1. AA&A spring 20021

2. 2 Radioactive dating—Generalities

3. AA&A spring 20023 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

4. AA&A spring 20024 Radioactive decay: known 1/2-life C14—5568 versus 5730 years U238 —> Th230 —> Pb206 –More complex—long chain –Many 1/2 lives involved –BUT we know and understand the scheme More precisely, a known time dependence

5. AA&A spring 20025 Radioactive decay: known 1/2-life O21 —> F21 —> Ne21 –More complex—short chain –Two 1/2 lives involved –BUT we know and understand the scheme! More precisely, a known time dependence

6. AA&A spring 20026 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

7. AA&A spring 20027 THEN! Starting the clock 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 compelling argument for initial values of those numbers (or that ratio)

8. AA&A spring 20028 Starting the clock C14—drops by 1/2 every 5730 years: t = T 1/2 log 2 (R 0 /R t ) U238/Th230—complicated but calculable

9. AA&A spring 20029 Useful range of ages? Critical parameter is some half-life! Works “best” at ages ~T 1/2 –6,000 years for C14 –75,000 years for U238/Th 230 In trouble for ages > ~(6-10) T 1/2 –The exponential decay is hard to fight against In trouble for ages < ~[(1-few)/100] T 1/2 –Depends on details of decay scheme and method

10. AA&A spring 200210 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