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Author Topic: What is the evidence for the existence of protons and neutrons?  (Read 11866 times)

Offline mathew_orman

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Is there an experiments proving existence of  neutrons and protons?
« Last Edit: 25/08/2015 16:45:02 by chris »


 

Offline PmbPhy

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Re: Structure of an atom
« Reply #1 on: 25/08/2015 08:47:14 »
Quote from: mathew_orman
Is there an experiments proving existence of  neutrons and protons?
No. But there is endless amounts of experiments which are consistent with their existence and which therefore has led us to accept their existence as a fact. The science of physics is not about "proving" anything. For more on this please see: http://www.newenglandphysics.org/common_misconceptions/DSC_0002.MOV

Here's an example of a device which leads us to accept the existence of atoms:
https://en.wikipedia.org/wiki/Scanning_tunneling_microscope

In that page are photos in which you can actually "see" the atoms on the surface of the material being examined.
 

Offline alancalverd

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Re: Structure of an atom
« Reply #2 on: 25/08/2015 11:13:54 »
If you define a proton as a stable particle of mass 1 and charge 1, and a neutron as an uncharged particle of mass 1.008 that decays into a proton, an electron and an antineutrino, then yes, the interactions of these particles with mesoscopic objects are observed. Neutron diffraction is particularly good fun as you can extract colllimated and monochromatic neutrons from a reactor by using a rotating disc with a curved channel to select only neutrons with a specific velocity.
 

Online evan_au

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Geiger & Marsden showed that there is a tiny, positively charged nucleus within the much larger atom. This is evidence for protons, which give the nucleus its positive charge. 

Chadwick received the Nobel prize for showing that neutral particles were emitted when alpha particles struck light elements like beryllium.

The existence of different isotopes of an element occurs because the identity of an element is determined by the number of protons & electrons. The number of neutrons can vary over a small range, which affects the mass of the atom, but does not change the element.

The atomic bomb dropped on Japan 70 years ago only works because the neutral neutron is not affected by the intense electrical repulsion of the many protons in the nucleus of uranium nuclei.
 

Offline mathew_orman

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Re: Structure of an atom
« Reply #4 on: 27/08/2015 13:27:29 »
Yes, evidence of atoms exist...
 

Offline chiralSPO

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Re: Structure of an atom
« Reply #5 on: 27/08/2015 14:00:43 »
Yes, evidence of atoms exist...

and evidence of all sorts of subatomic particles, including protons and neutrons.

I think some of the most important evidence of protons and neutrons comes from nuclear magnetic resonance (NMR, also what's used in MRI https://en.wikipedia.org/wiki/Nuclear_magnetic_resonance ). The magnetic behavior of an atom's nucleus is very much consistent with the nucleus being composed of one or more spin-1/2 particles that each have a mass of approximately 1 au (the neutral ones are a little heavier than the positive ones). By comparing the magnetic behavior of different isotopes of a single element, it becomes quite apparent.
 

Offline dlorde

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Another way to look at it is that protons and neutrons were proposed to explain experimental observations of the behaviour of the atomic nucleus. The Experimental evidence led to the idea of protons and neutrons, not the other way around.
 

Offline mathew_orman

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If a neutron, which is a hypothetical particle, has no known force field, it should not be possible to detect it.
Also how are neutrons able to stay together so close if the calculated mutual repulsive force is so large?
« Last Edit: 28/08/2015 08:41:36 by mathew_orman »
 

Offline Colin2B

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If a neutron, which is a hypothetical particle, has no known force field, it should not be possible to detect it.
Many chemicals and particles can be detected by their reaction to other materials.
There are quite a few reactions that will detect neutrons eg He gas bombarded with neutrons produces an isotope of He and a γ ray of a specific energy.
Neutron detection is well established in a lot of industries so it would be worth searching the net as there must be a lot of info out there
 

Offline PmbPhy

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If a neutron, which is a hypothetical particle, has no known force field, it should not be possible to detect it.
Also how are neutrons able to stay together so close if the calculated mutual repulsive force is so large?
Regarding your question about proving something in science. Science is not about proving things. It's about forming hypotheses and testing what can be deduced from those hypotheses. All we get from testing is whether the observations are consistent with the hypotheses and theories formed from them. For more on this please see:
http://www.newenglandphysics.org/common_misconceptions/DSC_0002.MOV

Regarding your comment above regarding it being a "hypothetical particle, has no known force field." A neutron does have a "force field." It interacts with nucleons via the strong force. While the charge of a neutron is zero it does have a non-zero magnetic moment. It also has a tiny electric dipole moment because its composed of quarks, which are charged.

We can also account for their existence from nuclear reactions. See:
https://en.wikipedia.org/wiki/Neutron_detection#Basic_physics_of_neutron_detection
 

Offline UltimateTheory

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Re: Structure of an atom
« Reply #10 on: 28/08/2015 13:36:36 »
Quote from: mathew_orman
Is there an experiments proving existence of  neutrons and protons?

Quantum particles can be seen in Cloud Chamber for instance.
https://en.wikipedia.org/wiki/Cloud_chamber

Charged particle is leaving trace.

We can detect kinetic energy of incoming particle, it's correlated to length of trace.
Positively charged particle spin in different direction than negatively charged particle in applied external electric and/or magnetic fields.
If particle is decaying to other charged particled, trace is "ending" and new branches are appearing from that location.

Unstable isotopes proton-rich, are decaying by emission of free proton (or positron and neutrino).
While unstable isotopes neutron-rich are decaying mostly by emission of free neutron (or electron and antineutrino).

For further info read
https://en.wikipedia.org/wiki/Proton_emission
https://en.wikipedia.org/wiki/Neutron_emission
https://en.wikipedia.org/wiki/Beta_decay
https://en.wikipedia.org/wiki/Positron_emission

Alpha particles are Helium-4 atoms. They're emitted in alpha decay process by heavy unstable isotopes, such as Uranium-238.
It's decaying to Thorium-234
U-238 -> Th-234 + He-4 + 4.27 MeV energy released

If such highly accelerated alpha particle will hit f.e. Deuterium (Hydrogen with one proton, one neutron), it'll give it missing energy 2.22 MeV, and there will be disintegration:
H-2 + 2.22 MeV -> p+ + n0
to free proton and free neutron.
Again, it can be seen in Cloud Chamber (proton).
Neutron within 15 minutes (~10 minutes half-life) will decay to:
n0 -> p+ + e- + Ve + 0.782 MeV

If neutron will be captured by stable isotope of precisely picked up element, and after A+1 it'll become radioactive,
it'll decay emitting f.e. electron,
which we will be able to detect and count.
f.e.
Deuterium turns to Tritium
H-2 + n0 -> H-3
Tritium is unstable isotope (but with quite high half-life 12.32 years)
H-3 -> He-3 + e- + Ve + 18.6 keV
it'll decay to Helium-3 atom, emitting electron.
Once again we can detect either Helium (second the lightest gas after all),
or detect electron,
if electron accelerated by 18.6 keV energy will hit some material there will be blink and photons from that location,
can be detected by photomultipliers.

« Last Edit: 28/08/2015 13:40:47 by UltimateTheory »
 

Offline UltimateTheory

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You can build your own particle detector for something like 20-50 usd.
http://www.ultimate-theory.com/en/2014/6/8/how-to-build-cloud-chamber-particle-detector

There are videos showing how it works in practice.
 

Offline mathew_orman

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So, if I understand correctly, neutrons stay together by mutual strong force and simultaneously make the protons also stay within together?
Also this means that strong force only acts on protons and neutrons but not on orbiting electrons?
 

Online evan_au

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Quote from: mathew_orman
strong force only acts on protons and neutrons but not on orbiting electrons?
Yes

Quote
strong force ...make the protons & neutrons stay together?
Yes. See https://en.wikipedia.org/wiki/Strong_interaction

Quote
neutrons stay together by mutual strong force

I don't recall hearing of anyone producing a subatomic particle consisting of two or more neutrons (and no protons).
That doesn't mean it is impossible (or that I have heard of everything).

A single neutron by itself is unstable. I am guessing that two neutrons together might also be unstable?
 

Offline mathew_orman

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Now, if neutrons do not affect electrons then what stops an electron falling down onto the nucleus as being attracted by protons?
« Last Edit: 30/08/2015 12:01:12 by mathew_orman »
 

Online evan_au

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Quote from: evan_au
I don't recall hearing of anyone producing a subatomic particle consisting of two or more neutrons (and no protons).
Now I am in a country which permits access to well-known search engines, I see that a dineutron consisting of two neutrons is a very short-lived state which has been shown to exist transiently in some nuclear interactions.

So the strong nuclear force is not enough, by itself, to hold neutrons together.

Particles consisting solely of more than 2 neutrons are currently not thought to exist (or at least, claims of their existence have not been able to be reproduced).
See: https://en.wikipedia.org/wiki/Neutronium#Neutronium_and_the_periodic_table
 

Online evan_au

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Quote from: mathew_orman
Now, if neutrons do not affect electrons then what stops an electron falling down onto the nucleus as being attracted by protons?
Niels Bohr proposed that electrons can take on only certain specific energy levels. There is a minimum energy level for each atom, and the electron cannot approach the nucleus closer than this.

This simple model of an atom was later superseded by more accurate models of an electron cloud surrounding the nucleus, developed by Heisenberg & Schroedinger.
 

Offline mathew_orman

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Proposed models would not hold an electron from falling onto the nucleus...
Orbiting electrons act as repelling force for atoms and there is no force system that would hold electrons in stable orbits for what ever reason or to satisfy the proposed models...
 

Offline chiralSPO

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Do not try to think of electrons as little particles orbiting a nucleus that is another particle--It's a trap!

You must think of these systems in a quantum mechanical way, not a classical mechanical way. On this scale, it is best to think of electrons in terms of their wave-like nature. Because electrons are so light (low mass) their de Broglie wavelengths are much, much larger than the wavelengths of the nucleus, and therefore, even when the electrons and nucleus share the same center of mass, the electron is much more diffuse, and the nucleus much more concentrated.


I think it is best to think of every electron in an atom as "stuck to the nucleus." The electrons are trapped in the atom, and can only be removed with a significant amount of energy, or can be stolen by another nucleus (or collection of nuclei), given the right circumstances.
 

Offline mathew_orman

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Quantum mechanics does not support continuity of motion and there is no evidence of instantaneous transfer of matter...
 

Offline chiralSPO

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Quantum mechanics does not support continuity of motion and there is no evidence of instantaneous transfer of matter...

Actually, there is plenty of evidence for quantum tunneling (if that's what you mean by instantaneous transfer of matter), your computer wouldn't work without it! Here is one example that I am quite familiar with, that involves comparing the tunneling rate of a hydrogen atom vs that of a deuterium atom: https://en.wikipedia.org/wiki/Kinetic_isotope_effect#Tunneling
 

Online evan_au

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Quote from: mathew_orman
Proposed models would not hold an electron from falling onto the nucleus...
On the contrary, these quantum models of the atom & electron were developed to explain the observed fact that the electron's average distance from the nucleus doesn't fall to zero.

In Heisenberg's model, the mass of the electron had an equivalent wavelength. Like vibrations on a guitar string, the electron could only exist in certain orbits which were an integer number of wavelengths. The smallest orbit is 1 wavelength long, ie the electron cannot fall into the nucleus. (Schroedinger's model gives an even better explanation, but you have to be a supercomputer to solve it for most atoms.)

But there is another case which is instructive: the Muon has the same charge as an electron, but has 207 times the mass. In Heisenberg's model, this means that the Muon wavelength is 207 times smaller than an electron. It is possible to replace an electron in an atom by a Muon; the minimum orbital radius of the Muon is 207 times smaller than an electron - but it still has a minimum average radius around the nucleus.

If you do a Muon substitution in heavy Hydrogen (Deuterium gas), the two Deuterium nuclei are brought close enough so that tunnelling sometimes causes nuclear fusion events, despite the Muon's minimum orbital radius!
 

Offline mathew_orman

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There is no model of motion of such structures as proposed by QED...
At present time we can created 3D simulations of any structures in motion but not a single atomic structure containing electrons spinning around the nucleus exists...
Not even a gold foil at room temperature...
 

Online evan_au

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In my simple understanding of modern quantum theory, it describes the energy that particles can have, and the probabilities of finding them in some particular location, but it does not tell you how they get there. Stronger than that, it states that you cannot define an exact path by which the particle reached that position.

This supersedes simple high-school analogies such as "An atom has electrons circling the nucleus like planets orbit around the Sun". This analogy provides a reasonable mental image of a massive nucleus surrounded by low-mass electrons. But it can also produce incorrect images such as thinking that an electron spins around the nucleus. Like any analogy, it has its limitations.

 
 

Offline chiralSPO

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not a single atomic structure containing electrons spinning around the nucleus exists...

That's true. Because, as others have pointed out, electrons DON'T spin around the nucleus. Therefore it is a good thing that models based on that assumption fail.
 

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