Secondary sources for neutrons and positrons

Although neutrons are a unique tool, to elicit the secrets of matter different tasks require different "types" of neutrons.

Adapt wave lengths to tasks

Neutrons as such are indistinguishable. If they are to be adapted to a specific task, it is only possible to adjust their energy and wavelength or magnetic properties. In order to define the location of atoms in a metallic compound, wave lengths of the same dimensions are necessary, i.e. 0.1 nm. To measure the globular structure of a protein, neutrons on a nanometre scale are necessary. Silicon can be very well doped for the semiconductor industry using thermal rather than fast neutrons. In the same way, in everyday life the thickness of a human hair cannot be determined using a metre stick, but is quite possible with a micrometer.

Distinguish neutrons according to their energy

It is usual to distinguish neutrons according to their energy. Depending on their energy, they are referred to as highly energetic, fission, hot, thermal, cold or ultra-cold neutrons. Moreover, it is possible through the application of suitable magnetic fields to synchronize the magnetic properties of neutrons and thus to measure the magnetic fields in the interior of solid bodies. One then speaks of polarized neutrons.

The energy of the neutrons is adjusted via moderators of different temperature. Since, in physics, the energy variables, temperature (E = kB × T), wavelength (E = ħ × w) and speed (E = ½ mn × v2) are only different descriptions of the same phenomenon, the terms are often used interchangeably. A distinction is generally made between the following energy groups:

Equivalence of energy, temperature, wavelength and speed of neutrons. The limits are generally not well-defined.
Description Energy Temperature Wavelength Speed
High energetic neutrons >20 MeV
Fission neutrons 2 MeV
40 – 103 meV 2300 K 0,05 nm 5 km/s
Thermal neutrons 3 – 150  meV 300 K

0,2 nm

2,2 km/s
Cold neutrons 0,1 – 20 meV

25 K

0,2 - 25 nm 600 m/s
Ultra cold neutrons 10-6 – 0,01 meV mK 10 - 1000 nm 5 m/s

In order to provide neutrons with energies suitable for scientific use, the FRM II has at its disposal various so-called secondary sources. Their common feature is that they are largely located at the maximum of the thermal neutron flux, some 30 cm from the center of the moderator tank of the reactor.

Full details can be found eg in (pdf). Experimental facilities