Thread: What Exactly Is A Neutron?

I've read countless websites that describe the neutron simply as 'a basic subatomic particle' and then go on to describe the features of it. I haven't been able to find a description of a neutron that has really clicked in my head. Let me try to boil my curiosity down into a few questions:

- Where exactly do neutrons come from? As in, are there a finite number of them out there in the galaxy that have always been around and continuously bond and split with protons, or can they be converted into other forms of energy and, likewise, be created from energy?

- This probably ties into question 1. Floating out in the cosmos are uncountable amounts of protons/hydrogen waiting to turn into heavier elements. Stars and other hot, massive objects fuse these protons with neutrons and other protons to make helium et al. Where are the neutrons supplied to these fusion reactors from? Are they also simply floating through the vaccum of space with the protons, waiting to be converted into something more complex?

- A proton by itself is a hydrogen ion, and massive amounts of hydrogen particles can form gas and react with other matter. Does an individual neutron have any particularly special properties or reactions? If you gathered a massive amount of neutrons together, would they form a gas like protons? Since they have mass, can they be compressed into a liquid like hydrogen?

a neutron is a bunch of quarks (3)

Originally Posted by Frenchi

- Where exactly do neutrons come from? As in, are there a finite number of them out there in the galaxy that have always been around and continuously bond and split with protons, or can they be converted into other forms of energy and, likewise, be created from energy?

Neutrons as a type of particle come from the same place or event that everything else came from, wherever and whatever that was. There are a finite but not fixed number of them and although many came into being (as part of the small fraction of non-hydrogen matter) about 13.73 billion years ago with everything else, there are new ones created all the time in the nucleosynthesis (fusion) process of stars. They are not just out there in the galaxy but they are inside of you as well. Yes they can be coverted into other forms of energy and can be created from other forms of energy too.

Originally Posted by Frenchi

- This probably ties into question 1. Floating out in the cosmos are uncountable amounts of protons/hydrogen waiting to turn into heavier elements. Stars and other hot, massive objects fuse these protons with neutrons and other protons to make helium et al. Where are the neutrons supplied to these fusion reactors from? Are they also simply floating through the vaccum of space with the protons, waiting to be converted into something more complex?

Neutrons are produced in the fusion process under the very high temperature and pressures inside a star. The simplest such fusion process is where four protons become two protons, two neutrons, two positrons, two neutrinos and some extra energy in the form of electromagnetic radiation and kinetic energy. This can only hapen because the the two protons and two neutrons form a helium nuclei which (even when we add in the two positrons and two neutrinos) has a lower total energy than the four protons.

Originally Posted by Frenchi

- A proton by itself is a hydrogen ion, and massive amounts of hydrogen particles can form gas and react with other matter. Does an individual neutron have any particularly special properties or reactions?

A neutron by itself actually has a higher energy than a proton, electron and antineutrino by themselves, which is why a neutron by itself is unstable and will decay into these. So no, neutrons are not just floating around in space waiting to be converted into something more complex, they only exist out there in atoms together with protons as products of stellar fusion, except for a brief time (half-life of 10.3 minutes).

Originally Posted by Frenchi

If you gathered a massive amount of neutrons together, would they form a gas like protons? Since they have mass, can they be compressed into a liquid like hydrogen?

There is one way to make neutrons stable other than by combining them with protons in atomic nuclei. This happens in a neutron star (also called a pulsar) which is a collapsed star (after running out fuel) with too much mass to become a white dwarf and not enough to become a black hole. A neutron star with the mass of the sun would only be about 20 km across (the size of a city) compared to the white dwarf which would be about the size of the earth. So to answer your question neutrons can be compressed into a form (called neutronium) that is much denser than any kind of atomic matter whether hydrogen or uranium, and in this form (that of a neutron star) they will be stable, but in any less dense form like a gas they are going to decay rather quickly.

Thank you.

Originally Posted by mitchellmckain

The simplest such fusion process is where four protons become two protons, two neutrons, two positrons, two antineutrinos and some extra energy...

Could you clarify this statement for me? You mention that four protons become two protons and two neutrons (as well as a handful of other things), and then go on to say that neutrons actually have higher energy states than protons, which introduces a conservation of energy dilemma. Are the neutrons that are created and bound to form helium in this process at a lower energy state than a neutron by itself? In addendum to that, are the two remaining protons also at a lower energy state than they began at?

To append another question, I have read that neutron detection is particularly difficult due to a number of factors. Considering this, by what method do we deduce the number of neutrons in a particular atom to determine its element or isotope?

Originally Posted by Frenchi

Thank you.

Originally Posted by mitchellmckain

The simplest such fusion process is where four protons become two protons, two neutrons, two positrons, two antineutrinos and some extra energy...

Could you clarify this statement for me? You mention that four protons become two protons and two neutrons (as well as a handful of other things), and then go on to say that neutrons actually have higher energy states than protons, which introduces a conservation of energy dilemma. Are the neutrons that are created and bound to form helium in this process at a lower energy state than a neutron by itself? In addendum to that, are the two remaining protons also at a lower energy state than they began at?

To append another question, I have read that neutron detection is particularly difficult due to a number of factors. Considering this, by what method do we deduce the number of neutrons in a particular atom to determine its element or isotope?

First, I made some corrections this morning. It four protons become two protons, two neutrons, two positrons, two neutrinos and some extra energy, but the neutron has more energy than a proton, electron and antineutrino together and therefore decays into these. That's what comes of writing this off the top of my head late at night.

The thing is that Helium nuclei has LESS energy than its component parts separately! The binding energy is negative. This is what makes the helium atom stable for otherwise it would decay and fly apart. It is what makes the stars work for otherwise they would soak up energy rather than produce energy.

Originally Posted by http://www.newton.dep.anl.gov/askasci/chem00/chem00319.htm

The rest mass of the proton, neutron, and electron in AMU is: 1.007276470, 1.008665012, 0.00054858026, respectively. So the sum of 2 protons, 2 neutrons, and 2 electrons is 4.0322980; however, the measured mass of helium(4) is 4.00260.

Each larger atom has a more negative binding energy per nucleon until you get to iron with 26 protons and 30 neutrons then the binding energy per nucleon starts to become less negative. This is why stars can produce elements up to iron on the periodic chart in processes that also produce energy, but producing elements higher on the periodic chart would be at a loss of energy compared to iron, and so these are generally thought to be created in supernova explosions, rather than in regular stellar fusion processes.

Originally Posted by mitchellmckain

Each larger atom has a more negative binding energy per nucleon until you get to iron with 26 protons and 30 neutrons then the binding energy per nucleon starts to become less negative.

So in order for protons and neutrons to bind to a nucleus, at least in these lighter elements, they sacrifice energy, and hence mass according to e=mc2? Forgive me if the answer is staring me in the face here, but what does the lost energy/mass become? The positron, two neutrinos, and 'extra energy' you mentioned earlier?

I can accept, mathematically, that a negative binding energy would contribute to the total energy of the atom and cause it to be less massive; however I have some difficulty understanding the nature of 'negative binding energy'. I would like to research this further, if you have any recommended readings.

Originally Posted by Frenchi

Originally Posted by mitchellmckain

Each larger atom has a more negative binding energy per nucleon until you get to iron with 26 protons and 30 neutrons then the binding energy per nucleon starts to become less negative.

So in order for protons and neutrons to bind to a nucleus, at least in these lighter elements, they sacrifice energy, and hence mass according to e=mc2?

No they gain energy, in the sense that mass energy from the free particles must be converted to some other form of energy.

Originally Posted by Frenchi

Forgive me if the answer is staring me in the face here, but what does the lost energy/mass become?

The missing mass energy is that extra energy coming out of the fusion process which comes out in the form of heat and radiation. This is why stars are hot and bright.

Originally Posted by Frenchi

I can accept, mathematically, that a negative binding energy would contribute to the total energy of the atom and cause it to be less massive; however I have some difficulty understanding the nature of 'negative binding energy'. I would like to research this further, if you have any recommended readings.

This is not that strange because gravity works the same way. Earth and spacecraft 100,000 miles apart has more energy than Earth and space craft only 4,000 miles apart. In this case the difference is called gravitational potential energy and it is also more negative the closer to gravitating objects are to each other. nucleosynthesis is the same only it is the strong force that is involved.

What happens when an object falls to the earth is that this increasingly negative gravitational potential energy means that potential energy has to be converted to something else, in this case it is the energy of motion and so the object accelerates at 32 ft/sec or 9.8 meters/sec (near the surface of the earth).

If you want to read something why not try the Wikipedia articles on fusion and binding energy.

Thanks Mitchell, it's starting to come together in my head. Allow me to ask a few more questions to make sure I'm understanding this.

When two protons fuse, one proton changes into a neutron and releases a positron, a neutrino, and extra energy. According to Wikipedia, the atomic mass of a deuteron is 2.014102 u, while the sum of its free constituent parts is 2.01649 u, giving a binding energy of 0.002388 u, or 2.224 MeV. Since the fusion reaction releases only 0.42 MeV of extra energy ( I assume this is in the form of heat? ), then the other 1.804 MeV is going into the mass of the positron and the neutrino?

Where does the visible light from the plasma come from in this reaction? The positron annihilates with an electron to release two gamma ray photons, but I can't imagine gamma rays are something desirable to look at =)

So we now have a bound system that has given out some of its energy, but this is not in violation of energy conservation because we can simply put 2.224 MeV (the binding energy) back into the deuteron through some other process to split it into its composite pieces again, correct?

Originally Posted by Frenchi

I've read countless websites that describe the neutron simply as 'a basic subatomic particle' and then go on to describe the features of it. I haven't been able to find a description of a neutron that has really clicked in my head. Let me try to boil my curiosity down into a few questions:

- Where exactly do neutrons come from? As in, are there a finite number of them out there in the galaxy that have always been around and continuously bond and split with protons, or can they be converted into other forms of energy and, likewise, be created from energy?

- This probably ties into question 1. Floating out in the cosmos are uncountable amounts of protons/hydrogen waiting to turn into heavier elements. Stars and other hot, massive objects fuse these protons with neutrons and other protons to make helium et al. Where are the neutrons supplied to these fusion reactors from? Are they also simply floating through the vaccum of space with the protons, waiting to be converted into something more complex?

- A proton by itself is a hydrogen ion, and massive amounts of hydrogen particles can form gas and react with other matter. Does an individual neutron have any particularly special properties or reactions? If you gathered a massive amount of neutrons together, would they form a gas like protons? Since they have mass, can they be compressed into a liquid like hydrogen?

It was the greatest and most destructive thing to science, I ever saw. Neutrons were never proven, using the same principles that were used to isolate elements on the periodic table, elements 1 through 86.

The claim was that with this new particle that science could advance much further, and so much more quickly that it would be worth it, to over look the fact that the neutron obviously does not check out, as real.

At this time real scientists were shunned. And poor scientists were advanced with big government money. I mean big money from the government. And the neutron became a particle, in the minds of multi subatomic particle, scientists.
The older Universal Scientists, who were not readily understood, and very hard to find, came forward and said stop, you are like spoiled children in a very neat candy shop.

And the multi subatomic particle scientists acted much like spoiled children in a candy shop. Willy Wonker and the chocolate factory come to mind.

From there it degraded into something so unscientific and cult like that, if you don't see it, maybe you should not be in the field of science.

NASA is claiming that with 100 billion dollars, and a lot of multi subatomic particle scientists, they might put us on the moon.

No one would believe the tiny sum of money that put us on the moon originally. And inflation does not explain it.



Sincerely,


William McCormick

Originally Posted by Frenchi

When two protons fuse, one proton changes into a neutron and releases a positron, a neutrino, and extra energy. According to Wikipedia, the atomic mass of a deuteron is 2.014102 u, while the sum of its free constituent parts is 2.01649 u, giving a binding energy of 0.002388 u, or 2.224 MeV. Since the fusion reaction releases only 0.42 MeV of extra energy ( I assume this is in the form of heat? ), then the other 1.804 MeV is going into the mass of the positron and the neutrino?

The .42MeV extra energy is most likely to be carried away in the form of kinetic energy (which is a source of heat). But you should know that this is only one of the fusion reactions in the sun and it is the one that produces the least energy I think.

But you are confusing two different equations. The 2.224MeV is from a proton and a neutron while the .42 is from two protons. Recalculating for the two protons gives .001548u which converts to 1.4417MeV then subracting the positron .511MeV gives .9307MeV. Then all that is missing from the equation is .511Mev for if you subtract this from the .9307 you get .42MeV. Is the fact that this is equal to the mass of an electron just a coincidence?

There is some ambiguity about what the equation in Wikipedia actually means. For example, is the .42MeV in the equation
2 protons -> deterium + elecron +neutrino + .42MeV
just the energy excess over the mass energies? If so then something does not add up, but I don't think this is the case. I think it is more likely that the .42MeV is the excess energy over that which is required for the reaction to occur.

You must realize that the whole thing is a bit more complicated than that equation makes it appear because this is a reaction that will only ocurr at high temperatures, which means that there is kinetic energy that must go into the reaction as well to overcome the coulomb barrier between the protons.

Solar netrinos do carry between 0 and 20MeV depending on the interaction they are produced by. So it is my guess that the .511MeV we found missing in the Wikipedia equation is carried away by the neutrino. I would say that it is the minimum kinetic energy to be carried away by all the resultants but since the neutron has a nearly insignificant mass, it is naturally going to carry away nearly all of the kinetic energy, due to conservation of momentum.

Originally Posted by Frenchi

Where does the visible light from the plasma come from in this reaction? The positron annihilates with an electron to release two gamma ray photons, but I can't imagine gamma rays are something desirable to look at =)

While the neutrinos have a very low cross section (especially at .511MeV) and are likely to carry away all the energy they start with, the same is NOT true of the gamma rays.

Originally Posted by Frenchi

So we now have a bound system that has given out some of its energy, but this is not in violation of energy conservation because we can simply put 2.224 MeV (the binding energy) back into the deuteron through some other process to split it into its composite pieces again, correct?

That is correct.