AA&A spring 2002 2

Radioactive dating—Generalities

AA&A spring 2002 3

Four EssentialsMust 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

AA&A spring 2002 4

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

AA&A spring 2002 5

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

AA&A spring 2002 6

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

AA&A spring 2002 7

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)

AA&A spring 2002 8

Starting the clock

C14—drops by 1/2 every 5730 years: t = T1/2 log2 (R0/Rt)

U238/Th230—complicated but calculable

AA&A spring 2002 9

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

AA&A spring 2002 10

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