Physics 362 – week 13 Modern Physics Seminars
How to measure the Lifetime of an Isotope • Several measurements over a period of time • Single measurement of radioactivity and mass
Dating with radioactive nuclei • Dating of fossils • Dating of rocks
Carbon Nuclear Data Three principal isotopes of carbon occur naturally 12C - 98.89% 13C - 1.11% 14C - (radioactive) 0.00000000010%.
Carbon-14 Decay 14C 14N + e- + ne t1/2=5715 yr MC14=14.003242 u MN14=14.003074 u Q=0.016 MeV
Carbon Dating Libby, Anderson and Arnold (1949) • t has been measured in the laboratory • We can measure Nin the sample • If we know N0 we can derive t
Measuring Carbon-14 activity The sample is disintegrated and the activity is measured using a radiation detector t1/2=5715 yr Only good up to about 50,000 yr
Dating of rocks and meteorites Potassium-Argon: 40K 40Ar + e+ + ne;t1/2=1.28 109 yr Rubidium-Strontium: 87Rb 87Sr + e- + ne; t1/2=4.8 1010 yr Samarium-Neodymium: 147Sm 143Nd + a; t1/2=10.5 1010 yr Lutetium-Hafnium: 176Lu 176Hf + e- + ne; t1/2=3.8 1010 yr Rhenium-Olmium: 187Re 187Ol + e- + ne; t1/2=4.2 1010 yr
Determining N and N0 We measure the ratio between parent and daughter abundances: How do we evaluate N0-N?
Potassium-Argon Argon is a gas. Whenever rock is melted to become magma or lava, the argon tends to escape. Once the molten material hardens, it begins to trap the new argon produced since the hardening took place. In this way the potassium-argon clock is clearly reset when an igneous rock is formed. In nearly all of the dating methods, except potassium-argon and the associated argon-argon method, there is always some amount of the daughter product already in the rock when it cools.