Podsumowanie wiadomości z fizyki jądrowej

Before you start, do the following.

Prepare short statements explaining the following concepts.

1. Atomic nucleus.
2. Isotopes.
3. Mass defect.
4. Binding energy.
5. Nuclear radiation.
6. Radiation dose.
7. Half‑life.
8. The law of radioactive decay.
9. Nuclear transformations.
10. Nuclear reactions.
11. Nuclear fission.
12. Chain reaction.
13. Nuclear power plant.
14. Thermonuclear fusion.
15. Radiation detection.
16. Radioactivity in technology and medicine.

Nuclear physics deals with the structure of the atomic nucleusnucleusnucleus, investigates the processes occurring in it and processes involving atomic nuclei. No less important are the applications of nuclear reactions in science and technology.

1. Atomic nucleus

The atomic nucleus is the high density region located in the centre of the atom. The atomic nucleus is several orders of magnitude smaller than the size of the atom itself. It has a positive charge.

The atomic nucleus is made up of protons and neutrons called nucleons. Protons have a positive charge and neutrons are electrically neutral. Both particles have similar masses.

The composition of the atomic nucleus is symbolically written as:

where:
X - chemical symbol of the element,
Z - atomic number indicating the number of protons in the nucleus,
A - mass number equal to the number of nucleons (total number of protons and neutrons) in the nucleus.

The number of neutrons is the difference between mass and atomic numbers, or A - Z.

2. Isotopes

Isotopes are different forms of the same element that differ in the number of neutrons in the nucleus. The isotope nuclei have the same atomic number, while the different mass number.

The isotopes of a given element have the same chemical properties, but different physical properties.

Hydrogen is the only element whose natural isotopes have their own names: protium (1H), deuterium (2H) and tritium (3H).

Protium, the most commonly occurring hydrogen isotope, consists of a proton and an electron. Protium and deuterium are the stable isotopes. Third isotope tritum is radioactive.

3. Mass defect

The original mass of the nucleusnucleusnucleus is smaller than the sum of the masses of its individual components. A mass defect or otherwise a mass deficit, which is the difference between the sum of the masses of the nucleus constituents and its mass, is equivalent to the energy released during the formation of the nucleus.

The mass deficit is calculated using the formula:

where:
$m_p$ - is thr mass of proton,
$m_n$ - is the mass of the neutron,
$m_j$ - is the mass of the nucleus.

4. Binding energy

There are attractive nuclear forces acting between the nucleons in the nucleus (known as strong interactions), which do not depend on the electric charge. They have very large magnitude but a short range. As a result of these forces, the nucleons are strongly bound to each other.

The nuclear binding energy is responsible for the difference between the actual mass of the nucleus and the sum of the masses of the its constituents. It is the energy necessary to keep the nucleus together.

According to Einstein's law of the equivalence of mass and energy, the binding energy of the nucleus is:

where:
Δm - is the mass deficit,
c - the speed of light.

For the light atomic nuclei the nuclear binding energy increases with the mass number.

5. Nuclear radiation

The nuclei of some atoms are unstable. These atoms (primary isotopes) undergo spontaneous decay creating a more stable atom (a descendant isotope) and emit radiationradiationradiation. The substances emitting radiation are called radioactive.

There are three types of nuclear radiation.

• Alpha (α) - are made up of 2 protons and 2 neutrons. They have a positive charge equal to twice the elementary charge and are identical to helium nuclei. They have a low penetration ability. You can stop them with a piece of paper.

• Beta (β) - beta particles have a negative charge equal to the elementary charge and a mass of 1/2000th of the mass of the proton. They are electrons (or positrons) produced in the nucleusnucleusnucleus as a result of a radioactive decay called beta decay (they are not electrons from atomic shells). They are very light and move quickly. It can be stopped by a thin aluminium plate.

• Gamma (γ) - gamma rays are electromagnetic waves, not particles. They do not have mass or charge. Gamma radiationradiationradiation has the highest penetrating power. Low energy gamma rays penetrate through air, paper or a thin layer of metal. High energy rays can be stopped only by a few centimetres of lead or a few meters of concrete.

6. Radiation dose

The quantities and units used to measure radioactivity and its effects.

- Radioactivity (A) refers to the amount of ionizing radiationradiationradiation released by a given substance. It represents the number of atoms decaying over a given period of time. The unit in the SI system is becquerel (Bq).

where:
N - is the number of decays,
t - time unit.

- Radiation exposure describes ionization of air due to radiation. The unit is a $\frac{coulomb}{kg}$ ($\frac{C}{kg}$).

- Absorbed dose (D) refers to the amount of radiation absorbed by an object or person. The unit in the SI system is gray (Gy).

where:
E - radiation energy absorbed by the body,
m - body mass.

The traditional dose unit is rad, 1 Gy = 100 rad.

- Effective dose describes the amount of radiationradiationradiation absorbed by humans, corrected by the type of radiation, described by the weight ratio of radiation and the effect on particular organs. The unit in the SI system is the sievert (Sv).

7. Half‑life

The rate of radioactive decay is measured using the concept of half‑life.

The half‑life is the time required for the radioactivity of a given isotope to be reduced by half and is denoted as T or TIndeks dolny 1/2_1/2. After two half‑lives the size of the sample is quartered, after third half‑life an eighth of atoms is left intact and so on. The half‑life does not depend on the age of the nuclei or the sample size.

Radioactive decay as a function of time is exponential.

8. The law of radioactive decay

The law of radioactive decay describes the statistical behaviour of a large number of nuclides. It says that:

where:
N(t) - is the total amount of radioactive untransformed nuclei at a time t,
∆N - is the number of radioactive nuclei decayed in the time ∆t.

The probability of a nuclear transformation is different for each radioactive nucleusnucleusnucleus and can be expressed by the decay constant λ. The unit of the decay constant is sIndeks górny -1^-1.

The radioactivity of an object is measured by the number of nuclear decays it emits each second - the more it emits, the more radioactive it is.

The decay rate is known as the activity of a particular sample and is defined as a number of decays at any given moment.

The basic unit of activity is the becquerel (Bq).

9. Nuclear transformations

In the process of radioactive decay, the decaying nucleus is called the parent nucleus, and the process product is called the daughter nucleus. The law of radioactive displacements, also known as Soddy and Fajans law, describes the relationship between the parent nucleus and the daughter nucleus in terms of atomic number and mass number.

During nuclear transformations, the following is met.

• The principle of number of nucleons conservation - the sum of the number of nucleons in all decay products is equal to the number of nucleons before decay.

• The principle of charge conservation - the sum of charges in the products of decay is the same before the decay as after the decay.

• The principle of mass and energy - the sum of masses and energy after the decay is the same as before the decay.

α decay

In α decay, the new element has an atomic number less by 2 and a mass number less by 4 than the parent radioisotope. The α  decay can be expressed as:

β decay

In the β decay (the emitted particle is an electron or positon), the mass number remains unchanged, while the atomic number increases or decreases by 1 relative to the parent radioisotope. The β‑decay can be expressed as:

γ decay

When the radioactive nucleus emits γ radiationradiationradiation, only the energy level of the nucleus changes, and the atomic number and mass number remain the same.

During the decay of α or β, the daughter nucleus is usually in an excitedexcitedexcited state. Return to the ground state is associated with the emissionemissionemission of γ‑radiation.

10. Nuclear reactions

In nuclear processes in which two nuclei or nucleons collide, different products than the initial particles are produced. This process is called a nuclear reactionnuclear reactionnuclear reaction. The nuclear reaction does not occur spontaneously, but two particles must collide.

The nuclear reaction, as in the case of nuclear decays, can be presented by a balanced equation:

where:
X - target nucleus,
a - bombarding particle (projectile),
Y - final nucleus,
b - produced particle (ejectile).

11. Nuclear fissionfissionfission

Nuclear fission is a type of nuclear reactionnuclear reactionnuclear reaction in which the nucleus is divided into smaller fragments of smaller mass. The fissionfissionfission process produces free neutrons and gamma rays. In this process, a large amount of energy is released.

Nuclear fission is carried out in nuclear energy reactors.

Example of the reaction:

12. Chain reaction

During the fissionfissionfission reaction, neutrons are released. They may hit other fissile nuclei and cause them to split. Even more neutrons are then released, which in turn can split more nuclei. This is called a chain reaction. The chain reaction in nuclear reactors is controlled to stop it going too fast. The chain reaction is ongoing so long as the fissile nuclei are present in the sample.

13. Nuclear power plant

Nuclear power plants perform a similar function as solid fuel power plants - their task is to supply energy. In the case of a nuclear power plant, energy is released during chain reaction. This energy, created in the reactor, is used to convert water into steam, which in turn drives the turbine rotors. As a result, electricity is generated.

A nuclear reactor is a basic element of a nuclear power plant in which a controlled chain reaction takes place.

The most important elements of a nuclear reactor are:

• fuel - i.e. fissile material, e.g. enriched uranium, plutonium,

• moderator - a substance weakly absorbing neutrons, whose task is to slow them down, for example, heavy water, graphite,

• control rods and safety rods - made of substances strongly absorbing neutrons, e.g. cadmium, boron,

• coolant - a substance that discharges heat from the reactor core, e.g. water, liquid sodium.

14. Thermonuclear fusionfusionfusion

Nuclear fusion is a type of nuclear reactionnuclear reactionnuclear reaction where two light nuclei collide together to form a single, heavier nucleusnucleusnucleus. This nucleus is unstable and decay into more stable daughter products. In this process, according to the principle of mass‑energy equivalence the energy is released because the mass of the new nucleus is less than the sum of the colliding masses.

Some possible fusion reactions.

$D+T\to {}_{}{}^{4}He+n+17,58MeV$

$D+T\to {}_{}{}^{4}He+n+3,27MeV$

$D+D\to T+p+4,03MeV$

$D+{}_{}{}^{3}He\to {}_{}{}^{4}He+p+18,35MeV$

$p+{}_{}{}^{11}B\to 3{}_{}{}^{4}He+8,7MeV$

Thermonuclear fusionfusionfusion processes take place:

• in the interiors of stars, also in our Sun,

• in hydrogen bombs,

• in thermonuclear reactors (so far only experimental, devoid of industrial applications).

15. Radiation detection

Devices for detecting and recording nuclear (ionizing) radiationradiationradiation are called particle detectors. Most often these are devices that use the phenomena:

• gas ionization (ionization chamber, Wilson chamber, bubble chamber, Geiger‑Müller counter),

• excitation of certain substances to emit light (scintillation counter),

• chemical reaction (photographic emulsion).

16. Radioactivity in technology and medicine

Artificial radioactivity has found wide application in many areas.

• Power engineering - power plants, nuclear batteries (used, for example, for pacemakers).

• Medicine - radioisotopes, markers for diagnostic tests, treatment of cancerous diseases (cobalt bomb), accelerators.

• Science - determining the age of archaeological finds by radiocarbon dating method (14C‑dating), activation analysis (a very sensitive method of studying the elemental composition of a sample).

• Technique - atomic engines of ships and spacecraft, precise thickness gauges, glowing paints, smoke detectors.

• Industry - detection of defects in the elements of aircraft engines, sterilization of food and medical equipment, control of the expiry date of products.

Remember

• Nuclear physics is the branch of physics dealing with the structure of the atomic nucleus, investigates the processes occurring in it and processes involving atomic nuclei.

Exercises

Glossary

Keywords

emissionemissionemission

fissionfissionfission

ionisationionisationionisation

nuclear reactionnuclear reactionnuclear reaction

radiationradiationradiation