Neutrons, Protons, Electrons and Neutron Stars

[OneOnOne1162]

Let me preface this by saying I'm not a physicist, or physics student.

 

Now that we have that out of the way, I've been playing a game called Elite Dangerous and seeing a neutron star there reminded me that while I've looked up plenty on black holes, I'd never really looked into neutron stars very much. So I was looking up the composition of a neutron star, etc. and I read (on this website http://science.nationalgeographic.com/science/space/solar-system/neutron-stars/) thata neutron star's birth involves " A neutron star's almost incomprehensible density causes protons and electrons to combine into neutrons." Now, is this correct? How do protons and electrons combine into neutrons? Why don't they normally bind into neutrons while inside of an atom together? Just lack of pressure condensing them? And finally, is this how all neutrons ever are formed?

[Sensei]

And finally, is this how all neutrons ever are formed?

In laboratory free neutrons are created by bombarding f.e. Deuterium by high energy alpha particle.

[math]D^+ + 2.22 MeV \rightarrow p^+ + n^0[/math]

(because Deuterium has the lowest energy needed to split it)

 

There are some other unstable isotopes that after bombarding them with high energy particles are releasing free neutrons.

F.e. Uranium-235 while fission release ~3 new free neutrons, after bombarding it by 1 free neutron.

 

Neutron is unstable particle, with half-life ~10 minutes, mean-life ~15 minutes. So soon after creation, they decay back to:

[math]n^0 \rightarrow p^+ + e^- + \bar{v}_e + 0.782 MeV[/math]

[OneOnOne1162]

In laboratory free neutrons are created by bombarding f.e. Deuterium by high energy alpha particle.

[math]D^+ + 2.22 MeV \rightarrow p^+ + n^0[/math]

(because Deuterium has the lowest energy needed to split it)

 

There are some other unstable isotopes that after bombarding them with high energy particles are releasing free neutrons.

F.e. Uranium-235 while fission release ~3 new free neutrons, after bombarding it by 1 free neutron.

 

Neutron is unstable particle, with half-life ~10 minutes, mean-life ~15 minutes. So soon after creation, they decay back to:

[math]n^0 \rightarrow p^+ + e^- + \bar{v}_e + 0.782 MeV[/math]

 

Well, but those are atoms, right? Doesn't that mean that they simply release a neutron that was already present in their atomic structure?

[imatfaal]

Let me preface this by saying I'm not a physicist, or physics student.

 

Now that we have that out of the way, I've been playing a game called Elite Dangerous and seeing a neutron star there reminded me that while I've looked up plenty on black holes, I'd never really looked into neutron stars very much. So I was looking up the composition of a neutron star, etc. and I read (on this website http://science.nationalgeographic.com/science/space/solar-system/neutron-stars/) thata neutron star's birth involves " A neutron star's almost incomprehensible density causes protons and electrons to combine into neutrons." Now, is this correct? How do protons and electrons combine into neutrons? Why don't they normally bind into neutrons while inside of an atom together? Just lack of pressure condensing them? And finally, is this how all neutrons ever are formed?

 

They combine by reverse beta decay aka electron capture

 

[latex]p + e^- \rightarrow n + v_e[/latex]

 

It is energetically favourable for electrons and protons to combine to neutrons only due to the massive pressure due to the intense gravitational force - a "gas" of neutrons is higher "density" (ie more are packed in to a given volume) than the plasma of protons and electrons.

 

This is because the electrons will resist being put too close together due to the exclusion principle - that two half spin fermion particles cannot share the same quantum space. Neutrons also resist due to the fermionic exclusion principle - but at the energy levels considered the wavelength of the neutron is much much shorter than that of the electron which very simplistically means that the space required for a neutron to be happy is much much smaller than the space for an electron to be happy.

 

At even higher gravitational pressures the neutrons get forced together too much and it is hypothesised that they might split into their component quarks which again can be packed even tighter. Again simplifying; This does not continue - at higher gravitational pressure the matter is so dense that the mass is packed within a radius that is lower than the schwartzchild radius - and from this point you have a black hole; we cannot know what is inside a black hole as there is a one way barrier at the event horizon

[swansont]

Compared to a free proton and electron, the mass energy of a neutron is higher. That's why a free neutron decays, and why electrons don't spontaneously combine with protons to form neutrons. The electron capture process described by imatfaal only happens in certain isotopes where it is energetically favorable.

[OneOnOne1162]

 

They combine by reverse beta decay aka electron capture

 

[latex]p + e^- \rightarrow n + v_e[/latex]

 

It is energetically favourable for electrons and protons to combine to neutrons only due to the massive pressure due to the intense gravitational force - a "gas" of neutrons is higher "density" (ie more are packed in to a given volume) than the plasma of protons and electrons.

 

This is because the electrons will resist being put too close together due to the exclusion principle - that two half spin fermion particles cannot share the same quantum space. Neutrons also resist due to the fermionic exclusion principle - but at the energy levels considered the wavelength of the neutron is much much shorter than that of the electron which very simplistically means that the space required for a neutron to be happy is much much smaller than the space for an electron to be happy.

 

At even higher gravitational pressures the neutrons get forced together too much and it is hypothesised that they might split into their component quarks which again can be packed even tighter. Again simplifying; This does not continue - at higher gravitational pressure the matter is so dense that the mass is packed within a radius that is lower than the schwartzchild radius - and from this point you have a black hole; we cannot know what is inside a black hole as there is a one way barrier at the event horizon

 

That (and swansont's answer) answers most of my questions. Though I was wondering if you know the answer to my last question too: Is this how all neutrons were originally formed?

[imatfaal]

 

That (and swansont's answer) answers most of my questions. Though I was wondering if you know the answer to my last question too: Is this how all neutrons were originally formed?

 

No it is not how all neutrons are formed. Just as a single counter example that you will already be aware of - Positron Emission (as in PET scan)

 

This is when a proton decays into neutron, a positron and an electron neutrino:

 

[latex] p \rightarrow n + v_e + e^+ [/latex]

 

This is however a poor example in that it is very very similar to the initial example (the absence of an electron IS the existence of a positron) - but it is one that crops up in everyday life through the medical application.

 

I am not sure if we truly understand the part of baryogenesis in which the bulk of the universe's neutron and protons (and the antiparticles) are formed a fraction of a second after the big bang - we definitely do not understand where all the antiparticles went to. We do think that the reason we have so many neutrons (which should otherwise have rapidly decayed) is that they can form deuterons (a proton and a neutron - ie a deuterium nucleus) which are stable in the prevalent conditions at the time

[OneOnOne1162]

 

No it is not how all neutrons are formed. Just as a single counter example that you will already be aware of - Positron Emission (as in PET scan)

 

This is when a proton decays into neutron, a positron and an electron neutrino:

 

[latex] p \rightarrow n + v_e + e^+ [/latex]

 

This is however a poor example in that it is very very similar to the initial example (the absence of an electron IS the existence of a positron) - but it is one that crops up in everyday life through the medical application.

 

I am not sure if we truly understand the part of baryogenesis in which the bulk of the universe's neutron and protons (and the antiparticles) are formed a fraction of a second after the big bang - we definitely do not understand where all the antiparticles went to. We do think that the reason we have so many neutrons (which should otherwise have rapidly decayed) is that they can form deuterons (a proton and a neutron - ie a deuterium nucleus) which are stable in the prevalent conditions at the time

 

Alright, thanks for the answers.

[Enthalpy]

Protons and neutrons transform in an other rather easily. Look at a table of the isotopes: for 80 protons+neutrons, the number of neutrons can vary by +-3 typically, and beyond that, radioactivity brings the nucleus to the favourable proportion, which is around 1 neutron per proton, or rather 1.5 for heavy nuclei. If the mismatch is big, the decay gets very quick.

 

This kind of transformation happens by electron capture, electron emission or positron emission. ⁴⁰K does all three.

https://en.wikipedia.org/wiki/Potassium-40

 

An isolated neutron happens to be on the wrong side of the favourable proportion. It's radioactive and decays into a proton. Normally you don't find an isolated neutron.

 

I can't tell whether the neutrons in primordial helium and lithium were formed by proton collision that created these elements or before - nor even if such a question is meaningful.

 

On Earth, about every neutron is in a nucleus that was formed in a previous star and spread in an explosion

https://en.wikipedia.org/wiki/Nucleosynthesis

so many of these neutrons result from the transformation of a proton, the exception being those from primordial nuclides like helium that were incorporated in said star at its birth.