Generation of free Neutrons
To study solids with neutron beams (which are used here as matter waves) researchers need free neutrons with a defined energy, focused onto a sample of interest. The strength of the interaction between neutrons and matter is very weak. To collect any information one needs a huge amount of neutrons passing through the chosen sample. Behind the sample, one has to detect the scattered neutrons flying in various directions. Currently two types of neutron sources with sufficient neutron flux exist: research reactors and spallation neutron sources.
In a research reactor, like in any other nuclear reactor, the main nuclear reaction is the fissioning of uranium. Every nuclear fission reaction produces more than two free neutrons. An outgoing free neutron can induce the next nuclear fission reaction, which produces again new free neutrons, continuing the so-called chain reaction. A fission reaction takes only place with slow neutrons (energies of meV); therefore, the neutrons must be strongly decelerated by a factor of 100,000 from the initially high energies (in the range of MeV), by a moderator, which is mostly water.
The second type of neutron source, the spallation neutron source, uses a different type of nuclear reaction to generate free neutrons: Protons are accelerated in a linear accelerator to very high energies (approximately 1 to 2 GeV, GeV = 109 eV!) and are focused onto a "target" containing heavy atoms. In order to avoid material problems, because of the large amount of energy deposited in the target, the target material should be liquid. It can consist of mercury or a mixture of liquid bismuth and lead. A proton, which collides with an atomic nucleus, transports its kinetic energy into the nucleus, which "boils" and falls apart into smaller fragments and many free neutrons. This is called the spallation reaction. The outgoing neutrons must again be decelerated to lower energies to use them as matter waves of appropriate wavelength in experiments. This production method of free neutrons is not related to a chain reaction and from the security point of view therefore a much superior system, since the nuclear reaction is stopped just by turning off the accelerator.
A detailed description of the projected European Spallation neutron Source ESS may be found at www.ess-europe.de.
Texts: Th. Wilpert , Ch. Schulz
