons to trace out a circle just 0.1 micrometres in
radius. This would allow physicists to obtain extremely short
pulses of synchrotron radiation.

‘During fission, a nucleus goes through a short-lived transition
state when it stretches before breaking into smaller pieces’,
Kaplan told PhysicsWeb. ‘This is the kind of process we could
observe by scattering zeptosecond pulses off the nucleus’.

The lasetron technique would also generate magnetic fields of up
to 106 tesla – a hundred billion times stronger than the magnetic
field of Earth. This would allow astrophysicists to study fields as
strong as those that exist in space. According to Kaplan, these
magnetic fields are within reach. ‘The lasers are available now,
although it might take a couple of years to develop the
experimental set-up and devices to measure such fields’, he says.

But Kaplan admits that it is likely to be a decade before
zeptosecond pulses are achieved. He points out that their
calculations are just the first step, and that – currently – nobody
even knows how to measure such short bursts.

Katie Pennicott is Editor of PhysicsWeb

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