Discovery of the proton. Discovery of the neutron. Discovery of the proton and neutron Discovery of the proton who

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ABSTRACT

"discovery of the neutron"

Cadet Smirnov S.V. completed and defended an essay with a grade___from__.__2014

2014

Neutron

What do we know about the neutron?

Neutromn (from Latin neuter - neither one nor the other) is a heavy elementary particle that has no electric charge. The neutron is a fermion and belongs to the class of baryons. Neutrons (along with protons) are one of the two main components of atomic nuclei; The common name for protons and neutrons is nucleons.

DISCOVERY OF THE NEUTRON

In 1930, V. A. Ambartsumyan and D. D. Ivanenko showed that the nucleus cannot, as was believed at that time, consist of protons and electrons, that electrons emitted from the nucleus during beta decay are born at the moment of decay, and that in addition to protons, some neutral particles must be present in the nucleus.

In 1930, Walter Bothe and G. Becker, working in Germany, discovered that when high-energy alpha particles emitted by polonium-210 hit certain light elements, especially beryllium or lithium, radiation with unusually high penetrating power was produced. At first it was thought that it was gamma radiation, but it turned out that it has a much greater penetrating power than all known gamma rays, and the results of the experiment cannot be interpreted in this way. An important contribution was made in 1932 by Irene and Frédéric Joliot-Curie. They showed that if this unknown radiation hits paraffin or any other hydrogen-rich compound, high-energy protons are produced. This in itself did not contradict anything, but the numerical results led to inconsistencies in the theory. Later that same year, 1932, the English physicist James Chadwick conducted a series of experiments in which he showed that the gamma ray hypothesis was untenable. He suggested that this radiation consisted of uncharged particles with a mass close to the mass of a proton, and conducted a series of experiments that confirmed this hypothesis. These uncharged particles were called neutrons, from the Latin root neutral and the usual suffix for particles, on. In the same 1932, D. D. Ivanenko and then W. Heisenberg suggested that the atomic nucleus consists of protons and neutrons.

JAMES CHADWICK

English physicist James Chadwick was born in Bollington, near Manchester. He was the eldest of four children of John Joseph Chadwick, a laundry owner, and Ann Mary (Knowles) Chadwick. After graduating from a local primary school, he entered Manchester Municipal High School, where he excelled in mathematics. In 1908, Chadwick entered the University of Manchester, intending to study mathematics, but due to a misunderstanding, he was interviewed for physics. Too modest to point out a mistake, he listened carefully to the questions asked of him and decided to change his specialization. Three years later he graduated from the university with honors in physics.

In 1911, Chadwick began postgraduate work under the supervision of Ernest Rutherford in the physical laboratory in Manchester. It was at this time that experiments on the scattering of alpha particles (which were regarded as charged helium atoms) passed through a thin metal foil led Rutherford to propose that the entire mass of an atom was concentrated in a dense, positively charged nucleus surrounded by negatively charged electrons, which, as is known, , have a relatively low mass. Chadwick received his master's degree from Manchester in 1913, and in the same year, having won a scholarship, he went to Germany to study radioactivity under Hans Geiger (Rutherford's former assistant) at the State Institute of Physics and Technology in Berlin. When the First World War began in 1914, Chadwick was interned as an English citizen and spent more than 4 years in a civilian camp at Ruhleben. Although Chadwick suffered from harsh conditions that undermined his health, he took part in the scientific society created by his fellow sufferers. The group's activities received support from some German scientists, including Walter Nernst, whom Chadwick met while interned.

Chadwick's discovery

neutron particle chadwick alpha

Chadwick returned to Manchester in 1919. Shortly before this, Rutherford had discovered that bombardment with alpha particles (which were now regarded as helium nuclei) could cause the nitrogen atom to decay into lighter nuclei of other elements. A few months later, Rutherford was chosen as director of the Cavendish Laboratory at Cambridge University, and he invited Chadwick to follow him. Chadwick received a Wolleston Fellowship at Gonville and Caius College, Cambridge, and was able to work with Rutherford to continue experiments with alpha particles. They found that bombarding nuclei often produced what appeared to be nuclei of hydrogen, the lightest of the elements. The hydrogen nucleus carried a positive charge equal in magnitude to the negative charge of the corresponding electron, but had a mass approximately 2 thousand times greater than the mass of the electron. Rutherford later called it the proton. It became clear that the atom as a whole was electrically neutral, since the number of protons in its nucleus was equal to the number of electrons surrounding the nucleus. However, this number of protons did not agree with the mass of atoms, with the exception of the simplest case of hydrogen. To resolve this discrepancy, Rutherford proposed in 1920 the idea that nuclei could contain electrically neutral particles, which he later called neutrons, formed by the combination of an electron and a proton. The opposing view was that atoms contain electrons both outside and inside the nucleus and that the negative charge of nuclear electrons simply cancels out some of the charge on protons. Then the protons of the nucleus would make a full contribution to the total mass of the atom, and their total charge would be just enough to neutralize the charge of the electrons surrounding the nucleus. Although Rutherford's suggestion that a neutral particle existed was respected, there was still no experimental confirmation of this idea.

Chadwick received his doctorate in physics from Cambridge in 1921 and was elected to the academic council of Gonville and Caius College. Two years later he became deputy director of the Cavendish Laboratory. Until the end of the 20s. he investigated such atomic phenomena as the artificial disintegration of nuclei of light elements under bombardment by alpha particles and the spontaneous emission of beta particles (electrons). During this work, he pondered how the existence of the Rutherford neutral particle could be confirmed, but the decisive research that made this possible was carried out in Germany and France.

In 1930, German physicists Walter Bothe and Hans Becker discovered that bombarding certain light elements with alpha particles produced radiation with a special penetrating power, which they mistook for gamma rays. Gamma rays first became known as radiation produced by radioactive nuclei. They had greater penetrating power than X-rays because they had a shorter wavelength. However, some of the results were puzzling, especially when beryllium was used as a bombardment target. In this case, radiation in the direction of movement of the incident flux of alpha particles had greater penetrating power than backward radiation. Chadwick suggested that beryllium emits a stream of neutral particles rather than gamma rays. In 1932, French physicists Frederic Joliot and Irene Joliot-Curie, studying the penetrating ability of beryllium radiation, placed various absorbing materials between the bombarded beryllium and the ionization chamber, which served as a radiation recorder. When they used paraffin (a substance rich in hydrogen) as an absorber, they found an increase, not a decrease, in the radiation coming out of the paraffin. The test led them to the conclusion that the increase in radiation was due to protons (hydrogen nuclei) knocked out of the paraffin by penetrating radiation. They proposed that protons were knocked out by collisions with quanta (discrete units of energy) of unusually powerful gamma rays, much as electrons were knocked out by X-rays (the Compton effect) in an experiment pioneered by Arthur H. Compton.

Chadwick quickly repeated and expanded the experiment carried out by the French couple and found that a thick lead plate did not have any noticeable effect on the radiation of beryllium, without attenuating it or generating secondary radiation, which indicated its high penetrating power. However, the paraffin again gave an additional flux of fast protons. Chadwick performed a test that confirmed that these were indeed protons and determined their energy. He then showed that, by all indications, it was extremely unlikely that collisions of alpha particles with beryllium could produce gamma rays with sufficient energy to knock protons out of paraffin at such a rate. So he abandoned the gamma ray idea and focused on the neutron hypothesis. Having accepted the existence of the neutron, he showed that as a result of the capture of an alpha particle by a beryllium nucleus, the nucleus of the element carbon can be formed, and one neutron is released. He did the same with boron, another element that generated penetrating radiation when bombarded with alpha rays. An alpha particle and a boron nucleus combine to form a nitrogen nucleus and a neutron. The high penetrating ability of the neutron flux arises because the neutron does not have a charge and, therefore, when moving in a substance, it is not influenced by the electric fields of atoms, but interacts with nuclei only in direct collisions. A neutron also requires less energy than a gamma ray to knock out a proton, since it has greater momentum than a quantum of electromagnetic radiation of the same energy. The fact that beryllium radiation in the forward direction turns out to be more penetrating can be associated with the preferential radiation of neutrons in the direction of the pulse of the incident flux of alpha particles.

Chadwick also confirmed Rutherford's hypothesis that the mass of a neutron must be equal to the mass of a proton by analyzing the exchange of energy between neutrons and protons knocked out of matter, as if we were talking about the collision of billiard balls. Energy exchange is especially efficient since their masses are almost the same. He also analyzed the tracks of nitrogen atoms hit by neutrons in a condensation chamber, an instrument invented by C.T.R. Wilson. The steam in the condensation chamber condenses along an electrified path, which is left by the ionizing particle when interacting with steam molecules. The path is visible, although the particle itself is invisible. Since the neutron does not directly ionize, its trace is not visible. Chadwick had to establish the properties of the neutron from the track left after a collision with a nitrogen atom. It turned out that the mass of a neutron is 1.1% greater than the mass of a proton.

Experiments and calculations done by other physicists confirmed Chadwick's findings, and the existence of the neutron was quickly accepted. Shortly thereafter, Werner Heisenberg showed that the neutron cannot be a mixture of a proton and an electron, but is an uncharged nuclear particle - the third subatomic, or elementary, particle to be discovered. Chadwick's proof of the existence of the neutron in 1932 fundamentally changed the picture of the atom and paved the way for further discoveries in physics. The neutron also had a practical use as an atom destroyer: unlike a positively charged proton, it is not repelled when approaching the nucleus.

Confession

“For his discovery of the neutron,” Chadwick was awarded the Nobel Prize in Physics in 1935. “The existence of the neutron has been completely established,” said Hans Pleyel of the Royal Swedish Academy of Sciences in his acceptance speech, “leading to a new concept of atomic structure that better fits the distribution of energy within atomic nuclei. It became obvious that the neutron forms one of the building blocks of which atoms and molecules are made, and therefore the entire material Universe.”

Chadwick moved to the University of Liverpool in 1935 to establish a new center for nuclear physics research. In Liverpool, he oversaw the modernization of university equipment and supervised the construction of a cyclotron - a facility for accelerating charged particles. When World War II began in 1939, the British government asked Chadwick whether a nuclear chain reaction was possible, and he began using the Liverpool cyclotron to investigate this possibility. The following year he joined the Mod Committee, a small select group of eminent British scientists who drew optimistic conclusions about Britain's ability to build an atomic bomb, and became the coordinator of experimental atomic weapons programs in Liverpool, Cambridge and Bristol. Subsequently, however, Britain decided to join the American nuclear weapons program and sent its nuclear scientists to the United States. From 1943 to 1945, Chadwick coordinated the efforts of British scientists working on the Manhattan Project (a secret program to create an atomic bomb).

Chadwick returned to the University of Liverpool in 1946. Two years later he retired from active scientific work and became head of Gonville and Caius College. In 1958 he moved to North Wales with his wife Eileen, before marrying Stewart-Brown, whom he married in 1925. They returned to Cambridge in 1969 to be closer to their twin daughters. Chadwick died 5 years later in Cambridge.

In addition to the Nobel Prize, Chadwick received the Hughes Medal (1932) and the Copley Medal (1950) of the Royal Society, the US Government Merit Medal (1946), the Franklin Medal of the Franklin Institute (1951) and the Guthrie Medal of Physics Institute in London (1967). Ennobled in 1945, he held honorary degrees from nine British universities and was a member of many scientific societies and academies in Europe and the United States.

Used Books

1.http://ru.wikipedia.org

2. http://hirosima.scepsis.ru

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When beryllium is bombarded with α-particles emitted by radioactive polonium, strong penetrating radiation arises that can overcome such an obstacle as a layer of lead 10 -20 thick cm. This radiation was observed almost simultaneously with Chadwick by the Joliot-Curie spouses Irene and Frederic (Irene is the daughter of Marie and Pierre Curie), but they assumed that these were high-energy γ-rays. They discovered that if a paraffin plate is placed in the path of beryllium radiation, the ionizing ability of this radiation increases sharply. They proved that beryllium radiation knocks out protons from paraffin, which are present in large quantities in this hydrogen-containing substance. Based on the free path of protons in air, they estimated the energy of γ-quanta capable of imparting the required speed to protons during a collision. It turned out to be huge - about 50 MeV.

In his experiments, J. Chadwick observed in a cloud chamber the tracks of nitrogen nuclei that had collided with beryllium radiation. Based on these experiments, he made an estimate of the energy of the γ-quantum capable of imparting the speed observed in the experiment to nitrogen nuclei. It turned out to be 100-150 MeV. The γ quanta emitted by beryllium could not have such enormous energy. On this basis, Chadwick concluded that it is not massless γ quanta that are emitted from beryllium under the influence of α particles, but rather heavy particles. Since these particles were highly penetrating and did not directly ionize the gas in the Geiger counter, they were therefore electrically neutral. This proved the existence of the neutron, a particle predicted by Rutherford more than 10 years before Chadwick’s experiments.

Hydrogen, 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 produced by the Big Bang have not yet decayed. The proton mass is 1.627*10-27 kg or 938.272 eV. More often this value is expressed in electronvolts.

The proton was discovered by the “father” of nuclear physics, Ernest Rutherford. He put forward the hypothesis that the nuclei of atoms of all chemical elements consist of protons, since their mass exceeds the nucleus of a hydrogen atom by an integer number of times. Rutherford performed an interesting experiment. At that time, the natural radioactivity of some elements had already been 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 it was a proton. Further experiments in a Wilson bubble chamber confirmed his assumption. So in 1913, a new particle was discovered, but Rutherford’s hypothesis about the composition of the nucleus turned out to be untenable.

Discovery of the neutron

The great scientist found an error in his calculations and put forward a hypothesis about the existence of another particle that is part of the nucleus and has almost the same mass as a proton. Experimentally, he could not detect it.

This was done in 1932 by the English scientist James Chadwick. He conducted an experiment in which he bombarded beryllium atoms with high-energy alpha particles. As a result of the nuclear reaction, a particle was emitted from the beryllium nucleus, later called a neutron. For his discovery, Chadwick received the Nobel Prize three years later.

The mass of a neutron really differs little from the mass of a 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 the high Coulomb potential barrier and penetrate into the structure of the nucleus.

The proton and neutron have quantum properties (they can exhibit the properties of particles and waves). Neutron radiation is used for medical purposes. High penetrating ability allows this radiation to ionize deep-seated tumors and other malignant formations and detect them. In this case, the particle energy is relatively low.

The neutron, unlike the proton, is an unstable particle. Its lifetime is about 900 seconds. It decays into a proton, an electron and an electron neutrino.

In 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 of 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 assumed that this was hard 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 that 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 the protons flying forward. The protons knocked out of the 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 the atomic nucleus of any chemical element must contain a nucleus

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, based on 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 “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 a greater number of protons than in the atomic nucleus of hydrogen.

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, it 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.