Here is an excellent slide show from a presentation at North Carolina State University by Huffman:
From http://neutron.physics.ncsu.edu/SummerSchool/Huffman.pdf
[I have taken the "fair use" liberty of abstracting the essence of many of the slide texts here]:
'What are UCNs" ? [UCN = Ultra Cold Neutrons]
- Neutrons with energies less than or equal approximately to 200 neV (n = nano)
- They reflect from certain surfaces at all angles of incidence
- They can be stored using material, gravitational, and/or magnetic interactions
[next slide bottom lines to "They can be polarized 100 %" eg. via a magnetic field f]
[further slides skipped here]
"General Things to Keep in Mind"
- Strong Interaction: nuclear scattering V ~ 100 neV
- Magnetic Field Interaction V ~ 100 neV / T (T is teslas)
- Gravitation Field Interaction V ~ 100 neV / m
- 100 neV ~ 1 mK ~ 3-5 m / s
UCN density 1000 per cubic centimeter, AT THE SOURCE!
-Liouville's Theorem: The volume of a phase space occupied by a group of particles is constant if the particles move under the action of forces that are derivable from a potential
- Therefore the density at the source IS the limiting density!
[More slides skipped]
[Citation of an article by V. V. Nesvizhevsky in Physics of Atomic Nuclei, V. 65 (3) pp. 400-408 (2002) "Interaction of Neutrons with Nanoparticles" --- one strong conclusion of which is that such interactions may cool neutrons further depending on specific conditions.
Extensive discussion of moderation of hot neutrons, and the application of d' Liouville's Theorem to analysis of this process.
Various cold neutron sources discussed and/or diagrammed. Including a proton beam into a lead target.
Neutron cooling by phonon emission from a liquid helium II bath and many other techniques.... enough for technicians, engineers and non-specialists at least to "get some ideas", it appears]
"UCN Transport"
Can basically treat UCN as an ideal gas ... with following caveats:
- UCN collisions with the walls of a container are mostely elastic and specular. Inelastic scattering will result in "heating" and thus loss of neutrons.
- Densities are low enough such that neutron=neutron collisions don't occur. Randomization of the velocity direction comes purely from non-specular scattering.
- UCN densities decay during storage (neutron lifetime and losses from wall collisions, etc.)
- The motion of UCN is affected by gravity.
"UCN Detection"
Typical detectors use materials with large neutron absorption cross sections (ie. lithium, 3He, cadmium) with detection of the reaction products.
3He and BF3 detectors have the absorbing element as part of charge multiplication medium
Deposition of materials on surfaces of solid-state detectors
Detection of the neutron decay products
[several slides skipped, among which "gravity" is shown as a useful UCN acceleration field]
"UCN Storage"
Ultimate limit on storage times limited by the neutron's beta-day liftime: Tn = 886 s
Storage time depends heavily on the material, temperature, and state of the surface of the container
Dominant escape mechanisms include nuclear absorption and energy gain from upscattering
[Next slide]
Early Measurements UCN]
Plagued by large "anomalous" losses: typical initial bottle lifetimes were of the order 10 s !
Initially, the culprit was quite elusive: minimal changes observed for both the temperature dependence and material choice.
Problem turned out to be the preparation of the surface: H surface contamination was serious effect.
[Next slide]
Surface Improvements
Bottle lifetimes approaching the theoretical predictions have now been obtained by careful preparation of the surfaces.
Techniques for cleaning the surfaces include:
- discharge cleaning (esp. w/ deuterium)
- sputtering with fresh metallic surfaces
- coating with fluorinated oil while under vacuum
[slides skipped]
Storage discussed, with a "Neutron Storage Bottle -- made of permanent magnets diagrammed and discussed via a slide.
Other "bottles" discussed.
References given.