TEXT 6. RADIOACTIVITY
Ernest Rutherford1 studied the invisible radiations. He discovered that they consisted of two types of charged particles and one type of ray. The negatively charged particles were called beta-particles. They were actually electrons. But what were the other particles? To find out2 Rutherford carried out more experiments. He discovered that particles, called alpha-particles, were actually positively charged nuclei of helium atoms.
Further study showed that the particles move at tremendous speeds. Alpha-particles travel at 10,000 to 20,000 miles per second. Beta-particles move at even higher speeds. Gamma rays, which travel at the speed of light, were found to be similar to X rays. Gamma rays have the greatest penetrating power of the three types of radiations. Ordinary aluminum foil can stop alpha-particles, but it requires an aluminum sheet 1 centimeter thick to stop beta-particles. The gamma rays, given off by unstable atomic nuclei, have a penetrating power which is 10,000 times greater than of alpha-particles.
A number of natural elements and all transuranium elements are naturally radioactive. People who work with radioactive materials must have special protection because radiation exposure can damage body tissues. Special badges, containing photographic film sensitive to radiations, are worn. The film is examined at regular intervals to determine the amount of radiation to which the people have been exposed. This is a safety measure to prevent harmful effects of radiation. Radioactive elements lose mass as they give off particles and rays. At first, it was thought that a radioactive element would continue to produce the same amounts of energy forever. However, it was later discovered that a radioactive element loses mass. This could only mean that the nucleus of the radioactive element was undergoing change. Every radioactive element breaks down at a certain rate that is characteristic for it. This decay rate for the radioactive elements is described in terms of the half-life of an element. Half-life is the time it takes for one-half of the atoms of a radioactive element to decay.
Radium, for example, has a half-life of 1600 years. That means that 50 per cent of all the atoms in a given amount of radium will break down in 1600 years. Then, half of the remaining radium atoms will break down in the next 1600 years, and so on. The nuclear decay continues until all the radium atoms have changed into stable lead atoms. Some radioactive elements have a half-life of thousands of years. Others have a half-life measured in minutes or seconds.
Radioactivity can be detected with special instruments. Particles and rays given off by radioactive elements may cause the uncharged atoms of a gas through which they pass to become electrically charged. An atom that becomes electrically charged is known as an ion. An ion has properties different from those of uncharged atoms. The atoms of a gas may become either positively or negatively charged. This depends on whether the radioactive particles remove or add electrons from the atoms. This process in which the atoms of a gas become electrically charged is called ionization.
When radiations pass through an instrument, called a cloud chamber3, they produce streams of ionized atoms. These attract molecules of the surrounding gas, causing them to condense into visible fog trails4. By counting the number of tiny droplets in each track, scientists have determined the speed of the radioactive particles.
1) What did Ernest Rutherford study?
2) How were the negatively charged particles called?
3) At what speed do alpha-particles move?
4) Do radioactive elements lose mass as they give off particles and rays?
5) How can radioactivity be detected?
1. Ernest Rutherford (1871 - 1937) - Эрнст Резерфорд (английский физик, 1871 - 1937)
2. to find out - для того, чтобы выяснить
3. chamber - камера
4. trail - след
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