Ultra Cold Neutrons

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.

An important "bottom line" to the Ultra Cold Neutron slideshow referenced and abstracted above, are its conclusions that UCN / ULM neutrons do not necessarily interact with elemental matter. It appears that UCN do interact rapidly with hydrogen when it is present, as it would be in many types of container walls. Under appropriate conditions UCNs have a mean lifetime to beta decay of 886 seconds (just under 15 minutes). Plenty of time for laboratory manipulation.

Thermalization of UCNs (adduced as a criticism of W-L) is an interesting phenomenon. Strong lattice structures may not easily contribute to UCN velocity, since the interactions will only be with the lattice nuclei, velocities of which are very low. Recall that specific heat is very much an electronic phenomenon-- and this relationship is in fact the cause and hence putative mechanistic source of "heavy electrons". Bound electrons have only very limited interactions with slow neutrons, to the best of my current knowledge.

I'm not here to defend Widom-Larsen or WLS theory, but I have long advocated examining that theory using simple experiments that would easily show the presence of ULM / UCN neutrons hypothesized by those folks--- experiments that should also strongly disprove that theory if such ultra cold neutrons are not found. I have discussed here before a simple way to separate and analyze ULM / UCN neutrons that might otherwise "immediately" interact near their site of origin. The method is low cost and straightforward.

@scuromio

Interesting. Do you recall what energy ranges were detectable in this way?

I highly recommend the slide show referenced to any who have an interest in ultra slow neutrons. If nothing else, it is a surprising story and perhaps diverges from that promoted in Widom-Larsen theory..... but perhaps not.

AlainCo:

Why were they blocked? Who is said to have blocked them? Do you, Alain, have any idea what the security or other issue might have been?

The technique I proposed does not require any use of Bonner spheres. For low cost implementation it would require that the neutron source in question be fairly compact and easily powered by batteries. My idea still requires a recognized target such as BF3, He3 etc.

Thanks for your interest and ongoing efforts, by the way.

@axil,

Yes, that seems clear. Only if the neutrons are associating with, or contained in, a solid matrix or otherwise "bottled" as in the Huffman presentation, would they likely escape rapid thermalization.

scuromio:

If it (Bonner spheres and associated equipment) are cheap and easy, then great. The level of inferences between the events and the resulting analyzed data should be first order and very direct to compete with my proposal.

Continuing axil;

With regard to your, [Axil's], concern above: "The energy of
the plasma would transfer to at least some of those neutrons to make
them detectable. To make this ultra low energy neutron idea work, both
the neutron and its target atom both need to be stationary."

There
is an easy alternative to this issue. I have mentioned it here before,
but most here and those at another "members only" forum either don't see
the point, don't understand it, dismiss it out of some prejudice. It
appears that I may have to take up the task of demonstrating it myself
as a slow-moving 70-something "elder". I am increasingly willing to do
this in view of the ongoing lack of outside interest, and in view of the
importance of the issue. Scuromio's familiarity with Bonner spheres is a
refreshing nuance here, but currently I see them as more complex and
less fundamentally connected to the underlying phenomena than my
approach is likely to be.