Uploaded: 7 years ago
Contributor: Guest
Category: Environmental Biology
Type: Lecture Notes
Tags: energy, protons, nucleus, radioactive, fission,
*
nuclear, neutrons, radiation, fusion, massive, penetrating, unstable, nuclear, stable,
*
radiation
Rating:
N/A
|
Filename: Lec11.ppt
(2.37 MB)
Credit Cost: 3
Views: 153
Last Download: N/A
|
Transcript
*
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!
|
|