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Transcript
Radioactivity Unit Outline 1. Modern Quiz/Pasta Lab 2. Take up research assign, Decay equations, series Blue book p. 940 P19, 20, 21, 22, 25, 27, 29 Red book p. 631 #13, p. 642 #1, 2, 3, 4 3. E = mc2, Geiger Counter demo Blue book p. 939 Q1, 2, 3, 5, 8, 11*, 12*, 13*, 18, 19 (must do 11-13!!) Red book p. 621 #5-8, p. 624 Concept Review, p. 636 R2, 3, 5, 10, A1, 3, 5, P 4, 5, 6, 7 4. Fission and Fusion Assignment Lesson #1 Introduction to Nuclear Physics References: Chapters 30 and 31 in both books Radiation and radioactivity are a part of our everyday lives. Radioactivity results from instability within atomic nuclei, which causes them to decay, or split apart. This is happening all the time, all around us. Flying on an airplane will actually give you a similar dose of radiation to getting a medical X-ray done (within about 1000 times), since you are closer to the sun and space while flying. Smoking cigarettes or being exposed to a lot of second-hand smoke also gives you a significant dose of radiation. Radioactive substances and radiation are routinely used for medical reasons (cobalt “bomb” for cancer therapy or iodine treatment for thyroid problems). Different types of food are routinely irradiated to stave off spoiling. Radiation sickness and other bad effects only usually occur when people are exposed to unusually high doses of radiation, and/or over long periods of time. Glow-in-the-dark objects are made from radioactive substances. The nuclei in the material are excited by light that falls on them (usually ultraviolet light). They reach a higher energy metastable (“sort-of-stable”) state, from which they decay some time later. During the decay, they emit photons of certain smaller frequencies, which we can then see, and the atoms return to their lower-energy stable state. This makes them glow in the dark. There are 3 types of radiation that can be emitted by nuclei: alpha (), beta () and gamma (). Each of these is associated with a certain type of nuclear decay. Nuclei decay because (a) there is a lower energy state available to them; and/or (b) the strong nuclear force can no longer hold all the protons and neutrons together (for large nuclei), and the Coulomb (electrostatic) repulsion force of the protons takes over. Alpha radiation An particle (wave) consists of two protons and two neutrons (a 4He nucleus). Note that this is a positive particle. Generally alpha radiation has little energy although it is heavy. Ex. Write the equation for the alpha decay of Uranium 238. 238 234 92 U 90Th parent daughter + alpha particle + energy This process is called transmutation since one type of nucleus changes into another type of nucleus. Note that the number of nucleons (particles in nucleus = protons + neutrons) is conserved (conservation of nucleon number), and energy is also conserved. The decay happens spontaneously, and there is no way to predict when it will happen. General equation: A A4 Z N Z 2 N ' Beta Radiation A beta particle/wave is a high speed electron (or positron). These are higher energy than but less than . There are 3 types of beta radiation: - decay (electron emitted): 32 32 15 P16 S e parent daughter + electron + antineutrino + energy An electron is emitted from a neutron in the nucleus, changing the neutron into a proton. Charge, nucleon number and energy are all conserved. Generally A A Z N Z 1 N 'e + decay (positron emitted): 22 22 11 Na10 Ne e parent daughter + positron + neutrino + energy A positron is emitted from a proton in the nucleus, changing the proton into a neutron. Again, charge, nucleon number and energy are all conserved. Generally A A Z N Z 1 N 'e Electron capture (EC) by nucleus: 7 4 Be e 37 Li parent + electron daughter + neutrino + energy An inner electron is captured by the nucleus, changing a proton into a neutron. The rest of the electrons fall down to fill the space. Again, charge, nucleon number and energy are all conserved. Generally A ZN e Z A1 N ' Gamma Radiation Very high energy (frequency) photons (no charge) are emitted by nuclei in an excited state. These photons have a great deal of energy, and are even more powerful than X rays. The NUCLEI are excited, not the electrons. This generally happens as a result of previous nuclear decays, and sometimes by collisions with other particles. 12 * 12 Ex. 6 C 6 C Excited nucleus ground state nucleus + gamma particle + energy Generally A * A Z X Z X Type Alpha Power/Energy Low Particle (charge) 4 He nucleus (+) Beta Medium - Electron (-) + Positron (+) EC Orbital electron (-) Gamma High Photon (none) A ZN Equation(s) A-4Z-2N’ + 42He + A A Z N Z 1 N 'e A A Z N Z 1 N 'e A A Z N e Z 1 N ' A * A Z X Z X Decay Series Often an unstable nucleus can decay in two or more different ways, and can produce another unstable nucleus as its daughter. Then this new nucleus can also decay in several ways. This creates a decay series, or chain, finally ending with a stable element. (see Blue Book p. 933) Recommended problems: Blue book p. 939 Q1, 2, 3, 5, 8, 11*, 12*, 13*, 18, 19 (must do 11-13!!) Red book p. 621 #5-8, p. 624 Concept Review, p. 636 R2, 3, 5, 10, A1, 3, 5, P 4, 5, 6, 7 Lesson #2 Energy-Mass Equivalence **********Not doing this demo in 2003 – Sorry, No Time!!****************** Geiger Counter Demo Explain source, source, ( source) What are the particles? Write the decay equations? 1. Distance (GAMMA SOURCE) a. Count clicks for 1 min at distance of infinity, 60 cm, 50 cm, …, 10 cm b. Plot 2. Shielding (BETA SOURCE) a. At fixed distance, count clicks for 1 min with shields of same thickness if possible (foil, paper, lead, glass,…) ************************************************************** Energy-Mass Equivalence E = mc2 is one of the most used and least understood expressions in science. It relates energy to mass. It applies for all processes, but is most noticeable for nuclear reactions. Binding energy (or mass defect) of a nucleus. Stable nuclei do not spontaneously fall apart, but they are constantly repelled from each other due to Coulomb repulsion. If we want to blow them apart, we must add energy. The amount of energy needed to separate the particles is called the binding energy of the nucleus. Ex. Determine the mass defect and binding energy of carbon 12. Individual particles 6p + 6n + 6e =6(1.67 × 10-27 kg) + 6(1.67 × 10-27 kg) + 6(9.11 × 10-31 kg) =6(1.00728 u) + 6(1.008665 u) + 6(0.000549 u) =12.098964 u or 2.00903 10-26 kg [see Appendix F in Blue book, or p. 721 in Red book for these values] Group 12.0000 u × 1.6605 × 10-27 kg/u =1.9926 10-26 kg These have different masses, so they must have different energies. The binding energy is E = mdiff c2 ={(2.00903-1.9926) 10-26 kg}{2.9979 108 m/s}2 =1.47663 10-11 J or 148 MeV Note: conversion factor between amu and MeV is 931.5 MeV/1 amu Ex. A Uranium 236 nucleus undergoes alpha decay. Determine the amount of energy released during this process, using the tables in the back of your text. Equation: 236 232 192 U 90Th Energy before: 236.045562 u (from back of book) Energy after: 232.038051 u + 4.002602 u + So =236.045562-232.038051-4.002602 u =0.004909 u or 0.02244754 MeV Recommended problems Blue book p. 940 P19, 20, 21, 22, 25, 27, 29 Red book p. 631 #13, p. 642 #1, 2, 3, 4 Fission and Fusion Assignment 1. Explain the difference between nuclear fission and nuclear fusion. Give examples. 2. Show an example of a decay equation for a fission reactor. 3. What is a chain reaction? Critical mass? Where and by whom was the first sustained chain reaction achieved? 4. Explain how the reaction inside a fission reactor works. 5. Provide definitions for each of the following, and explain what they do in a fission reactor. Control rods Slow neutrons Heavy water Enriched uranium 6. Explain what Canada’s contribution to fission reactor technology was (try “CANDU reactor”) and why it is/was important. 7. What was the Manhattan Project? Describe it briefly. 8. What is your opinion about nuclear power? Outline the risks and benefits, then make a decision about whether we (Canada, the world) should continue to develop and use our nuclear fission reactor technology. Nuclear Physics Problem Set / Review #1 1. Calculate a. The approximate size of a 212Pb nucleus. b. The atomic mass number of an atom with a nuclear radius of 2.74 10-15 m. (Remember that A must be a whole number, so round off!) 2. Determine the number of protons and neutrons in each of the following kinds of nuclei. a. 16O b. 35Cl c. 234Th d. 1H 3. Write equations for the following nuclear decays: [the answers at the bottom of the page give just the daughter nuclei.] a. alpha decay of 230Th b. beta (–) decay of 212Pb c. beta (+) decay of 46Cr 4. Determine what type of decay causes the following transmutations: a. 226Ra 222Rn + _____ ? b. 214Bi 214Po + _____ ? c. 239Np 239U + _____ ? 5. In #3 and #4 above, which reactions involved the emission of a neutrino? Which involved the emission of an antineutrino? 6. Write the equation for the electron capture of chromium 51. 7. The mass of a Beryllium 7 nucleus is 1.1652 10-26 kg. a. What is the difference between the mass of the nucleus and the mass of its constituent particles, in kg? b. What is the binding energy of this nucleus, in J? c. What is the binding energy per nucleon of this nucleus in J? 8. If the binding energy of a lithium 6 nucleus is 4.98118 10-12 Joules, what is the mass of the nucleus in amu? 9. **The mass spectrometer is a device used to measure the masses of different types of particles in the same compound. The particles are separated using a magnetic field that is perpendicular to their direction of travel. Singly charged ions (+1e) pass through a region of magnetic field 0.250 T and electric field 7.00 103 V/m, where the electric and magnetic forces balance each other. (The fields are perpendicular to each other.) They then enter the region where the magnetic field is the same, but the electric field is turned off. If the particles are deflected by a radius of 8.12 10-3 m, what is the mass of each ion in kg? [Try to find the speed first, then use forces to find the mass.] Answers: 1.) a) 7.1 10-15 m, b) A = 12 2.) a) 8 p, 8 n, b) 17 p, 18 n, c) 90 p, 144 n, d) 1 p, 0 n 3.) a) 226Ra, b) 212Bi, c) 46V 4.) a) alpha, b) beta (), c) beta (+) 5.) neutrino: 3c & 4c, antineutrino: 3b & 4b 6.) 51Cr + e- 51V + e+ 7.) a) 6.31 10-29 kg, b) 5.68 10-12 J, c) 8.11 10-13 J 8.) 6.01493 amu -26 9.) 1.16 10 kg