Radioactive Decay.notebook Radioactive Decay Unstable Nuclides Goals April 16, 2012 Define and relate the terms radioactive decay and nuclear radiation Describe the different types of radioactive decay and their effects on the nucleus Define the term halflife and explain how it relates to the stability of the nucleus Explain how artificial radioactive nuclides are made, and discuss their significance There are 265 known stable nuclides on Earth There many other nuclides that are unstable in that they undergo radioactive decay Decay is spontaneous (No outside energy input is needed.) Products are a lighter element and nuclear radiation Lighter element may or may not be stable Radiation can be particles (like alpha and beta) or electromagnetic radiation (like gamma rays) Aug 14:09 PM Types of Radioactive Decay 5 major varieties Alpha Emission Occurs almost exclusively with heavy nuclei Reduces both the number of protons and neutrons in the nucleus Releases an alpha particle (He nucleus) Aug 14:11 PM Beta Emission Nuclide is above the band of stability (too many neutrons) Neutron "splits" into a proton and an electron (beta particle) Positron Emission Nuclides is below the band of stability (too many protons) Proton "splits" into a neutron and a positron Positron has the mass of an electron with a positive charge Aug 14:18 PM Electron Capture Nuclide is below the band of stability Electron is captured from an inner orbital Electron combines a proton to make a neutron Atomic number goes down forming a new element Aug 14:21 PM Halflife Time required for half of the atoms in any sample to decay Can measured from fractions of a second to several billion years Measure of stability High halflife has more stability Low halflife has less stability Gamma Emission Usually happens in conjunction with other forms of decay Gamma radiation is released High energy and dangerous to human life Existence supports the nuclear shell model; provides proof of excited states in the nucleus Aug 14:37 PM Aug 14:44 PM 1 Radioactive Decay.notebook Calculations with Halflife Halflife can be used to calculate the amount remaining in a decaying sample. Need to figure out how many halflives have expired in a given length of time. Another variation is to find the halflife of a substance given the time and final mass April 16, 2012 Example Phosphorus32 has a halflife of 14.3 days. How many milligrams of phosphorus32 remain after 57.2 days if you start with 4.0 mg of the isotope? Given: t1/2 = 14.3 days t = 57.2 days 1. Divide t/t1/2: Initial mass: 4.0 mg 57.2/14.3 = 4 halflives 2. Multiply the initial mass by (0.50)n , where n is the number of halflives. 4.0 mg (0.5)4 = 0.25 mg Aug 14:52 PM 1 The halflife of polonium210 is 138.4 days. How many milligrams of polonium210 remain after 415.2 days if you start with 2.0 mg (answer in mg)? Aug 14:56 PM Example A sample contains 4.0 mg of uranium238. After 4.46 x 109 years, the sample will contain 2.0 mg of uranium238. What is the halflife of uranium238? Given: 1. Initial mass: 4.0 mg Final mass: 2.0 mg t = 4.46 x 109 years Divide the initial mass by the final mass. 4.0 / 2.0 = 2 2. Set answer equal to 2n, where n = the number of halflives, and solve for n. 2 = 2n; 3. n = 1 Divide the time elapsed by n. 4.46 x 109 / 1 = 4.46 x 109 years. Aug 16:31 PM 2 A sample contains 16 mg of polonium218. After 12 minutes, the sample will contain 1.0 mg of polonium 218. What is the halflife of polonium218? (answer in minutes) Aug 16:51 PM Aug 16:34 PM Implications of Halflife Halflife plays a major in determining appropriate roles for storage of radioactive materials Longer halflives need longer storage times Yucca Mountain near Las Vegas, NV National storage site for nuclear waste with high halflives Halflife does not give information of how dangerous the waste is What is emitted from the decay is more important γ radiation Aug 310:23 AM 2 Radioactive Decay.notebook April 16, 2012 Artificial Transmutation Decay Series Sometimes a radioactive decay can result in an unstable nuclide that continues to decay. Called a decay series Series will continue until a stable nuclide is reached Can have multiple pathways Parent nuclide the heaviest nuclide in the series Also the first nuclide It is possible to have elements made by artificial transmutation. Start with a stable nuclide Bombard with various particles to create new nuclides Transuranium elements (Z > 92) are all made by artificial transmutation Early discoveries were in the fallout of atomic bombs, then repeated in particle accelerators The entire 7th period has been filled out with the confirmed discover of element 117 in 2010. Daughter nuclides nuclides in the series that are produced by the decay of the parent nuclide U238 decay series has 18 daughter nuclides Stable nuclide is Pb206 Aug 17:29 PM Aug 17:37 PM 3
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