Glossary for the World Directory of Cold and Ultracold Neutron Sources


Cold Neutron Source (CNS)

a facility producing cold neutron beams from the thermal neutrons generated by a neutron source.

Cold Neutrons

free neutrons in the energy range below 20 meV

Cold Neutron Wave Length

from 0.2 nm (energy 20 meV) to more than 2 nm (0.2 meV)

Cold Neutron Velocity

from 2000 m/s (energy 20 meV) to less than 200 m/s (0.2 meV)

Cold Neutron Temperature

from 220 K (energy 20 meV) to less than 3 K (0.2 meV)

Cold Neutron Moderator

a liquid or solid which is cooled to below 100 K in order to shift the spectrum of incoming thermal neutrons to lower energies. A good moderator has a high scattering cross section combined with low absorption cross section (examples: liquid hydrogen, solid methane).

Cold Neutron Gain

a measure of the performance of the CNS, in fact the ratio of the neutron flux given by the operating CNS to the flux if the CNS were not there. This ratio is wavelength dependant. It can reach values well above 100 at very long wavelengths.

Heat Removal

a lot of heat is deposited by nuclear radiation in the cold neutron moderator, the latter being usually close to the radioactive neutron source. Powerful refrigeration at very low temperatures and efficient heat transport from the moderator to the refrigerator are needed, see thermal siphon.

Neutron Source

a neutron generator like the core of a nuclear fission reactor (mostly giving a steady state neutron output), or the target of a high energy proton accelerator (mostly for pulsed neutron beams), see Spallation Source.

Neutron Guide

an evacuated channel, up to 100 m long, containing and confining the neutron beam. The colder the neutrons the better they are transported in such a guide.

Spallation Source

a dedicated target station at a high power proton accelerator for neutron production by spallation of heavy nuclei like Zr, W, Pb, Hg, or U. Cold neutron beams are obtained by placing a cold moderator in the vicinity of the target.

Thermal Siphon

the heat developed in the cold moderator by nuclear radiation can be removed either by direct contact with the refrigerator gas (usually He) or by convection in the moderator fluid, the latter extending from the active moderator volume to a heat exchanger. The convection can be forced (circulator) or natural. The natural convection loop is called a thermal siphon.

Ultracold Neutrons (UCN)

neutrons with energies below about 100 neV. They totally reflect at certain walls, i.e. one can keep them in a bottle until they decay.

Ultracold Neutron Source

Although UCN are present in CNS (at a rather low density), the development of dedicated UCN Sources is underway, the cold moderator being e.g. solid deuterium below 10 K or superfluid helium below 1 K.