What is a neutron? What are its structure, properties and functions? Neutrons are the largest of the particles that make up atoms, the building blocks of all matter.
Atomic structure
Neutrons are found in the nucleus, a dense region of the atom also filled with protons (positively charged particles). These two elements are held together by a force called nuclear. Neutrons have a neutral charge. The positive charge of the proton is matched with the negative charge of the electron to create a neutral atom. Even though the neutrons in the nucleus do not affect the charge of the atom, they still have many properties that affect the atom, including the level of radioactivity.
Neutrons, isotopes and radioactivity
A particle that is located in the nucleus of an atom is a neutron that is 0.2% larger than a proton. Together they make up 99.99% of the total mass of the same element and may have different numbers of neutrons. When scientists refer to atomic mass, they mean average atomic mass. For example, carbon typically has 6 neutrons and 6 protons with an atomic mass of 12, but it is sometimes found with an atomic mass of 13 (6 protons and 7 neutrons). Carbon with atomic number 14 also exists, but is rare. So, atomic mass for carbon averages to 12.011.
When atoms have different numbers of neutrons, they are called isotopes. Scientists have found ways to add these particles to the nucleus to create larger isotopes. Now adding neutrons does not affect the charge of the atom since they have no charge. However, they increase the radioactivity of the atom. This can lead to very unstable atoms that can discharge high levels energy.
What is the core?
In chemistry, the nucleus is the positively charged center of an atom, which consists of protons and neutrons. The word "kernel" comes from the Latin nucleus, which is a form of the word meaning "nut" or "kernel". The term was coined in 1844 by Michael Faraday to describe the center of an atom. The sciences involved in the study of the nucleus, the study of its composition and characteristics are called nuclear physics and nuclear chemistry.
Protons and neutrons are held together by the strong nuclear force. The electrons are attracted to the nucleus, but move so fast that their rotation occurs at some distance from the center of the atom. The nuclear charge with a plus sign comes from protons, but what is a neutron? This is a particle that has no electrical charge. Almost all the weight of an atom is contained in the nucleus, since protons and neutrons have much more mass than electrons. The number of protons in an atomic nucleus determines its identity as an element. The number of neutrons indicates which isotope of the element the atom is.
Atomic nucleus size
The nucleus is much smaller than the overall diameter of the atom because the electrons can be further away from the center. A hydrogen atom is 145,000 times larger than its nucleus, and a uranium atom is 23,000 times larger than its center. The hydrogen nucleus is the smallest because it consists of a single proton.
Arrangement of protons and neutrons in the nucleus
The proton and neutrons are usually depicted as being packed together and evenly distributed into spheres. However, this is a simplification of the actual structure. Each nucleon (proton or neutron) can occupy a specific energy level and range of locations. While the nucleus can be spherical, it can also be pear-shaped, spherical, or disc-shaped.
The nuclei of protons and neutrons are baryons, consisting of smallest ones called quarks. The attractive force has a very short range, so protons and neutrons must be very close to each other to be bound. This strong attraction overcomes the natural repulsion of charged protons.
Proton, neutron and electron
A powerful impetus in the development of such science as nuclear physics, was the discovery of the neutron (1932). We should thank for this the English physicist who was a student of Rutherford. What is a neutron? This is an unstable particle that, in a free state, can decay into a proton, electron and neutrino, the so-called massless neutral particle, in just 15 minutes.
The particle gets its name because it has no electrical charge, it is neutral. Neutrons are extremely dense. In an isolated state, one neutron will have a mass of only 1.67·10 - 27, and if you take a teaspoon densely packed with neutrons, the resulting piece of matter will weigh millions of tons.
The number of protons in the nucleus of an element is called the atomic number. This number gives each element its unique identity. In the atoms of some elements, such as carbon, the number of protons in the nuclei is always the same, but the number of neutrons can vary. An atom of a given element with a certain number of neutrons in the nucleus is called an isotope.
Are single neutrons dangerous?
What is a neutron? This is a particle that, along with the proton, is included in However, sometimes they can exist on their own. When neutrons are outside the nuclei of atoms, they acquire potential dangerous properties. When they move at high speeds, they produce deadly radiation. So-called neutron bombs, known for their ability to kill people and animals, yet have minimal effect on non-living physical structures.
Neutrons are a very important part of the atom. High density These particles combined with their speed gives them extreme destructive power and energy. As a result, they can alter or even tear apart the nuclei of the atoms they strike. Although a neutron has a net neutral electrical charge, it is composed of charged components that cancel each other with respect to charge.
A neutron in an atom is a tiny particle. Like protons, they are too small to be seen even with electron microscope, but they are there because this is the only way to explain the behavior of atoms. Neutrons are very important for the stability of an atom, but outside its atomic center they cannot exist for long and decay on average in only 885 seconds (about 15 minutes).
Chapter first. PROPERTIES OF STABLE NUCLEI
It was already said above that the nucleus consists of protons and neutrons bound by nuclear forces. If we measure the mass of a nucleus in atomic mass units, it should be close to the mass of a proton multiplied by an integer called the mass number. If the charge of a nucleus is a mass number, this means that the nucleus contains protons and neutrons. (The number of neutrons in the nucleus is usually denoted by
These properties of the kernel are reflected in symbolic notation, which will be used later in the form
where X is the name of the element whose atom the nucleus belongs to (for example, nuclei: helium - , oxygen - , iron - uranium
The main characteristics of stable nuclei include: charge, mass, radius, mechanical and magnetic moments, spectrum excited states, parity and quadrupole moment. Radioactive (unstable) nuclei are additionally characterized by their lifetime, type of radioactive transformations, energy of emitted particles and a number of other special properties, which will be discussed below.
First of all, let's consider the properties of the elementary particles that make up the nucleus: proton and neutron.
§ 1. BASIC CHARACTERISTICS OF THE PROTON AND NEUTRON
Weight. In units of electron mass: proton mass, neutron mass.
In atomic mass units: proton mass, neutron mass
In energy units, the rest mass of a proton is the rest mass of a neutron.
Electric charge. q is a parameter characterizing the interaction of a particle with an electric field, expressed in units of electron charge where
All elementary particles carry an amount of electricity equal to either 0 or The charge of a proton The charge of a neutron is zero.
Spin. The spins of the proton and neutron are equal. Both particles are fermions and obey Fermi-Dirac statistics, and therefore the Pauli principle.
Magnetic moment. If we substitute the proton mass into formula (10), which determines the magnetic moment of the electron instead of the electron mass, we obtain
The quantity is called nuclear magneton. It could be assumed by analogy with the electron that the spin magnetic moment of the proton is equal to However, experience has shown that the proton’s own magnetic moment is greater than the nuclear magneton: according to modern data
In addition, it turned out that an uncharged particle - a neutron - also has a magnetic moment that is different from zero and equal to
The presence of a magnetic moment in a neutron and so great importance magnetic moment of the proton contradict assumptions about the point nature of these particles. A number of experimental data obtained in last years, indicates that both the proton and the neutron have a complex inhomogeneous structure. At the center of the neutron there is a positive charge, and at the periphery there is a negative charge equal in magnitude distributed in the volume of the particle. But since the magnetic moment is determined not only by the magnitude of the flowing current, but also by the area covered by it, the magnetic moments created by them will not be equal. Therefore, a neutron can have a magnetic moment while remaining generally neutral.
Mutual transformations of nucleons. The mass of a neutron is 0.14% greater than the mass of a proton, or 2.5 times the mass of an electron,
In a free state, a neutron decays into a proton, electron and antineutrino: Its average lifetime is close to 17 minutes.
A proton is a stable particle. However, inside the nucleus it can turn into a neutron; in this case the reaction proceeds according to the scheme
The difference in the masses of particles on the left and right is compensated by the energy imparted to the proton by other nucleons in the nucleus.
A proton and a neutron have the same spins, almost the same masses, and can transform into each other. It will be shown later that the nuclear forces acting between these particles in pairs are also identical. Therefore, they are called by a common name - nucleon and they say that a nucleon can be in two states: proton and neutron, differing in their relationship to the electromagnetic field.
Neutrons and protons interact due to the existence of nuclear forces that are non-electrical in nature. Nuclear forces owe their origin to the exchange of mesons. If we depict the dependence of the potential energy of interaction between a proton and a low-energy neutron on the distance between them, then approximately it will look like the graph shown in Fig. 5, a, i.e. it has the shape of a potential well.
Rice. 5. Dependence of potential interaction energy on the distance between nucleons: a - for neutron-neutron or neutron-proton pairs; b - for a proton-proton pair
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The charge of a neutron is zero. Consequently, neutrons do not play a role in the amount of charge on the nucleus of an atom. The serial number of chromium is also equal to the same value.
The charge of a proton is qp e The charge of a neutron is zero.
It is easy to see that in this case the charge of the neutron is zero, and the charge of the proton is 1, as expected. We obtain all baryons included in two families - eight and ten. Mesons consist of a quark and an antiquark. The line denotes antiques; their electric charge differs in sign from the charge of the corresponding quark. The pi meson does not include a strange quark; pi mesons, as we have already said, are particles with strangeness and spin equal to zero.
Since the charge of a proton is equal to the charge of an electron and the charge of a neutron is equal to a bullet, then if you turn off the strong interaction, the interaction of a proton with electromagnetic field And it would be the usual interaction of a Dirac particle - Yp / V. The neutron would have no electromagnetic interaction.
Designations: 67 - charge difference between electron and proton; q - neutron charge; qg is the absolute value of the electron charge.
The nucleus consists of positively charged elementary particles - protons and charge-free neutrons.
The basis modern ideas The structure of matter is based on the statement about the existence of atoms of matter, consisting of positively charged protons and uncharged neutrons, forming a positively charged nucleus, and negatively charged electrons rotating around the nucleus. The energy levels of electrons, according to this theory, are discrete in nature, and the loss or acquisition of some additional energy by them is considered as a transition from one allowed energy level to another. In this case, the discrete nature of electronic energy levels causes the same discrete absorption or emission of energy by an electron during the transition from one energy level to another.
We assumed that the charge of an atom or molecule is completely determined by the scalar sum q Z (q Nqn, where Z is the number of electron-proton pairs, (q qp - qe is the difference between the charges of an electron and a proton, A is the number of neutrons, and qn is the charge of a neutron.
The charge of a nucleus is determined only by the number of protons Z, and its mass number A coincides with the total number of protons and neutrons. Since the charge of a neutron is zero, there is no electrical interaction according to Coulomb's law between two neutrons, or between a proton and a neutron. At the same time, an electrical repulsive force acts between the two protons.
Further, within the limits of measurement accuracy, not a single collision process has ever been recorded in which the law of conservation of charge was not observed. For example, the inflexibility of neutrons in uniform electric fields allows us to consider the charge of a neutron as equal to zero with an accuracy of 1 (H7 of the charge of an electron.
We have already said that the difference in the magnetic moment of a proton from one nuclear magneton is an amazing result. Even more surprising (It seems that there is a magnetic moment in a neutron that has no charge.
It is easy to verify that these forces cannot be reduced to any of the types of forces discussed in the previous parts of the physics course. In fact, if we assume, for example, that gravitational forces act between nucleons in nuclei, then it is easy to calculate from the known masses of the proton and neutron that the binding energy per particle will be negligible - it will be 1036 times less than that observed experimentally. The assumption about the electrical nature of nuclear forces also disappears. Indeed, in this case it is impossible to imagine a stable nucleus consisting of one charged proton and no neutron charge.
The strong bond that exists between nucleons in the nucleus indicates the presence of special, so-called nuclear forces in atomic nuclei. It is easy to verify that these forces cannot be reduced to any of the types of forces discussed in the previous parts of the physics course. In fact, if we assume, for example, that gravitational forces act between nucleons in nuclei, then it is easy to calculate from the known masses of the proton and neutron that the binding energy per particle will be negligible - it will be 1038 times less than that observed experimentally. The assumption about the electrical nature of nuclear forces also disappears. Indeed, in this case it is impossible to imagine a stable nucleus consisting of one charged proton and no neutron charge.
→Many people know well from school that all substances consist of atoms. Atoms, in turn, consist of protons and neutrons that form the nucleus of atoms and electrons located at some distance from the nucleus. Many have also heard that light also consists of particles - photons. However, the world of particles is not limited to this. To date, more than 400 different elementary particles are known. Let's try to understand how elementary particles differ from each other.
There are many parameters by which elementary particles can be distinguished from each other:
- Weight.
- Electric charge.
- Lifetime. Almost all elementary particles have a finite lifetime, after which they decay.
- Spin. It can be considered, very approximately, as a rotational moment.
A few more parameters, or as they are commonly called in the science of quantum numbers. These parameters do not always have a clear physical meaning, but they are needed to distinguish some particles from others. All these additional parameters are introduced as some quantities that are preserved in the interaction.
Almost all particles have mass, except photons and neutrinos (according to the latest data, neutrinos have mass, but so small that it is often considered zero). Without mass particles can only exist in motion. All particles have different masses. The electron has the smallest mass, not counting the neutrino. Particles called mesons have a mass 300-400 times greater than the mass of an electron, a proton and a neutron are almost 2000 times heavier than an electron. Particles that are almost 100 times heavier than a proton have now been discovered. Mass (or its energy equivalent according to Einstein’s formula:
is preserved in all interactions of elementary particles.
Not all particles have an electric charge, which means that not all particles are capable of participating in electromagnetic interaction. All freely existing particles have an electric charge that is a multiple of the electron charge. In addition to freely existing particles, there are also particles that are only in a bound state, we will talk about them a little later.
Spin, like other quantum numbers, is different for different particles and characterizes their uniqueness. Some quantum numbers are conserved in some interactions, some in others. All these quantum numbers determine which particles interact with which and how. 
Life time is also very important characteristic particles and we will consider it in more detail. Let's start with a note. As we said at the beginning of the article, everything that surrounds us consists of atoms (electrons, protons and neutrons) and light (photons). Where then are hundreds more? various types elementary particles. The answer is simple - everywhere around us, but we don’t notice it for two reasons.
The first of them is that almost all other particles live very short, approximately 10 to the minus 10 power of seconds or less, and therefore do not form such structures as atoms, crystal lattices, etc. The second reason concerns neutrinos; although these particles do not decay, they are subject only to weak and gravitational interactions. This means that these particles interact so little that they are almost impossible to detect.
Let us visualize how well a particle interacts. For example, the flow of electrons can be stopped by a fairly thin sheet of steel, on the order of a few millimeters. This will happen because the electrons will immediately begin to interact with the particles of the steel sheet, will sharply change their direction, emit photons, and thus lose energy quite quickly. This is not the case with the neutrino flow; they can pass through the Earth almost without interactions. And therefore it is very difficult to detect them.
So, most particles live very a short time, after which it disintegrates. Particle decays are the most common reactions. As a result of decay, one particle breaks up into several others of smaller mass, and they, in turn, decay further. All decays obey certain rules - conservation laws. So, for example, as a result of decay, electric charge, mass, spin and a number of other quantum numbers must be conserved. Some quantum numbers may change during decay, but also subject to certain rules. It is the decay rules that tell us that the electron and proton are stable particles. They can no longer decay subject to the rules of decay, and therefore they are the ones that end the chains of decay.
Here I would like to say a few words about the neutron. A free neutron also decays into a proton and an electron in about 15 minutes. However, this does not happen when the neutron is in the atomic nucleus. This fact can be explained different ways. For example, when an electron and an extra proton from a decaying neutron appear in the nucleus of an atom, a reverse reaction immediately occurs - one of the protons absorbs an electron and turns into a neutron. This picture is called dynamic equilibrium. It was observed in the universe on early stage its development shortly after the big bang.
In addition to decay reactions, there are also scattering reactions - when two or more particles interact simultaneously, and as a result one or more other particles are obtained. There are also absorption reactions, when two or more particles produce one. All reactions occur as a result of strong weak or electromagnetic interactions. Reactions due to strong interaction are the fastest; the time of such a reaction can reach 10 minus 20 seconds. The speed of reactions occurring due to electromagnetic interaction is lower; here the time can be about 10 minus 8 seconds. For weak interaction reactions, the time can reach tens of seconds and sometimes years.
At the end of the story about particles, let's talk about quarks. Quarks are elementary particles that have an electrical charge that is a multiple of a third of the charge of an electron and that cannot exist in a free state. Their interaction is arranged in such a way that they can only live as part of something. For example, a combination of three quarks of a certain type forms a proton. Another combination produces a neutron. A total of 6 quarks are known. Their different combinations give us different particles, and although not all combinations of quarks are allowed by physical laws, there are quite a lot of particles made up of quarks.
Here the question may arise: how can a proton be called elementary if it consists of quarks? It’s very simple - the proton is elementary, since it cannot be split into its component parts - quarks. All particles that participate in the strong interaction consist of quarks, and at the same time are elementary.
Understanding the interactions of elementary particles is very important for understanding the structure of the universe. Everything that happens to macro bodies is the result of the interaction of particles. It is the interaction of particles that describes the growth of trees on earth, reactions in the interior of stars, radiation from neutron stars, and much more.
| Probabilities and Quantum Mechanics | > |
A proton is a stable particle from the class of hadrons, the nucleus of a hydrogen atom.
It is difficult to say which event should be considered the discovery of the proton: after all, as a hydrogen ion, it has been known for a long time. The creation of a planetary model of the atom by E. Rutherford (1911), the discovery of isotopes (F. Soddy, J. Thomson, F. Aston, 1906-1919), and the observation of hydrogen nuclei knocked out of nuclei by alpha particles played a role in the discovery of the proton nitrogen (E. Rutherford, 1919). In 1925, P. Blackett received the first photographs of proton traces in a cloud chamber (see Nuclear Radiation Detectors), confirming the discovery of the artificial transformation of elements. In these experiments, the β-particle was captured by a nitrogen nucleus, which emitted a proton and turned into an oxygen isotope.
Together with neutrons, protons form the atomic nuclei of all chemical elements, and the number of protons in the nucleus determines the atomic number of a given element. A proton has a positive electric charge equal to the elementary charge, i.e., the absolute value of the charge of the electron. This has been tested experimentally with an accuracy of 10-21. Proton mass mp = (938.2796 ± 0.0027) MeV or ~ 1.6-10-24 g, i.e. a proton is 1836 times heavier than an electron! WITH modern point From a perspective, the proton is not a true elementary particle: it consists of two u-quarks with electric charges +2/3 (in units of elementary charge) and one d-quark with electric charge -1/3. Quarks are interconnected by the exchange of other hypothetical particles - gluons, quanta of the field that carries strong interactions. Data from experiments in which the processes of electron scattering on protons were considered indeed indicate the presence of point scattering centers inside protons. These experiments are in a certain sense very similar to Rutherford's experiments that led to the discovery of the atomic nucleus. Being a composite particle, the proton has final dimensions~ 10-13 cm, although, of course, it cannot be represented as hard ball. Rather, the proton resembles a cloud with a fuzzy boundary, consisting of created and annihilated virtual particles. The proton, like all hadrons, participates in each of the fundamental interactions. So. strong interactions bind protons and neutrons in nuclei, electromagnetic interactions bind protons and electrons in atoms. Examples of weak interactions are the beta decay of a neutron or the intranuclear transformation of a proton into a neutron with the emission of a positron and neutrino (for a free proton such a process is impossible due to the law of conservation and transformation of energy, since the neutron has a slightly larger mass). The proton spin is 1/2. Hadrons with half-integer spin are called baryons (from the Greek word meaning "heavy"). Baryons include the proton, neutron, various hyperons (?, ?, ?, ?) and a number of particles with new quantum numbers, most of which have not yet been discovered. To characterize baryons, a special number was introduced - the baryon charge, equal to 1 for baryons, - 1 - for antibaryons and O - for all other particles. The baryon charge is not a source of the baryon field; it was introduced only to describe the patterns observed in reactions with particles. These patterns are expressed in the form of the law of conservation of baryon charge: the difference between the number of baryons and antibaryons in the system is conserved in any reactions. The conservation of the baryon charge makes it impossible for the proton to decay, since it is the lightest of the baryons. This law is empirical in nature and, of course, must be tested experimentally. The accuracy of the law of conservation of baryon charge is characterized by the stability of the proton, the experimental estimate for the lifetime of which gives a value of no less than 1032 years.
At the same time, theories that combine all types of fundamental interactions predict processes leading to the disruption of the baryon charge and the decay of the proton. The lifetime of a proton in such theories is not very accurately indicated: approximately 1032 ± 2 years. This time is enormous, it is many times longer than the existence of the Universe (~ 2*1010 years). Therefore, the proton is practically stable, which made the formation of chemical elements and ultimately the emergence of intelligent life possible. However, the search for proton decay is now one of the most important problems in experimental physics. With a proton lifetime of ~ 1032 years in a volume of water of 100 m3 (1 m3 contains ~ 1030 protons), one proton decay per year should be expected. All that remains is to register this decay. The discovery of proton decay will be important step To correct understanding unity of the forces of nature.
Neutron is a neutral particle belonging to the class of hadrons. Discovered in 1932 by the English physicist J. Chadwick. Together with protons, neutrons are part of atomic nuclei. The electric charge of a neutron qn is zero. This is confirmed by direct measurements of the charge from the deflection of a neutron beam in strong electric fields, which showed that |qn|<10-20e (здесь е -- элементарный электрический заряд, т. е. абсолютная величина заряда электрона). Косвенные данные дают оценку |qn|< 2?10-22 е. Спин нейтрона равен 1/2. Как адрон с полуцелым спином, он относится к группе барионов. У каждого бариона есть античастица; антинейтрон был открыт в 1956 г. в опытах по рассеянию антипротонов на ядрах. Антинейтрон отличается от нейтрона знаком барионного заряда; у нейтрона, как и у протона, барионный заряд равен +1.Как и протон и прочие адроны, нейтрон не является истинно элементарной частицей: он состоит из одного u-кварка с электрическим зарядом +2/3 и двух d-кварков с зарядом - 1/3, связанных между собой глюонным полем.
Neutrons are stable only in stable atomic nuclei. A free neutron is an unstable particle that decays into a proton (p), electron (e-) and electron antineutrino. The neutron lifetime is (917?14) s, i.e. about 15 minutes. In matter, neutrons exist in free form even less due to their strong absorption by nuclei. Therefore, they occur in nature or are produced in the laboratory only as a result of nuclear reactions.
Based on the energy balance of various nuclear reactions, the difference between the masses of the neutron and proton was determined: mn-mp(1.29344 ±0.00007) MeV. By comparing it with the proton mass, we obtain the neutron mass: mn = 939.5731 ± 0.0027 MeV; this corresponds to mn ~ 1.6-10-24. The neutron participates in all types of fundamental interactions. Strong interactions bind neutrons and protons in atomic nuclei. An example of the weak interaction is the beta decay of a neutron.
Does this neutral particle participate in electromagnetic interactions? The neutron has an internal structure, and with general neutrality, there are electric currents in it, leading, in particular, to the appearance of a magnetic moment in the neutron. In other words, in a magnetic field, a neutron behaves like a compass needle. This is just one example of its electromagnetic interaction. The search for the electric dipole moment of the neutron, for which an upper limit was obtained, gained great interest. Here, the most effective experiments were carried out by scientists from the Leningrad Institute of Nuclear Physics of the USSR Academy of Sciences; The search for the neutron dipole moment is important for understanding the mechanisms of violation of invariance under time reversal in microprocesses.
Gravitational interactions of neutrons were observed directly from their incidence in the Earth's gravitational field.
A conventional classification of neutrons according to their kinetic energy is now accepted:
slow neutrons (<105эВ, есть много их разновидностей),
fast neutrons (105?108eV), high-energy (> 108eV).
Very slow neutrons (10-7 eV), which are called ultracold neutrons, have very interesting properties. It turned out that ultracold neutrons can be accumulated in “magnetic traps” and their spins can even be oriented in a certain direction there. Using magnetic fields of a special configuration, ultracold neutrons are isolated from the absorbing walls and can “live” in the trap until they decay. This allows many subtle experiments to study the properties of neutrons. Another method for storing ultracold neutrons is based on their wave properties. Such neutrons can simply be stored in a closed “jar”. This idea was expressed by the Soviet physicist Ya. B. Zeldovich in the late 1950s, and the first results were obtained in Dubna at the Institute of Nuclear Research almost a decade later.
Recently, scientists managed to build a vessel in which ultracold neutrons live until their natural decay.
Free neutrons are able to actively interact with atomic nuclei, causing nuclear reactions. As a result of the interaction of slow neutrons with matter, one can observe resonance effects, diffraction scattering in crystals, etc. Due to these properties, neutrons are widely used in nuclear physics and solid state physics. They play an important role in nuclear energy, in the production of transuranium elements and radioactive isotopes, and find practical application in chemical analysis and geological exploration.
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