Лекция 2. Ядерная физика

Dimensions, composition and charge of the atomic nucleus.

Mass and charge numbers

E. Rutherford:

Conclusions from the results of experiments on the passage of  -particles with energy of several MeV through thin (d = 1 μm) gold foils

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1) The atom consists of a positively charged nucleus and surrounding electrons.

2) The approximate dimensions of the atomic nucleus are 10-14-10-15 m (linear dimensions of the atom are  10-10 m)

D. D. Ivanenko  ,  V. Heisenberg:

The atomic nucleus consists of elementary particles -  protons and neutrons  (the proton-neutron model of the nucleus)

The proton (  p  )

A particle with a positive charge, in absolute value equal to the charge of the electron and rest mass  mp  (  mp =  1.6726  10-27kg  1836  tons,  where  m   e  is the electron mass)

The neutron (  n  )

Neutral particles with a mass resting  TN   (TN =  1,6749  10-27kg  1839  TE).

Protons and neutrons are called  nucleons  (the Latin nucleus is the nucleus).

The total number of nucleons of the atomic nucleus is called the  mass number  A.

The mass of the nucleus (and of the atom) is measured in atomic units of mass.

For 1 atomic mass unit (amu), 1/12 part of the mass of the neutral carbon atom 12C:

1 amu = (1/12) (12 /  NA  ) = 1 / 6.022  1023 = 1.66  10-27kg.

The atomic nucleus is characterized by the  charge  Ze, where  Z  is the  charge number of the  nucleus. The charge number is equal to the number of protons in the nucleus and the ordinal number corresponds to the chemical element in the Periodic Table of Mendeleev's elements.

The kernel is denoted by a symbol similar to the neutral atom:,

Where

X is a symbol of a chemical element,

Z  is the number of the atom (the number of protons of the nucleus),

A is the  mass number (the number of nucleons of the nucleus).

The proton-neutron model of the nucleus

The proton-electronic structure of the nucleus:

The mass  A  number of the nucleus is equal to the number of protons in the nucleus, and the difference between the mass number and the number of electrons must be equal to the charge number.

+ the model is consistent with the values ​​of isotopic masses and charges.

- the model contradicts the values ​​of the spins and magnetic moments of the nuclei, the binding energy of the nucleus, and so on.

- The model is not compatible with the uncertainty relation.

The kernel charge determines:

the number of protons in the nucleus;

number of electrons in the atom.

Number of electrons

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The distribution of electrons over states in an atom

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Chemical properties of atoms

The nuclear charge determines the specificity of this chemical element, i.e. the number of electrons in the atom, the configuration of their electron shells, the magnitude and nature of the intra-atomic electric field.

Kernels with the same  Z  , but different  A  (ie with different neutron numbers  N = A-Z  ) are called  isotopes,  and nuclei with the same  A  but different  Z  isobars.

For example, hydrogen (  Z  = 1) has three isotopes: - protium (  Z  = 1,  N  = 0), - deuterium (  Z  = 1,  N  = 1), tritium (  Z  = 1,  N  = 2), tin - ten isotopes, and so on.

In most cases, isotopes of the same chemical element have the same chemical and almost identical physical properties (except, for example, isotopes of hydrogen), which are determined mainly by the structure of the electron shells, which is the same for all isotopes of this element. Kernels can serve as an example of nuclei-isobars. Currently, there are more than 2500 nuclei, differing either  Z  , or  A,  or both.

Nuclei, in one of which the number of protons is equal to the number of neutrons in the other, are called  mirror nuclei:;  ; .

The radius of the nucleus is determined by the empirical formula:

R = R  0  A  1/3

where  R  0  =  (1,3  1,7) 10-15 m.

The volume of the nucleus is proportional to the number of neutrons in the nucleus.

The density of nuclear matter is the same for all nuclei; (  1017 kg / m3).

The mass defect and the binding energy of the nucleus

The mass of the nuclei can be determined very accurately with the help of mass spectrometers, measuring instruments separating by electric and magnetic fields charged particle beams (usually ions) with different specific charges  Q / m.

Mass spectrometric measurements showed that the  mass of the nucleus is less than the sum of the masses of its constituent nucleons.  But since any change in mass must correspond to a change in energy, then, therefore, a definite energy must be released during the formation of the nucleus. The law of conservation of energy also implies the opposite: to separate the nucleus into its constituent parts, it is necessary to expend the same amount of energy that is released during its formation.

The energy that must be expended to split the nucleus into individual nucleons is called the  binding energy of the nucleus  .