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Comment by phkahler | Hacker News Reader

How does one contain neutrons in a "bottle"?

Neutrons with low enough kinetic energy (< 300 neV) are reflected by some materials due to the so-called Fermi pseudo potential. These ultracold neutrons can therefore be stored in an actual bottle made from an appropriate material (such as Nickel or Aluminum). Another approach is to use magnetic fields to create a potential well (neutrons have a magnetic moment) and to rely on gravity to keep them in that well. Ultracold neutrons behave very much like ping-pong balls and cannot jump very high (less than a few meters) due to their low velocities.

are reflected by some materials due to the so-called Fermi pseudo potential

Cool! I learned something new today!

Ultracold neutrons behave very much like ping-pong balls and cannot jump very high

Most of the time. My understanding is that they can tunnel potential barriers, but this isn't that likely to happen in these traps, so it shouldn't affect the results.

Anything can tunnel through a potential barrier, the question is how big the barrier is, and what probabilities you expect from this event. When the barrier is big enough, it basically doesn't happen.

Correct. https://en.wikipedia.org/wiki/Quantum_tunnelling

Magnetic fields and lots of cooling.

https://www.nature.com/articles/47444

Ah, but how do you capture a neutral particle with a magnetic field?

(As I understand, they're a little bit magnetic because their constituent quarks are charged.)

> As I understand, they're a little bit magnetic because their constituent quarks are charged.

More than a little bit. The neutron's magnetic moment is about 2/3 the magnitude of the proton's, and of opposite sign.

Hmmm, if neutron's are of the opposite sign, is this what keeps the electrons repelled enough stay on the "outside" in orbit?

No. The gross structure of an atom is only determined by the electrostatic interaction between the charge of the electrons and of the protons in the nucleus. The interaction between the magnetic moments of the electrons and of the nucleus (which is due to both protons and neutrons) leads to hyperfine structure on a much smaller energy scale.

> if neutron's are of the opposite sign, is this what keeps the electrons repelled enough stay on the "outside" in orbit?

No, but it does affect which spin states of nuclei have the lowest energy. For example, the ground state (lowest energy state) of a deuterium nucleus has spin 1--spin of the proton and neutron are parallel--because the opposite signs of the proton and neutron magnetic moments make that state have a lower energy (more tightly bound).

How does it even have sign? I thought it was a vector.

The sign refers to the direction that the vector points relative to the particle's spin vector. The proton's magnetic moment is positive, meaning it points in the same direction as the proton's spin. The neutron's magnetic moment is negative, meaning it points in the opposite direction to the neutron's spin.

The magnet inside your compass isn't charged either. Magnetic fields act on magnetic moments, not on charge (unless the charge is moving which gives you a Lorentz force).

> (unless the charge is moving which gives you a Lorentz force)

Isn't this always the case, though? I mean, two objects not moving relative to each other is a very specific state and not something that can be held in practice without a negative feedback loop on their relative velocities.

> unless the charge is moving which gives you a Lorentz force

Also spin

Exactly, everything's magnetic if you have a large enough magnet. I'm more interested in how they limit such a field =P

LANL has a lot of high-level description. You can use neutron-reflective material, or magnetic fields.

https://lansce.lanl.gov/facilities/ultracold-neutrons/experi...

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