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Phy107Spr06Lect32

# Phy107Spr06Lect32 - E x a m 3 W e d A p r 1 9 C o v e rs Q...

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1 Wed, Apr. 12, 2006 Phy107 Lecture 32 1 Exam 3: Wed. Apr. 19 Covers Quantum Physics, Solids, Nuclear Physics HW11: Griffith Chap 19: Q9, Q11, Q13, E6, E8, E12 From last time… • Radioactive decay: alpha, beta, gamma • Radioactive half-life • Decay type understood in terms of number neutrons, protons. • Understand in terms of weak interaction, Quark internal structure. Wed, Apr. 12, 2006 Phy107 Lecture 32 2 14 C to 12 C ratio 14 C has a half-life of ~6,000 years, continually decaying back into 14 N. Steady-state achieved in atmosphere, with 14 C: 12 C ratio ~ 1:1 trillion (1 part in 10 12 ) As long as biological material alive, atmospheric carbon mix ingested (as CO 2 ), ratio stays fixed. After death, no exchange with atmosphere. Ratio starts to change as 14 C decays Wed, Apr. 12, 2006 Phy107 Lecture 32 3 Carbon-dating question The 14 C: 12 C ratio in a fossil bone is found to be 1/4 that of the ratio in the bone of a living animal. The half-life of 14 C is 5,730 years. What is the approximate age of the fossil? A. 17,200 years B. 7,640 years C. 22,900 years D. 11,460 years Since the ratio has been reduced by a factor of 4, two half-lives have passed. 2 x 5,730 years = 11,460 years Wed, Apr. 12, 2006 Phy107 Lecture 32 4 Other carbon decays Lightest isotopes of carbon are observed to emit a particle like an electron, but has a positive charge! •This is the antiparticle of the electron. •Called the positron . Wed, Apr. 12, 2006 Phy107 Lecture 32 5 Antimatter Every particle now known to have an antiparticle. Even antimatter has been generated. Matter and antimatter annihilate when in close proximity. Photons are created so that energy is conserved. Wed, Apr. 12, 2006 Phy107 Lecture 32 6 What is going on? 14 C has more neutrons than the most stable form 12 C. So it decays by electron emission, changing neutron into a proton. Other isotopes of carbon have fewer neutrons Decays by emitting positron , changing proton into neutron.

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2 Wed, Apr. 12, 2006 Phy107 Lecture 32 7 Decay question 20 Na decays in to 20 Ne, a particle is emitted? What particle is it? Na atomic number = 11 Ne atomic number = 10 A. Alpha B. Electron beta C. Positron beta D. Gamma 20 Na has 11 protons, 9 neutrons 20 Ne has 10 protons, 10 neutrons So one a proton (+ charge ) changed to a neutron (0 charge) in this decay. A positive particle had to be emitted. Wed, Apr. 12, 2006 Phy107 Lecture 32 8 Gamma decay So far Alpha decay: alpha particle emitted from nucleus Beta decay: electron or positron emitted Both can leave the nucleus in excited state Just like a hydrogen atom can be in an excited state Hydrogen emits photon as it drops to lower state. Nucleus also emits photon as it drops to ground state This is gamma radiation But energies much larger, so extremely high energy photons. Wed, Apr. 12, 2006 Phy107 Lecture 32 9 Radioactive decay summary Alpha decay Nucleus emits alpha particle (2 neutrons + 2 protons) Happens with heavy nuclei only Beta decay Nucleus emits electron (beta-) or positron (beta+) – Internally, neutron changes to proton (beta-), or proton changes to neutron (beta+) Gamma decay Nucleus starts in internal excited state
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Phy107Spr06Lect32 - E x a m 3 W e d A p r 1 9 C o v e rs Q...

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