In the history of physics, the discovery of neutrons has experienced a tortuous and dramatic cognitive process. It passed by the famous experimental physicist Aurio Curie, which brought many regrets to the couple. Today, revisiting this period of history still gives us a lot of inspiration.
The discovery of neutrons is inseparable from people's exploration of nuclear structure.
1. Discovery of Proton and Proton Electron Model Hypothesis of Nucleus
19 1 1 year, British physicist Rutherford put forward the nuclear structure model of atoms according to the results of scattering experiments of α particles. This model was supported and developed by Bohr, and was quickly recognized by physicists. Since then, a series of questions have been posed to physicists: What is the nucleus made of? Does the nucleus still have a structure? Can it be subdivided?
19 19, Rutherford did the experiment of bombarding nitrogen nuclei with α particles released by radium, and found protons, which realized the artificial evolution of nuclei for the first time. At that time, people realized that the basic particles were only protons, electrons and photons, so in the 1920s, it was generally believed that the nucleus was composed of protons and electrons, and it was assumed that a nucleus with an atomic weight of n and an atomic number of z should be composed of n protons and n-z electrons, and then form neutral atoms with z orbital electrons. This is the "proton-electron" model of the nucleus. However, this assumption encountered some difficulties. One of the difficulties: what is the state of electrons in the nucleus. In the "proton-electron" model, electrons exist in the nucleus as individuals. When N=238, the radius of the nucleus is estimated to be 8.7 cm, while the classical radius of the electron is 2.8 cm. As a part of an individual, electrons are almost the same as the whole nucleus! This is unimaginable; The second difficulty: according to the uncertainty principle proposed by Heisenberg 1927, if an electron is confined in a very small nucleus, its momentum will be very uncertain, so it cannot stay in the nucleus for more than a fraction of a second; The third difficulty: it contradicts many-body statistics and spin theory in quantum mechanics. 1925, Uhlenbeck and Goodschmidt proposed that electrons have spins, and its quantum number is equal to 1/2, and the quantum number of protons is also equal to 1/2. Therefore, for a nitrogen nucleus, because it has 14 protons and 7 electrons, the total number of spins of these particles should be a fraction. However, experiments show that. For the latter two difficulties, many famous physicists suspect that quantum mechanics is not applicable to the interior of the nucleus, and there is no doubt that the "proton-electron" model itself has problems.
2. Rutherford's prediction about "neutron"
When the last two difficulties of the "proton-electron" model did not appear, Rutherford pointed out (1920) that if a proton and an electron were regarded as a complex and a particle, the theoretical contradiction could be solved, and the "proton-electron" complex should be electrically neutral. He predicted: "Under certain conditions, an electron may combine more closely with the hydrogen nucleus, thus forming a neutral dipole. This kind of atom will have very unusual properties. Its external electric field will actually be equal to zero unless it is very close to its core. Therefore, it can pass through matter freely. It may be difficult to detect its characteristics with a spectroscope, and it is impossible to keep it in a closed container. On the other hand, it should easily enter the atomic structure, or combine with the nucleus or be split by the strong field of the nucleus. " Rutherford claimed: "The existence of such atoms seems to be crucial to explain the composition of heavy element nuclei". In the early 1920s, researchers in Cavendish Laboratory tried to detect the formation of this hypothetical "neutron" by introducing a strong current into a hydrogen discharge tube, but failed.
3. Bert Beryllium Radiation Experiment
1930, German physicists Bert (W.W.G. Bothe, 189 1 ~ 1957) and H.Becker bombarded light elements, especially beryllium, and found that beryllium emitted a ray with low intensity but strong penetration. This radiation will not deflect (and therefore will not be charged) in both electric and magnetic fields. After penetrating a 2 cm thick lead plate, the radiation intensity only decreased by 13%. At that time, this kind of radiation was called beryllium radiation. According to the research of all kinds of radiation found at that time, neither alpha ray nor beta ray has such strong penetrating power. The only thing that can penetrate the lead plate and is uncharged is gamma rays, so the two physicists mistakenly thought that they had discovered high-energy gamma rays. According to the fact that the intensity of this ray weakens after passing through the lead plate, they calculated that the energy of this ray is about 10 MeV.
4. Curie's experiment of beryllium radiation bombarding paraffin.
1932, Mr. and Mrs. Iorio Curie repeated Bert's beryllium radiation experiment. Their experimental conditions are very good and they have a strong radiation source, so it is easy to get the same results as Bert. In order to measure the absorption of beryllium radiation by substances, they put all kinds of substances between beryllium plate and radiometer. Unexpectedly, it was found that when paraffin was placed in the path of beryllium radiation, the number of particles recorded by the radiometer not only did not decrease, but was much more than that without paraffin. After identification, they found that protons flew out of paraffin. This shows that beryllium radiation has produced protons from paraffin. According to the velocity of protons, they calculated that the energy of this ray is 50 MeV, which is far from the above 10 MeV. However, Aurio Curie and his wife still follow Bert's wrong thinking, and they interpret this phenomenon as Compton scattering of photon homoons. 193265438+1October 18, Aurio and Curie published their experimental results and comments. Because of their contempt for theory, they lost an opportunity to discover neutrons in vain.
5. chadwick discovered neutrons.
Aurio and Curie's papers spread to Britain, and the British physicist chadwick read their papers and told Rutherford the contents of the papers. It is said that Rutherford shouted "I don't believe it" when he heard their explanation, and neither did chadwick. After some thinking, he immediately realized that the recoil proton with such great energy could never be the result of photon collision, but probably the result of the collision of "neutral particles" predicted by Rutherford ten years ago. He used polonium and beryllium as radioactive sources and bombarded hydrogen, helium, nitrogen and other elements with this new ray. It is found that the properties of this kind of ray are different from those of ordinary rays. Generally speaking, when radiation strikes a substance, the greater the density of the substance, the more it absorbs. The nature of this kind of ray is just the opposite, and the lower the density, the easier it is to be absorbed. When chadwick bombarded hydrogen atoms with this kind of ray, he found that hydrogen nuclei were ejected, which indicated that this kind of ray was a particle stream with a certain mass. Because this particle flow is uncharged, the electric field and magnetic field have no influence on it, so its mass cannot be calculated by its trajectory in magnetic field or electric field. Chadwick believes that when the particle passes through a substance, it will collide with the nucleus in the substance elastically, so as to transfer energy to the nucleus, make the collided nucleus move, and measure the speed of the touched nucleus, so that the mass of the particle can be calculated according to the conservation of momentum and energy. By bombarding hydrogen atoms and nitrogen atoms, he calculated that the mass of this particle is almost equal to the mass of protons, and he called the particle of this ray "neutron"
6. The significance of neutron discovery
The discovery of neutrons has had a great and far-reaching impact on the development of nuclear physics. Neutron is a brand-new particle. Its discovery makes it possible to establish a nuclear model without the participation of electrons, and also solves the problem of whether quantum mechanics is applicable to the interior of the nucleus. Shortly after the discovery of neutrons, the famous physicist Heisenberg published a paper pointing out that quantum mechanics is also applicable to the interior of the nucleus, and pointed out that the nucleus is composed of protons and neutrons. Because the neutron is uncharged, there is no Coulomb repulsion between it and the nucleus, so it can reach all the nuclei and become the most effective tool to promote the evolution of the nucleus. The discovery of neutrons also promoted the study of nuclear force and promoted the development of particle physics.
7. Some revelations
The concept of "neutron" was first put forward by Rutherford to solve the difficulties faced by theory, and was later discovered in experiments. One of the reasons for chadwick's success is that he thought about the concept of neutron. Before that, he tried to generate neutrons by strong discharge or other methods, but all failed, so when they appeared, he could immediately find them clearly and convincingly. The Curies in Iori Yagami, because they were not prepared for this, apparently appeared in their experiments, but they didn't know it. As Aurio said, "If my husband and I had listened to Rutherford's Beckley speech, we wouldn't have let chadwick beat us to it." This also reflects how necessary it is to exchange academic ideas in scientific research.