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Free neutrons are not stable. But inside a magnetic botlle they become stable. Why?

Japha Nakhão
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    They don't. Where did you get the idea that they do? – John Doty Jul 08 '22 at 12:25
  • Neutrons are stored in magnetic bottles, so that to use later to measure they lifetime: https://www.quantamagazine.org/neutron-lifetime-puzzle-deepens-but-no-dark-matter-seen-20180213/ – Japha Nakhão Jul 08 '22 at 12:32
  • Not "later". The neutrons are decaying all the time. The time that a neutron survives in the bottle before decaying is an estimate of the neutron lifetime. Averaging the results from many neutrons yields a precise estimate. – John Doty Jul 08 '22 at 12:43
  • Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. – Community Jul 08 '22 at 12:47
  • Then there is no need the bottle must be magnetic. Why does a magnetic bottle is used? – Japha Nakhão Jul 08 '22 at 12:52
  • John Doty, what procedure is used for the production of the free neutrons, stored in the magnetic bottles? – Japha Nakhão Jul 08 '22 at 13:13
  • Since neutrons react with all materials, a magnetic bottle is the only kind of bottle that can hold them. – John Doty Jul 08 '22 at 13:39
  • The free neutrons come from nuclear reactions, either fission in a reactor or fusion from an ion beam hitting a target. Such sources make many neutrons. I don't know the details of the rest of this experiment, but I believe some variant of the following procedure must have been used. First, slow the neutrons down to thermal energies (on average) using a moderator like polyethylene. A few of the resulting neutrons will have much less than the average energy. Those may be trapped in the magnetic bottle. – John Doty Jul 08 '22 at 13:50
  • @JohnDoty Ultra-cold neutrons can be trapped in material bottles. The criterion is that repulsive “optical potential” of the bottle’s walls is larger than the neutron’s kinetic energy. For energies below 100 nano-eV, lots of materials become available. – rob Jul 08 '22 at 14:35
  • @rob Aren't neutron interactions with the walls a problem? Total internal reflection involves evanescent waves, which enter the reflecting material, and may be absorbed. Reflective neutron optics are established technology, but can this be used for storage at a loss rate sufficiently low for a lifetime experiment? – John Doty Jul 08 '22 at 17:29
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    @JohnDoty Yes, wall losses are a problem. One solution is to coat the walls using a material with low capture cross section, like $\rm D_2O$ ice or a vacuum product called "Fomblin oil." The magnetic traps were invented to reduce wall losses. A recent review is: Wietfeldt, F.E.. (2018). Measurements of the Neutron Lifetime. doi: 10.3390/atoms6040070. – rob Jul 08 '22 at 17:46

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The first line of the page you linked contradicts your statement:

When physicists strip neutrons [...], put them in a bottle, [...] neutrons radioactively decay in 14 minutes and 39 seconds, on average.

They are not stored for their lifetime to be measured later. Rather, their lifetime is measured in the bottle.

Jeffrey
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  • Look how ultra cold neutrons are stored: "The production, transportation and storage of UCN is currently motivated by their usefulness as a tool to determine properties of the neutron and to study fundamental physical interactions. Storage experiments have improved the accuracy or the upper limit of some neutron related physical values" https://en.wikipedia.org/wiki/Ultracold_neutrons Therefore, the ultra cold neutrons are stored and later transported, which means that they are free neutron stored in the magnetic bottles, and therefore they are stable during the time they are in the bottle. – Japha Nakhão Jul 08 '22 at 13:32
  • Therefore, as free neutrons inside magnetic bottles are transported to later be used in experiments, then obviously free neutrons are stable inside magnetic bottles. And the question is: WHY IS A NEUTRON STABLE INSIDE A MAGNETIC BOTLLE ???????? – Japha Nakhão Jul 08 '22 at 13:44
  • Obviously, the starting point for measuring the lifetime of the neutron stored in a magnetic bottle begins when the conditions inside the bottle are changed, so that to allow to begin the process of neutron's decay, inside the bottle, where the conditions were changed. – Japha Nakhão Jul 08 '22 at 13:54
  • Those experiments are done in short timeframes and they account for ongoing loss of neutrons in their bottles. I'm afraid the two "obviously" stable comments don't hold. – Jeffrey Jul 08 '22 at 14:11
  • AS CONCLUSION: It seems there is not any explanation (by considering the current model of neutron composed by three quarks, ddu) from which can be answered the following question: "WHY IS A NEUTRON STABLE INSIDE A MAGNETIC BOTLLE ???????. And this suggests that the current model of neutron ddu can be wrong. And that's why there is neutron PUZZLE: a difference of 8 seconds between the decay of neutrons in the bottle experiments and the decay of neutrons emitted by a nucleus, impossible to be explained by considering the structure of the neutron according to the Standard Model. – Japha Nakhão Jul 08 '22 at 14:14
  • No, Jeffrey, the UCN are produced in a laboratory, and later transported to other laboratories, where the neutrons are used in experiments, so that to study the properties of the neutron. – Japha Nakhão Jul 08 '22 at 14:16
  • @JaphaNakhão I have been adjacent to the design, operation, and analysis of a number of UCN experiments. When the UCN are produced outside of the trap volume, as at the ILL turbine or the deuterium moderator at LANL, the distance traveled by the UCN is meters, not hundreds of meters. – rob Jul 08 '22 at 14:32
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    @JaphaNakhão I'm afraid you're misreading that source. Neutrons are not transported long distances after being trapped in a magnetic bottle, precisely because they either decay or escape the bottle by some means. You appear to be under the impression that neutrons are placed in a bottle and then shipped by mail to some other laboratory and used in an experiment some days later, which is not the case. – J. Murray Jul 08 '22 at 14:32
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    In addition, the "bottles" typically weigh several tons, and any vibration in the walls is a mechanism for transferring heat to the UCN and removing them from the trap by "upscattering." The bottles are vibration-isolated from the floor, which requires them to be stationary. – rob Jul 08 '22 at 15:03
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A magnetic neutron bottle is a class of experiments for measuring the lifetime of the free neutron.

A neutron in a trap may leave the trap by radioactive decay, with time constant $t_\text{decay}$; by interacting with some residual gas in the vacuum, with time constant $t_\text{capture}$; by interacting with the wall of the trap, with time constant $t_\text{wall}$; and probably by some other processes which I’ve forgotten. The total time in the trap obeys

$$ \frac1t=\frac1{t_1}+\frac1{t_2}+\cdots $$

for all of the independent loss processes. The fastest loss mechanism “wins,” but only the weak decay lifetime is really interesting. So you want the interactions with the residual vacuum and the walls to contribute as little as possible to neutron losses.

The “trap” works because a neutron in a magnetic field has a dipole energy of about 50 nano-eV per tesla, and the neutron’s spin is unlikely to flip relative to the field direction if the neutron’s travel through the field is “adiabatic.” If the neutron’s kinetic energy is below about 100 nano-eV (“ultra-cold,” in the jargon), then one of the spin states cannot enter tesla-scale magnetic field regions, and you can therefore prevent your neutrons from actually touching the walls of your trap.

rob
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