In the early radioactive research, Rutherford has found that the radiation emitted by radioactive substances actually belongs to different kinds. He called positively charged alpha rays, negatively charged beta rays and electromagnetic waves unaffected by magnetic fields gamma rays. 19 10, Rutherford bombarded atoms with alpha particles and discovered the existence of atomic nuclei. Thus, the nucleation model of atoms is established.
If an atom has a nucleus, what is the nucleus made of? Because an atom is electrically neutral, it must be positively charged, and its charge is the same as that of electrons outside the nucleus. 19 14 years, Rutherford bombarded hydrogen with cathode rays. As a result, the electrons of hydrogen atoms are knocked out and become positively charged cations, which are actually the nuclei of hydrogen. Rutherford speculated that this is an anode ray as opposed to a cathode ray, which has a charge unit and a mass unit. Rutherford named it proton.
19 19, Rutherford bombarded nitrogen atoms with accelerated high-energy alpha particles, and found that nitrogen nuclei were knocked out of protons and nitrogen atoms became oxygen atoms. This may be the first time that human beings have really changed one element into another, and the dream of an alchemist has come true for the first time in thousands of years. But the transmutation of this element has no practical value for the time being, because only one of hundreds of thousands of particles was hit by high-energy particles. By 1924, Rutherford had produced protons from the nuclei of many light elements, further confirming the existence of protons.
After the discovery of electrons and protons, people initially speculated that the nucleus was composed of electrons and protons, because both α particles and β particles were emitted from the nucleus. But Rutherford's student Moselle (1887- 19 15) noticed that the number of positive charges carried by the nucleus is equal to the atomic number, but the atomic weight is greater than the atomic number, which shows that if the light of the nucleus is composed of protons and electrons, its mass will be insufficient, because the mass of electrons can be ignored. Based on this, Rutherford speculated that there might be electrically neutral particles as early as 1920.
Chadwick (1891-1974), another Rutherford student, is looking for this kind of electrically neutral particle in cavendish's laboratory. He has been designing an acceleration method to make protons get high energy, thus hitting the nucleus and looking for evidence about neutral particles. 1929, he is going to bombard beryllium atom, because it will not release protons under the impact of alpha particles, and may split into two alpha particles and a neutron.
At the same time, German physicist Porter and his student Becker have left first. Since 1928, they have been doing bombardment experiments of beryllium nuclei. The results show that when α particles bombard it, it can emit extremely penetrating rays, and the rays are electrically neutral. But they think it's a special kind of gamma ray. In France, Madame Curie's son-in-law and daughter Aurio Curie are doing similar experiments. As soon as Potter's results were published, they further confirmed it, but they also mistakenly thought that the new ray was a kind of gamma ray.
The year is 1932. After seeing the experimental results of German and French colleagues, chadwick realized that this new ray was probably the neutron he had been looking for for for years. He immediately started the experiment and published the paper "Neutrons may exist" in less than a month. He pointed out that gamma rays have no mass and it is impossible to knock protons out of the nucleus. Only those particles that have roughly the same mass as protons have this possibility. Secondly, chadwick measured the mass of neutrons by the cloud chamber method, and confirmed that neutrons are indeed electrically neutral. This is how neutrons are discovered. Iorio-Curie later said that if they had listened to Rutherford's lecture in France in 1932, they would not have missed this important discovery, because Rutherford had just talked about his conjecture about the existence of neutrons. Chadwick won the 1935 Nobel Prize in Physics for discovering neutrons.