Proton + electron = neutron?

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1. Jun 2, 2015

Stephanus

This the answer that I have from Chalnoth in my other thread about nuclear fusion inside the sun (or star in main sequence).

And after iron burning in the core of massive star, the star explodes and leaves a neutron star (or a black hole) behind.

Is producing neutron in P+P reaction chains and producing neutron star a similar process?

2. Jun 2, 2015

ChrisVer

The interaction is an inverse beta decay (or electron capture)
$pe^- \rightarrow n \nu_e$
and it's what happens at the collapsing core of the sun before forming a neutron star. (sidenote: Not all protons or electrons have to disappear)

3. Jun 2, 2015

4. Jun 2, 2015

ChrisVer

Also check here:
http://en.wikipedia.org/wiki/Proton–proton_chain_reaction
where it gives the reactions...
For the pp I branch (temperatures 10-14 MK) you don't have any electron capture processes vs pp II branch (temperatures 14-23MK) you have electron capture processes (Berylium gets an electron to give the Lithium)

5. Jun 2, 2015

Stephanus

Yes, I do remember it ChrisVer.
It's just that I realized in P + P reaction P + P -> Deuterium
It is different I think with He3 + He 3 -> Two Hydrogens + He 4.
There's no change of particle here, but in P + P there is a change particle. Proton becomes a Neutron. So I remember back then when I ask that previous question about neutron star. Is it the same process.
In neutron star it is pressure, right?
And in P + P reaction it is collision (and pressure), right?
Because there's also fusion reaction in tokamaks, but in atmospheric pressure.

6. Jun 2, 2015

ChrisVer

that change is just a usual beta+ decay and not an electron capture.

Both are due to pressure...

i am not sure if Tokamaks have achieved fusion.

7. Jun 2, 2015

Stephanus

http://education.jlab.org/qa/particlemass_02.html

It seems that proton is heavier than neutron.
Is He4 heavier than 4 Hydrogens?

Last edited by a moderator: May 7, 2017
8. Jun 2, 2015

Stephanus

Haven't they? Perhaps for 10 minutes then dies I think. ITER is designed to sustained the reaction much longer.

9. Jun 2, 2015

ChrisVer

the proton is lighter than the neutron, and that's why a free proton cannot decay to a neutron.

no... Nuclei are not "balls" made up of constituents you can sum up and get their mass. There is a difference between summing over the neutron/proton masses and the nucleus mass, and that's because the nucleus is a bound system.

Last edited: Jun 2, 2015
10. Jun 2, 2015

Stephanus

Thanks for your invaluable help ChrisVer.
Wikipedia says "Comparing the mass of the final helium-4 atom with the masses of the four protons reveals that 0.007 or 0.7% of the mass of the original protons has been lost"
The proton is lighter than the neutron. As you say and many links in internet and my text book in high school. Okay...
Or a SINGLE proton is lighter than a SINGLE neutron?
Or somehow 2P+2N (as in He4) if combined together becomes even more lighter than 4 single protons?

11. Jun 2, 2015

Staff: Mentor

They have.

Fusion needs high pressure and/or high temperature - both are important. We cannot reproduce the pressure in the interior of the sun in tokamaks, but we can reach higher temperatures.
Right. It does not make sense to ask about the mass of a single proton or neutron inside a nucleus. That is a meaningless concept.

12. Jun 2, 2015

ChrisVer

That's what it happens... Again I'm repeating that the mass of the Helium is not $2m_p + 2m_n$, but less than that... The difference is due to the binding energy of the four nucleons within the Helium nucleus.

The proton (or hydrogen nucleus) has a mass of around 1.007825u
So 4 protons have a total mass of 4.031300u....

The Helium has a mass of 4.00260u....

Take the difference 4p-He: 4.031300u-4.00260u =0.02870u (or 26.72 MeV )
That's positive so $M_{4p} > M_{He}$
And so that's why it says that the protons lost some of their mass when they got combined into the Helium nucleus.

Making a check of the percentage: $\frac{\Delta M}{M_{4p}} = \frac{0.02870}{4.031300}=0.00711929154 \approx 0.007$ or 0.7%

Last edited: Jun 2, 2015
13. Jun 2, 2015

ChrisVer

OK, thanks for that :)

14. Jun 2, 2015

ChrisVer

I don't understand this question.
A proton is lighter than a neutron... it can decay to a neutron only by finding from somewhere the needed energy [and it does so when it is within a bound state like a nucleus, and so you have nuclei that can undergo beta+ decay]

15. Jun 2, 2015

Stephanus

I mean the mass of a positive ion of Hydrogen is lighter compared to the mass of a single free neutron.

16. Jun 2, 2015

ChrisVer

You are rephrasing the word "proton" in a complicated way here

and yes... a free "positive ion of Hydrogen" has a mass $m_p$ while a neutron has $m_n$...and we already agreed that $m_n> m_p$.
But still, two free "positive ions of Hydrogen" are lighter (mass=$2m_p$) than two free neutrons (mass=$2m_n$)... (that's why I had problem with the word "single" but I'd prefer "free").

But for example (from the wiki article on the pp chains) the diproton which is not "free", is heavier than a deuterium...

17. Jun 2, 2015

Stephanus

Okay, okay I understand now. Really do.
Thank you everybody.
Can I add a question here?
Beta decay.
I can understand (if not imagine) beta decay in P+P reaction chain in the Sun.
Are you saying that the whole iron core, 1.44 solar mass I think, decays in 1 seconds to become a big ball of neutrons?
I mean tritium decays in 12 year, is it?
And Carbon 14 decays more than 5000 years.
But if somehow P + P decays becomes a deuterium in more than 1 second, I can imagine. At least, they say the proces for P + P becomes He4 in the sun takes more than 100 000 years. Forgot how long.
But in neutron star, does beta decay take place in 1 second?

Last edited: Jun 2, 2015
18. Jun 2, 2015

Stephanus

FREE is the word. I understand now. Thanks.
It means a big different than the proton in nucleus.

19. Jun 2, 2015

ChrisVer

I don't know about the time scales to be honest...
However when the core collapses beyond the white dwarf stage, it gets heated up... this heat up provides enough energy for:
$e^-+ p+1.36MeV \rightarrow n \nu_e$
As I have already explained, the existing electron/proton degeneracy blocks the neutron from decaying back and makes it stable within the core.
As collapse keeps on going and temperatures keep on growing (more free energy), any surviving atomic nuclei will undergo the inverse beta decay, with a peak appearing for the iron at ~3.7MeV.
So you get suddenly too many neutrons in the core. After that point the collapse is getting slowed down from free-fall by the neutrino pressure.
When neutrons are enough to become degenerate, the collapse is halt and immediately you go back into equillibrium ...this immediate transition produces an outward going shockwave (being also boosted by the neutrino pressure) because of the bounce of the infalling matterial over the neutron degenerate core. And that shock is the supernova.

20. Jun 2, 2015

Stephanus

Thanks ChrisVer, it's good enough for me.