Atomic Armor for accelerator technology: 2D Shield for photocathodes

X-rays aren’t just for investigating broken
bones in people. Materials scientists use x-rays and other particles to look at the
structure of solid materials. This is because the wavelength of x-rays is
roughly the size of an atom, making it an excellent tool to measure the most detailed
information on the atomic structure of the solid. Although, when we use x-rays for medical diagnosis,
we want to use the lowest possible number of x-rays to minimise any side-effects. However, for solids, getting the best quality
images and most information means that we need to make the x-rays as bright as possible
and this involves using big machines known as accelerators. There are a handful of x-ray accelerator facilities
worldwide, including a future facility at Los Alamos National Laboratory. These facilities use electron accelerator
technologies to produce the large number of x-rays needed to high resolution movies of
atomic structure and dynamics. At the heart of an accelerator is the source
that produces the particles. One type of source is known as a photocathode,
which is a material that emits electrons when it absorbs photons. These electrons are then accelerated to near
speed of light, often over distances of kilometres, using a series of accelerating cavities or
structures. Once the electrons have reached sufficiently
high energy, they are used to generate X-rays using one or more methods depending upon application. Generating the electrons to start this process
is very demanding on the photocathode, which starts to break down under such extreme conditions
by reacting with gases or through heat damage. The more electrons that are produced, the
more x-rays that can be made but this usually causes even faster degradation of the photocathode. Drs Hisato Yamaguchi and Nathan Moody, at
Los Alamos National Laboratory, have a solution to this, that will likely allow the design
of brighter, better accelerators. They have made very thin sheets of graphene
that can be wrapped around the cathode. Because they are atomically thin, electrons can pass
through, but the photocathode is shielded from damage by any residual gases. This is similar to covering a metal surface
with a protective layer of another material to prevent rusting. The team’s findings mean that the photocathodes
can run at much higher efficiencies, without reducing their lifetimes and we can make accelerators
that produce more x-rays. Dr Yamaguchi and his team are excited about
the possibilities these protective graphene shields bring. By finding a way to grow these cathodes onto
the graphene substrates, this work shows significant new potential for an array of processes to
improve photocathode lifetimes, something that has been a challenge for the scientific
community, and will improve x-ray sources for use in materials characterisation, medical
applications and fundamental physics.

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