Ran Cob1e, Age 14, Of Graham, N.C., for his question:

 What is radioactive half life?     

The half life is a measurement of radioactive decay. It is the period of time In which half a supply of radioactive material will decay and become a different substance. Each of the different radioactive substances decays at its own rate, and nothing we have of can speed up or slow down its rate of decay. Each radioactive material has its own half life.     Every hospital keeps a supply of radium for treating certain patients. Pure Radium is a silvery white metallic element. The hospital uses salty crystals of Radium compounds, usually radium chlorine or radium bromide. The element radium and its compounds are highly radioactive, which means that its atoms are breaking apart and becoming atoms of something different      The cost of radium is about $25,000 a gram, and it takes 32 grams to equal one ounce. And every gram of this costly material is steadily decaying. In exactly 1620 years, half of every gram of radium will no longer be radium. Sme of the Radium will break into smaller atoms of radon gas and some will be given off as Radioactive energy.     We say that the half life of radium is 1620 years because, after this time, half of every gram of radium has decayed. You might suppose that the other half gram decays in the next 1620 years, but this does not happen. The half life of the half gram of radium is still 1620 years. After this time, one quarter gram has decayed and one quarter gram remains. In the next 1620 years, the quarter gram is reduced by half, and so the original supply of radium goes on getting smaller and smaller by halves.     The half life of uranium 238 is four and a half billion years. The atoms of Radon which form as radium decays have a half life of 3.82 days. Scene radioactive materials have half lives measured in millionths of a second. Each radioactive substance decays at its own rate, and its half life cannot be changed. Any atom of radium can decay at any time. But the fabulous numbers of tiny Atoms obey the law of averages. Of the teeming atoms in a pinch of radium, only a certain number explode each second, which is why decay proceeds at a fixed rate.     Uranium 238 heads a long chain of radioactive decay. Its atoms break into Smaller and smaller atoms until they become lead, which is not radioactive. In the process of decay, the atoms lose alpha and beta particles and give forth searing streams of radioactive energy.