Exercise 1. The following sentences (A-F) summarize the four paragraphs of Reading 3-A.Read the sentences and then match them to the paragraphs of Reading 3-A,1-4.

UNIT 3

Reading 3-А

Radiation and Radioactivity

"Life on earth has developed with an ever present background of radiation. It is not something new, invented by the wit of man: radiation has always been there."

Eric J. Hall

1. As is the case with so many discoveries, the discovery of the phenomenon of radioactivity was purely accidental. It was discovered in 1896 by the French physicist Henri Becquerel (1852-1908), who was interested at that time in the phenomenon of fluorescence. Не found that compounds of uranium emitted rays that gave an impression on а photographic plate covered with black paper. These rays were able to pass through thin sheets of metal and other substances that are opaque to light. Becquerel called these rays 'radiation'.

2. It is now known that radiation is emitted by the atoms themselves. The radiation may be in the form of particles, such as neutrons, alpha particles, and beta particles, or waves of energy, such as gamma and X-rays. The property of certain atoms to emit radiation is called radioactivity. Such atoms are called radioactive, and the process in which they spontaneously disintegrate or decay emitting both particles and energy is known as radioactive decay. This process occurs because unstable isotopes tend to transform into different, more stable atoms.

3. As unstable isotopes decay and stabilize themselves in this way, a radioactive material becomes less radioactive. Atoms in a radioactive substance decay at a characteristic rate. Each radioactive substance has its own 'half-life'. The half-life is the time taken for half of the atoms of a radioactive substance to decay. Half-lives can range from a millionth of a second to millions of years depending on the element concerned. After one half-life the level of radioactivity of a substance is halved, after two half-lives it is reduced to one quarter, after three half-lives to one-eighth, and so on. Take iodine-131, which has a half-life of eight days. If we start off with a billion atoms of I-131, it will be prac­tically all gone within three months. On the other hand, uranium-238 has a half-life of about 4.6 billion years, which is about the lifetime of the earth. Starting with a billion atoms of uranium, it takes 4. 6 billion years for half of them to become stable.

4. This leads to a very important fact about radioactivity. If a substance has a long half-life, it will not give off much radiation in a second - it will have 'low activity’. In general, substances with very long half-lives are not very radio­active. On the other hand, substances with very short half-lives are very radioactive but are soon gone. The most dangerous ones are those in between, such as caesium (half-life 28 years), which are radioactive enough to cause a problem, but have a long enough half-life to remain radioactive for a con­siderable period of time.

Exercise 1. The following sentences (A-F) summarize the four paragraphs of Reading 3-A.Read the sentences and then match them to the paragraphs of Reading 3-A,1-4.

A. The radioactivity of a substance depends on its half-life.

B. Radiation and radioactivity are not the same thing.

C. The discovery of radioactivity was purely accidental.

D. Each radioactive isotope has its own characteristic half-life which can range from a millionth of a second to millions of years.

Exercise 2. Match the two parts of the sentences. Look at Reading 3-A to help you.

1. The property of certain atoms to emit radiation...

2. The radiation may be in the form of particles,...

3. The process in which radioactive atoms spontaneously disintegrate or decay...

4. This process occurs because unstable isotopes...

5. The half-life is the time...

6. Substances with very long half-lives are not very radio­active, …

A..... emitting both particles and energy is known as radioactive decay.

B..... tend to transform into different, more stable atoms.

C..... taken for half of the atoms of a radioactive substance to decay.

D..... while substances with very short half-lives are very radioactive but are soon gone.

E..... such as neutrons, alpha particles, and beta particles, or waves of energy, such as gamma and X-rays.

F. ….is called radioactivity.

Exercise 3. Classify the nouns below according to their suffixes. Copy them out.

Radiation, measurement, fluorescence, emission, impression, ability, passage, property, disintegration, stability, equipment, difference, excitation, tendency, dependence, density, voltage, requirement, existence, brevity, absorption, achievement, deflection, invention, isolation, substance, radioactivity, storage, development, accuracy.

Exercise 4. Use Reading 3-A to find the English equivalents for the following Russian phrases.

Как часто бывает с открытиями; интересоваться явлением; элементы и соединения; соединения урана; оставлять отпечаток на; проходить сквозь тонкие листы; непроницаемый для света; энергия в виде волн; радиоактивный распад; распадаться с характерной скоростью; находиться в диапазоне от… до…; данный элемент; вызывать проблему; оставаться радиоактивным; значительный период времени.

Exercise 5. Read the text again and answer the questions below.

1. When was the phenomenon of radioactivity discovered? Who made this discovery?

2. How was the discovery made?

3. Where does radiation come from?

4. In what form can radiation be emitted?

5. What is radioactivity? What is the difference between radiation and radioactivity?

6. What is meant by radioactive decay?

7. Why does the process of radioactive decay occur?

8. What happens when unstable isotopes decay?

9. What is meant by the half-life of a radioactive substance?

10. What does the half-life of a radioactive substance depend on?

11. Which is more radioactive: substances with very long half-lives or those with very short half-lives?

12. What radioactive substances are the most dangerous ones?

Reading 3-B

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