View Full Version : Neutron formation

2007-Feb-21, 11:57 PM
So, when a large star collapses into a neutron star, it becomes much denser than an atomic nucleus forcing protons and electrons together forming neutralized particles, neutrons, corrrect?

Protons are made of two up quarks and a down quark, and neutrons are made of two downs and an up, so why does adding a lepton change the structure of these particles, or are the particles still in proton structure just neutralized of their positive charge?

2007-Feb-22, 03:20 AM
when a large star collapses into a neutron star, it becomes much denser than an atomic nucleus
I always think of it as BECOMING an enormous atomic nucleus.

Actually, although the core is a load of neutrons, the less dense outer layers are comprised of normal atoms.

why does adding a lepton change the structure of these particles
Adding the lepton basically changes an up to a down. Since an up has charge 2/3 and down has -1/3, charge is still conserved.

2007-Feb-22, 01:39 PM
A schematic for neutron decay is at www.particleadventure.org/frameless/npe.html

In it the correct sequence theoretically is shown. The neutron's up quark converts to a down and emits a W-, mediator of the weak force. The W immediately disintegrates into an electron and an electron-type antineutrino. So the neutron yields three net particles. The electron binds to the newly created proton, and the antineutrino radiates away at c as it is massless.
The reverse process, the proton becoming a neutron does not really involve adding a single lepton..(electron).. to convert the proton's charge to zero. Nor does it involve the unlikely scenario of the three particles superimposing to create a neutron in reverse.(Though that's not strictly forbidden...it is endothermic,also)
In particle physics' world of conservation laws, absorption of a particle is equivalent to emission of the same anti-particle....so the proton takes the equivalent route....an energetic proton emits a W+, converting an up to a down, and becoming a neutron as it does. The W+ immediately emits a positron and an electron-type neutrino. The positron annihilates the previously existing electron in two opposing or three coplanar gamma rays totalling 1.022 Mev.
This is why in core collapse supernova, there is a GRB, and a prompt neutrino burst over ~ 10 seconds, as the neutron star forms. Equations of state for neutron stars allow all kinds of models, and continue to evolve. They aren't just neutrons as RobA suggests.

2007-Feb-22, 02:42 PM
as allways very educating to read, thanks 88

2007-Feb-22, 05:44 PM
as allways very educating to read, thanks 88

You're welcome, Satori. I labored under the same incorrect scenario for a long time, as a rookie science teacher. The weak interaction is very subtle to understand. I credit Isaac Asimov with squaring away the misconceptions.
One of the truly odd things is that our transient gamma ray bursts are energetic enough to cause stray neutrons to wander through the environment, with thermal ones sticking to nuclei nilly-willy. One of the consequences to this is radioactive transmutations in the biosphere. Like it or not, your eye color, or other personal phenotype may be due not only to cosmic rays causing C-14 in the upper atmosphere, and then moving into the food chain, but a distant GRB in the past. The stars do have something to say about human fates.:shifty: Pete.

2007-Feb-22, 07:06 PM
Hello trinitree88,
it is funny, but i have never heard, that those GRB were peeked at (or even essential ) 1 MeV (not that i had sought for any detailed decription or understanding though )
I also must confess, that I am still not in the full picture as to how such a gamma photon (hitting the atmosphere) would (precisely ) produce a stray neutron (of high momentum) ( if you have pair building in the presence of a proton and you would have the digestion of the electron by that same particle, then you would get a neutron , ok, but why would that leave the nucleus at a high velocity? )
Also is my personal horizon not wide enough to follow you strictamente to the arcane consequences for our human phenotypicality ....you would have to expand on that one, and i would indeed ask you too really do it...

2007-Feb-22, 11:11 PM
Satori; The energy of a gamma ray burst is spread over many values. Typical bursts have an average value per photon, but some of them are much higher than the average. This high end "tail" of the distribution of energies is where they come to ~ 10 Mev. That happens to be the binding energy for most nucleons (protons or neutrons). If such a gamma ray strikes an atmospheric nucleus, or one closer to the surface, (and they do)...nucleons are liberated.
The protons are short-lived, pick up a stray electron, and become a hydrogen atom.
The neutrons are another story. Fast moving neutrons are useful to cause fission in U-238. Slow-moving (called "thermal" neutrons cause their speeds are near room temperature velocity distributions)...are very "sticky" to nuclei, and are used in fissioning U-235, and reactor grade uranium. But, that's not their fate when they evolve in a gamma ray burst. They meander your room while you eat or sleep and cause radioactive isotopes to form. They cause little glitches in DNA chains, the way C-14 does. If such a glitch occurred thousands of years ago, it may be the reason you have brown eyes, or a mole on your elbow...hence stars affecting human fates. Pete.

2007-Feb-23, 10:44 AM
well ...........you were alluding then to the mechanism of mutation in general....I thought there might be some specific cause for the ...say developement of eye coulors, that I would have not been aware of...and that would have struck me as strange !
I know that the thermic, slow neutrons of the overall distribution spectrum have a greater cross section ( counter intuitively ) if it comes to nuclear transformations.....but would they realy wander around to the very inside of your cosy little home? Wouldn't the bulk of all those interactions happen way out in the atmosphere ? And are there estimates as to what degree GRBs are contributing to the total balance of nuclear transphormations in the atmosphere ? Isn't the formation rate of C14 supposed to be an almost constant ....how does that square in? Is ther an sensible effect on the gauging of our nuclear decay time mesuring methods ?

2007-Feb-24, 06:21 PM
Satori. I can only answer a few of these. If neutrons are freed up, yes they can wander a bit through your house, it's one of the reasons why nuclear reactor tours are prohibited to those under 18 years of age. Years ago I toured the Seabrook nuke facility and my underclass students were banned. A few questions to the Health Physics Officer, and the presence of a slow stream of neutrons was part of the answer. I've also toured other reactors, same credo. Pete.