Final Exam. Evaluations. From last time: Alpha radiation. Beta decay. Decay sequence of 238 U

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1 Evaluations Please fill out evaluation and turn it in. Written comments are very helpful! Lecture will start 12:15 Today, evaluate Prof. Rzchowski If you weren t here Tuesday, also evaluate Prof. Montaruli today. Final Exam Fri, Dec 21, at 7:45-9:45 am in Ch equation sheets allowed (HAND WRITTEN!) About 40% on new material Rest on topics of exam1, exam2, exam3. Study Tips: 1. Download blank exams and take them. 2. Download blank quizzes and take them. 3. Look through group problems. 4. Look through lab question sheets. 5. Make up an exam question, explain solution to your study group. Thu, Dec Phy208 Lect29 1 Thu, Dec Phy208 Lect29 2 From last time: Alpha radiation From last time Radioactive decay: alpha, beta, gamma Radioactive half-life Today: More about beta, gamma, decay Medical uses of radiation Nuclear fission Alpha particle: (2 protons, 2 neutrons) Piece of atom (alpha particle) ejected from heavy nucleus Thu, Dec Phy208 Lect29 3 Thu, Dec Phy208 Lect29 4 Decay sequence of 238 U But what are these? α decay decreases by one increases by one Electron (beta particle) emitted Beta decay Thu, Dec Phy208 Lect29 5 But nucleus has only neutrons & protons. Thu, Dec Phy208 Lect29 6 1

2 Beta decay Nucleus emits an electron (negative charge) Must be balanced by a positive charge appearing in the nucleus. Changing particles Neutron made up of quarks. One of the down quarks changed to an up quark. New combination of quarks is a proton. This occurs as a neutron changing into a proton Thu, Dec Phy208 Lect29 7 Thu, Dec Phy208 Lect29 8 beta decay example 14 6 C " 14 7 N + e # 8 neutrons 6 protons 7 neutrons 7 protons Other carbon decays Lightest isotopes of carbon emit positron antiparticle of electron, has positive charge! 14 nucleons 14 nucleons 6 positive charges = = 7 positive charges + 1 electron + 1 negative charge This is antimatter Used in radioactive carbon dating. Half-life 5,730 years. 9 neutrons 9 protons 10 neutrons 8 protons + e + Thu, Dec Phy208 Lect29 9 Thu, Dec Phy208 Lect29 10 Antimatter Every particle has an antiparticle. Antimatter (anti-atoms) has been formed. Matter and antimatter annihilate Photons are created, conserving energy, momentum. Thu, Dec Phy208 Lect29 11 Thu, Dec Phy208 Lect

3 Positron Emission Tomography - PET Emission Detection Ring of detectors Gamma Photon #2 Isotope Max. Positron Range (mm) Short-lived radioactive tracer isotope emits positron Positron annihilates with nearby electron e + + e - 2γ 18 F C Ga 9.0 Two 511 kev gamma rays are produced Thu, Dec Phy208 Lect29 82 Rb 16.5 They fly in opposite directions (to conserve momentum) 13 If detectors receive gamma rays at the approx. same time, we have a detection Nuclear physics sensor and electronics Thu, Dec Phy208 Lect29 14 Radioactive tracers Worked on radioactivity as student with Ernest Rutherford. Lodged in nearby boarding home. Suspected his landlady was serving meals later in the week recycled from the Sunday meat pie. His landlady denied this! Detect ionizing radiation Geiger counter dehevesy described his first foray into nuclear medicine: The coming Sunday in an unguarded moment I added some radioactive deposit [lead-212] to the freshly prepared pie and on the following Wednesday, with the aid of an electroscope, I demonstrated to the landlady the presence of the active deposit in the soufflé. George de Hevesy Electroscope Thu, Dec Phy208 Lect29 15 Thu, Dec Phy208 Lect29 16 Decay of 60 Co This is one of sources used in the lab. Decays by electron emission, as predicted. But decays to an excited state. Photons emitted as excited state drops to its ground state. Gamma decay Alpha decay (alpha particle emitted), Beta decay (electron or positron emitted), can leave nucleus in excited state Nucleus has excited states just like hydrogen atom Emits photon as it drops to lower state. Nucleus also emits photon as it drops to ground state This is gamma radiation Extremely high energy photons Ni Ni Thu, Dec Phy208 Lect29 17 Thu, Dec Phy208 Lect

4 Cancer Radiation Therapy 50-60% of cancer patients treated with radiation Goal: disable cancerous cells without hurting healthy cells Typically X and γ-rays ( 60 Co) from 20 KV to 25 MV Radiation Levels rad (radiation absorbed dose) amount of radiation to deposit 0.01 J of energy in 1 kg of absorbing material RBE (relative biological effectiveness = # rads of x-rays that produces same biological damage as 1 rad of radiation being used rem (roentgen equivalent in man) = dose in rem = dose in rad x RBE Ground 0.30 rem/yr Mercury rem/yr Apollo rem/yr MIR Station 34.9 rem/yr Space Station 36.5 rem/yr Radiation type RBE X-rays 1 Gamma rays 1 Beta particles 1-2 Alpha particles Safe limit ( US gov ) 0.50 rem/yr Thu, Dec Phy208 Lect29 19 Thu, Dec Phy208 Lect29 20 Your exposure in the lab 60 Co has an activity of 1 µcurie " 3.7 #10 4 decays/s Each decay: 1.3MeV photon emitted Assume half are absorbed by a 10kg section of your body for 2 hours. Energy absorbed = 1.3"10 6 ev ( )( 3600s/hr) 1.6 "10 #19 J /ev ( ) 1 ( "104 decays/s) 2hr ( 2.8 "10 #5 J /10kg) 1rad /( 0.01J /kg) What is your dose in mrem? ( ) = 3 mrad = 3 mrem ( ) = 2.8 "10 #5 J A. 0.5 rem B. 0.3 rem C. 0.1 rem D rem E rem Thu, Dec Phy208 Lect29 21 Decay summary Alpha decay He nucleus (2 protons, 2 neutrons) Nucleus loses 2 protons, 2 neutrons Beta - decay Electron Neutron changes to proton in nucleus Beta + decay Positron Proton changes to neutron in nucleus Gamma decay Nucleus emits photon as it drops from excited state Thu, Dec Phy208 Lect29 22 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. Nuclear binding energes 56 Fe is most stable Move toward lower energies by fission or fusion. Energy released related to difference in binding energy. Nuclear fission Thu, Dec Phy208 Lect29 23 Thu, Dec Phy208 Lect

5 Fission Fission occurs when a heavy nucleus breaks apart into smaller pieces. Does not occur spontaneously, but is induced by capture of a neutron Chain reaction If neutrons produced by fission can be captured by other nuclei, fission chain reaction can proceed. Thu, Dec Phy208 Lect29 25 Thu, Dec Phy208 Lect29 26 Neutron capture How much energy? When neutron is captured, 235 U becomes 236 U Only neutron # changes, same number of protons. Nucleus distorts and oscillate, eventually breaking apart (fissioning) Binding energy/nucleon ~1 MeV less for fission fragments than for original nucleus This difference appears as energy. Binding energy /nucleon Energy/nucleon released by fission Fission fragments Mass number 235 Uranium Thu, Dec Phy208 Lect29 27 Thu, Dec Phy208 Lect29 28 Energy released 235 U has 235 total nucleons, so ~240 MeV released in one fusion event. 235 U has molar mass of ~235 gm/mole So 1 kg is ~ 4 moles = 4x(6x10 24 )=2.5x10 25 particles Fission one kg of 235 U Produce ~6x10 33 ev = Joules 1 kilo-ton = 1,000 tons of TNT = 4.2x10 12 Joules This would release ~250 kilo-tons of energy!!! Chain reaction suggests all this could be released almost instantaneously. The critical mass An important detail is the probability of neutron capture by the 235 U. If the neutrons escape before being captured, the reaction will not be self-sustaining. Neutrons need to be slowed down to encourage capture by U nucleus The mass of fissionable material must be large enough, and the 235 U fraction high enough, to capture the neutrons before they escape. Thu, Dec Phy208 Lect29 29 Thu, Dec Phy208 Lect