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Lecture 12

EATS1010 - The Dynamic Earth and Space Geodesy Lecture 12

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Earth, Space Science and Engineering
ESSE 1010
Gary Jarvis

Lecture 12 Geochronology • Radioactive age dating o Based on spontaneous radioactive decay of unstable isotopes o What’s a decay? o What’s an isotope? o What’s an atom? o What’s the point? Time scale for polar wandering and geomagnetic reversals  rates of plate spreading Nucleus • 99.98% of mass is concentrated in the Nucleus • Properties of the nucleus is what’s radioactive • A carbon atom has 6 neutron and 6 protons 12 Atomic mass (protons + neutrons) • atomic number (number of protons 6 • some isotopes have different number of neutrons but always have the same amount of protons o e.g. C66P 7N 14 o e.g. C66P 8N • If the amount of protons change, then the element changes Mass Charge Proton 2000M e +1 Neutron 2000M e 0 Electron 1M e -1 How does an atom decay? • Emission of Alpha and Beta particles o Alpha particle: 2 protons + 2 neutrons  When the alpha particles is spat out, it loses 2 protons so it then becomes an element 2 atomic numbers down o Beta particle: 1 electron  When the beta particles comes out, we end up with 1 more proton  A neutron can be thought of as equal to a proton + an electron  What can happen is a neutron can turn into a proton and spit out the electron, a beta particle  There is now a new element since there is an additional proton  The opposite can also happen, an electron can shoot into the nucleus and fuse with a proton to form a neutron, once again changing the element • K-capture: o Capture of an inner electron • Emission of γ (gamma)-radiation (no effect on Z) Radioactive Decay Delta Z (change in Delta A (change in atomic number) atomic mass) Alpha-Emission -2 -4 Beta-Emission
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