Neutron scattering facilities

Figure 1: The two main methods of neutron production. 1): Traditional nuclear reactors make use of production of neutrons for maintaining the chain reaction; surplus of neutrons can be used for neutron scattering. 2): Protons accelerated into the GeV regime can split heavy nuclei with a large neutron surplus, creating free neutrons among the reaction products.

Neutron sources with flux densities adequate for neutron scattering investigations of materials are based on one of two principles, also illustrated in Figure xx--CrossReference--fig:production--xx:

• Fission: A high continuous flux of neutrons is produced in the core of a conventional fission reactor.
• Spallation: A pulsed production of neutrons is obtained by bombarding a target of heavy elements with high-energy particles, typically accelerated protons.

Table 1: Common naming of neutron energy ranges, and typical origin of neutrons with these energies. The "standard" thermal energy is 25 meV, corresponding to \(\lambda_{\rm th} = 1.798\) Å, or \(v_{\rm th} = 2200\) m/s.

Common to both types of sources is that neutrons are moderated to "thermal" or "cold" velocities close to the source and then transported to the neutron scattering instruments in neutron guide systems. For the conventional naming of neutron energy intervals, see Table xx--CrossReference--tab:neutron_naming--xx.

Both fission and spallation neutron sources are built as dedicated facilities, each hosting tens of instruments. All major sources are user facilities, meaning that they serve a research community much larger than the staff affiliated with the facilities. Typically, user experiments are selected through a competitive proposal system.

At the time of writing, more than twenty neutron facilities are in operation worldwide. The most important being the reactor source ILL, Grenoble, France, and the spallation source ISIS, Oxfordshire, UK. However, the traditional European dominance in this field was in the late 00's challenged by the new and powerful spallation sources: Spallation Neutron Source SNS, Oak Ridge, USA, and Japan Proton Accelerator Research Complex J-PARC, Tokai, Japan. For this reason, it was in 2009 decided to build the European Spallation Source (ESS), in Lund, Sweden. After some years of preparation work, the actual construction was initiated in the summer of 2014. The source is expected to be operational in 2022, with first user experiments ultimo 2013, and to reach full power with 16 running instruments in 2028.

A list of the most significant neutron sources worldwide is given at the Neutron sources and moderators page.