4. Neutron Scattering

Neutron Research Facilities – Where Do the Neutrons Come From?

Before diving into how neutrons are used to learn about different characteristics of matter, it should be discussed where the neutrons used for neutron scattering actually come from. There are currently around 30 active neutron research facilities scattered around the globe, each one containing the means to generate thermal neutrons for experimental research. These neutrons are usually produced by either research reactors specifically engineered for releasing neutrons or by a process known as spallation, both of which will now be described.

In essence, research reactors are just like most other nuclear reactors and depend highly on the process of fission. In these fission processes, a nucleus, usually U-235 or Pu- 239 , captures a neutron and then splits into fragments including neutrons with kinetic energy, beta radiation, and photons. Each fission reaction produces on average 2.5 neutrons, one of which ends up being used to incite further fission, a half of which is produced delayed and is essential for reactor control, and the last one actually collected for experimental research.

As an alternative to the use of research reactors, some neutron research facilities that have a high-powered particle accelerator can produce neutrons in a process called nuclear spallation. Here, accelerated high-energy protons are directed toward a dense and high-mass- number target such as tungsten, uranium, tantalum, or mercury whereby the collision leaves the nuclei of the target in a highly excited state. The array of products that emerge from this interaction includes various high-energy neutrons and protons that proceed to collide with other yet-to-be excited target nuclei along with lower energy evaporation neutrons that end up being collected for scattering experiments. In terms of yield, for each proton-target nuclei collision, an average of 20 neutrons are produced with the majority of those being evaporation ones.

For neutron production in both the cases of research reactors and spallation, the immediate low-energy neutrons outputted still possess too much energy for practical experimental usage. Thus, a moderation stage is necessary in order to slow them down to become so-called thermal neutrons. Typically this involves surrounding the neutron source with a large volume of an apt moderator such as heavy water, water, or beryllium where fast neutrons will enter and gradually lose their energy in a series of collisions with the nuclei of the moderators.

It should be mentioned that at many neutron research facilities today, there are also cold neutron sources in which the moderator used is liquid hydrogen, which allows neutrons to cool to cryogenic temperatures for sub-room temperature experiments. After tens of collisions per neutron, the resulting thermal neutrons leak out of the moderator through a designated beam tube and are ready to be utilized for scattering experiments.

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