In modern experimental and applied physics, neutrons play an important role. With their help, it was possible to release energy atomic nucleus in the process of nuclear fission and create powerful sources of energy. Since the neutron is an uncharged particle, the Coulomb barrier does not prevent its penetration into the nucleus. This provides special opportunities for using the neutron to study nuclear structures and reactions.
The history of the discovery of the neutron is very characteristic of the development paths nuclear physics at all. Rutherford, back in 1920, based on general considerations, predicted the existence of a particle with a mass approximately equal to the mass of a proton, and even outlined some of its properties.
In 1930, Bothe and Becker, irradiating a plate with particles, observed some kind of radiation that acted on the counter. This “something” could not be -particles, since the ranges of the -particles were less than the thickness of the plate used. Since this radiation was weakly absorbed by lead, it was natural to consider it y-rays.
In 1932, Joliot and Curie repeated the experiment with In the path of unknown radiation, they placed paraffin and observed protons knocked out of the paraffin. The energy of the protons turned out to be equal. It was suggested that a nuclear photoelectric effect occurs. From general laws kinematics, it can be shown that protons of such energy could be knocked out of the nucleus due to the nuclear photoelectric effect only if the energy of the primary ones exceeded But by this time it was already clear that the nucleus was characterized by energy levels of the order of only a few units and therefore the nuclei emitting could not have an excited level with energy equal to Thus, the question of the source of such hard energy was not resolved.
Chadwick, guided by Rutherford's idea, analyzed the results of the experiments of Bothe and Becker, Joliot and Curie and suggested that the new penetrating radiation does not consist of photons, but of heavy neutral particles. By observing nitrogen recoil nuclei in a cloud chamber, resulting from the interaction of new radiation with nitrogen, and recoil protons formed in paraffin, Chadwick was the first to determine the mass of the neutron, which turned out to be approximately equal to the mass of the proton.
Let us consider the laws of conservation of energy and momentum, from which the value of the neutron mass was first obtained. If we assume that neutrons knock out recoil protons from paraffin and consider the collision of a neutron with a proton as elastic scattering, then we can write for a head-on collision, when the speed acquired by the proton is maximum:
where is the mass of the neutron; neutron speed before collision; neutron speed after collision; proton mass and speed.
Here two equations contain three unknown quantities: (the speed of a proton is determined by its path). Therefore, additional experience is required. To obtain the third equation, the experiment on nitrogen is repeated with the same neutrons (the mass of the nitrogen nucleus and the maximum recoil energy of the nitrogen nucleus collided with by the neutron is determined. It is equal to. The recoil energy of the proton is equal to. Therefore, it is possible to determine the velocities of protons and nitrogen nuclei by solving the equations jointly for the velocities of the nuclei return, we'll get
>> Discovery of the neutron
§ 103 DISCOVERY OF THE NEUTRON
The most important stage in the development of atomic nuclear physics was the discovery of the neutron in 1932.
Artificial transformation of atomic nuclei. For the first time in human history, the artificial transformation of nuclei was carried out by Rutherford in 1919. This was no longer an accidental discovery.
Since the core is very stable, and neither high temperatures, no pressure, no electromagnetic fields do not cause the transformation of elements and do not affect the rate of radioactive decay, then Rutherford suggested that very high energy is needed to destroy or transform the nucleus. The most suitable carriers of high energy at that time were alpha particles emitted from nuclei during radioactive decay.
The first nucleus to undergo artificial transformation was that of the nitrogen atom. By bombarding nitrogen with high-energy particles emitted by radium, Rutherford discovered the appearance of protons - the nuclei of the hydrogen atom.
In the first experiments, registration of protons was carried out using the scintillation method, 1 and their results were not sufficiently convincing and reliable. But a few years later, the transformation of nitrogen was observed in a cloud chamber. About one -particle for every 50,000 -particles emitted by the radioactive drug in the chamber is captured by a nitrogen nucleus, which results in the emission of a proton. In this case, the nitrogen nucleus turns into the nucleus of an oxygen isotope:
Figure 13.9 shows one photograph of this process. On the left you can see a characteristic “fork” - a branching of the track. The thick trace belongs to the oxygen nucleus, and the thin trace belongs to the proton. The remaining -particles do not undergo collisions with nuclei, and their tracks are straight. Other researchers discovered transformations under the influence of -particles of fluorine, sodium, aluminum, etc. nuclei, accompanied by the emission of protons. Nuclei of heavy elements found at the end periodic table, did not experience transformations. Obviously, due to the large electric (positive) charge, the particle could not get close to the nucleus.
1 Scintillation is a flash that occurs when particles hit a surface coated with a layer of a special substance, for example a layer of zinc sulfide.
Joliot-Curie Frederic (1900-1958)- French scientist and progressive public figure. Together with his wife Irene, he discovered artificial radioactivity in 1934. The work of the Curies on the study of beryllium radiation under the influence of -particles was of great importance for the discovery of neutrons. In 1939, with his colleagues, he first determined the average number of neutrons emitted during the fission of the nucleus of a uranium atom, and showed the fundamental possibility of chain nuclear reaction with the release of energy.
Discovery of the neutron. In 1932, the most important event for all nuclear physics took place: the neutron was discovered by Rutherford’s student, the English physicist D. Chadwick.
When bombarded with beryllium particles, protons did not appear. But some strongly penetrating radiation was discovered that could overcome such an obstacle as a lead plate 10-20 cm thick. It was assumed that these were high-energy rays.
Irène Joliot-Curie (daughter of Marie and Pierre Curie) and her husband Frederic Joliot-Curie discovered that if a paraffin plate is placed in the path of the radiation generated by the bombardment of beryllium particles, the ionizing ability of this radiation increases sharply. They correctly assumed that the radiation knocks out protons from the paraffin plate, which are present in large quantities in such a hydrogen-containing substance. Using a cloud chamber (the experimental diagram is shown in Figure 13.10), the Joliot-Curie spouses discovered these protons and estimated their energy based on their path length. According to their data, if protons were accelerated as a result of collisions with -quanta, then the energy of these quanta should have been enormous - about 55 MeV.
Chadwick observed in a cloud chamber the tracks of nitrogen nuclei colliding with beryllium radiation. According to his estimate, the energy of -quanta capable of imparting the velocity κ to nitrogen nuclei, which was detected in these observations, should have been 90 MeV. Similar observations of tracks of argon nuclei in a cloud chamber led to the conclusion that the energy of these hypothetical quanta should be 150 MeV. Thus, considering that nuclei come into motion as a result of collisions with massless particles, researchers came to an obvious contradiction: the same quanta had different energies.
It became obvious that the assumption about the emission of beryllium quanta, i.e., massless particles, is untenable. Some fairly heavy particles fly out of beryllium under the influence of -particles. After all, only in collisions with heavy particles could protons or nuclei of nitrogen and argon receive the high energy that was observed experimentally. Since these particles had great penetrating power and did not directly ionize the gas, they were therefore electrically neutral. After all, a charged particle interacts strongly with matter and therefore quickly loses its energy.
The new particle was called a neutron. Its existence was predicted by Rutherford more than 10 years before Chadwick’s experiments. From the energy and momentum of the nuclei colliding with neutrons, the mass of these new particles was determined. It turned out to be slightly larger than the mass of a proton - 1838.6 electron mass instead of 1836.1 for a proton. It was eventually established that when β-particles enter beryllium nuclei, the following reaction occurs:
Here is the symbol for the neutron; its charge is zero, and relative mass- about one.”
A neutron is an unstable particle: a free neutron decays in about 15 minutes into a proton, an electron and a neutrino - a massless neutral particle.
An elementary particle - a neutron - has no electrical charge. The mass of a neutron is approximately 2.5 electron masses greater than the mass of a proton.
Explain why, in a central collision with a proton, the neutron transfers all the energy to it, but in a collision with a nitrogen nucleus, only part of it.
Myakishev G. Ya., Physics. 11th grade: educational. for general education institutions: basic and profile. levels / G. Ya. Myakishev, B. V. Bukhovtsev, V. M. Charugin; edited by V. I. Nikolaeva, N. A. Parfentieva. - 17th ed., revised. and additional - M.: Education, 2008. - 399 p.: ill.
Calendar and thematic planning in physics, video on physics online, Physics and astronomy at school download
Lesson content lesson notes supporting frame lesson presentation acceleration methods interactive technologies Practice tasks and exercises self-test workshops, trainings, cases, quests homework discussion questions rhetorical questions from students Illustrations audio, video clips and multimedia photographs, pictures, graphics, tables, diagrams, humor, anecdotes, jokes, comics, parables, sayings, crosswords, quotes Add-ons abstracts articles tricks for the curious cribs textbooks basic and additional dictionary of terms other Improving textbooks and lessonscorrecting errors in the textbook updating a fragment in a textbook, elements of innovation in the lesson, replacing outdated knowledge with new ones Only for teachers perfect lessons calendar plan for the year methodological recommendations discussion programs Integrated LessonsIn 1920, Rutherford conjectured about the existence of a neutral elementary particle formed as a result of the merger of an electron and a proton. To conduct experiments to detect this particle in the thirties, J. Chadwick was invited to the Cavendish Laboratory. The experiments took place over many years. Using an electrical discharge through hydrogen, free protons were produced, which bombarded the nuclei various elements. The calculation was that it would be possible to knock the desired particle out of the nucleus and destroy it, and indirectly record the acts of knocking out from the tracks of proton and electron decays.
In 1930, Bothe and Becker irradiated a- beryllium particles discovered radiation of enormous penetrating power. Unknown rays passed through lead, concrete, sand, etc. At first it was supposed to be tough x-ray radiation. But this assumption did not stand up to criticism. When observing rare acts of collision with nuclei, the latter received such a large return that to explain it it was necessary to assume an unusually high energy of X-ray photons.
Chadwick decided that in the experiments of Bothe and Becker, the neutral particles he was trying to detect flew out of beryllium. He repeated the experiments, hoping to detect leaks of neutral particles, but to no avail. No tracks were found. He put aside his experiments.
The decisive impetus for the resumption of his experiments was the paper published by Irène and Frédéric Joliot-Curie on the ability of beryllium radiation to knock protons out of paraffin (January 1932). Taking into account the results of Joliot-Curie, he modified the experiments of Bothe and Becker. The diagram of his new installation is shown in Figure 30. Beryllium radiation was produced by scattering a- particles on a beryllium plate. A paraffin block was placed in the radiation path. It was discovered that radiation knocks protons out of paraffin.
We now know that the radiation from beryllium is a stream of neutrons. Their mass is almost equal to the mass of a proton, so neutrons transfer most of their energy to protons flying forward. The protons knocked out of paraffin and flying forward had an energy of about 5.3 MeV. Chadwick immediately rejected the possibility of explaining the knocking out of protons by the Compton effect, since in this case it was necessary to assume that the photons scattered on protons had a huge energy of about 50 MeV(at that time the sources of such high-energy photons were not known). Therefore, he concluded that the observed interaction occurs according to the scheme
Joliot-Curie reaction (2)
In this experiment, not only were free neutrons observed for the first time, it was also the first nuclear transformation - the production of carbon by the fusion of helium and beryllium.
Task 1. In Chadwick's experiment, the protons knocked out of paraffin had the energy 5.3 MeV. Show that for protons to acquire such energy during photon scattering, it is necessary that the photons have energy 50 MeV.
After it was discovered that substances consist of molecules, and those in turn - of atoms, physicists were faced with a new question. It was necessary to establish the structure of atoms - what they are made of. His students also took on the solution to this difficult problem. They discovered the proton and neutron at the beginning of the last century
E. Rutherford already had assumptions that an atom consists of a nucleus and electrons revolving around it at enormous speed. But what the nucleus of an atom consists of was not entirely clear. E. Rutherford proposed the hypothesis that in the composition of the atomic nucleus of any chemical element there must be a core
It was later proven by a series of experiments that resulted in the discovery of the proton. The essence of E. Rutherford's experimental experiments was that nitrogen atoms were bombarded with alpha radiation, with the help of which some particles were knocked out of the nitrogen atomic nucleus.
This process was recorded on photosensitive film. However, the glow was so weak, and the sensitivity of the film was also low, so E. Rutherford suggested that his students, before starting the experiment, spend several hours in a row in a dark room so that their eyes could see barely noticeable light signals.
In this experiment, from the characteristic light traces, it was determined that the particles that were knocked out were the nuclei of hydrogen and oxygen atoms. E. Rutherford's hypothesis, which led him to the discovery of the proton, was brilliantly confirmed.
E. Rutherford proposed to call this particle a proton (translated from Greek language"protos" means first). In this case, we must understand this in such a way that the atomic nucleus of hydrogen has such a structure that only one proton is present in it. This is how the proton was discovered.
It has a positive electric charge. In this case, it is quantitatively equal to the charge of the electron, only the sign is opposite. That is, it turns out that the proton and electron seem to balance each other. Therefore, all objects, since they consist of atoms, are initially uncharged, and they receive an electric charge when an electric field begins to act on them. The structure of the atomic nuclei of various chemical elements may contain more protons than in the hydrogen atomic nucleus.
After the discovery of the proton was made, scientists began to understand that the nucleus of an atom of a chemical element consists not only of protons, since, conducting physical experiments with the nuclei of the beryllium atom, they discovered that there were four units in the nucleus, while in general core mass - nine units. It was logical to assume that another five units of mass belong to some unknown particles that do not have an electric charge, since otherwise the electron-proton balance would be disrupted.
A student of E. Rutherford, he conducted experiments and was able to detect elementary particles that flew out of the atomic nucleus of beryllium when they were bombarded with alpha radiation. It turned out that they do not have any electrical charge. The absence of charge was discovered due to the fact that these particles did not react to. Then it became clear that the missing element of the structure of the atomic nucleus had been discovered.
This particle discovered by D. Chadwick was called the neutron. It turned out that it has the same mass as a proton, but, as already mentioned, has no electrical charge.
In addition, it was confirmed experimentally that the number of protons and neutrons is equal to the serial number of a chemical element in the periodic table.
In the Universe you can observe objects such as neutron stars, which are often the final stage in the evolution of stars. Such neutron stars are very dense.
Description of the video lesson
An atom consists of a nucleus and an electron shell. The nucleus contains two types of nucleons - protons and neutrons. In 1919, Rutherford, while studying the physics of the atomic nucleus, was the first in the history of mankind to carry out the artificial transformation of nuclei, which served as the impetus for new discoveries. He suggested that very high energy is needed to destroy or transform the nucleus, because the nucleus is very stable and is not affected by high temperatures, pressure, and electromagnetic fields. Rutherford was also able to experimentally verify that temperature, pressure and the electromagnetic field do not affect the rate of radioactive decay of the nucleus, the carriers of which at that time were considered to be alpha particles emitted from nuclei during radioactive decay. Rutherford's experience was as follows. The nitrogen atom was bombarded by high-energy α particles emitted by radium. As a result, the appearance of protons - the nuclei of the hydrogen atom - was discovered. Proton registration was carried out using the scintillation method. The results obtained needed to be confirmed. This was accomplished several years later by observing the transformation of nitrogen in a cloud chamber. Then scientists concluded about the transformation of the nitrogen nucleus:
EN 14 -7 into the nucleus of the oxygen isotope 17 - 8 and at the same time a proton is emitted - a hydrogen atom АШ 1 1. To carry out this transformation, one α particle out of every 50,000 α particles emitted by the radioactive drug in the cloud chamber is captured by the nitrogen nucleus . The photograph of this process shows the branching of the track. The thick trace belongs to the oxygen nucleus, and the thin trace belongs to the proton. The tracks of the remaining α particles are straight, so they do not collide with nitrogen nuclei. Similar experiments on the transformation of the nuclei of one element into the nuclei of another under the influence of α-particles were successfully carried out with the nuclei of fluorine, sodium, aluminum and other elements. In all cases, protons were also emitted. Problems arose only with the nuclei of heavy elements, which are at the end of the periodic table. They did not experience transformations, because the alpha particle could not get close to the nucleus, because it has a large electrical positive charge.
In 1932, Rutherford's student, English physicist James Chadwick, discovered the neutron. He bombarded beryllium with alpha particles. In this case, protons did not appear, but highly penetrating radiation was discovered that could overcome a lead plate 10-20 cm thick. Chadwick suggested that these were high-energy γ-rays. The French scientific couple Frederic and Irene Joliot-Curie also worked in the same direction. They discovered artificial radioactivity in 1934. The results of their experiments on studying the radiation of beryllium under the influence of α-particles had great value for the discovery of neutrons. The study of the atomic nucleus did not end there, but only flared up with greater force. In 1939, Joliot-Curie and his colleagues proved the possibility of a nuclear chain reaction with the release of energy and determined the average number of neutrons emitted during the fission of the nucleus of a uranium atom. Continuing their experiments, the Joliot-Curie couple discovered that if a paraffin plate is placed in the path of the radiation generated when beryllium is bombarded with alpha particles, the ionizing ability of this radiation quickly increases, because the radiation knocks out protons from the paraffin plate, which are abundant in this hydrogen-containing substance . Protons were detected using a cloud chamber, and their energy was estimated from their path length. In their opinion, protons were accelerated as a result of collisions with -quanta having enormous energy - about 55 MeV (megaelectronvolt).
1 megaelectronvolt (MeV) is 1 million electronvolts. If we compare with a temperature of 1 eV of approximately 11 6040C, Chadwick, observing in a cloud chamber the tracks of nitrogen nuclei that have experienced a collision with beryllium radiation, argued that the energy of -quanta capable of imparting speed to nitrogen nuclei should be 90 MeV, and for argon nuclei the energy of these hypothetical -quanta should be 150 MeV. The results of these experiments indicated that nuclei, as a result of collisions with massless particles, begin to move, and the same quanta will have different energies. This led scientists astray, since it turned out that the assumption about the emission of massless particles - quanta by beryllium is incorrect, i.e., from beryllium under the influence of - particles some other rather heavy particles fly out, which, when colliding with protons or nuclei of nitrogen and argon, could get more energy. In addition, these particles, having great penetrating ability, did not ionize the gas, but were electrically neutral, since a charged particle quickly loses its energy as a result of interaction with matter.
This particle was called a neutron. The mass of neutrons was determined by the energy and momentum of the nuclei colliding with them. It turned out to be slightly larger than the mass of a proton - 1838.6 electron mass instead of 1836.1 for a proton. The mass of a neutron exceeds the mass of a proton by 1.94 MeV, that is, more than 2.5 masses or, more simply, 1840 times more than an electron. Therefore, they say that almost the entire mass of an atom is concentrated in its nucleus.As a result of the entry of -particles into beryllium nuclei, a reaction occurs that converts beryllium into carbon and releases a neutron.A neutron is an unstable elementary particle that has no electrical charge. EN one zero - symbol of the neutron; the charge is zero and the relative mass is one. A free neutron decays into a proton, an electron, and a neutrino—a massless neutral particle—in about 15 minutes. The mass of a neutron is approximately 2.5 electron masses or 1840 times greater than the mass of a proton. Neutron research. Shapiro and Estulin in 1955, carrying out direct measurements of the neutron charge by the deflection of a beam of thermal neutrons in an electrostatic field, determined that the neutron charge is less than 6 times 10 to the minus 12 power of the electron charge e. Having checked the measurement results under the best conditions of beam collimation by reflection from mirrors they received: the charge is equal to the sum or difference of minus one point nine tenths and three point seven tenths multiplied by 10 to minus 18 degrees of the charge of the electron, i.e. the charge on the neutron was not detected.
It is very difficult to observe the decay of neutrons as they pass through matter. However, it can be observed in a vacuum; for this it is necessary to use intense beams of slow neutrons.
The half-life of the neutron was determined in 1950. According to Robson, it turned out to be 9-25 minutes. In subsequent works by Robson, a refined value of the period was given: 12.8 ± 2.5 minutes.
In 1967, Christensen and other scientists carried out new measurements of the half-life of the neutron, and found that the half-life was equal to: 650 plus or minus 10 seconds. The average lifetime τ (tau) is related to the half-life by the relation: The half-life is equal to the product of the lifetime of a tau neutron times natural logarithm two, calculating the natural logarithm of two, we get the half-life equal to 0.69 times the lifetime. Thus, the average lifetime of τ (tau) is 940 plus or minus 15 seconds, or about 10 to the third power of a second.
Now neutrons are very widely used. IN nuclear reactors When heavy uranium nuclei fission under the influence of neutrons, very large energy is released. However, this process must be controlled, since the amount of energy can be so great that it will lead to an explosion. Therefore, nuclear power plants use moderators of this process.
The question arises: why use neutrons and radioactive uranium? The answer is simple. The use of uranium helps save the earth's fuel resources, although it also requires additional costs to ensure safety.
IN modern world Scientists are trying to find new uses elementary particles- electrons, neutrons and protons. These are colliders, fast neutron reactors.
