In the previous article, we have talked about the discovery process of electrons, the measurement of electron charge values, and the size of the mass and volume of atoms.
All of this work was done before 1906, when Rutherford announced the discovery of the nucleus in 1911, and in 1913, team member Sleef measured the nuclear charge of the nucleus and found that the nuclear charge of the nucleus was the same as its position in the periodic table.
At this point we know that the hydrogen nucleus has one unit of nuclear power charge, and that helium has two, all the way up to uranium 92, and its core has 92 nuclear power charges.
And according to the concept of relative atomic mass, we can also see that the mass of the atom is about an integer multiple of the mass of the hydrogen atom, so at this time people speculate that the atomic nucleus is composed of hydrogen nuclei and electrons.
For example, in the hydrogen atom, there is only one hydrogen nucleus, so its unit charge number is 1, the relative atomic mass is about 1, then the helium atom should have four hydrogen nuclei and two electrons, so as to ensure that the electron cancels out two units of nuclear power charge, leaving two, and can also ensure that the mass of the helium atom is 4 times the mass of the hydrogen atom.
According to this rule, it could be arranged all the way to the element uranium, which has a relative atomic mass of 238 and an atomic number of 92, and supposedly has 146 electrons in its core.
This was the idea of the structure of the atomic nucleus before the discovery of neutrons in 1932, and one of Rutherford's discoveries during this period more or less validated this idea.
As recently as 1917, Rutherford discovered that bombarding some light nuclei with α particles could cause the nuclei of light atoms to split, and the phenomenon it first observed was this, once he coated the surface of some metals with radioactive radium, and found that there was a flash of light on the nearby zinc sulfide fluorescent screen.
You might be wondering, what's so strange about this phenomenon, it must be that the α particles released by radium hit the fluorescent screen, which caused the flash. But things are not so simple, Rutherford found that the fluorescent screen is placed in a position that exceeds the distance α particles fly in the air.
This shows that the particles hit on the fluorescent screen are not α particles, are they β particles? This thing flies much farther than α particles, and the electromagnetic field test found that this particle is a hydrogen nucleus, which is what we now call protons.
Where did the proton come from? In order to confirm the source of the hydrogen nucleus, Rutherford redesigned the experiment, he let the α particles through the nitrogen, and found that α particles can knock out a proton in the nitrogen nucleus.
So the above is the earliest discovery of nuclear fission by human beings, but also the process of discovering protons, in fact, protons do not need to be found, people already know that there is such a thing, that is, hydrogen nuclei, as I just said, people still use this thing to build other atomic nuclei.
Rutherford's experiment was nothing more than a reaffirmation of the previous view, and the discovery of radioactive decay in 1906 β particles were also shot out of the nucleus, so the above evidence shows that the nucleus is made of protons and electrons. Moreover, Rutherford also proposed the concept of neutrons in 1920, believing that he was a complex of protons and electrons, and that the electrical neutrality was 1 relative to atomic mass. Obviously this is not the same as what we call neutrons today.
This idea went on for decades, and in 1932, an unexplainable phenomenon was discovered that wavered above the idea, which led to the discovery of neutrons.
This discovery is also related to α particles, in 1930, physicists Bot and Becker found that using α particles to bombard beryllium, the rays it emits are much stronger than the penetration of protons and electrons, and can not be deflected by electromagnetism, so they speculated that according to previous experience, this must be an electromagnetic wave similar to γ rays.
By 1932, Ilena and Jolio Curie, The daughter and son-in-law of Marie Curie, used beryllium rays to bombard paraffin wax, a hydrogen-rich substance with many hydrogen atoms, and they discovered that beryllium rays could shoot out the protons in paraffin wax.
This is not surprising, but to their surprise, the speed of the protons emitted by the beryllium rays is very high, the speed means kinetic energy, kinetic energy is energy, they calculated, it turned out that the energy is not conserved in this process, if the beryllium ray is an electromagnetic wave, its collision with the proton belongs to Compton scattering, according to the kinetic energy of the proton we can calculate the energy of the electromagnetic wave, and it turns out that the energy carried by the beryllium ray, that is, the electromagnetic wave they think is carried by the electromagnetic wave is 10 times the energy carried by α the particles that produce it. With this result, the Curies did not doubt the nature of beryllium rays, but suspected that energy might not be conserved at the microscopic level, allowing them to miss a major discovery.
Chadwick, knowing this, told Rutherford, who was working in Rutherford's laboratory, and since Rutherford had predicted the existence of a central composite particle and had also punched protons out of a light nucleus, it was only natural that it was this neutral complex particle that had been punched out α of the beryllium nucleus.
Chadwick found that after the interaction of beryllium rays with hydrogen nuclei, helium nuclei, and nitrogen nuclei, these nuclei have a recoil phenomenon, like the elastic collision between two billiard balls, and the simple exchange of kinetic energy between them is more certain of the idea that beryllium rays are neutral mass streams.
The next most important thing is how to determine some properties of this neutral particle? Chadwick uses this method, which first allows beryllium rays to hit hydrogen nuclei, then measures the recoil velocity of hydrogen nuclei, which is about 3.3 × 10^7 m/s, and then uses the same beryllium rays to hit nitrogen nuclei, which have a recoil velocity of 4.7 × 10^6 m/s, and the relative atomic mass ratio of nitrogen nuclei and hydrogen nuclei is about 14.
From this relationship, Chadwick calculated that the relative atomic mass of the beryllium rays measured was about 1.16, which was not much different from the relative atomic mass of the neutral composite particle predicted by the teacher, of course, this deviation is still relatively large, mainly because the recoil velocity he measured was not accurate.
In February 1932, Chadwick published the above findings, in the paper Chadwick called this particle a neutron, but in his mind, like the teacher, he also believed that neutrons are composite particles of protons and electrons, which are not as basic as protons.
But there is a way to prove whether the neutron is a composite of protons and electrons, and that is Einstein's mass-energy relationship, which is that the internal energy of the complex particle, or the mass of the composite particle, must be less than the mass of its components, which means that now that you have two buns, and you wear them together, the mass of them must be less than the mass of the two buns, and if it is greater than or equal, the combination of the two buns is unstable, or it is difficult to combine, because the system always tends to run to a lower energy state.
That in 1934, Chadwick and Godhaber, they used gamma rays to knock open a deuterium nucleus, got protons and neutrons, and measured neutrons, slightly greater than the mass of protons plus electrons, so neutrons are not a composite of protons and electrons, then some people will say, is not less neutrinos, nor is it, although the decay products of neutrons are protons, electrons and neutrinos, but this does not mean that neutrons are made of those three things.
Since then, people have believed that neutrons are as basic as protons, but then the problem is more difficult, and humans have thought about such a problem at this time, neutrons are not charged, can not counteract the electrostatic repulsion of protons, then what is its use in the nucleus? What is the force that is fighting against the electrostatic repulsion of the protons and gluing the nucleus together?
As early as 1932, Heisenberg published a paper trying to answer this question, arguing that protons and neutrons were tied to the nucleus of an atom by exchanging electrons.
In this process a neutron will emit an electron, become a proton, and then the proton will absorb this electron into a neutron, and when the charge is exchanged, it will also exchange energy and momentum, resulting in an exchange force, and from this description it can be seen that Heisenberg still regards neutrons as a composite of protons and electrons, so its theory must be wrong.
And in 1936, physicists Mohr Tuv, Heidenberg, and Hofstadter found that there was a very strong interaction force between protons and protons when studying the scattering experiment between protons and protons, and the collision cross-section caused by this force was obviously greater than the electromagnetic force, so this showed that the force between protons and protons was not electromagnetic, and the strength of this force between protons and protons, and between protons and neutrons, was the same.
So the final conclusion is that the nuclear force has no relationship with the charge, and the intensity of the nuclear force on protons and neutrons shows that the proton and neutron are twin brothers.
Although this conclusion rejects Heisenberg's conjecture about nuclear forces, it validates one of the ideas in Heisenberg's paper that protons and neutrons are different manifestations of the same particle.
Because in Heisenberg's paper, it proposes a completely new quantum number that describes different manifestations of protons and neutrons, the same as spin, which is proposed by analogy with the concept of spin, we know that particles have spin properties, it has three orientations in space, x, y, z, in each orientation there are two directions of projection, for example, on the z-axis the spin of the electron can face up and down, that is, +1/2 and -1/2, respectively.
In order to distinguish between the two different states of protons and neutrons, Heisenberg abstracted an isotopic spin space, in which the isospinic spin of protons and neutrons is 1/2, and the difference between them is that on the third component of the isospinic spin space, that is, i3, the projection of the proton isospin, that is, the orientation is upward, that is, +1/2, and the orientation of the neutron isospine is -1/2.
For a simple understanding, we can directly think that the isotropic spin space of protons and neutrons can only take two directions, up and down. It's that simple.
In the isospinic spin space, the proton and the neutron are the same particle, but the spin orientation is different, we rotate the proton to become a neutron, and the neutron becomes a proton when it rotates.
The concept of this isomorphic spin is very important in strong interactions, and the creation of the Young Mills gauge field is also related to the isomorphic spins of protons and neutrons, which we will talk about later when we talk about the forces of elementary particles.
If you don't already understand the concept of isomorphic spins, let me give you another example, you don't think of isospines as something spinning, physicists are sometimes very abstract when naming names, because a lot of things are mathematical concepts.
We can analogize the isospine into a charge, for the charge we can think that it has two values, and the isospin has two orientations, the charge can take positive and negative in the abstract charge space, and the isocentric spin in the abstract isospine space can be in two directions, up and down.
Now that I understand, I will mention the isospine in the next article. In addition to the nuclear force problem, there is another problem, so since there are no electrons in the neutron, where do the β rays emitted?
For this problem, in 1934 Fermi proposed a new force, that is, what we now call a weak force, which briefly elaborated on the radioactivity of β, but it would take decades for humans to completely tame the weak force and understand how it transmits the force.
By this time, human beings seem to have mastered something, and it feels like they are about to touch the truth, and you see that protons and neutrons have found that through them they can explain all the elements, plus electrons, and it seems that no other elementary particles are needed anymore, and our world can be built.
The rest of the work is to explain the nuclear force and the weak force, but the real situation is that humans have discovered the tip of the iceberg of the particle world, and from the next lesson, you will find that all kinds of unknown particles with super strange names are very large, and I can guarantee that no one can remember the names of these particles.