All these tasks were completed before 1906. Rutherford announced the discovery of atomic nuclei in 19 1 year. 19 13, team member Slif measured the nuclear charge of the nucleus and found that the nuclear charge of the nucleus was the same as its position number in the periodic table.
At this time, we know that the hydrogen core has one unit of nuclear charge, and then there are two helium, up to uranium element 92, and its core has 92 nuclear charges.
Moreover, according to the concept of relative atomic mass, we can also see that the mass of atoms is about an integer multiple of the mass of hydrogen atoms, so people guess that the nucleus is composed of hydrogen nuclei and electrons.
For example, there is only one hydrogen nucleus in a hydrogen atom, so its unit charge number is 1, and its relative atomic mass is about 1. Then there should be four hydrogen nuclei and two electrons in the helium atom, so as to ensure that the electrons can cancel out the nuclear charge of two units, and the remaining two helium atoms have four times the mass of hydrogen atoms.
This idea is not bad, it was quite useful at that time. According to this law, it can be discharged to uranium. The relative atomic mass of uranium is 238 and the atomic number is 92. According to the idea, its core should have 146 electrons.
This is the idea of nuclear structure before neutron discovery in 1932, and a discovery by Rutherford during this period more or less verified this idea.
19 17, Rutherford found that bombarding some light nuclei with alpha particles can lead to light nuclei splitting. The first thing he observed at that time was this. On one occasion, he applied radioactive radium to the surface of some metals and found that the zinc sulfide screen nearby flashed.
You may find this phenomenon strange. To be sure, the alpha particles released by radium hit the fluorescent screen, causing the flash. But it's not that simple. Rutherford found that the position of the fluorescent screen exceeded the flight distance of alpha particles in the air.
Does this mean that the particles hitting the fluorescent screen are not alpha particles, but beta particles? The flying distance of this thing is much farther than that of the alpha particle, and when measuring the electromagnetic field, it is found that this particle is a hydrogen nucleus, which is what we now call a proton.
How did the protons come from? Rutherford redesigned the experiment to confirm the source of hydrogen nuclei. He let alpha particles pass through nitrogen and found that alpha particles can knock out a proton in the nitrogen nucleus. Rutherford was sure it was a hydrogen nucleus.
So the above is the first nuclear fission discovered by human beings, and it is also the process of discovering protons. In fact, protons don't need to be discovered. People already know that there is such a thing, that is, the hydrogen nucleus. As I said just now, people also use this thing to build other nuclei.
So Rutherford's experiment is nothing more than reaffirming the previous point of view, plus 1906 people found that beta particles in radioactive decay are also shot out of the nucleus, so the above evidence shows that the nucleus is composed of protons and electrons. Moreover, Rutherford put forward the concept of neutron in 1920, thinking that it is a compound of protons and electrons, and the relative atomic mass of electric neutrality is 1. Obviously, this is different from what we call neutrons today.
This idea has been going on for decades, until 1932, an unexplained phenomenon was discovered, which led to the discovery of neutrons.
This discovery is also related to alpha particles. 1930, physicists Bert and Becker found that when beryllium was bombarded with alpha particles, the radiation it released was much stronger than the penetration ability of protons and electrons, and it could not be deflected by electromagnetic waves, so according to previous experience, it must be an electromagnetic wave similar to gamma rays.
1932, Elena and Madame Curie's daughter-in-law Joliot-Curie bombarded paraffin with beryllium rays. Paraffin wax is a substance rich in hydrogen, with many hydrogen atoms. They found that beryllium rays can defeat protons in paraffin wax.
This is not surprising, but to their surprise, the speed of protons generated by beryllium rays is very high, which means that kinetic energy, kinetic energy is energy. When they calculate, they find that energy is not conserved in this process. If beryllium ray is electromagnetic wave, then its collision with proton belongs to Compton scattering. According to the kinetic energy of protons, we can calculate the energy of electromagnetic waves, and it turns out that the energy carried by beryllium rays, that is, the energy carried by electromagnetic waves, is the energy carried by α particles that produce them. After obtaining this result, the Curies did not doubt the essence of beryllium rays, but suspected that energy might not be conserved at the microscopic level, which made them miss an important discovery.
When chadwick knew about it, he told Rutherford about it. At that time, he was working in Rutherford Laboratory. Because Rutherford had predicted the existence of central composite particles before and played with protons of light nuclei, it was natural for him to guess that α particles played from beryllium nuclei might be neutral composite particles.
Chadwick found that after beryllium rays interacted with hydrogen nuclei, helium nuclei and nitrogen nuclei, these nuclei had recoil phenomenon, just like the elastic collision between two billiard balls, they simply exchanged kinetic energy, which further confirmed the viewpoint that beryllium rays were neutral mass flow.
Most importantly, how to determine some properties of this neutral particle? The method adopted by chadwick is as follows: firstly, the hydrogen nucleus is hit by beryllium rays, and then the recoil speed of the hydrogen nucleus is measured, which is about 3.310.7 m/s, and then the nitrogen nucleus is hit by the same beryllium rays, and the recoil speed of the nitrogen nucleus is 4.710.6 m/s, and the relative atomic mass ratio of the nitrogen nucleus to the hydrogen nucleus is about 14.
According to this relationship, chadwick calculated that the relative atomic mass of beryllium ray is about 1. 16, which is different from the relative atomic mass of neutral composite particles predicted by the teacher. Of course, this deviation is still relatively large, mainly because the recoil speed he measured is not accurate.
1932 February, chadwick announced the above findings. In the paper, chadwick called this kind of particle neutron, but in his mind, like the teacher, he also thought that neutron is a composite particle of proton and electron, which is not as basic as proton.
But there is a way to prove whether a neutron is a compound of protons and electrons, and that is Einstein's mass-energy relationship. That is to say, the internal energy of composite particles, or the mass of composite particles, must be less than the mass of its components. That is to say, now you have two buns, and if you wear them together, their mass must be less than the mass of these two buns. If it is greater than or equal to it, the combination of two steamed buns is unstable or difficult.
Then in 1934, chadwick and Goldhaber smashed a deuterium with gamma rays, and got protons and neutrons. The measured neutron mass is slightly greater than that of protons and electrons, so the neutron is not a compound of protons and electrons. Some people will say, is there a neutrino missing? No, although the decay products of neutrons are protons, electrons and neutrinos, it doesn't mean that neutrons are those three.
Since then, people believe that neutrons are as basic as protons, but the next question is more difficult. At this time, human beings have thought about such a problem. Neutrons have no charge and cannot counteract the electrostatic repulsion of protons. What is its use in the nucleus? What is the force that binds the nuclei together against the electrostatic repulsion of protons?
As early as 1932, Heisenberg published a paper to try to answer this question. He believes that protons and neutrons are combined with the nucleus by exchanging electrons.
In this process, a neutron will release an electron and become a proton, and then the proton will absorb this electron and become a neutron. When exchanging charges, energy and momentum are also exchanged, resulting in an exchange force. It can be seen from this description that Heisenberg still regards neutrons as a compound of protons and electrons, so his theory must be wrong.
Moreover, in 1936, physicists Moore Taft, Heidelberg and hofstadter found that there is a very strong interaction between protons, and the collision cross section caused by this force is obviously larger than the electromagnetic force, so this shows that the interaction between protons is not electromagnetic force, but the intensity of this interaction is between protons and protons, as well as between protons and neutrons.
So the final conclusion is that nuclear force has nothing to do with charge, and the strength of nuclear force on protons and neutrons shows that protons and neutrons are twin brothers.
Although this conclusion denies Heisenberg's conjecture about nuclear force, it verifies a point in Heisenberg's paper that protons and neutrons are different forms of the same particle.
Because in Heisenberg's paper, it puts forward a brand-new quantum number, which describes the different forms of protons and neutrons. They are all spins, and this quantum number is put forward by analogy with the concept of spin. We know that particles have spin properties, and there are three directions in space, X, Y and Z, and each direction has two directions of projection. For example, the spin of electrons on the Z axis can be upward or downward, which are+1/ respectively.
Heisenberg abstracted an isospin space in order to distinguish two different states of protons and neutrons. In isospin space, the isospin of proton and neutron is 1/2. The difference between them is that in the third component of isospin space, that is, I3, the projection of proton isospin, that is, the orientation is upward, that is,+1/2, and the neutron isospin.
For simple understanding, we can directly think that the isospin space of protons and neutrons can only take two directions, upward and downward. It's that simple.
In isospin space, protons and neutrons are the same particle, but their spin orientations are different. We turn protons into neutrons and neutrons into protons.
The concept of isospin is very important in strong interaction, and the generation of Young Mills gauge field is also related to the isospin of protons and neutrons, which we will talk about later when we talk about the force of elementary particles.
If you haven't understood the concept of isospin, I'll give you another example. Don't think that isospin is a rotating thing. Physicists sometimes give it an abstract name because many things are mathematical concepts.
We can compare isospin to electric charge. For charge, we can think that it has two values, while isospin has two orientations. Charge can take positive and negative values in the abstract charge space, and isospin can take up and down directions.
Now that you understand, I will mention isospin again in a later article. In addition to nuclear power, there is another problem. Since there are no electrons in neutrons, where did the beta rays come from?
For this problem, Fermi put forward a new force in 1934, which is what we call weak force now, and briefly expounded β radioactivity. However, it will take decades for human beings to completely tame the weak force and understand how it transmits the force.
By this time, human beings seem to have mastered something and feel that they are about to touch the truth. Look at protons and neutrons. Through them, all elements can be explained. Plus electrons, it seems that our world can be established without any other elementary particles.
The remaining task is to explain nuclear force and weak force, but the real situation is that human beings have just discovered the tip of the iceberg of the particle world. Starting from the next class, you will find all kinds of unknown particles with super strange names, and the number is very large. I can guarantee that no one can remember all the names of these particles.