Lecture 365/2/05 Tuesday 2:30 TSB 006. What does it mean to be radioactive?

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Lecture 36 5/2/05 Tuesday 2:30 TSB 006
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Transcript of Lecture 365/2/05 Tuesday 2:30 TSB 006. What does it mean to be radioactive?

  • Slide 1
  • Lecture 365/2/05 Tuesday 2:30 TSB 006
  • Slide 2
  • What does it mean to be radioactive?
  • Slide 3
  • Radioactive emissions Alpha particle () Helium nucleus Mass: 2264222 Atomic #: 88286
  • Slide 4
  • Radioactive emissions Beta particle () electron neutron electron () + proton Mass: 2390239 Atomic #: 92-193
  • Slide 5
  • Gamma () Radioactive emissions
  • Slide 6
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  • positron ( + ) electron proton positron () + neutron Mass: 2070207 Atomic #: 84+183
  • Slide 8
  • Radioactive Electron capture Electron + proton neutron Mass: 707 Atomic #: 4-13
  • Slide 9
  • Rates of radioactive decay Decay is not affected by temperature, pressure, or state of chemical combination N 0 = # of radioactive nuclei at t = 0 N t = # of radioactive nuclei at t = t 1 K = decay constant t = time
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  • If you start with 1.5 mg of tritium 3 H, how much is left after 49.2 years? t 1/2 =12.3 years
  • Slide 13
  • Belt of stability (empirically derived) Belt of stability ends at element 83 elements 84 protons are radioactive Type of decay nuclei above belt of stability (high N/Z) Beta emission 131 I 131 Xe + decreases N/Z nuclei below the belt of stability (low N/Z) loss of positron of electron capture 11 C 11 B + + or 81 Rb + e 81 Kr increases N/Z nuclei with atomic # 84 alpha emission 238 U 234Th +
  • Slide 14
  • Belt of stability
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  • Carbon dating in atmosphere t 1/2 =5730 years Ratio of 14 C/ 12 C constant until death, then 14 C/ 12 C decreases At death, 14 C has about 14 disintegrations per minute/gram (dpm/g)
  • Slide 16
  • Carbon-14 limitations 1. assume 14 C in atmosphere is constant 2. cant date object < 100 years old 3. accuracy only 100 years 4. only good back to ~ 40, 000 years