Slide 2
Proton discovery
After the creation of the nuclear model of the atom, the question of the composition of the atomic nucleus became one of the main issues in nuclear physics. What does an atomic nucleus consist of? One of the main characteristics of an atomic nucleus is its electric charge. Accurate measurements of the electric charge of atomic nuclei were made in 1913 by G. Moseley. Henry Gwyn Jeffries Moseley The electric charge of the atomic nucleus q is equal to the product of the elementary electric charge e and the serial number Z of the chemical element in the table D.I. Mendeleev: q \u003d Z · e.
Slide 3
The first particle that is part of atomic nuclei was discovered in 1919 by E. Rutherford, studying the interaction of α-particles with the nuclei of nitrogen atoms. Ernest Rutherford K - source of alpha particles; E - transparent screen coated with zinc sulfide; Inside the vessel is nitrogen gas. The source was moved away at a distance at which alpha particles did not reach the screen; But flashes were recorded on the screen.
Slide 4
Conclusion: some other charged particles knocked out α-particles from nitrogen nuclei. Studies of the action of electric and magnetic fields on particles knocked out of nitrogen nuclei have shown that these particles have a positive elementary charge and their mass is equal to the mass of the hydrogen nucleus. These particles are called protons. We denote the proton or then this reaction can be written as follows: + → + q \u003d + 1e, m \u003d 1a.e.m
Slide 5
Continuing experiments with boron, fluorine, sodium, and a number of other elements, E. Rutherford discovered that the α particle also knocks protons out of these nuclei. Conclusion: the nuclei of atoms of all elements contain protons. Contradiction: suppose that the nucleus consists only of protons, then \u003d 4, but \u003d 1 amu, then \u003d 4 amu, but it is 9 amu Conclusion: the nucleus contains another particle that does not have a charge.
Slide 6
Proof of the existence of a neutron.
In 1930, German scientists V. Bothe and G. Becker found that when beryllium is irradiated with α particles, radiation of an unknown nature arises that can pass through thick layers of lead with less attenuation than even x-ray or γ-radiation. Bothe and Becker decided that they received very hard gamma rays. In 1932, French scientists F. and I. Joliot-Curie found out that these rays hardly ionize the air through which they pass. But if paraffin is placed on their way, then the ionizing ability of the rays increases sharply. They suggested that this radiation knocks protons out of a paraffin plate.
Slide 7
In the same 1932, the English physicist D. Chadwick (an employee of E. Rutherford) suggested that when irradiating beryllium with α particles, a stream of neutral particles with a mass approximately equal to the mass of the proton is emitted. The name neutron comes from lat. neutron - neither one nor the other, i.e. having no positive or negative charge. James Chadwick Chadwick's experiments were experimental evidence of the existence of neutrons. q \u003d 0, m \u003d 1a.e.m
Slide 8
The structure of the atomic nucleus
Soviet physicist D. D. Ivanenko and V. Heisenberg proposed a proton-neutron model of the nucleus. According to this model, the atomic nucleus of any substance consists of protons and neutrons. A proton and a neutron are two charge states of a nuclear particle called a nucleon. The number of protons in the nucleus is equal to the number of electrons in the atom;
Slide 9
Isotopes
Studies of atomic nuclei have shown that most chemical elements are a mixture of atoms with the same charge number, but with different masses. All of them have the same chemical properties. Atoms with the same nuclear charges, but with different masses, are called element isotopes. The name isotope comes from the Greek. isos is the same, topos is the place, i.e. these are chemicals that occupy the same place in the DI table Mendeleev.
Slide 10
Isotope nuclei differ in the number of neutrons. For example, hydrogen has three isotopes: protium - the nucleus consists of one proton, deuterium - the nucleus consists of one proton and one neutron, tritium - the nucleus consists of one proton and two neutrons. Uranium has 12 isotopes with mass numbers from 228 to 239.
Slide 11
The atomic masses of chemical elements in the periodic table are expressed as fractional numbers due to the fact that they have isotopes. For example: an average of 75 atoms with a mass of 35 amu per 100 chlorine atoms and 25 atoms with a mass of 37 amu, so the average mass: \u003d\u003d 35.5a.mu
Slide 12
Use of isotopes
As a way to control the wear of piston rings in internal combustion engines. Radioactive isotopes allow us to judge the diffusion of metals and processes in blast furnaces. Powerful gamma radiation of radioactive preparations is used to study the internal structure of metal castings in order to detect defects in them. In industry:
Slide 13
One of the most prominent studies was the study of metabolism in organisms. Radioactive isotopes are used in medicine for both diagnosis and therapeutic purposes. Radioactive sodium, introduced in small amounts into the bloodstream, is used to study blood circulation, iodine is intensively deposited in the thyroid gland, especially in case of bazedovy disease. Intense gamma radiation of cobalt is used in the treatment of cancer (cobalt gun). In medicine:
Slide 14
Irradiation of plant seeds leads to a marked increase in yield. They cause mutations in plants and microorganisms, which leads to the appearance of mutants with new valuable properties. The microorganisms used in the production of antibiotics are obtained. For the control of harmful insects and for the conservation of food. To find out which phosphorus fertilizer is better absorbed by the plant. In agriculture:
Slide 15
In archeology: To determine the age of ancient objects of organic origin.
Slide 16
The method of "labeled atoms": Based on the fact that the chemical properties of radioactive isotopes do not differ from the properties of non-radioactive isotopes of the same elements.
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After Rutherford discovered the atomic nucleus, as well as numerous experiments to study radioactivity in detector devices, it became clear that the nuclei of atoms, like the atoms themselves, have a complex structure.
It has now been firmly established that the atomic nuclei of various elements consist of particles of two types, −− protons and neutrons. But first things first.
So, the first to put forward the theory that the nucleus of atoms of all chemical elements includes the nucleus of a hydrogen atom was Ernest Rutherford. He also gave the name to this particle - the proton, which in Greek means the first, the main one. The basis for this assumption was that the masses of atoms of chemical elements exceed the mass of a hydrogen atom by an integer number of times.
It is noteworthy that for the first time the proton was observed in his experiments by Joseph Thomson in 1907. He even managed to measure the ratio of his charge to mass. But since the technical capabilities of that time did not make it possible to find out, then this is a particle, then Thomson called it simply - an H-particle.
It was only in 1919 that Rutherford discovered that Thomson H particles are the nuclei of hydrogen atoms that are present in the fission products of the atomic nuclei of many chemical elements.
So, this year Ernest Rutherford set the experience of studying the interaction of the nucleus of a nitrogen atom with an alpha particle. The device consisted of a vacuum chamber in which a source of alpha particles was located. The camera window was covered with metal foil. Its thickness was selected so that alpha particles could not penetrate through it. Outside the window was a screen coated with zinc sulphide and a microscope. With its help, it was possible to observe scintillations in those places of the screen where heavy charged particles fell.
Under normal conditions, rare flashes of alpha particles leaked through the screen were observed on the screen. But when the chamber was filled with low-pressure nitrogen, multiple light flashes appeared on the screen, which indicated the appearance of a stream of some particles capable of penetrating through the foil.
Rutherford even managed to measure the mean free path of these particles — 28 centimeters, which coincided with the estimate of the mean free path of H-particles previously observed by Thomson. But since the observation was carried out by the scintillation method, it was impossible to say with certainty that the particles observed were protons.
It was possible to verify that the nucleus of the hydrogen atom actually flew out of the nucleus of the nitrogen atom only a few years later, when this reaction was carried out in the Wilson chamber.
You can see straight lines diverging in a fan in the photo. These are traces of α particles that flew through the chamber without experiencing collisions with the nuclei of nitrogen atoms. But the trace of one α-particle bifurcates, forming the so-called “fork”. At this bifurcation point of the track, an α-particle interacted with the nucleus of a nitrogen atom, resulting in the formation of a nucleus of an oxygen and hydrogen atom. The fact that these nuclei are formed was revealed by the nature of the curvature of the tracks when the Wilson chamber was placed in a magnetic field.
According to modern measurements, the mass of a proton is approximately equal to one atomic unit of mass, and its positive charge is equal to one elementary charge, that is, it is exactly equal in magnitude to the charge of an electron.
Such a coincidence of the charges of two particles not alike, is surprising and remains one of the fundamental mysteries of modern physics.
Subsequently, the experiment was repeated with a number of other gaseous substances. And in all cases, it was found that protons are knocked out of their nuclei by α particles. This gave reason to assume that proton is included into the composition of the nuclei of all chemical elements.
But the discovery of the proton did not give a complete answer to the question about the composition of the atomic nucleus. The fact is that if the nuclei of atoms consisted only of protons, then serious contradictions would arise. For example, suppose that the nucleus of a carbon atom consists only of protons. Since the proton charge is equal to one elementary charge, the total number of protons in a given nucleus should be equal to the charge number, in our case 6.
And since the mass of the proton is approximately equal to one atomic unit of mass, the atomic mass of carbon should be equal to 6 atomic units of mass.
However, in reality, the atomic mass of cesium is 12 atomic mass units. Therefore, in addition to protons, some other particles must also be part of the nucleus.
In 1920, Ernest Rutherford hypothesized that there is a rigidly bound compact proton-electron pair in the nucleus, which is an electrically neutral formation - a particle with a mass approximately equal to the mass of the proton. He even coined her name, the neutron.
In 1913, Soviet physicists Viktor Amazaspovich Ambratsumyan and Dmitry Dmitrievich Ivanenko showed that the nucleus cannot exist from protons and electrons, and that electrons arising from β decay are born immediately at the moment of decay.
Thus, although it was beautiful, but, as it turned out, Rutherford's erroneous theory was not confirmed.
After 10 years, the German scientist Walter Bothe and his student Herbert Becker found that α-particles emitted by radioactive polonium, falling on beryllium and lithium, form radiation with very high penetrating power. Initially, scientists thought that open beryllium radiation is a stream of gamma rays.
In 1932, Irene and Frederic Joliot-Curie, studying beryllium radiation, showed that when it hits paraffin, it knocks out high-energy protons from it. This in itself did not contradict anything, but the numerical results led to inconsistencies in theory.
In the same year, the Englishman James Chadwick, a student of Rutherford, investigated the new radiation in Wilson's chamber. In particular, he studied the tracks of nitrogen nuclei that experienced a collision with beryllium radiation. On the basis of his experiments, he estimated the energy of a gamma-ray quantum, which can tell the nuclei of nitrogen the speed observed in the experiment. It turned out that it was so large that gamma quanta emitted by beryllium could not have it. Based on this, James Chadwick found that during the bombardment of beryllium with α particles, it is not massless gamma rays that are emitted from it, but rather heavy particles with great penetrating power. And since they did not ionize the gas in the Geiger counter, they were electrically neutral. He also managed to estimate the mass of a new particle by its interaction with other particles. It turned out that it is slightly larger than the mass of the proton.
So the neutron was discovered - a particle predicted by Rutherford more than 10 years before the experiments of Chadwick.
3 material reinforcement.
In conclusion, we note thatalmost immediately after the discovery of the neutron, the Russian scientist Dmitry Dmitrievich Ivanenko and, independently of it, the German physicist Werner Karl Heisenberg put forward the hypothesis of the proton-neutron structure of atomic nuclei, which was fully confirmed by subsequent studies. But we will talk about this in one of the following lessons.
The story of the discovery of the neutron begins with Chadwick's unsuccessful attempts to detect neutrons in electrical discharges in hydrogen (based on the aforementioned Rutherford hypothesis). Rutherford, as we know, carried out the first artificial nuclear reaction by bombarding atomic nuclei with a-particles. This method also managed to carry out artificial reactions with the nuclei of boron, fluorine, sodium, aluminum and phosphorus. In this case, long-range protons flew out. In the future, it was possible to split the nuclei of neon, magnesium, silicon, sulfur, chlorine, argon and potassium. These reactions were confirmed by the experiments of Viennese physicists Kirsch and Petterson (1924), who also claimed that they managed to split the nuclei of lithium, beryllium and carbon, which Rutherford and his collaborators failed to do.
A discussion erupted in which Rutherford disputed the fission of these three cores. Recently, O. Frisch suggested that the results of the crowns are explained by the participation in the observations of students who sought to “please” the leaders and saw flashes where they were not there.
In 1930, Walter Bothe (1891-1957) and G. Becker bombarded beryllium with α-particles of polonium. At the same time, they found that beryllium, as well as boron, emit strongly penetrating radiation, which they identified with hard γ-radiation.
And in January 1932, Irene and Frederic Joliot-Curie reported at a meeting of the Paris Academy of Sciences the results of studies of radiation discovered by Bothe and Becker. They showed that this radiation "is capable of releasing protons in hydrogen-containing substances, giving them a high speed."
These protons were photographed by them in a Wilson chamber.
In the next report, made on March 7, 1932, Irene and Frederic Joliot-Curie showed photographs of traces of protons in Wilson’s chamber knocked out of paraffin by beryllium radiation.
Interpreting their results, they wrote: “Assumptions about elastic collisions of a photon with a nucleus lead to difficulties, consisting, on the one hand, in that this requires a quantum with significant energy, and, on the other hand, in that this process takes place too often. Chadwick suggests that radiation excited in beryllium consists of neutrons - particles with a unit mass and zero charge. ”
The results of Joliot-Curie endangered the law of conservation of energy. In fact, if we try to interpret the Joliot-Curie experiments on the basis of the presence of only known particles in nature: protons, electrons, photons, then the explanation of the appearance of long-range protons requires the creation of photons in beryllium with an energy of 50 MeV. In this case, the photon energy turns out to depend on the type of recoil nucleus used to determine the photon energy.
Chadwick resolved this collision. He placed a beryllium source in front of the ionization chamber, into which protons knocked out from a paraffin plate fell. Having absorbing aluminum screens between the paraffin plate and the chamber, Chadwick found that beryllium radiation knocks protons with energies up to 5.7 MeV from paraffin. To communicate such energy to protons, the photon must itself have an energy of 55 MeV. But the energy of the nitrogen recoil nuclei observed with the same beryllium radiation turns out to be 1.2 MeV. In order to transfer such energy to nitrogen, the radiation photon must have an energy of at least 90 MeV. The law of conservation of energy is incompatible with the photon interpretation of beryllium radiation.
Chadwick showed that all difficulties are removed if we assume that beryllium radiation consists of particles with a mass equal to approximately the mass of a proton and a zero charge. He called these particles neutrons. Chadwick published an article on his results in the Proceedings of the Royal Society for 1932. However, a preliminary note on the neutron was published in the Nature issue of February 27, 1932. Later, I. and f. Joliot-Curie in a number of works 1932-1933. confirmed the existence of neutrons and their ability to knock protons out of light nuclei. They also established the emission of neutrons by the nuclei of argon, sodium and aluminum when irradiated with a-rays.
According to the generally accepted model, the nuclei of atoms of any chemical element consist of protons and neutrons. These tiny particles were discovered at different times. Each of the discoveries brought scientists one step closer to the use of nuclear energy.
Proton discovery
A proton is the nucleus of a hydrogen atom, an element that has the simplest structure. It has a positive charge and an almost unlimited lifetime. It is the most stable particle in the universe. The protons formed as a result of the Big Bang have not yet decayed. The mass of the proton is 1.627 * 10-27 kg or 938.272 eV. More often this value is expressed in electron volts.
The proton was discovered by the "father" of nuclear physics, Ernest Rutherford. He hypothesized that the nuclei of atoms of all chemical elements consist of protons, since in mass they exceed the nucleus of a hydrogen atom by an integer number of times. Rutherford delivered an interesting experience. In those days, the natural radioactivity of certain elements was already discovered. Using alpha radiation (alpha particles are high-energy helium nuclei), the scientist irradiated nitrogen atoms. As a result of this interaction, a particle flew out. Rutherford suggested that this is a proton. Further experiments in the Wilson bubble chamber confirmed his assumption. So in 1913, a new particle was discovered, but Rutherford's hypothesis on the composition of the nucleus proved to be untenable.
Neutron discovery
The great scientist found a mistake in his calculations and put forward the hypothesis of the existence of another particle that is part of the nucleus and has almost the same mass as the proton. Experimentally, he could not find her.
This was done in 1932 by the English scientist James Chadwick. He set up an experiment in which he bombarded beryllium atoms with high-energy alpha particles. As a result of a nuclear reaction, a particle, subsequently called a neutron, flew out of the beryllium nucleus. For his discovery, Chadwick won the Nobel Prize three years later.
The neutron mass does not really differ much from the mass of the proton (1.622 * 10-27 kg), but this particle does not have a charge. In this sense, it is neutral and at the same time capable of causing fission of heavy nuclei. Due to the lack of charge, a neutron can easily pass through a high Coulomb potential barrier and penetrate into the structure of the nucleus.
The proton and neutron possess quantum properties (they can exhibit the properties of particles and waves). Neutron radiation is used for medical purposes. High penetrating power allows this radiation to ionize and detect deep tumors and other malignant tumors. In this case, the particle energy is relatively small.
A neutron, unlike a proton, is an unstable particle. Her life time is about 900 seconds. It decays into a proton, electron, and electron neutrino.
Sir James Chadwick (photo posted in the article) - an English physicist, Nobel Prize laureate, who became famous after the discovery of the neutron. This fundamentally changed the physics of that time and allowed scientists to create new elements, and also led to the discovery of nuclear fission and its use for military and peaceful purposes. Chadwick was part of a group of British scientists who during the Second World War helped the United States develop an atomic bomb.
James Chadwick: A Short Biography
Chadwick was born in Bollington, Cheshire, England on October 20, 1891, in the family of John Joseph and Ann Mary Knowles. He studied at the local primary and Manchester municipal secondary schools. At sixteen he received a scholarship from the University of Manchester. James intended to study mathematics, but mistakenly attended introductory lectures on physics and entered this specialty. At first, he had concerns about his decision, but after the first year of training, he found the course more interesting. Chadwick was enrolled in a class where he studied electricity and magnetism, and later the teacher assigned James a research project on the radioactive element of radiation.
Early research
James Chadwick graduated from the University in 1911 and continued to work with Rutherford on the absorption of gamma radiation, receiving a master's degree in 1913. The supervisor contributed to the appointment of a research scholarship that required work elsewhere. He decided to study in Berlin with Hans Geiger, who visited Manchester at the time when James received a master's degree. During this period, Chadwick established the existence of a continuous spectrum of beta radiation, which discouraged researchers and led to the discovery of neutrinos.
Camp voucher
Shortly before World War I, when hostilities became inevitable, Geiger warned Chadwick to return to England as soon as possible. James was bewildered by the advice of a travel company and stayed in a German prisoner of war camp until the end of the war. Over the five years of his imprisonment, Chadwick managed to negotiate with the guards and conduct elementary fluorescence studies.
Work in the Cavendish Laboratory
James Chadwick, whose biography in physics had every chance of ending in 1918, thanks to the efforts of Rutherford, returned to science and confirmed that he was equal to the atomic number. In 1921, he was awarded a research internship at the Cambridge College of Gonville and Keyes, and the following year became Rutherford's assistant at the Cavendish Laboratory.
Working every day, he still found time for research, the direction of which was generally proposed by Rutherford. Chadwick and his fellow prisoner Charles D. Ellis then continued their studies at Trinity College and at Rutherford, exploring the transmutation of elements bombarded with alpha particles (helium nuclei). A research team in Vienna reported results that were inconsistent with the data obtained by the Cavendish Laboratory, the correctness of which was skillfully defended by further experiments by Chadwick and his colleagues.
In 1925, James married Eileen Stuart-Brown. The couple had twin daughters.
In the mid-1920s, James Chadwick conducted experiments on the scattering of alpha particles fired at targets from metals, including gold and uranium, and then helium itself, whose core has the same mass as alpha particles. Scattering turned out to be asymmetric, and Chadwick explained this in 1930 as a quantum phenomenon.
Neutron discovery
As early as 1920, Rutherford suggested the existence of an electrically neutral particle called a neutron to explain the existence of hydrogen isotopes. It was believed that this particle consisted of an electron and a proton, but the emission of such a composition was not detected.
In 1930, it was found that during the bombardment of light nuclei by alpha rays emitted by polonium, penetrating radiation without electric charge occurred. It was supposed to be gamma rays. However, when using a beryllium target, the rays turned out to be many times more penetrating than when using other materials. In 1931, Chadwick and his colleague Webster suggested that neutral rays actually indicated the existence of a neutron.
In 1932, a couple of researchers Irene Curie and Frederic Joliot showed that the radiation of beryllium was more penetrating than reported by previous researchers, but they also called it gamma rays. James Chadwick read the report and immediately set to work on calculating the mass of a neutral particle, which could explain the latest results. He used the radiation of beryllium to bombard various elements and found that the results are consistent with the effect of a neutral particle with a mass almost identical. This was an experimental confirmation of the existence of a neutron. In 1925, Chadwick received the Nobel Prize in Physics for this achievement.
From neutron to nuclear reaction
The neutron quickly became a tool of physicists who used it to penetrate the atoms of elements and transform them, so positively charged nuclei did not repel it. Thus, Chadwick prepared the way for the fission of uranium-235 and the creation of nuclear weapons. In 1932, for this important discovery, he was awarded the Hughes medal and in 1935 the Nobel Prize. Then he found out that Hans Falkenhagen discovered the neutron simultaneously with him, but was afraid to print his results. The German scientist modestly refused the offer to share the Nobel Prize, which James Chadwick made to him.
The discovery of the neutron allowed the creation of transuranium elements in laboratories. This was the impetus for the discovery by the Nobel Prize winner Enrico Fermi of nuclear reactions caused by delayed neutrons, and the discovery by the German chemists Otto Hahn and Strassmann of nuclear fission, which resulted in the creation of nuclear weapons.
Work on the atomic bomb
In 1935, James Chadwick became a professor of physics at the University of Liverpool. Based on the results of the 1940 Frisch-Peierls memorandum on the feasibility of creation, he was appointed to the MAUD committee, which investigated this issue in more detail. In 1940, he visited North America with the Tizard mission to establish cooperation in nuclear research. After returning to the UK, he decided that nothing would come of it until the war was over.
In December of that year, Francis Simon, who worked at MAUD, found the opportunity to separate the uranium-235 isotope. In his report, he outlined the cost estimate and technical specifications for creating a large enterprise for Later Chadwick wrote that only then he realized that it was not only possible, but also inevitable. From that moment he had to start taking sleeping pills. James and his group generally supported the bomb from U-235 and approved its separation by diffusion from the isotope U-238.
Life summary
Soon, he went to Los Alamos, the headquarters of the Manhattan Project, and along with Niels Bohr gave valuable advice to the developers of atomic bombs dropped on Hiroshima and Nagasaki. Chadwick James, whose discoveries radically changed the course of human history, was knighted in 1945.
At the end of World War II, he returned to his post in Liverpool. Chadwick resigned in 1958. After spending ten years in North Wales, he returned to Cambridge in 1969, where he died on July 24, 1974.