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. |