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Chapter 24 : Nuclear Reactions and Their Applications 24.1 Radioactive Decay and Nuclear Stability

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Chapter 24 : Nuclear Reactions and Their Applications 24.1 Radioactive Decay and Nuclear Stability Chapter 24 : Nuclear Reactions and Their Applications 24.1 Radioactive Decay and Nuclear Stability 24.2 The Kinetics of Radioactive Decay 24.3 Nuclear Transmutation: Induced Changes in Nuclei 24.4 The Effects of Nuclear Radiation on Matter 24.5 Applications of Radioisotopes 24.6 The Interconversion of Mass and Energy 24.7 Applications of Fission and Fusion Comparison of Chemical and Nuclear Reactions Chemical Reactions Nuclear Reactions 1. One substance is converted to 1. Atoms of one element typically another, but atoms never change change into atoms of another. identity. 2. Orbital electrons are involved as 2. Protons, neutrons, and other bonds break and form; nuclear particles are involved; orbital particles do not take part. electrons rarely take part. 3. Reactions are accompanied by 3.Reactions are accompanied by relatively small changes in energy relatively large changes in energy and no measurable changes in mass. and measurable changes in mass. 4. Reaction rates are influenced by 4. Reaction rates are affected by temperature, concentration, number of nuclei, but not by catalysts, and the compound in temperature, catalysts, or the which an element occurs. compound in which an element Table 24.1 occurs. A + 0 ZX A - mass number = p + n Z - number of protons(atomic no.) = p+ Number of neutrons(N) in a nucleus N = A - Z 238 0 92U N = 238 - 92 = 146 of n 238 - + 92U has 92e 92 p 235U - atomic bomb Subatomic elementary particles 0 Electron -1 e 1 Proton 1 p 1 Neutron 0n Nuclide - nuclear species with specific numbers of two types of nucleons Nucleons - made up ofprotons and neutrons Properties of Fundamental Particles • Particle Symbol Charge Mass • (x10-19 Coulombs) (x 10-27 kg) • Proton p +1.60218 1.672623 • Neutron n 0 1.674929 • Electron e -1.60218 0.0005486 NUCLEAR STABILITY Modes of Radioactive Decay 4 2+ • Alpha decay–heavy isotopes: 2He or • Beta decay–neutron rich isotopes: e- or • Positron emission–proton rich isotopes: • Electron capture–proton rich isotopes: x-rays • Gamma-ray emission( )–Decay of nuclear excited states • Spontaneous fission–very heavy isotopes Fig. 24.1 (p. 1046) Alpha Decay–Heavy Elements 238 234 • U Th + a + e 9 t1/2 = 4.48 x 10 years • 210Po 206Pb + a + e t1/2 = 138 days • 256Rf 252No + a + e t1/2 = 7 ms • 241Am 237Np + a + e t1/2 = 433 days Beta Decay–Electron Emission • n p+ + b- + Energy • 90Sr 90Y + b- + Energy t1/2= 30 years • 14C 14N + b- + Energy t1/2= 5730 years • 247Am 247Cm + b- + Energy t1/2= 22 min • 131I 131Xe + b- + Energy t1/2 = 8 days Electron Capture–Positron Emission p+ + e- n + Energy = Electron capture p+ n + e+ + Energy = Positron emission 51Cr + e- 51V + Energy t1/2 = 28 days 7Be 7Li + b+ + Energy t1/2 = 53 days 177Pt + e- 177Ir + Energy t1/2 = 11 s 144Gd 144Eu + b+ + Energy t1/2 = 4.5 min Fig. 24.2 Number of Stable Nuclides for Elements 48 through 54 Atomic Number of Element Number (Z) Nuclides Cd 48 8 In 49 2 Sn 50 10 Sb 51 2 Te 52 8 I 53 1 Xe 54 9 Table 24.3 (p. 1049) Distribution of Stable Nuclides • Protons Neutrons Stable Nuclides % • Even Even 157 58.8 • Even Odd 53 19.9 • Odd Even 50 18.7 • Odd Odd 7 2.6 Total = 267 100.0% (c.f. Table 24.4, p. 1050) The 238U Decay Series Fig. 24.3 CHEM 1332 Exam 1 is Friday Feb. 11 at 5:30 PM Room 117 SR1 Web site http://www.uh.edu/~chem1p User ID = chem1332 Password = ggw85p Predicting the Mode of Decay Unstable nuclide generally decays in a mode that shifts its N/Z ratio toward the band of stability. 1. Neutron-rich nuclides undergo ß decay, which converts a neutron into a proton, thus reducing the value of N/Z. 2. Proton-rich nuclides undergo positron decay or electron capture, thus increasing the value of N/Z. 3. Heavy Nuclides Z > 83 undergo alpha decay, thus reducing the Z and N values by two units per emission. 243 4 239 24.26 95Am 2He + 93Np 239 0 239 93Np -1ß + 94Pu 239 4 235 94Pu 2He + 92U 235 4 231 92U 2He + 90Th Kinetics of Radioactive Decay The decay rate or activity(A) = - N/ t N = Change in number of nuclei t = change in time SI unit of radioactivity is the becquerel(Bq), defined as one disintegration per second(d/s): 1Bq = 1 d/s 1 curie(Ci) equals the number of nuclei disintegrating each second in a 1g of radium- 226. 1 Ci = 3.70 X 1010 d/s For a large collection of radioactive nuclei,the number decaying per unit time is proportional to the number present: Decay rate (A) a N or A = kN k = decay constant The larger the value of k, the higher is the decay rate. A = -DN/Dt = kN The activity depends only on N raised to the first order so radioactivity decay is a first- order process. We consider the number of nuclei than their concentration. Half-life of Radioactive Decay t1/2 The half-life (t1/2) of a nuclide is the time it takes for half the nuclei present to decay. 14 14 0 6C 7N + -1ß The number of nuclei remaining is halved after each half-life. Natural Decay Series of Existing Isotopes 40K 40Ar 9 t1/2 = 1.29 x 10 years 232 Th 208 Pb 10 t1/2 = 1.4 x 10 years 235U 207 Pb 8 t1/2 = 7 x 10 years 238U 206 Pb 9 t1/2 = 4.5 x 10 years Natural Decay Series for Uranium-238 238U 234 Th 234Pa 234U 230 Th 226Ra 222Rn 218Po 214Pb 218At 214Bi 210 Tl 214Po 210Pb 206Hg = a decay 210Bi 206Tl = b- decay 210 Po 206Pb 238U: 8 decays and 6 decays leaves you with 206Pb Natural Decay Series for Uranium-235 235U 231 Th 231Pa 227Ac 223Fr 219At 215Bi 227 Th 223Ra 219Ra 215Po 211Pb = decay 215At 211Bi 207 Tl = decay 211Po 207Pb 235U: 8 decays and 4 decays leaves you with 207Pb Natural Decay Series for Thorium-232 232 Th 228Ra 228Ac 228 Th 224Ra 220Rn 216Po 212Pb = decay 212Bi 208Tl = decay 212Po 208Pb 232 Th: 7 decays and 4 decays leaves you with 208Pb Fig. 24.A Decrease in Number of 14C Nuclei Over Time Fig. 24.4 ln Nt / No = - kt ln No / Nt = kt No is the number of nuclei at t = 0 and Nt is the number of nuclei remaining at any time t. To calculate the half-time ; set Nt = 1/2 N0 ln No / (1/2No) = k t 1/2 ; t 1/2 = ln2 / k The half-life is not dependent on the number of nuclei and is inversely related to the decay constant. Large k short t 1/2. Decay Constants (k) and Half-lives (t1/2) of Beryllium Isotopes Nuclide k t1/2 7 -2 4Be 1.30 x 10 /day 53.3 day 8 16 -17 4Be 1.0 x 10 /s 6.7 x 10 s 9 4Be Stable 10 -7 6 4Be 4.3 x 10 /yr 1.6 x 10 yr 11 -2 4 Be 5.02 x 10 /s 13.8 s Table 24.5 (p. 1054) Radioisotopic Dating 14 1 14 1 7N + 0n 6C + 1p 12C / 14C ratio is constant in living organisms. Dead organism 12C / 14C ratio increases. 14 14 14 0 C decays 6C 7N + -1ß Radiocarbon Dating for Determining the Age of Artifacts Fig. 24.5 Nuclear Transmutation 14 4 1 17 7N + 2He 1H + 8O Notation 14N (a , p) 17O 27 4 1 30 13Al + 2He 0n + 15P Notation 27Al(a , n) 30P Fig. 24.6A The Cyclotron Accelerator Fig. 24.7 Formation of Some Transuranium Nuclides Reaction Half-life of Product 239 4 240 1 1 94Pu + 2He 95Am + 1H + 2 0n 50.9 h 239 4 242 1 94Pu + 2He 96Cm + 0n 163 days 244 4 245 1 1 96Cm + 2He 98Bk + 1H + 2 0n 4.94 days 238 12 246 1 92U + 6C 98Cf + 4 0n 36 h 253 4 256 1 99Es + 2He 101Md + 0n 76 min 252 10 256 1 98Cf + 5B 103Lr + 6 0n 28 s Table 24.6 (p. 1059) Fig. 24.8 Units of Radiation Dose rad = Radiation-absorbed dose The quantity of energy absorbed per kilogram of tissue: 1 rad = 1 x 10-2 J/kg rem = Roentgen equivalent for man The unit of radiation dose for a human: 1 rem = 1 rad x RBE RBE = 10 for a RBE = 1 for x-rays, g-rays, and b’s Examples of Typical Radiation Doses from Natural and Artificial Sources–I Source of Radiation Average Adult Exposure Natural Cosmic radiation 30-50 mrem/yr Radiation from the ground From clay soil and rocks ~25-170 mrem/yr In wooden houses 10-20 mrem/yr In brick houses 60-70 mrem/yr In light concrete houses 60-160 mrem/yr Radiation from the air (mainly radon) Outdoors, average value 20 mrem/yr In wooden houses 70 mrem/yr In brick houses 130 mrem/yr In light concrete houses 260 mrem/yr Internal radiation from minerals in tap water and daily intake of food ~40 mrem/yr ( 40K, 14C, Ra) Table 24.7 (p. 1062) Examples of Typical Radiation Doses from Natural and Artificial Sources–II Source of Radiation Average Adult Exposure Artificial Diagnostic x-ray methods Lung (local) 0.04-0.2 rad/film Kidney (local) 1.5-3.0 rad/film Dental (dose to the skin) £ 1 rad/film Therapeutic radiation treatment locally £ 10,000 rad Other sources Jet flight (4 hr) ~1 mrem Nuclear tests < 4 mrem/yr Nuclear power industry < 1 mrem/yr Total Average Value 100-200 mrem/yr Table 24.7 (p.
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