Radioactive Decay Atomic Physics Becquerel In 1896 Henri Becquerel - uranium compounds would fog photographic plates as if exposed to light. - a magnetic field could deflect the radiation that caused the fogging. 1
Pierre and Marie Curie Marie and Pierre Curie investigated further. they found thorium is also radioactive and discovered two new elements: radium and polonium Marie coined the term radioactivity. Three Radiation Types Alpha: the emission of a helium nucleus (an alpha particle): Beta: the emission of a high energy electron: Gamma: the emission of a high energy photon: 2
Radiation in magnetic field Heads Up! Guaranteed exam Question Conservation In Nuclear Reactions Charge: Net charge is constant: total charge of the reactants = total charge of the products. Atomic mass number: The total A (nucleons) for the products = total A for the reactants. 3
Example A. Is this reaction possible? 1. Charge: (p + are +) +90 = + 88 + + 2 (charge is balanced) 2. Atomic mass number: 230 = 226 + 4 (N is balanced) The reaction is possible. Example B. Is this reaction possible? 1. Charge: neutron + 27 + 28 + 0 (charge not balanced) 2. Atomic mass number: 60 = 60 + 1 (N not balanced) The reacon is not possible. 4
1. Alpha Decay alpha (α) spontaneous emission of an alpha particle by a large, unstable nucleus. occurs when F e within a nucleus is as great or slightly greater than the strong nuclear force during the decay, momentum, energy ( mass energy) and charge are conserved Alpha Decay consists of 2 protons and 2 neutrons cannot penetrate more than a sheet of paper ionize atoms they collide with low biological hazard Eg) Mass is conserved 238=238 Charge is conserved 92=92 5
Note: In a large nucleus, the F e pushing p + apart is nearly as strong as the attractive strong force, making the nucleus unstable. An alpha particle (2 p + and 2 n ) is a very stable configuration, so alpha particles are emitted, not individual p + or n. In general, Z decreases by 2, and A decreases by 4. Alpha reactions In general: Example: Parent element Daughter element 6
Example Write the alpha decay process for polonium-210. Name the parent and daughter elements. Parent element: polonium Daughter element: lead Energy in alpha decay Mass defect, and mass-energy equivalence can be used to determine the maximum E k an emitted alpha particle will have. 7
Example Write the alpha decay process for the decay of radium-226. Use the data on page 881 to determine the mass defect in the reaction, and its energy equivalence. This energy will be the maximum E k an alpha particle could have in the reaction. Solution: Decay reaction Parent element: daughter element: radium radon 8
Solution: Mass defect Note: if Δm were negave, this means no energy was released, so the decay would not happen. Energy equivalence Most of this energy will be E k of the alpha particle. 9
2. Beta Decay spontaneous emission of a high energy electron A. Beta Negative Decay A neutron emits a beta particle (an e ) and becomes a proton. penetrate up to 3mm of aluminum, 500 sheets of paper Produces burns to skin or tissue Beta Negative Decay Z increases by 1, and A does not change. The transformation of a neutron into a proton involves the weak nuclear force and antimatter Beta particle: 10
May 30 10:25 AM B. Beta Positive Decay 11
Examples: Beta Negative Decay: Beta Positive Decay: May 30 10:30 AM 12
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