The riddle of the neutron

• bobie
In summary, the force that makes a neutron eject an electron is the weak force, and it is strongest when the distance between the electron and neutron is small. The speed of the extranged electron and neutrino is unknown, but it is probably very close to the speed of light.

bobie

Gold Member
What is the force that makes a neutron eject an electron, and how strong is it?

hi bobie! welcome to pf!
bobie said:
What is the force that makes a neutron eject an electron, and how strong is it?

a neutron can't eject an electron

if you mean the force when a neutron "decays" to a proton an electron and a neutrino …

that would be the weak force

1 person
Thanks, tiny-tim, for correcting my wording,
I just wanted to stress the fact that in order to "repel " an electron you must overcome the electric attraction, which at that distance, is extremely strong.
I couldn't find details, but wiki says the process requires less than 1 MeV, and that weak force acts between electrons and neutrinos.

Could you kindly explain exactly what happens when an electron is "captured" by a proton and when it is "freed"?
Is the neutrino formed inside the neutron when the electron is captured? and how?
Is 0.8 MeV enough to compensate the Coulomb force?
What is the speed of the extranged electron and neutrino?
Why does it take so long (15 minutes) for the weak force to act?

I am much obliged

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i don't see anything there about electrons being "captured"

can you please quote the passage you don't understand, and why?

There are no captured electrons.
In a beta decay of a neutron, the electron is created in the decay process. It has enough energy to get separated from the newly formed proton (that's why the decay happens at all). The sum of the kinetic energies of all 3 particles (electron, neutrino, proton) is ~782 keV - the coulomb interaction is taken into account in that value. The process does not need any energy source.

The weak interaction is not like a classical force - it can create and destroy particles, that's why the term "weak interaction is better. The neutron has a long lifetime as the interaction is weak (literally).

What is the speed of the extranged electron and neutrino?
The neutrino is (nearly always) very close to the speed of light, the electron can be everything from "very slow" to a significant fraction of the speed of light.

Thanks,
when I said captured i was referring to the formation of a neutron: in a nucleus a proton captures an electron forming a neutron , is that wrong?
If a neutron is freed from the nucleus it decays into a proton emitting the electron plus a neutrino.

You say the electron is created in the process, but an electron ought to be already in a proton so that it can have no charge, is that wrong too?

when I said captured i was referring to the formation of a neutron: in a nucleus a proton captures an electron forming a neutron , is that wrong?
That can happen in some isotopes (it is a rare process, however). The electron is destroyed in the process.

You say the electron is created in the process, but an electron ought to be already in a proton so that it can have no charge, is that wrong too?
No, there is no "electron in a proton".

bobie said:
Thanks,
when I said captured i was referring to the formation of a neutron: in a nucleus a proton captures an electron forming a neutron , is that wrong?
If a neutron is freed from the nucleus it decays into a proton emitting the electron plus a neutrino.

You say the electron is created in the process, but an electron ought to be already in a proton so that it can have no charge, is that wrong too?

the capturing of an electron that you are referring to is often known as K(or L) capture

bobie said:
when I said captured i was referring to the formation of a neutron: in a nucleus a proton captures an electron forming a neutron , is that wrong?

yes (wrong): the electrons round a nucleus have nothing to do with it

a neutron will "decay" (with a 15-minute half-life) if there are no electrons around it

(inside a nucleus, a neutron is much more stable)

when a neutron "decays", the energy produced is sufficient to create a proton an electron and a neutrino, all at the same time

(when a proton inside a nucleus "decays" into a neutron, it can be either by "electron capture" or by "positron emission", see http://en.wikipedia.org/wiki/Electron_capture and http://en.wikipedia.org/wiki/Positron_emission)​

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tiny-tim said:
(inside a nucleus, a neutron is much more stable)
That depends on the nucleus. Helium-6, for example, decays to Li-6 with a half-life of less than a second.

If there is no negative particle insaide a neutron, how come it has no positive charge?

If a neutron decays into a proton and an electron , what prevents you from saying that a neutron is made up by a proton and an electron?
How can an electron be created in the process without a positron being created too?

It's tempting to think of a neutron being composed of a proton and an electron somehow bound together but its more involved than that. During the capture reaction the quark structure is changed from that of the proton(up,up,down)to that of the neutron(up,down,down) .All conservation rules, including that of charge, apply

bobie said:
If there is no negative particle insaide a neutron, how come it has no positive charge?

There are no electrons inside a neutron. It's well established that a neutron is made up of an up quark (charge +2/3e) and two down quarks (each with charge -1/3e), which have a total charge of zero. In beta decay, a down quark converts to an up quark and an electron and antineutrino are created.

mfb said:
the electron can be everything from "very slow" to a significant fraction of the speed of light.
Even if the electron is created in the process, once it comes to exist it is attracted by the proton by the Coulombs law, and there is no other interaction between them, right?...now, how come they acquire different speeds?
Again, could you expand on the process?...
.. as soon as a neutron is free the process of decay starts creating an electron and a neutrino, these two interact and the neutrino expels the electron? ...and who expels the neutrino at the speed of light? the electron?

jtbell said:
There are no electrons inside a neutron. It's well established that a neutron is made up of an up quark (charge +2/3e) and two down quarks (each with charge -1/3e), which have a total charge of zero. In beta decay, a down quark converts to an up quark and an electron and antineutrino are created.
And so there is no positron in a proton an up-quark is equivalent to a positron?

Please look at the section on neutron decay (who knows, maybe a diagram makes it easier to understand).

http://hyperphysics.phy-astr.gsu.edu/hbase/particles/proton.html

The down quark changes flavor via the weak interaction to an up quark. There is no electron and there is no proton bound together to form a neutron.

Zz.

bobie said:
.. as soon as a neutron is free the process of decay starts creating an electron and a neutrino …

and a proton, yes

except it doesn't "start to" create it, it isn't a gradual process, it's a very sudden one, that could happen at any time, but tends to take 15 minutes …

it isn't decay like wood decays, it's more like someone running about in a mine-field
… these two interact and the neutrino expels the electron? ...and who expels the neutrino at the speed of light? the electron?

the electron and the neutrino do not interact

after they are both created together, they have nothing to do wiht each other (the neutrino goes away at almost the speed of light anyway)

(and the electron manages to escape from the proton because the electron is created with a lot of energy and momentum)

tiny-tim said:
after they are both created together, they have nothing to do wiht each other (the neutrino goes away at almost the speed of light anyway)

(and the electron manages to escape from the proton because the electron is created with a lot of energy and momentum)
That is what I do not understand: wiki says that weak force is an interactions between leptons, and here we have only one electron and a neutrino, if they do not interact where is the weak force? who starts the process of decay?.

Zapper says the down quark is changed to an up quark via the weak force, is it an interaction between quarks?.. and the neutrino zips off instantly via interaction with a quark? ... and what gives the neutrino the huge KE and direction?

bobie said:
That is what I do not understand: wiki says that weak force is an interactions between leptons, and here we have only one electron and a neutrino, if they do not interact where is the weak force? who starts the process of decay?.

Er... Hello? Electrons and neutrinos are BOTH LEPTONS. so what is the problem here?

Zapper says the down quark is changed to an up quark via the weak force, is it an interaction between quarks?

I have no idea what this means.

This thread is a bit frustrating because I'm seeing that we keep having to make several steps backwards. Obviously, I'm wrong in this case. Having a diagram of this process DID NOT make it any clearer.

Zz.

ZapperZ said:
Er... Hello? Electrons and neutrinos are BOTH LEPTONS. so what is the problem here?
I am sorry if I cannot make myself clear, but I was commenting tiny-tim's ".. electron and neutrino have nothing to do with each other".

It was kind of you to give me that link, but a diagram doesn't help me. I 'd like to know the details of the process:
a free neutron changes one of its quark from down flavour to up
the interaction between what starts this process?
what supplies the energy for the process, is the mass of the up quark different from the mass of a down quark? what mass of the neutron is lost in the process?

You can look up the masses of ALL the quarks very easily!

Zz.

bobie said:
If there is no negative particle insaide a neutron, how come it has no positive charge?

If a neutron decays into a proton and an electron , what prevents you from saying that a neutron is made up by a proton and an electron?
There is no electron inside a proton just as there is no butterfly inside of a caterpillar. The Neutron transforms into a electron + proton + anti-neutrino.
How can an electron be created in the process without a positron being created too?

That's why the anti-neutrino is produced. There is no need to produce an positron if you can produce an anti-neutrino instead. (As long as charge is conserved, which it is in this case)

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bobie said:
That is what I do not understand: wiki says that weak force is an interactions between leptons

it is very unhelpful if you claim to quote from wikipedia but you do not provide a link

the weak interaction is an interaction between fermions (which include neutrons protons and quarks), see http://en.wikipedia.org/wiki/Weak_interaction
who starts the process of decay?

the neutron is inherently unstable, nothing starts it "decaying"
... and what gives the neutrino the huge KE and direction?

the neutrino's KE is not substantially larger than that of the proton or the electron

all three particles have a substantial KE

but since the rest-mass of the neutrino is hugely smaller than that of the proton or the electron, its speed is hugely larger (for a similar KE)

bobie said:
And so there is no positron in a proton an up-quark is equivalent to a positron?

The quarks are elementary particles on their own right. No reason to compare them to positrons.

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Thanks, you have been very helpful,
what baffles me is the fact that an electron is created by pair production: electron- positron.

Now, how can you create a pair electron-antineutrino? it has no charge and its mass is billions of billions time smaller than the electron's, it's extremely unbalanced!

bobie said:
… an electron is created by pair production: electron- positron.

pair production (ie creating two particles out of pure energy) is not the only possibility

creating four (or more) particles out of pure energy (or creating three particles out of one particle plus energy) also happens

when you create particles out of pure energy, everything has to balance

so if you only create two particles, and one is an electron with charge -1 and lepton number 1, then obviously the other particle must have charge 1 and lepton number -1, ie it must be a positron

but if you create more than two particles, the charge 1 can be in one particle (eg proton), and the lepton number -1 can be in another particle (eg antineutrino, which I've been calling neutrino for short)
Now, how can you create a pair electron-antineutrino? it has no charge and its mass is billions of billions time smaller than the electron's, it's extremely unbalanced!

[they aren't created as a pair, but as part of a four, but anyway …]

mass is energy, energy is mass

rest-mass is not total energy

the rest-masses don't have to balance

only the total energies (and momentums) have to balance

(as i said before) although the neutrino's rest-mass is very small, the total energies can still balance if the neutrino has very high speed

bobie said:
Thanks, you have been very helpful,
what baffles me is the fact that an electron is created by pair production: electron- positron.

Now, how can you create a pair electron-antineutrino? it has no charge and its mass is billions of billions time smaller than the electron's, it's extremely unbalanced!

These two are created by different mechanisms!

Pair production is created out of conservation processes (energy, spin, momentum only). The electron and antineutrino in a beta decay are created via the weak interaction, i.e. there are other criteria and other particles involved. You'll never see just an electron-antineutrino pair being created out of a photon.

Zz.

1 person
Think of a neutrino as an electron without its charge. The electron - antineutrino pair is very balanced except for the charge unbalance. Since charge must be conserved, electron - antineutrino pair production cannot happen. When a neutron decays into a proton, a charge unbalance is created. That unbalance must be compensated elsewhere and that's why the electron - antineutrino pair becomes not only possible, bu necessary.

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?

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.

bobie said:
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 Schrodinger 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.

bobie said:
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.