18. Structure of a nucleus, nuclear forces. Energy of connection of nucleons.

Structure of a nucleus: The nucleus of atoms consists of elementary particles - protons and the neutrons, which are named nucleons. Protons and neutrons - independent particles in a free condition. Quantity Z of protons in a nucleus is equal to atomic number of an element. The mass number A is the integer number approximately equal to atomic mass of an element.The quantity of neutrons in nucleus is equal to: N = A - Z

Nucleons in a nucleus are connected by special forces of a mutual attraction - nuclear forces.

1. Nuclear forces is short-range

2. Nuclear forces is strong

3. Nuclear forces operate act between nucleons irrespective of their electric charge.

4. Nuclear forces have properties of saturation.

Energy of connection of nucleons: The energy of connection is a difference between general potential energy Еall free nucleons and potential energy Е_n of nucleons, which there are inside nucleus Е_c=Е-Е_n. Energy of connection is an energy, which is allocated at formation of a nucleus from free nucleons, or accordingly energy, which is spend if action of external forces to destroy a nucleus.

Nuclear binding energy is the energy required to split a nucleus of an atom into its components.

Nuclear binding energy is used to determine whether fission or fusion will be a favorable process.

The mass defect of a nucleus represents the mass of the energy binding the nucleus, and is the difference between the mass of a nucleus and the sum of the masses of the nucleons of which it is composed. The binding energy of nuclei is always a positive number, since all nuclei require net energy to separate them into individual protons and neutrons.

19. Radioactivity. Kinds of radioactive disintegrations.

Radioactive decay, also known as nuclear decay or radioactivity, is the process by which the nucleus of an unstable atom loses energy by emitting radiation, including alpha particles, beta particles, gamma rays and conversion electrons. A material that spontaneously emits such radiation is considered radioactive.

Alpha decay An alpha particle is identical to a helium nucleus, being made up of two protons and two neutrons bound together.

It initially escapes from the nucleus of its parent atom, invariably one of the heaviest elements, by quantum mechanical processes and is repelled further from it by electromagnetism, as both the alpha particle and the nucleus are positively charged.

The process changes the original atom from which the alpha particle is emitted into a different element.

Its mass number decreases by four and its atomic number by two. For example, uranium-238 will decay to thorium-234.

Sometimes one of these daughter nuclides will also be radioactive, usually decaying further by one of the other processes described below.

Beta decay Beta decay itself comes in two kinds: β⁺ and β^-.

β^- emission occurs by the transformation of one of the nucleus’s neutrons into a proton, an electron and an antineutrino. Byproducts of fission from nuclear reactors often undergo β^- decay as they are likely to have an excess of neutrons.

β⁺ decays is a similar process, but involves a proton changing into a neutron, a positron and a neutrino.

Gamma decay After a nucleus undergoes alpha or beta decay, it is often left in an excited state with excess energy.

Just as an electron can move to a lower energy state by emitting a photon somewhere in the ultraviolet to infrared range, an atomic nucleus loses energy by emitting a gamma ray.

Gamma radiation is the most penetrating of the three, and will travel through several centimetres of lead.

Beta particles will be absorbed by a few millimetres of aluminium, while alpha particles will be stopped in their tracks be a few centimetres of air, or a sheet of paper – although this type of radiation does the most damage to materials it hits.