Kevin Parker, age 12, of Rochester, New York, for his question:
What do scientists mean by a half life?
This odd expression sounds as though it belongs in biology, the science of living things. You might suspect that a half life has something to do with the age of a plant or animal. But no. The term half life belongs in physics, the dynamic science of nonliving matter and energy.
The term half life belongs in the field of radioactivity. It is used by atomic scientists and nuclear physicists. Experts who use or experiment with radioactive substances must know something about the half life. It tells how long they can depend upon this or that radioactive material. A half life is a period of time sometimes a split second, sometimes several billion years. It cannot be speeded up or slowed down and it works constantly in every sample of radioactive material. When a half life period of time is up, exactly half of the sample will have become a different substance.
A big hospital keeps a supply of radioactive radium. Day and night, at a fixed rate, a definite number of atoms in this supply break apart, releasing nuclear energy and fast flying atomic particles. This radioactive energy from the radium is used to treat certain diseases of bones and tissues. As the radioactive energy is spent, the radium atoms break into smaller atoms at a steady rate. The changing radium atoms become atoms of radon. In 1660 years, half of the supply of radium will be gone.
A hospital may pay $500,000 for one ounce of precious radium. It contains countless trillions of radium atoms but every second, one among every 100 billion atoms breaks apart. In 1660 years, half the radium atoms in the entire supply will decay for 1660 years is the half life of radioactive radium. In the next half life of 1660 years, half of the remaining radium will decay. Each following half life period reduces the remaining radium by half.
The radon atoms that are formed as radium decays also are radioactive. The half life period of radon is 3.85 days. Hence half the radon atoms in a radium sample decay in less than four days while new ones are formed. Both radium and radon are formed when still heavier atoms break down by radioactivity. They are two stages in a long series of decay that begins with radioactive uranium.
The big parent atoms of uranium start a series of breakdowns into smaller atoms. Each stage in the series produce a different radioactive substance having its own half life period. The half life of the parent uranium is 4 1/2 billion years. After this period, half the original uranium atoms become atoms of a lead isotope. The breakdown series ends with these lead atoms, for they are not radioactive.
In the earth's crust, traces of uranium are mixed with other minerals in more than 100 rocky ores. A sample of uranium contains fixed quotas of radium, radon and other substances that form during the long series of radioactive decay. Scattered through 1500 tons of uranium there is about one pound of radium atoms. In 1660 years, half these radium atoms decay but meantime more are added as bigger atoms continuously break apart.