Spallation is the process for producing neutrons by means of a particle accelerator and a heavy metal target. The ESS's 600-meter long linear accelerator will fire protons derived from hydrogen gas at a velocity just below the speed of light at a target made out of the metal tungsten.
The metal target absorbs the proton beam and transforms into fast neutrons. Which is basically just a really polite way of saying that the proton beam rips the target a new one which causes it to spill its guts all over the place, showering its environment with fast neutrons. To contain the extreme level of highly penetrating gamma and fast neutron radiation the target chamber is surrounded by a radiation shielding system, a 7000 ton sphere of steel. If that kind of talk doesn't get your heart racing I don't know what will! ;)
When the neutrons are slowed down they are, guided by beam lines, lead towards experimental stations where they allow us to see through matter on the smallest of scales. Because neutrons have no charge, they don't scatter on electrons and can penetrate deep into atoms and probe atomic nuclei directly, which is not possible with X-rays.
Two factors make neutrons especially interesting. With X-rays you only "see" the heavy elements, but with neutrons, which interact with light elements such as hydrogen and carbon, you can probe a wider range of materials, with applications in molecular biology, biomedical research, and even food science.
The second factor is that neutrons carry a magnetic moment. They interact with the magnetic moments of atoms and thus can assist researchers investigating materials like superconductors.
A big thanks to the more than a dozen European countries that are funding the project, especially Sweden and Denmark, the two biggest backers. If all goes well first light should be produced in 2019.
http://europeanspallationsource.se/science-using-neutrons
http://en.wikipedia.org/wiki/European_Spallation_Source
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