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Lead (Pb) has four stable isotopes: 204Pb, 206Pb, 207Pb, 208Pb. Lead-204 is entirely a primordial nuclide and is not a radiogenic nuclide. The three isotopes lead-206, lead-207, and lead-208 represent the ends of three decay chains: the uranium series (or radium series), the actinium series, and the thorium series, respectively. These series represent the decay chain products of long-lived primordial U-238, U-235, and Th-232, respectively. However, each of them also occurs, to some extent, as primordial isotopes that were made in supernovae, rather than radiogenically as daughter products. The fixed ratio of lead-204 to the primordial amounts of the other lead isotopes may be used as the baseline to estimate the extra amounts of radiogenic lead present in rocks as a result of decay from uranium and thorium. (See lead-lead dating and uranium-lead dating). The longest-lived radioisotopes are 205Pb with a half-life of ~15.3 million years and 202Pb with a half-life of ~53,000 years. Of naturally-occurring radioisotopes, the shortest half-life is 210Pb with a half-life of 22.20 years. The standard atomic mass (abundance-weighted average of the stable isotopes) is 207.2(1) u. Lead is the element with the heaviest stable isotope, 208Pb. (The more massive 209Bi, long considered to be stable, actually has a half-life of 1.9×1019 years). A total of 38 Pb isotopes are now known, including very unstable synthetic species. In its fully ionized state the isotope 205Pb also becomes stable. ==Lead-206== 206Pb is the final step in the decay chain of 238U, the "radium series" or "uranium series". In a closed system, over time, a given mass of 238U will decay in a sequence of steps culminating in 206Pb. The production of intermediate products eventually reaches an equilibrium (though this takes a long time, as the half-life of 234U is 245,500 years.) Once this stabilized system is reached, the ratio of 238U to 206Pb will steadily decrease, while the ratios of the other intermediate products to each other remain constant. Like most radioisotopes found in the radium series, 206Pb was initially named as a variation of radium, specifically radium G. It is the decay product of both 210Po (historically called radium F) by alpha decay, and the much more rare 206Tl (radium EII) by beta decay. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Isotopes of lead」の詳細全文を読む スポンサード リンク
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