Chapter 7 Nuclear Energy

• The nucleus of an atom normally remains intact
  • all chemistry occurs at the outer electrons
• Yet there is structure in the nucleus
  • Changes within the nucleus are possible
    • rarely occur because tremendous
      • energies are involved
    • we rarely can come up with the energy
      • required to breach the nucleus
      • (particle accelerators, nuclear reactors)
  • Also, unstable elements mostly vanished millions or billions of years ago

• When such changes do occur
  • they involve 1000-1,000,000 fold more energy than normal chemistry
  • we see this in high energy radiation
• • we can use the radiation to label and track chemicals
  • we can exploit the energy for explosions
    • (atomic and H-bombs)
  • we can control it as energy source (reactors)
  • at smaller scale we can use radioactivity to power remote objects ( NASA)

Spontaneous Changes

• Radioactive Elements
  • Nucleus transforms spontaneously
    • generally emits energy
    • much more than associated with light
• Rate varies from subsecond to millions of years
• Obviously natural radioactivity mostly is in the millions
  • but there are other cases
  • (remember carbon dating?)
• Follows a rule-- half life
  • half of the material vanishes in a period of time called the half life
  • half of what's left vanishes in equal period
    • now 1/4 left
    • then 1/8, 1/16, 1/32 ....

Einstein's famous E= mc²

• Einstein, 1905-1915, showed that mass and energy are related
• energy and mass are different forms of the same thing
  • not enough that you'd ever notice
  • exothermic reaction means tiny mass loss
    • absolutely unmeasurable as a mass change
  • however, if we cause a tiny change in mass
    • that's a fantastic energy difference

• example: two Deuterium nuclei (1p + 1 n)
  • weigh a little more than one He nucleus
    • also 2 protons, 2 neutrons
  • converting 2 ²H ---> ⁴He + energy
    • that's the kind of reaction in stars
    • stars could never find enough chemical energy--fuel to burn-- to produce that much heat

• likewise, splitting large atoms
• ²³⁵U + n -----------> ¹⁴¹Ba + ⁹²Kr + 3 n
• mass of {¹⁴¹Ba + ⁹²Kr + 3 n} < {U+n}
• ---> 2-3 smaller atoms + lots of energy

• chain reaction--
  • ²³⁵U + 1n -----------> ¹⁴¹Ba + ⁹²Kr + 3 n
  • those additional neutrons find more U atoms
    • if rapid, bomb
    • if slow, a reactor (power plant)

Radioactivity

• natural phenomenon, discovered 1989
• discovered after X-rays
  • some photographic film had gone bad
  • as if exposed to light or x-rays
  • traced to some U-salts in the same drawer

Three rather different types of radioactivity

• penetrate short distances in solids (metals, stone...)
  • actually better at it than X-rays
  • not deflected by magnets or electrical fields
    • so not charged particles
  • electromagnetic radiation

• travel through a few sheets of paper, few feet of air
• can deflect a little with a magnet of electric field
• evidence says negative particles, moving very fast
  • simply electrons, moving very fast
• they definitely emerge from the nucleus
  • but there aren't any electrons there...
  • usually during a n --> p + beta particle

• travel 1-2 cm in air
• can stop with a sheet of paper
  • such samples produce a trace of gas
  • gas looks just like Helium
  • alpha = (2p + 2 n) = ⁴He nucleus, traveling very fast
• nuclide must go from _xyZ to _x-2y-4Z

• unit called electron volt
• accelerate an electron with 1 volt
• 10 eV can pull an electron off an atom
• UV photon might be 4 eV
• radioactive materials -1 -100 MeV
  • that's why it travels through matter

Biological Damage

• remember how UV could break
  • chemical bonds, induce skin cancer
  • 1 photon, 1 molecule
• radiation can do likewise,
  • not just at the skin level
• 100 MeV gamma ray could produce
  • 10,000 10eV electrons
  • could disrupt 10-1000 molecules

Radiation effect tends to accumulate

• very low level radiation...
  • clearly species have survived it
• increasing levels...
  • see biological changes (white blood counts)
  • ... see long term statistical problems
    • (mainly increased cancer)
• high levels... clear radiation disease, illness, death
• alpha rays are more forgiving
  • equally dangerous (at same energy)
  • vanish if you stay 1 ft away
  • stopped by clothing
  • not likely to go beyond skin injury unless inhaled or ingested
• beta rays intermediate
  • relatively simple shielding can protect
• gamma rays
  • hard to shield
    • much of energy may pass through you
    • only part of the energy deposited in you

Measuring radioactivity

• oldest: photographic film darkens
  • slow, not very sensitive
• leakage of electrical charge
• a bit awkward, but useable
• Geiger counter
  • trick... each gamma or beta produces a few ions
  • 300-500 volts accelerats them
  • then collide and make more ions
  • so one photon (gamma) or beta ..... million electron pulse
    • amplify to make click
    • count clicks per second
• scintillation counter
  • gamma ray excites molecule
  • it emits visible light
  • measure with a photocell
    • (can make glow in the dark indicators)
    • (Radium watch dials)