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Radioactivity

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Radioactivity

  1. 1. Radioactivity
  2. 2. RadiationRadiation: The process of emittingenergy in the form of waves orparticles.Where does radiation come from?Radiation is generally producedwhen particles interact or decay.A large contribution of the radiationon earth is from the sun (solar) orfrom radioactive isotopes of theelements (terrestrial).Radiation is going through you atthis very moment! http://www.atral.com/U238.html
  3. 3. A. Definitions• Radioactivity – emission of high-energy radiation from the nucleus of an atom• Nuclide – nucleus of an isotope• Transmutation – process of changing one element into another via nuclear decay
  4. 4. IsotopesWhat’s an isotope?Two or more varieties of an elementhaving the same number of protons butdifferent number of neutrons. Certainisotopes are “unstable” and decay tolighter isotopes or elements.Deuterium and tritium are isotopes ofhydrogen. In addition to the 1 proton,they have 1 and 2 additional neutrons inthe nucleus respectively*.Another prime example is Uranium238, or just 238U.
  5. 5. RadioactivityBy the end of the 1800s, it was known that certainisotopes emit penetrating rays. Three types of radiationwere known: • Alpha particles (α) • Beta particles (β) • Gamma-rays (γ)
  6. 6. B. Types of Radiation• Alpha (α) 4 – helium nucleus 2 He 2+ paper Beta-minus (β-) 0 1- -1 e lead  electron Gamma (γ)  high-energy photon 0 concrete
  7. 7. C. Nuclear Decay• Why nuclides decay… – to obtain a stable ratio of neutrons to protons 39 19 K Stable 40 Unstable 19 K (radioactive)
  8. 8. C. Nuclear Decay TRANSMUTATION• Alpha Emission 238 92 U→ Th + He 234 90 4 2 Beta Emission 131 53 I→ 131 54 Xe + e 0 -1
  9. 9. Where do these particles come from ?These particles generally comefrom the nuclei of atomic isotopeswhich are not stable. The decay chain of Uraniumproduces all three of these formsof radiation. Let’s look at them in more detail…
  10. 10. Note: This is theatomic weight, whichis the number of Alpha Particles (α)protons plus neutrons Radium Radon + n p p n R226 Rn222 α (4He) 88 protons 86 protons 2 protons 138 neutrons 136 neutrons 2 neutrons The alpha-particle (α) is a Helium nucleus. It’s the same as the element Helium, with the electrons stripped off !
  11. 11. Beta Particles (β) Carbon Nitrogen + e- C14 N14 6 protons 7 protons electron 8 neutrons 7 neutrons (beta-particle)We see that one of the neutrons from the C14 nucleus“converted” into a proton, and an electron was ejected.The remaining nucleus contains 7p and 7n, which is a nitrogennucleus. In symbolic notation, the following process occurred: Yes, the same np+e (+ν) neutrino we saw previously
  12. 12. Gamma particles (γ)In much the same way that electrons in atoms can be in anexcited state, so can a nucleus. Neon Neon Ne20 Ne20 + 10 protons 10 protons gamma 10 neutrons 10 neutrons(in excited state) (lowest energy state) A gamma is a high energy light particle. It is NOT visible by your naked eye because it is not in the visible part of the EM spectrum.
  13. 13. Gamma Rays Neon Ne20 Neon Ne20 +The gamma from nuclear decay is in the X-ray/ Gamma ray part of the EM spectrum (very energetic!)
  14. 14. How do these particles differ ? Change in Change in Particle Mass atomic number number Gamma (γ) No change No change Increased by Beta (β) No change 1 Decreased Decreased Alpha (α) by 4 by 2
  15. 15. Rate of DecayBeyond knowing the types of particles which are emittedwhen an isotope decays, we also are interested in how frequentlyone of the atoms emits this radiation. A very important point here is that we cannot predict when aparticular entity will decay. We do know though, that if we had a large sample of a radioactivesubstance, some number will decay after a given amount of time. Some radioactive substances have a very high “rate of decay”,while others have a very low decay rate. To differentiate different radioactive substances, we look toquantify this idea of “decay rate”
  16. 16. Half-Life The “half-life” (h) is the time it takes for half the atoms of aradioactive substance to decay. For example, suppose we had 20,000 atoms of a radioactivesubstance. If the half-life is 1 hour, how many atoms of thatsubstance would be left after: #atoms % of atoms Time remaining remaining 1 hour (one lifetime) ? 10,000 (50%) 2 hours (two lifetimes) ? 5,000 (25%) 3 hours (three lifetimes) ? 2,500 (12.5%)
  17. 17. D. Half-life• Half-life (t½) – time it takes for half of the nuclides in a sample to decay Nuclear Decay 20 Example Half-lives 18 16 polonium-194 0.7 seconds Mass of Isotopes (g) 14 12 lead-212 10.6 hours 10 8 iodine-131 8.04 days 6 4 carbon-14 5,370 years 2 0 0 2 4 6 8 10 uranium-238 4.5 billion years # of Half-Lives
  18. 18. Half-life How much of a 20-g sample of sodium-24 would remain after decaying for 30 hours? Sodium-24 has a half-life of 15 hours.GIVEN: WORK:total time = 30 number of half-lives = 2hours 20 g ÷ 2 = 10 g (1 half-t1/2 = 15 hours life)original mass = 10 g ÷ 2 = 5 g (2 half-20 g lives)
  19. 19. Writing Nuclear Equations
  20. 20. Atomic number (Z) = number of protons in nucleus Mass number (A) = number of protons + number of neutrons = atomic number (Z) + number of neutrons Mass Number A ZX Element Symbol Atomic Number proton electron α particle 1 1 p or 1H 1 0 -1 e or -1β 0 4 2 He or 2α 4A 1 0 4Z 1 -1 2
  21. 21. Po decays by alpha emission. Write the balanced nuclear212equation for the decay of 212Po. alpha particle - 4 2 He or 2α 4 84Po 2 He + ZX 212 4 A 212 = 4 + A A = 208 84 = 2 + Z Z = 82 84 212 Po 2 4 He + 208Pb 82 23.1
  22. 22. Write Nuclear Equations!Write the nuclear equation for the betaemitter Co-60.60 0 60 Co e + Ni27 -1 28
  23. 23. Write Nuclear Equations!Write an equation to describe the beta decay of a lead-214nucleus to form a bismuth-214 nucleus. 214 Pb 0 e + 214 Bi 82 -1 83Write an equation to describe the alpha decay of aradium-226 nucleus to form a radon nucleus.
  24. 24. Summary Certain particles are radioactive and undergo decay. Radiation in nuclear decay consists of α, β, and γ particles The rate of decay is give by the radioactive decay law: After 5 lifetimes more than 99% of the initial particleshave decayed away. Subatomic particles usually have lifetimes which are fractions of a second…
  25. 25. A. F ission• splitting a nucleus into two or more smaller nuclei• some mass is converted to large amounts of energy 1 0 n+ 235 92 U→ 141 56 Ba + Kr + 3 n 92 36 1 0
  26. 26. A. F ission• chain reaction - self-feeding reaction
  27. 27. B. Fusion• combining of two nuclei to form one nucleus of larger mass• produces even more energy than fission• occurs naturally in stars
  28. 28. A. Nuclear Power• Fission Reactors Cooling Tower
  29. 29. A. Nuclear Power• Fission Reactors
  30. 30. A. Nuclear Power• Fusion Reactors (not yet sustainable)
  31. 31. A. Nuclear Power • Fusion Reactors (not yet sustainable) National Spherical Torus ExperimentTokamak Fusion Test Reactor Princeton University
  32. 32. A. Nuclear Power F F I U s S s I vs. i O o N n• 235U is limited • Hydrogen is abundant• danger of meltdown • no danger of meltdown• toxic waste • no toxic waste• thermal pollution • not yet sustainable
  33. 33. • Choose one of the following to investigate: – Irradiated Food – Radioactive Dating – Nuclear Medicine – Weapons of mass destruction – Chernobyl – Nuclear power future – Meltdowns/ leaks• Make a mini-poster to display what you have

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