The riddle of the neutron

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bobie
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Main Question or Discussion Point

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

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  • #2
tiny-tim
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hi bobie! welcome to pf! :smile:
What is the force that makes a neutron eject an electron, and how strong is it?
a neutron can't eject an electron :confused:

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

that would be the weak force :smile:
 
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  • #3
bobie
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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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tiny-tim
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which wikipedia page are you referrfing to?
 
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tiny-tim
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i don't see anything there about electrons being "captured" :confused:

can you please quote the passage you don't understand, and why? :smile:
 
  • #7
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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.
 
  • #8
bobie
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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?
 
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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".
 
  • #10
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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
 
  • #11
tiny-tim
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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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  • #12
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(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.
 
  • #13
bobie
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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?

Thanks for your patience!
 
  • #14
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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
 
  • #15
jtbell
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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.
 
  • #16
bobie
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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?
 
  • #17
bobie
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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?
 
  • #18
ZapperZ
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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.
 
  • #19
tiny-tim
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.. as soon as a neutron is free the process of decay starts creating an electron and a neutrino …
and a proton, yes :smile:

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 :redface:
… 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)
 
  • #20
bobie
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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?
 
  • #21
ZapperZ
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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.
 
  • #22
bobie
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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?

Thanks for your patience!
 
  • #23
ZapperZ
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You can look up the masses of ALL the quarks very easily!

Zz.
 
  • #24
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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?

Thanks for your patience!
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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  • #25
tiny-tim
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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 :redface:

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)
 

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