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Radioactive dating—Generalities

Radioactive dating—Generalities. Four Essentials. Must find or know each of these in order to use radioactive dating (reliably)! 1—decay scheme with known 1/2-lives 2—NOW! a reliable measurement of two numbers (or one ratio)

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Radioactive dating—Generalities

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  1. Radioactive dating—Generalities

  2. Four Essentials Must find or know each of these in order to use radioactive dating (reliably)! • 1—decay scheme with known 1/2-lives • 2—NOW! a reliable measurement of two numbers (or one ratio) • 3—THEN! a compelling argument for values of those numbers (or that ratio) when the clock “started” • 4—useful range of ages for assumed technique

  3. Radioactive decay: known 1/2-life More precisely, a known time dependence • C14—5568 versus 5730 years • U238 —> Th230 —> Pb206 • More complex—long chain • Many 1/2 lives involved • BUT we know and understand the scheme

  4. Radioactive decay: known 1/2-life More precisely, a known time dependence • O21 —> F21 —> Ne21 • More complex—short chain • Two 1/2 lives involved • BUT we know and understand the scheme!

  5. NOW! Reliable measurement two numbers or a ratio C14 dating— • counting technique: • C14 from beta activity (counting rate) • C12 from weight of sample • ASM (accelerator mass spectrometry) • C14 and C12 from the mass spectrometer • U238 —> Th230 —> Pb206 • U238 and Th 230 from alpha spectroscopy OR • Mass spectrometry

  6. THEN! Starting the clock compelling argument for initial values of those numbers (or that ratio) • C14 dating • All that stuff about generation of C14, atmospheric mixing, incomplete mixing in ocean, local high concentrations of “dead carbon,” … • U238 —> Th230 —> Pb206 • No Th in sea water bathing the corals: only U • No Th in seepage water as limestone cave formations develop: only U

  7. Starting the clock C14—drops by 1/2 every 5730 years: t = T1/2 log2 (R0/Rt) U238/Th230—complicated but calculable

  8. Useful range of ages? • Critical parameter is some half-life! • Works “best” at ages ~T1/2 • 6,000 years for C14 • 75,000 years for U238/Th 230 • In trouble for ages > ~(6-10) T1/2 • The exponential decay is hard to fight against • In trouble for ages < ~[(1-few)/100] T1/2 • Depends on details of decay scheme and method

  9. Must: • Know the nuclear physics • Measure two numbers now or (or one ratio) • Know how the clock started? • Not to try to date 150,000 years with C14

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