Atomic structure: what is a neutron? Measurements of the neutron lifetime by different methods still differ. What is a neutron?

The mass of a neutron can be determined in various ways. The first determination of m n was made by Chadwick by measuring the energy of recoil nuclei produced by the collision of neutrons with hydrogen and nitrogen nuclei. This method allowed us to determine only that the mass of the neutron is approximately equal to the mass of the proton.

The neutron has no charge, therefore the usual methods for determining the mass of atoms (mass spectroscopy, chemical methods) are not applicable to the neutron. All measurements of neutron mass were based on a method for analyzing the energy balance of various nuclear reactions involving neutrons. Soon after the discovery of the neutron, 11 B(α,n) 14 N and 7 Li(α,n) 10 B were used to determine its mass.

Currently, the difference in the masses of the proton and neutron has been determined quite accurately using the endoenergetic reaction 3 H+p→n+ 3 He and a method based on measuring the difference in the masses of the deuteron and the hydrogen molecule, as well as the binding energy of the deuteron. For the reaction 3 H(p,n) 3 He, the energy conservation law can be written as

where Q is the reaction energy, and the designations of atoms and particles should be understood as their rest energy. Using the relation for reaction energy

Q=(m 2 /(m 1 +m 2))*E T *(1-0.5(m 2 E T /((m 1 +m 2) 2 *c 2))), (2)

Where m 1 and m 2 are the masses of the proton and triton. The value Q=-(763.77±0.08) keV was found.

The difference between the masses of a neutron and a hydrogen atom can be obtained by knowing the maximum energy β -particles E β during tritium decay:

(m n -M H)c 2 =E β (1+m 0 /m 3)-Q+E H, (3)

where m 3 is the mass of the 3 He nucleus; m 0 – electron rest mass; E H – electron binding energy in a hydrogen atom; M H is the mass of the hydrogen atom, the antineutrino mass is assumed to be zero. By averaging the known data, the value for E β can be found (18.56 ± 0.05) keV. As a result, the difference between the masses of the neutron and proton turns out to be equal to δm n - p = (1293.0±0.1) keV.

One of the most accurate methods is based on the use of the reaction of radiative capture of thermal neutrons by protons:

If the proton is stationary, then the law of conservation of energy for this reaction

Tn, Td - kinetic energies of neutron and proton. At T n ≈ 0 (for example, for thermal neutrons the kinetic energy Tn = 0.025 eV) the kinetic energy of neutrons can be neglected. Based on the law of conservation of momentum for the kinetic energy of the deuteron, we can obtain the following expression; . Currently, the energy of γ quanta has been measured with great accuracy E γ = 2223.25 keV. Deuteron binding energy. Proton and deuteron masses m d And m p measured with good accuracy using a mass spectrometer, the estimate gives the value Td = 1.3 keV. From here we can calculate the mass of the neutron. The most accurate value of the neutron mass is (1981): m n = 939.5731(27) MeV. The error in the last two digits is indicated in parentheses.

The mass of a neutron is 1.293 MeV greater than the mass of a proton. Therefore the neutron is β -an active particle with a lifetime of 885.4 seconds. In the free state, neutrons are practically absent, except for a small amount produced under the influence of cosmic rays.

The process of β-decay of a free neutron can be represented as:

This process is energetically possible, since the total mass of particles included in the right side of the equation is less than the mass of the neutron. In the quark model, neutron decay is a consequence of the more fundamental process of d-quark transformation: d→u+e - + . Studying the β-decay of a free neutron makes it possible to obtain information about the weak interaction responsible for its decay. At the same time, the fact that the decay of an elementary particle is being studied makes it possible to get rid of the influence of nuclear effects on the decay process.

Measuring the lifetime of a neutron with respect to β decay provides valuable information for weak interaction physics, astrophysics and cosmology. In cosmology, the half-life of the neutron is directly related to the rate of helium formation in the initial period of the existence of the Universe. Knowledge of the half-life of the neutron is necessary for a correct understanding of the physical processes occurring in the Sun.

Electric charge of a neutron with a huge degree of accuracy (~10 -20 e, e- electron charge) is zero. The non-zero magnetic moment of a neutron indicates its internal structure. To study the structure of nucleons, it is necessary that the de Broglie wavelength (λ = 2 ћ/p) of the probing particles be small compared to the size of the nucleons. It turned out to be possible to fulfill these conditions using scattering of fast electrons (~100 MeV) on nucleons.

A neutron can have a dipole moment. This is possible if invariance with respect to time reversal does not hold in nature.

Although the neutron is generally neutral, it has a complex internal charge distribution, which is manifested in the interaction of neutrons with electrons.

We can summarize the first chapter.

A neutron is a neutral (z = 0) Dirac particle with spin and a negative magnetic moment (in units of nuclear magnetic moment), which mainly determines the electromagnetic interaction of the neutron. Just like the proton, the neutron is assigned a unit baryon charge Y n = +1 and positive parity P n =+1.

The neutron mass is m n = 1.00866491578 ± 0.00000000055 amu = 939.56633 ± 0.00004 MeV, which is 1.2933318 ± 0.0000005 MeV more than the proton mass. In this regard, the neutron is β -a radioactive particle. With life time τ = 885.4 ± 0.9(stat.) ± 0.4(syst.) sec it decays according to scheme (7). Here are the data from 2000.

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 the atomic mass for carbon averages out 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 result in very unstable atoms that can discharge high levels of 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 a 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 potentially 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. The high density of 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 an electron microscope, but they are there because that 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 of 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 such a large value of the magnetic moment in a proton contradict assumptions about the point nature of these particles. A number of experimental data obtained in recent years indicate 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

Atomic mass unit
Atomic mass unit

Atomic mass unit (a.u.m. or u) is a unit of mass equal to 1/12 of the mass of an atom of the carbon isotope 12 C, and is used in atomic and nuclear physics to express the masses of molecules, atoms, nuclei, protons and neutrons. 1 amu ( u) ≈ 1.66054 . 10 -27 kg. In nuclear and particle physics, instead of mass m use in accordance with Einstein's relation E = mc 2 its energy equivalent mc 2, and 1 electronvolt (eV) and its derivatives are used as a unit of energy: 1 kiloelectronvolt (keV) = 10 3 eV, 1 megaelectronvolt (MeV) = 10 6 eV , 1 gigaelectronvolt (GeV) = 10 9 eV, 1 teraelectronvolt (TeV) = 10 12 eV, etc. 1 eV is the energy acquired by a singly charged particle (for example, an electron or proton) when passing through an electric field of a potential difference of 1 volt. As is known, 1 eV = 1.6. 10 -12 erg = 1.6. 10 -19 J. In energy units
1 amu ( u)931.494 MeV. Proton (m p) and neutron (m n) masses in atomic mass units and in energy units are as follows: m p ≈ 1.0073 u≈ 938.272 MeV/ from 2, m n ≈ 1.0087 u≈ 939.565 MeV/s 2 . With an accuracy of ~1%, the masses of a proton and neutron are equal to one atomic mass unit (1 u).

The sizes and masses of atoms are small. The radius of the atoms is 10 -10 m, and the radius of the nucleus is 10 -15 m. The mass of an atom is determined by dividing the mass of one mole of atoms of the element by the number of atoms in 1 mole (N A = 6.02·10 23 mol -1). The mass of atoms varies within the range of 10 -27 ~ 10 -25 kg. Typically, the mass of atoms is expressed in atomic mass units (amu). For a.u.m. 1/12 of the mass of an atom of the carbon isotope 12 C is taken.

The main characteristics of an atom are the charge of its nucleus (Z) and mass number (A). The number of electrons in an atom is equal to the charge of its nucleus. The properties of atoms are determined by the charge of their nuclei, the number of electrons and their state in the atom.

Basic properties and structure of the nucleus (theory of the composition of atomic nuclei)

1. The atomic nuclei of all elements (except hydrogen) consist of protons and neutrons.

2. The number of protons in the nucleus determines the value of its positive charge (Z). Z- serial number of a chemical element in the periodic system of Mendeleev.

3. The total number of protons and neutrons is the value of its mass, since the mass of an atom is mainly concentrated in the nucleus (99.97% of the mass of the atom). Nuclear particles - protons and neutrons - are collectively called nucleons(from the Latin word nucleus, which means “kernel”). The total number of nucleons corresponds to the mass number, i.e. its atomic mass A rounded to the nearest whole number.

Cores with the same Z, but different A are called isotopes. Cores that, with the same A have different Z, are called isobars. In total, about 300 stable isotopes of chemical elements and more than 2000 natural and artificially produced radioactive isotopes are known.

4. Number of neutrons in the nucleus N can be found from the difference between the mass number ( A) and serial number ( Z):

5. The size of the kernel is characterized core radius, which has a conditional meaning due to the blurring of the core boundary.

The density of nuclear matter is of the order of magnitude 10 17 kg/m 3 and is constant for all nuclei. It significantly exceeds the densities of the densest ordinary substances.

The proton-neutron theory made it possible to resolve the previously arising contradictions in ideas about the composition of atomic nuclei and its relationship with the atomic number and atomic mass.

Nuclear binding energy is determined by the amount of work that needs to be done to split a nucleus into its constituent nucleons without imparting kinetic energy to them. From the law of conservation of energy it follows that during the formation of a nucleus the same energy must be released as must be expended during the splitting of the nucleus into its constituent nucleons. The binding energy of a nucleus is the difference between the energy of all the free nucleons that make up the nucleus and their energy in the nucleus.

When a nucleus is formed, its mass decreases: the mass of the nucleus is less than the sum of the masses of its constituent nucleons. The decrease in the mass of the nucleus during its formation is explained by the release of binding energy. If W sv is the amount of energy released during the formation of a nucleus, then the corresponding mass Dm, equal to

called mass defect and characterizes the decrease in the total mass during the formation of a nucleus from its constituent nucleons. One atomic mass unit corresponds to atomic energy unit(a.u.e.): a.u.e.=931.5016 MeV.

Specific nuclear binding energy w The binding energy per nucleon is called: w sv= . Magnitude w averages 8 MeV/nucleon. As the number of nucleons in the nucleus increases, the specific binding energy decreases.

Criterion for the stability of atomic nuclei is the ratio between the number of protons and neutrons in a stable nucleus for given isobars. ( A= const).

Nuclear forces

1. Nuclear interaction indicates that there are special nuclear forces, not reducible to any of the types of forces known in classical physics (gravitational and electromagnetic).

2. Nuclear forces are short-range forces. They appear only at very small distances between nucleons in the nucleus of the order of 10-15 m. The length (1.5 x 2.2)10-15 m is called range of nuclear forces.

3. Nuclear forces are detected charge independence: The attraction between two nucleons is the same regardless of the charge state of the nucleons - proton or nucleon. The charge independence of nuclear forces is evident from a comparison of binding energies in mirror cores. This is the name given to nuclei in which the total number of nucleons is the same, but the number of protons in one is equal to the number of neutrons in the other. For example, helium nuclei heavy hydrogen tritium - .

4. Nuclear forces have a saturation property, which manifests itself in the fact that a nucleon in a nucleus interacts only with a limited number of neighboring nucleons closest to it. This is why there is a linear dependence of the binding energies of nuclei on their mass numbers (A). Almost complete saturation of nuclear forces is achieved in the a-particle, which is a very stable formation.

Radioactivity, g-radiation, a and b - decay

1.Radioactivity is the transformation of unstable isotopes of one chemical element into isotopes of another element, accompanied by the emission of elementary particles, nuclei or hard x-rays. Natural radioactivity called radioactivity observed in naturally occurring unstable isotopes. Artificial radioactivity called the radioactivity of isotopes obtained as a result of nuclear reactions.

2. Typically, all types of radioactivity are accompanied by the emission of gamma radiation - hard, short-wave electric wave radiation. Gamma radiation is the main form of reducing the energy of excited products of radioactive transformations. A nucleus undergoing radioactive decay is called maternal; emerging subsidiary the nucleus, as a rule, turns out to be excited, and its transition to the ground state is accompanied by the emission of a g-photon.

3. Alpha decay called the emission of a-particles by the nuclei of some chemical elements. Alpha decay is a property of heavy nuclei with mass numbers A>200 and nuclear charges Z>82. Inside such nuclei, the formation of isolated a-particles occurs, each consisting of two protons and two neutrons, i.e. an atom of an element is formed, shifted in the table of the periodic system of elements D.I. Mendeleev (PSE) two cells to the left of the original radioactive element with a mass number less than 4 units(Soddy-Faience rule):

4. The term beta decay refers to three types of nuclear transformations: electronic(b-) and positronic(b+) decays, as well as electronic capture.

b-decay occurs predominantly in nuclei relatively rich in neutrons. In this case, the neutron of the nucleus decays into a proton, electron and antineutrino () with zero charge and mass.

During b-decay, the mass number of the isotope does not change, since the total number of protons and neutrons is maintained, and the charge increases by 1. Therefore, the atom of the resulting chemical element is shifted by the PSE one cell to the right from the original element, but its mass number does not change(Soddy-Faience rule):

b+- decay occurs predominantly in relatively proton-rich nuclei. In this case, the proton of the nucleus decays into a neutron, positron and neutrino ().

.

During b+ decay, the mass number of the isotope does not change, since the total number of protons and neutrons is maintained, and the charge decreases by 1. Therefore, the atom of the resulting chemical element is shifted by the PSE one cell to the left from the original element, but its mass number does not change(Soddy-Faience rule):

5. In the case of electron capture, the transformation consists of the disappearance of one of the electrons in the layer closest to the nucleus. A proton, turning into a neutron, “captures” an electron; This is where the term “electronic capture” comes from. Electronic capture, in contrast to b±-capture, is accompanied by characteristic X-ray radiation.

6. b-decay occurs in naturally radioactive as well as artificially radioactive nuclei; b+ decay is characteristic only of the phenomenon of artificial radioactivity.

7. g-radiation: When excited, the nucleus of an atom emits electromagnetic radiation of short wavelength and high frequency, which is more harsh and penetrating than x-rays. As a result, the energy of the nucleus decreases, but the mass number and charge of the nucleus remain unchanged. Therefore, the transformation of a chemical element into another is not observed, and the nucleus of the atom passes into a less excited state.