The chart given below lists all of the decay products of uranium-238 in their order of appearance. Uranium-238 is also referred to as ''depleted uranium''.
Each radioactive element on the list gives off either alpha radiation or beta radiation -- and sometimes gamma radiation too -- thereby transforming itself into the next element on the list. When uranium ore is extracted from the earth, most of the uranium is removed from the crushed rock during the milling process, but the radioactive decay products are left in the tailings. Thus 85 percent of the radioactivity of the original ore is discarded in the mill tailings.
Depleted uranium remains radioactive for literally billions of years, and over these long periods of time it will continue to produce all of its radioactive decay products; thus depleted uranium actually becomes more radioactive as the centuries and millennia go by because these decay products accumulate.
The horizontal bar beside the name of each decay product indicates the "half life" of that particular substance, measured on a logarithmic scale (each half-inch to the right represents multiplication by a factor of one thousand).
Lead-206, the last element on the list, is not radioactive. It does not decay, and therefore has no half-life.
The half-life of a radioactive element is the time it takes for half of its atoms to decay into something else. For example, the half-life of radium-226 is 1600 years (as indicated on the chart given above). Therefore, in 1600 years, one gram of radium-226 will turn into half a gram of radium-226 and half a gram of something else (the radioactive decay products of radium). After another 1600 years have elapsed, only a quarter of a gram of the original radium-226 will remain.
The quantity of any radioactive element will diminish by a factor of a thousand (1 000) during the span of 10 half-lives. Thus, in 16 000 years, one gram of radium-226 will decay into a milligram of radium-226 and 999 milligrams of other decay products. Similarly, in 760 000 years, one gram of thorium-230 will be reduced to a milligram (because of the 76 000 year half-life of thorium-230, which is indicated on the chart above).
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