The Nucleus: A Chemist’s View Chapter 21 The Nucleus: A Chemist’s View

Chapter 21: The Nucleus: A Chemists View 21.1 Nuclear Stability and Radioactive Decay 21.2 The Kinetics of radioactive Decay 21.3 Nuclear Transformations 21.4 Detection and Uses of Radioactivity 21.5 Thermodynamic Stability 21.6 Nuclear Fission and Nuclear Fusion 21.7 Effects of Radiation

Subatomic particle tracks in a bubble charger at CERN, the European particle physics laboratory in Geneva, Switzerland. Source: CERN, Geneva, Switzerland

Figure 21.1: Known nuclides

DISTRIBUTION OF STABLE NUCLIDES Protons Neutrons Stable Nuclides % Even Even 168 60.2 Even Odd 57 20.4 Odd Even 50 17.9 Odd Odd 4 1.4 Total = 279 99.9% (like table 21.1 , P 980)

PROPERTIES OF FUNDAMENTAL PARTICLES Particle Symbol Charge Mass (x10 -19 Coulombs) (x10-27kg) Proton P +1.60218 1.672623 Neutron N 0 1.674929 Electron e -1.60218 0.0005486

NUCLEAR STABILITY Modes of Radioactive Decay Alpha Decay - Heavy Isotopes - 42He+2- Beta Decay - Neutron Rich Isotopes - e - - 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

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 take part. 3. Reactions are accompanied by 3.Reactions are accompanied by relatively small changes in energy relatively large changes in and no measurable changes in mass. energy and often 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 nature of the temperature, catalysts, or the chemical substance. nature of the chemical substance.

Emission and Absorption of Light by Atoms Nucleus of atom Electron Light Emission occurs when an electron drops from a higher energy level to a lower one. Light Absorption by an atom moves an electron to a higher energy level.

Absorption and Emission of Light by The Nucleus Excited state Ground state Protons and Neutrons in the nucleus are moved up to excited states by the absorption of large amounts of energy, and they move from excited states back to the ground states by the emission of large amounts of energy! This energy is normally 106 times larger than the energy emitted by electron transfers around atoms, and is in the Gamma ray region of the electromagnetic spectrum.

Figure 12.1: Electromagnetic radiation has oscillating electric (E) and magnetic (H) fields in planes perpendicular to each other and to the direction of propagation.

Figure 12.2: The nature of waves

Figure 12.3: Classification of electromagnetic radiation

Figure 21.7: Schematic representation of a Geiger-Muller counter

Alpha Decay -Heavy Elements 238U 234Th +  + E T1/ 2= 4.48 x 10 9 yrs 210Po 206Pb +  + E T 1/ 2= 138 days 256Rf 252No +  + E T1/ 2= 7 msec 241Am 237Np +  + E T1/ 2= 433 days

Beta Decay - Electron Emission N P+ +  + Energy 90Sr 90Y +  + Energy T1/ 2= 30 yrs 14C 14N +  + Energy T1/ 2= 5730 yrs 247Am 247Cm +  + Energy T1/ 2= 22 min 131I 131Xe +  + 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 +  + Energy T1/2 = 53 days 177Pt + e - 177Ir + Energy T1/2 = 11 sec 144Gd 144Eu +  + Energy T1/2 = 4.5 min

Beta decay, e- 60Co 2.405 Mev 1.173 Mev Gamma ray 1.332 Mev Gamma Ray Emission, the Nuclear Particles in the Nucleus dropping from excited states to their ground states. Example is the decay of cobalt – 60 to excited states in nickel – 60, which then decay to the ground state of 60Ni. Beta decay, e- 60Co 2.405 Mev 1.173 Mev Gamma ray 1.332 Mev 1.332 Mev Gamma ray Ground state of 60Ni

Natural Decay Series of Existing Isotopes 40K 40Ar T1/2 = 1.29 x 109yrs 232 Th 208 Pb T1/2 = 1.4 x 1010yrs 235U 207 Pb T1/2 = 7 x 108yrs 238U 206 Pb T1/2 = 4.5 x 109yrs

Figure 21.2: Decay series

Natural Decay series for Uranium 238 238U 234 Th 234Pa 234U 230 Th 226Ra 222Rn 218Po 214Pb 218At 214Bi 210 Tl 214Po 210Pb 206Hg =  decay 210Bi 206Tl =  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 215At 211Bi 207 Tl 211Po 207Pb =  decay =  decay 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 212Bi 208Tl 212Po 208Pb =  decay =  decay 232 Th -- 7  decays and 4  decays leaves you with -- 208Pb

Figure 21.3: The decay of a 10.0 -g sample of strontium-90 over time.

Figure 21.4: change in the amount of Molybdenum - 99 with time

Figure 21.5: Schematic diagram of a cyclotron

Physicist works with a small cyclotron at the University of California at Berkeley. Source: Corbis

CERN, the world's largest particle accelerator, lies at the foot of the Jura Mountains near Geneva, Switzerland.

Figure 21.6: Diagram of a linear accelerator

Accelerator tunnel at Fermilab, a high-energy particle accelerator in Batavia, Illinois. Source: Fermilab Batavia, IL

Carbon-14 radioactivity is often used to date human skeletons found at archaeological sites Source: University of Pennsylvania Photo Archives

Figure 21.8: Consumption of Na131I Normal Thyroid An Enlarged Thyroid Source: Visuals Unlimited

Figure 21.9: Binding energy per nucleon as a function of mass number.

Units used for Nuclear Energy Calculations electron volt - (ev) The energy an electron acquires when it moves through a potential difference of one volt: 1 ev = 1.602 x 10-19J Binding energies are commonly expressed in units of megaelectron volts (Mev) 1 Mev = 106 ev = 1.602 x 10 -13J A particularly useful factor converts a given mass defect in atomic mass units to its energy equivalent in electron volts: 1 amu = 931.5 x 106 ev = 931.5 Mev

Binding Energy per Nucleon of Deuterium Deuterium has a mass of 2.01410178 amu. Hydrogen atom = 1 x 1.007825 amu = 1.007825 amu Neutrons = 1 x 1.008665 amu = 1.008665 amu 2.016490 amu Mass difference = Theoretical mass - actual mass = 2.016490 amu - 2.01410178 amu = 0.002388 amu Calculating the binding energy per nucleon: Binding Energy -0.002388 amu x 931.5 Mev / amu Nucleon 2 nucleons = Mev / nucleon =

Calculation of the Binding Energy per Nucleon for Iron- 56 The mass of Iron -56 is 55.934939 amu, it contains 26 protons and 30 Neutrons Theoretical Mass of Fe - 56 : Hydrogen atom mass = 26 x 1.007825 amu = 26.203450 amu Neutron mass = 30 x 1.008665 amu = 30.259950 amu 56.463400 amu Mass defect =Actual mass - Theoretical mass: 55.934939 amu - 56.463400 amu = - 0.528461 amu Calculating the binding energy per nucleon: Binding Energy - 0.528461 amu x 931.5 Mev / amu nucleon 56 nucleons = = Mev / nucleon

Calculation of the Binding Energy per Nucleon for Uranium - 238 The actual mass of Uranium - 238 = 238.050785 amu, and it has 92 protons and 146 neutrons Theoretical mass of Uranium 238: Hydrogen atom mass = 92 x 1.007825 amu = 92.719900 amu neutron mass = 146 x 1.008665 amu = 147.265090 amu 239.98499 amu Mass defect = Actual mass - Theoretical mass: 238.050785 amu - 239.98499 amu = - 1.934205 amu Calculating the Binding Energy per nucleon: Binding Energy -1.934205 amu x 931.5 Mev / amu mucleon 238 nucleons = = Mev / nucleon

Mass and Energy in Nuclear Decay - I Consider the alpha decay of 212Po T1/2 = 0.3 s 212Po 208Pb +  + Energy 211.988842 g/mol 207.976627 g/mol + 4.00151 g/mol Products = 207.976627 + 4.00151 = 211.97814 g/mol Mass = Po - Pb +  = 211.988842 211.97814 0.01070 g/mol E = mC2 = (1.070 x 10-5 kg/mol)(3.00 x 108m/s)2 = 9.63 x 1011 J/mol 9.63 x 1011 J/mol 6.022 x 1023 atoms/mol = ____________________ J/atom

Mass and Energy in Nuclear Decay - II The Energy for the Decay of 212Po is 1.60 x 10-12J/atom 1.60 x 10-12J/atom 1.602 x 10-19 J/ev = 1.00 x 107 ev/atom 10.0 x 106 ev 1.0 x 10-6 Mev atom ev x = 10.0 Mev/atom !!!!! The decay energy of the alpha particle from 212Po is = 8.8 Mev !!!!

Figure 21.10: Both fission and fusion produce more stable nuclides and are thus exothermic.

Figure 21.11: Upon capturing a neutron, the 235U nucleus undergoes fission to produce two lighter nuclides, free neutrons (typically three), and a large amount of energy.

Figure 21.12: Representation of a fission process in which each event produces two neutrons, which can go on to split other nuclei, leading to a self-sustaining chain reaction.

Figure 21.13: If the mass of the fissionable material is too small, most of the neutrons escape before causing another fission event; thus the process dies out.

Figure 21.14: Nuclear power plant

Breeder reactor at a nuclear power plant in St Breeder reactor at a nuclear power plant in St. Laurent-Des Eaux, France. Source: Stock Boston

A Uranium "button" for use as a fuel in a nuclear reactor.

Figure 21.15: Schematic of a reactor core

Neutron Induced Fission - Bombs and Reactors There are three Isotopes with sufficiently long half-lives and a significant fission cross-sections that are known to undergo neutron induced fission, and are useful in fission reactors, and Nuclear weapons. Of these only one exists on earth ( 235U which exists at an abundance of 0.72% of natural Uranium)and that is the isotope that we use in nuclear reactors for fuel and some weapons. The three isotopes are: 233U T1/2 = 1.59 x 105 years sigma fission = 531 barns 235U T1/2 = 7.04 x 108 years sigma fission = 585 barns 239Pu T1/2 = 2.44 x 105 years sigma fission = 750 barns

Breeding Nuclear Fuel There are two relatively common heavy Isotopes that will not undergo neutron induced fission, that can be used to make other Isotopes that do undergo neutron induced fission, and can be used as nuclear fuel in a nuclear reactor. Natural Thorium is 232 Th which is common in rocks. 232 Th + 10n 233 Th + Energy T1/2 = 22.3 min 233 Th 233Pa +  + Energy T1/2 = 27.0 days 233Pa 233U +  + Energy T1/2 = 1.59 x 105 yrs Natural Uranium is 238U which is common in rocks as well. 238U + 10n 239U + Energy T1/2 = 23.5 min 239U 239Np +  + Energy T1/2 = 2.36 days 239Np 239Pu +  + Energy T1/2 = 24400 years

A schematic diagram of the tentative plan for deep underground isolation of nuclear waste.

Disposal system Source: AP/Wide World Photos

Figure 21.16: Plot of energy versus the separation distance

Hydrogen Burning in Stars and Nuclear Weapons 1H + 1H 2H +  + 1.4 Mev 1H + 2H 3He + 5.5 Mev 2H + 2H 3He + 1n + 3.3 Mev 2H + 2H 3H + 1H + 4.0 mev 2H + 3H 4He + 1n + 17.6 Mev Easiest! Highest 2H + 3He 4He + 1H + 18.3 Mev cross section! 1H + 7Li 4He + 4He + 17.3 Mev

Helium Burning Reactions in Stars 12C + 4He 16O 16O + 4He 20Ne 20Ne + 4He 24Mg 24Mg + 4He 28Si 28Si + 4He 32S 32S + 4He 36Ar 36Ar +4He 40Ca

Image of a portion of the Cyngus Loop supernova remnant taken by the Hubble space telescope. Source: NASA

Positron Emission Tomography (PET) – A new and Important Tool in Imaging Research In the technique of positron Tomography, a positron emitting isotope Is included into a molecule that is incorporated into a chemical reaction. The positron emitted during the decay of the isotope will analite with an Electron and emit two 511 kev gamma rays that can then be detected, and the location of the decaying isotope isolated accurately. B+ + e- Energy Two Gamma rays at 180o 511 kev 511 kev The two gamma rays come away at 180o. e- + B+ Common Positron emitting Isotopes: 15O, T1/2 = 122s ; 18F, T1/2 = 1.83 hr 11C, T1/2= 20.3 min , 13N, T1/2 = 9.97 min , ETC

Positron Emission Tomograph The Tomograph is an instrument that is a ring of gamma ray detectors that react very fast to gamma rays, and by measuring the time each detector receives the signal one can locate the point of origin of the gamma ray to a precision of + 1 cm in a human being or any other physical object, with out any in vivo investigation. The detectors must have a capability of measuring up to + 250 ps per pulse. _ _

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  RBE = 1 for x-rays,  -rays, and ’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)

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 hrs) ~ 1 mrem Nuclear tests < 4 mrem/yr Nuclear power industry < 1 mrem/yr Total Average Value 100 - 200 mrem/yr

Examples of Typical Radiation Doses from Natural and Artificial Sources - III From Kotz & Percell, Freshman Chemistry text Smoke detectors 1 millirem/year Smoking Tobacco(1.5 packs a day) 9000 “ 10 Airline flights 3 “ Airline Crew 160 “ Hazards equivalent to the radiation dose of 10 mrem/year 3250 km travel by car 600 km travel by velocipede 150 km motorcycle 25 liters of wine 100 cigarettes smoked 1 diagonistic x-ray

Acute Effects of a Single Dose of Whole-Body Irradiation - I Dose Lethal Dose (rem) Effect Population (%) No. of Days 5 - 20 Possible late effect; possible --- --- chromosomal aberrations 20 - 100 Temporary reduction in --- --- white blood cells 50+ Temporary sterility in men --- --- (100+ rem = 1 yr duration) 100-200 “Mild radiation sickness”: vomiting, diarrhea, tiredness in a few hours Reduction in infection resistance Possible bone growth retardation in children

Acute Effects of a Single Dose of Whole-Body Irradiation - II Dose Lethal Dose (rem) Effect Population (%) No. of Days 300+ Permanent sterility in Women ---- --- 500 “Serious radiation sickness”: 50 - 70 30 marrow/intestine destruction 400 - 1000 Acute illness, early deaths 60 - 95 30 3000+ Acute illness, death in hours 100 2 to days