Measuring the Age of things (Astro 202 2/12/08) Nomenclature. Different Elements. Three Types of Nuclear Decay. Carbon 14 Decay.

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Electron Element: Number of Protons Carbon 12 6

neutrons 7 electrons Isotope: Number of Neutrons (Same

neutrons 6 electrons 6 electrons Three Types of

1 Measuring the Age of things (Astro 202 2/12/08) Nomenclature + Proton Different Elements Neutron Electron Element: Number of Protons Carbon 12 6 protons 6 neutrons 6 electrons Nitrogen 14 7 protons 7 neutrons 7 electrons Isotope: Number of Neutrons (Same No. of Protons) Same Element, Different Isotopes Note diagrams not to scale! Carbon 12 Carbon 14 6 protons 6 protons 6 neutrons 8 neutrons 6 electrons 6 electrons Three Types of Nuclear Decay Carbon 14 Decay! Helium Nucleus electron " neutrino Carbon 14 Nitrogen 14 electron neutrino # photon

2 Half-Life Half-Life 1/2 Half-Life Half-Life 1/4 1/8

3 Half-Life Example: Buried Trees from the End of the Ice Age found at sites around the edge of the Great Lakes. 1/16 Imagine we found a piece of wood which contains 100 nuclei of Carbon-14, which has a half-life of 6000 years. Assuming the wood did not gain or lose atoms to its environment, how many Carbon-14 nuclei would the object have: 6000 years ago? Imagine we found a piece of wood which contains 100 nuclei of Carbon-14, which has a half-life of 6000 years. Assuming the wood did not gain or lose atoms to its environment, how many Carbon-14 nuclei would the object have: 6000 years ago? years ago?

4 Imagine we found a piece of wood which contains 100 nuclei of Carbon-14, which has a half-life of 6000 years. Assuming the wood did not gain or lose atoms to its environment, how many Carbon-14 nuclei would the object have: 6000 years ago? years ago? years ago? Imagine we found a piece of wood which contains 100 nuclei of Carbon-14, which has a half-life of 6000 years. Assuming the wood did not gain or lose atoms to its environment, how many Carbon-14 nuclei would the object have: 6000 years ago? years ago? years ago? 800 Imagine we found a piece of wood which contains 100 nuclei of Carbon-14, which has a half-life of 6000 years. Assuming the wood did not gain or lose atoms to its environment, how many Carbon-14 nuclei would the object have: To figure out how old an object is using an unstable nucleus, we need to know how many nuclei it contains now and how many nuclei it contained originally years ago? years ago? years ago? 800 So, How old is this tree? How can we do this reliably?

space at speeds approaching the speed of light.

5 Ancient Trees and Modern trees obtain Carbon-14 from the same place.. Carbon-14 is produced by cosmic rays striking the atmosphere Cosmic rays are nuclei that move through space at speeds approaching the speed of light. All living organisms acquire carbon-14 from the atmosphere If the Carbon-14 content of the atmosphere has remained constant, then ancient trees have comparable amounts of Carbon-14 as modern trees

6 A 5,730 year old tree would have half the Carbon-14 of a modern tree By comparing Carbon-14 dates with independent age measurements, we can confirm the Carbon-14 content of the atmosphere has remained rather constant. A 11,460 year old tree would have one-quarter the Carbon-14 of a modern tree Data from Libby s 1960 Nobel Prize Lecture

Correcting Carbon-14 With Tree Rings Dendrochronology Variations in Carbon-14

These trees have roughly 1/4 of their original Carbon-14 remaining, so these

These trees date to the end of the last Ice Age, and give unique insights

. These variations can tell us something about the history of Earth s climate

7 Correcting Carbon-14 With Tree Rings Dendrochronology Variations in Carbon-14 content of the atmosphere can be detected and accounted for. These trees have roughly 1/4 of their original Carbon-14 remaining, so these tress are between 10,000 and 12,000 years old. These trees date to the end of the last Ice Age, and give unique insights into the shifting climate.. These variations can tell us something about the history of Earth s climate and solar system magnetic fields Data from radiocarbon.org INTCAL04 Leavitt et al in Radiocarbon 49(2) See also

How do we date earlier events, like the Chicxulub Impact crater Topography from NASA Radar There are

Stable Unstable Carbon-12 Carbon-14 Aluminum-27 Aluminum-26 Potassium-39 Potassium-40 Half-lives of

Carbon-14 5,730 years Aluminum-26 717,000 years Potassium-40 1,280,000,000 years Rubidium-87

8 How do we date earlier events, like the Chicxulub Impact crater Topography from NASA Radar There are many other types of unstable nuclei besides Carbon 14. Stable Unstable Carbon-12 Carbon-14 Aluminum-27 Aluminum-26 Potassium-39 Potassium-40 Half-lives of different nuclei. Carbon-14 5,730 years Aluminum ,000 years Potassium-40 1,280,000,000 years Rubidium-87 47,500,000,000 years Uranium ,000,000 years Uranium-238 4,468,000,000 years Potassium-40 has two ways it can decay 90% Potassium 40 Calcium 40 electron neutrino Potassium 40 Argon 40 10% Numbers from electron neutrino

9 Potassium-40 can be used to measure the age of melted rocks.. Potassium-40 decay in hot or molten rock Potassium-40 Argon-40 Calcium-40 Potassium-40 decay in a cold, solid rock The Rock Today Potassium-40 Argon-40 Calcium-40 Potassium-40 Argon-40 Calcium-40

The Rock Today Potassium-40 measurements of small particles of glass and melt from the impact date this event. Potassium-40 Argon-40 Calcium-40 Potassium 40 Calcium-40 The Original Rock Swisher et al.

10 The Rock Today Potassium-40 measurements of small particles of glass and melt from the impact date this event. Potassium-40 Argon-40 Calcium-40 Potassium 40 Calcium-40 The Original Rock Swisher et al in Science, 257 (954) Potassium-40 measurements of small particles of glass and melt from the impact date this event to / million years ago. Comparing the date of the impact with those of fossil deposits confirms the impact occurred close to the end of the age of the dinosaurs. Swisher et al in Science, 257 (954)

Summary points: Unstable nuclei are potentially powerful tools for determining the age of things, because whether a given nucleus decays or not depends

characterized by a parameter known as the half-life Summary points: In practice, to be able to use unstable nuclei to measure the age of some, we need to

Carbon-14 nuclei, which have a half-life of 5,730 years, are useful for dating objects that used to be part of living things that lived during the last

This is because the original Carbon-14 content of these organisms are comparable to that of similar organisms living today Potassium-40 nuclei, which have

11 Summary points: Unstable nuclei are potentially powerful tools for determining the age of things, because whether a given nucleus decays or not depends only on the interactions between the protons and neutrons in individual nuclei and because groups of unstable nuclei decay in a predictable, regular way characterized by a parameter known as the half-life Summary points: In practice, to be able to use unstable nuclei to measure the age of some, we need to know: (1) The half-life of the nucleus (2) The number of unstable nuclei in the object now (3) The number of unstable nuclei in the object originally Carbon-14 nuclei, which have a half-life of 5,730 years, are useful for dating objects that used to be part of living things that lived during the last ~50,000 years. This is because the original Carbon-14 content of these organisms are comparable to that of similar organisms living today Potassium-40 nuclei, which have a half-life of 1.28 billion years, are useful for measuring the age of ancient volcanic rocks that are millions or billions of years old. This is because molten rocks contain little Argon-40, so the amount of Argon- 40 in a rock is a measure of how much Potassium-40 decayed in the rock since it last solidified.

Unit 2 Lesson 3 Absolute Dating. Copyright Houghton Mifflin Harcourt Publishing Company

Unit 2 Lesson 3 Absolute Dating. Copyright Houghton Mifflin Harcourt Publishing Company It s About Time! How can the absolute age of rock be determined? Determining the actual age of an event or object in years is called absolute dating. Scientists often use radioactive isotopes to find the