What Causes a Neutron to Eject an Electron?

[bobie]

Thank you all for your help,
now allow me a last, stupid, question, what prevents you from saying that a neutron is made up by a proton and an electron stuck together and then, when the neutron is free, the electron is emitted along with a neutrino, leaving a regular proton?

 

[dauto]

This is not a stupid question. This possibility was considered back in the days before people knew how those things work. An electron stuck to a proton forms a hydrogen atom, not a neutron. If the electron was present there all along, there would be no need to produce an antineutrino to balance the electron production. In fact that neutrino production would be forbidden by lepton number and angular momentum conservations. No neutrino is produced when a hydrogen atom is ionized.

 

[ZapperZ]

Thank you all for your help,
now allow me a last, stupid, question, what prevents you from saying that a neutron is made up by a proton and an electron stuck together and then, when the neutron is free, the electron is emitted along with a neutrino, leaving a regular proton?

1. Solve the Schrödinger equation for a proton and an electron. The electron has no bound state THAT close to the proton.

2. We probed the nucleus. We see no signature of any electrons.

3. We know the content of a neutron very well, both from the Standard Model and from experiment. We understand why a bound neutron is more stable than an unbound neutron. This cannot be explained if a neutron is nothing more than a proton + electron.

4. Where did the antineutrino came from if it is just a proton+electron? After all, do you see an antineutrino when we break up a hydrogen atom into a proton and electron?

There is a whole series of theoretical and experimental observations here that will not fit in, especially within the Standard Model, if a neutron is a bound proton+electron.

Zz.

 

[Bill_K]

Thank you all for your help,
now allow me a last, stupid, question, what prevents you from saying that a neutron is made up by a proton and an electron stuck together and then, when the neutron is free, the electron is emitted along with a neutrino, leaving a regular proton?

Angular momentum. The neutron, proton and electron all have spin 1/2. You cannot make a spin 1/2 neutron by joining together two spin 1/2 particles.

 

[bobie]

Thanks, you've been very helpful.
The process lasts 15 min.: do you know exactly when the positive charge appears and how long after this the electron is emitted? , and, where does the extra mass of the boson comes from?

 

[tiny-tim]

The process lasts 15 min.

no, the process is instantaneous

it can happen at any time, including after 1 second or 1 year

it has a constant half-life, ie the probability of it happening in the next minute does not increase or decrease

that half-life is about 15 minutes, meaning that it always has a 50% chance of happening in the next 15 minutes

do you know exactly when the positive charge appears and how long after this the electron is emitted?

there is no gap, the process is instantaneous

and, where does the extra mass of the boson comes from?

what boson?

the neutron proton electron and neutrino are all fermions (not bosons)

 

[bobie]

what boson?

The diagram in the link showed neutron decaying into W-boson (100 times more massive than a proton) that in its turn decays into electron-antineutrino

 

[tiny-tim]

The diagram in the link …

you mean

?

that's a "Feynman diagram", it's not a picture of what happens, it's a diagram that summarises the mathematical equations which tell us which processes are possible, andwhat their probability is

since that particular diagram works mathematically, we know that the particular "decay" it represents is possible

other diagrams with more Ws (and other particles) also work for the same process: that diagram is the simplest one, and it calculates most of the probability

the W (boson) is called a virtual particle because it isn't real

the diagram shows a W^- being "lent" to the surrounding weak field ("emitted"), but it could equally well show a W⁺ being "borrowed" from the surrounding weak field ("absorbed")

the wikipedia article is misleading … it says that the neutron can either emit a W^- (which it says decays after a very short time), or absorb a W⁺ (which … what? … it doesn't say … how can any time be involved?)

… showed neutron decayng into a W …

no, it certainly doesn't show that, not even as a mathematical figment

(a neutron can't possibly decay into a W)

it shows the neutron decaying into the proton electron and neutrino

the W is shown as a "mediator" (a catalyst)

 

[bobie]

Tht diagram shows also the neutrino with an arrow pointing towards the proton, is that real or virtual?
What happens in reality? The neutron emits an electron and a neutrino , at what angle?
- why the speed of the electron can vary? why isn't it immediately attracted back by the Coulomb force?

 

[Dadface]

I think Bobie must be getting a bit confused. Just by googling you read stuff such as the W is a "virtual particle" or it can roughly be described as a "ripple in the field". At other places you read stuff such as it is "approximately 100 times more massive than the proton with a half life of about 10 to the minus 25 seconds" and "discovered in 1983 the W boson is a fundamental particle". This latter quote is from CERN (W Boson:Sunshine and Stardust)

 

[Dadface]

Tht diagram shows also the neutrino with an arrow pointing towards the proton, is that real or virtual?
What happens in reality? The neutron emits an electron and a neutrino , at what angle?
- why the speed of the electron can vary? why isn't it immediately attracted back by the Coulomb force?

The angle is such that the relevant conservation laws apply. The neutrino carries momentum and energy to keep these quantities in balance. Coulomb's law applies and as the particles separate there will be a conversion of KE to PE .Whether they subsequently approach or not depends on the original KE and other factors.

 

[tiny-tim]

Tht diagram shows also the neutrino with an arrow pointing towards the proton, is that real or virtual

well-spotted!

the backwards arrow is because it's regarded as an anti-particle

it's actually an anti-neutrino (and is represented as such with a bar over the letter $\nu$: $\bar{\nu}$)

anti-neutrinos are shown in Feynman diagrams as moving backwards in time

of course they don't in reality! … it's just a convention, and it helps with the maths

(electrons neutrons protons and up and down quarks are all regarded as particles, so they have forward arrows: their anti-particles are positrons anti-neutrons anti-protons and anti-up and anti-down quarks)

the end (initial and final) lines in a Feynman diagram represent real particles, the internal lines represent virtual particles

why isn't it immediately attracted back by the Coulomb force?

because it's moving too fast

 

[bobie]

Thanks, Dadface, right you are, it's really maddening for a neophite: force which is not a force at all , particles that are virtual, nay, fundamental etc., that's why I am really thankful to you all for clearing my douts.
Is this article reliable? http://en.wikipedia.org/wiki/Free_neutron#Free_neutron_decay ?

I asked you many times, but yet I do not understand: are not all neutrons identical? why a process can have different outcomes? if the decay energy is always 0.782 MeV how can the electron be emitted with different KE? , and sometimes there is also an extra γ ray: every single emitted electron should be slowed down by Coulomb law in the same way.

Moreover, at the distance of m x 10^-15 the force is certainly stronger than 0.78 MeV (I reckon is in the excess of millions of MeV, isn't it) so the electron should be resucked onto the proton

 

[mfb]

are not all neutrons identical? why a process can have different outcomes? if the decay energy is always 0.782 MeV how can the electron be emitted with different KE?

It is random, like many processes in quantum mechanics. The total energy of all three decay products is always the same and momentum is conserved as well, but it is random which part gets how much energy.

and sometimes there is also an extra γ ray

Random as well.

Moreover, at the distance of m x 10^-15 the force is certainly stronger than 0.78 MeV (I reckon is in the excess of millions of MeV, isn't it) so the electron should be resucked onto the proton

0.78 MeV is the energy the electron has at a large distance - the energy that is left after it moved away from the proton. And it is not millions of MeV, by the way.

 

[bobie]

0.78 MeV is the energy the electron has at a large distance - the energy that is left after it moved away from the proton. And it is not millions of MeV, by the way.

I thought that:
0.782 MeV is the available energy, the mass-energy the neutron loses (Mn- [Mp+Me+Mν]) and is tranformed into KE of the particles.
(If so,) the recoil of the proton and KE of neutrino is near to zero (say: 0.002), then 0.78 MeV (1.9 x 10^20 Hz) should be roughly the KE the electron can get anyway and the random variations may be only very small, considering the mass-span of the neutrino.
Am I wrong? the article I quoted says that sometimes the electron has less than a paltry 13-eV KE
and I read that sometimes it can get near C!

Since the electron is created near the neutron-proton at a distance of m x10^-14/15, how far can it go in such a strong electrostatic field , what is PE between mx10^-15 and 1m?

 

[mfb]

I thought that:
0.782 MeV is the available energy, the mass-energy the neutron loses (Mn- [Mp+Me+Mν]) and is tranformed into KE of the particles.

As pointed out multiple times, it is not.

The kinetic energy of the neutrino can be very small, very large or everything in between, there is no limit to it (apart from 0.78 MeV of course). The same is true for the electron (again, measured far away from the proton).

Since the electron is created near the neutron-proton at a distance of m x10^-14/15, how far can it go in such a strong electrostatic field , what is PE between mx10^-15 and 1m?

This is not really a useful model, as you cannot ignore quantum-mechanical effects here. The electron does not get created at a specific point in space, it is always a distribution over some volume.

 

[bobie]

The kinetic energy of the neutrino can be very small, .

What is the reason why mass-KE energy of the neutrino cannot be zero and the electron cannot take all available energy?

 

[mfb]

The kinetic energy of the neutrino can be as small as you like - "zero" as a single value has zero probability.
The electron does not get all the energy in this case, the proton has to get some as well to conserve momentum, but this is just a small fraction of the released energy.