Материал: Андрианова English Thesaurus of Student-Physicist 2013

The total mass of a nucleus is always less than the sum of the masses of its nucleons. Also, because mass is another manifestation of energy, the total energy of the bound system (the nucleus) is less than the combined energy of the separated nucleons. This difference in energy is called the binding energy of the nucleus and can be thought of as the energy that must be added to a nucleus to break it apart into its separated neutrons and protons.

Radioactivity

In 1896, Becquerel accidentally discovered that uranium salt crystals emit an invisible radiation that can darken a photographic plate even if the plate is covered to exclude light. After several such observations under controlled conditions, he concluded that the radiation emitted by the crystals was of a new type, one requiring no external stimulation. This spontaneous emission of radiation was soon called radioactivity. Subsequent experiments by other scientists showed that other substances were also radioactive.

Three types of radiation can be emitted by a radioactive substance: alpha (α) particles, in which the emitted particles are He nuclei; beta (β) particles, in which the emitted particles are either electrons or positrons; and gamma (γ) rays, in which the emitted "rays" are high-energy photons. A positron is a particle similar to the electron in all respects, except that it has a charge of + e. (The positron is said to be the antiparticle of the electron.)

The decay rate, or activity R, of a sample is defined as the number of decays per second. Another parameter that is useful for characterizing radioactive decay is the half-life. The half-life of a radioactive substance is the time it takes for half of a given number of radioactive nuclei to decay. The unit of activity R is the curie (Ci). The SI unit of activity is the becquerel (Bq): 1 Bq = 1 decay/s.

Radioactive nuclei decay spontaneously via alpha, beta, and gamma decay. If a nucleus emits an alpha particle (2He), it loses two protons and two neutrons. In order for alpha emission to occur, the mass of the parent must be greater than the combined mass of the daughter and the alpha particle. In the decay process, this excess mass is converted into energy of other forms and appears in the form of kinetic energy in the daughter nucleus and the alpha particle.

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When a radioactive nucleus undergoes beta decay, the daughter nucleus has the same number of nucleons as the parent nucleus, but the atomic number is changed by 1. In 1930 Pauli proposed that a third particle must be present to carry away the "missing" energy and to conserve momentum. Later, Enrico Fermi developed a complete theory of beta decay and named this particle the neutrino ("little neutral one") because it had to be electrically neutral and have little or no mass. In beta decay, an electron and an anti-neutrino are emitted or a positron and a neutrino are emitted.

Very often a nucleus that undergoes radioactive decay is left in an excited energy state. The nucleus can then undergo a second decay to a lower energy state - perhaps even to the ground state - by emitting one or more high-energy photons. The process is similar to the emission of light by an atom. An atom emits radiation to release some extra energy when an electron "jumps" from a state of high energy to a state of lower energy. The photons emitted in the process are called gamma rays, which have very high energy relative to the energy of visible light.

Natural radioactivity

Radioactive nuclei are generally classified into two groups: (1) unstable nuclei found in nature, which give rise to what is called natural radioactivity, and (2) nuclei produced in the laboratory through nuclear reactions, which exhibit artificial radioactivity.

Nuclear reactions

It is possible to change the structure of nuclei by bombarding them with energetic particles. Such changes are called nuclear reactions. Since the time of Rutherford, thousands of nuclear reactions have been observed, particularly following the development of charged-particle accelerators in the 1930s. These high-energy particles are used to create new particles whose properties are helping to solve the mysteries of the nucleus (and indeed, of the Universe itself).

Nuclear reactions in which there is a release of energy - that is, positive Q values - are said to be exothermic reactions. Reactions with negative Q values are called endothermic reactions. It can be shown that in order to conserve both energy and momentum, the incoming particle must have a minimum kinetic energy. This minimum value of the kinetic energy of the incoming particle is called the threshold energy.

Medical applications of radiation

Radiation absorbed by matter can cause severe damage. The degree and kind of damage depend on several factors, including the type and

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energy of the radiation and the properties of the absorbing material. Radiation damage in biological organisms is due primarily to ionization effects in cells. The normal function of a cell may be disrupted when highly reactive ions or radicals are formed as the result of ionizing radiation. Also, cells that do survive the radiation may become defective, which can lead to cancer.

Several units are used to quantify radiation exposure and dose. The roentgen (R) is defined as that amount of ionizing radiation which will produce 2.08 x 109 ion pairs in 1 cm3 of air under standard conditions. Radioactive particles can be used to trace chemicals participating in various reactions.

One of the most valuable uses of radioactive tracers is in medicine. Most medical applications of radiation require instruments to make quantitative measurements of radioactive intensity. Various devices have been developed to detect the energetic particles emitted when a radioactive nucleus decays: the Geiger counter; a semiconductor diode detector; a scintillation counter; Track detectors; a cloud chamber.

1.What steps of nuclear physics development do you know?

2.Name values which are used for description of properties of nuclei.

3.What three types of radiation are radiated by radioactive substance?

4.Into what two groups radioactive nuclei are generally classified?

5.How it is possible to change the structure of nuclei?

6.What examples of medical applications of radiation do you know?

Exercises

1. Rearrange the letters in the anagrams to form equivalents for the Russian words:

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2. Match the words in A with the words in B to form word combinations:

3.Fill in the gaps with the missing words from the list:

Roentgen, nuclear force, isotopes, half-life, nucleon

1.Some _____ don't occur naturally, but can be produced in the laboratory through nuclear reactions.

2.Nuclei are stable because of the presence of another, shortrange (about 2 fm) force: the _____, an attractive force that acts between all nuclear particles.

3.A _____ is the time it takes for half of a given number of nuclei to

decay.

4.The mass of the electron is much smaller than that of the lightest

_____, so we can approximate it as zero when we study nuclear decays and reactions.

5.The _____ is that amount of radiation which deposits 8.76 X 10~3 J of energy into 1 kg of air.

UNIT 30

NUCLEAR ENERGY AND ELEMENTARY PARTICLES

Read the following texts. Study the thesaurus on fig. 30, answer the questions after the texts

In this concluding Unit we discuss the two means by which energy can be derived from nuclear reactions: fission, in which a nucleus of large mass number splits into two smaller nuclei, and fusion, in which two light nuclei fuse to form a heavier nucleus. In either case, there is a release of large amounts of energy, which can be used destructively through bombs or constructively through the production of electric power. We end our study of physics by examining the known subatomic particles and the

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