Lec11

* Introductory Environment Science Lecture 11 Radioactivity, Radiation Nuclear Energy Fission Nuclear Power Fusion * Nuclear radioactivity Natural radioactivity Spontaneous emission of particles or energy from an unstable nucleus Discovered by Becquerel Three types of radioactive decay Alpha decay (He-nucleus) Beta decay (high energy electron) Gamma decay (high energy electromagnetic radiation) * Nuclear equations Atomic number = number of protons in nucleus Isotopes: same atomic number; different number of neutrons Mass number = number of nucleons (protons and neutrons) in nucleus Nuclear reactions Represented by balanced equations Charge conserved Mass number conserved * The nature of the nucleus Strong nuclear force Binds protons and neutrons Very short ranged, less than 10-15 m Overcomes proton-proton Coulomb repulsion Nuclear shell model Nucleon quantum energy levels Maximum stability for nucleon number = 2, 8, 20, 28, 50, 82 or 126 Band of stability * Generalizations - nuclear stability Atomic number > 83: unstable Nucleon number = 2, 8, 20, 28, 50, 82 or 126: added stability Pairs of protons and pairs of neutrons: added stability Odd number of both protons and neutrons less stable Neutron: proton ratios for added stability 1:1 in isotopes with up to 20 protons 1+increasing:1 with increasingly heavy isotopes * Types of radioactive decay Alpha emission Expulsion of helium nucleus Least penetrating: stopped by paper Beta emission Expulsion of an electron More penetrating: 1 cm of aluminum Gamma decay Emission of a high energy photon Most penetrating: 5 cm of lead * Radioactive decay series One radioactive nucleus decays to a 2nd, which decays to a 3rd, which… Three naturally occurring series Thorium-232 to lead-208 Uranium-235 to lead-207 Uranium-238 to lead-206 * Half-life Time required for 1/2 of a radioactive sample to decay Example: 1 kg of an unstable isotope with a one-day half-life After 1 day: 500 g remain After 2 days: 250 g remain After 3 days: 125 g remain U-238 decay series: wide half-life variation * Measurement of radiation Measurement methods Ionization counters Detect ions produced by radiation Example: Geiger counter Scintillation counters Rely on flashes of light produced as radiation strikes a phosphor * Radiation units Measured at the source Activity: number of disintegrations per unit time Units: Becquerel (SI unit), Curie, … Measured where absorbed Human exposure: rem SI unit: millisievert rad: radiation absorbed dose (unit = gray) Dosage related to effects on organism * Radiation exposure Natural radioactivity 100-500 mrem/yr Sources Cosmic rays from outer space Earth’s residual radioactivity Medical x-rays, TVs, … Consequences DNA disruption Free radical production Threshold versus linear exposure models * Nuclear energy Interconversion of mass and energy Mass defect Difference between masses of reactants and products Binding energy Energy required to break a nucleus into individual protons and neutrons Ratio: binding energy to nucleon number Iron-56 = most stable nucleus * Nuclear energy: Moving towards iron-56 (most stable) through fission or fusion * Nuclear fission Heavy nuclei splitting into lighter ones Chain reactions Possible when one reaction can lead to others One neutron in, two or more out Critical mass Sufficient mass and concentration to produce a chain reaction * Many possible fission fragments * Nuclear power plants Rely on controlled fission chain reactions Steel vessel contains fuel rods and control rods Full plant very intricate Containment and auxiliary buildings necessary Spent fuel rods Contain fissionable materials U-235, Pu-239 Disposal issues not settled * Nuclear power plants * Nuclear fusion Less massive nuclei forming more massive nuclei Energy source for Sun and other stars Requirements for fusion High temperature High density Sufficient confinement time Controlled fusion Magnetic confinement Inertial confinement * Source of nuclear energy Ultimately connected to origins of the Universe and the life cycles of stars Big Bang theory Incredibly hot, dense primordial plasma cools, creating protons and neutrons Continued cooling leads to hydrogen atoms which collapse gravitationally into 1st generation stars Stellar evolution Interior temperatures and densities suitable for fusion of heavy elements beyond hydrogen and helium Certain massive stars explode in supernovae, spreading heavy elements (some radioactive) Ultimate source: gravitational attraction!