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Understanding Radioactive Decay: Types, Principles, and Half-Life Calculations

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stamos
24 diapositives · 418 Views · Jul 22, 2014

This discussion delves into nuclear decay, exploring whether a nucleus undergoing radioactive decay must be older than its undamaged counterparts. It outlines various types of radioactive decay, including gamma, beta, and alpha decay, and elaborates on principles like energy conservation and half-life calculations. Additionally, it covers radiometric dating methods, especially carbon dating, alongside the physical properties and penetration depths of different radiations. Understanding these concepts is essential for grasping the nature of radioactive materials and their applications in fields like geology and archaeology.

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Understanding Radioactive Decay: Types, Principles, and Half-Life Calculations
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  1. Discussion question: If a nucleus undergoes radioactive decay it must be older than the other nuclei in the sample that haven’t decayed. • True • False

  2. Types of radioactive decay • Gamma • Beta (+,-, e.c.) • Alpha • Neutron emission • Fission • Fusion • http://ie.lbl.gov/education/isotopes.htm

  3. Gamma Decay Nucleus in excited state gives off energy as a photon

  4. Beta Decay – electron and positron decay Changes a proton to a neutron or a neutron to a proton β- 146C β+ 14162Sm 189F + e e.c.

  5. Alpha Decay--42He given off Alpha particle is a very low energy system / very tightly bound 22688Ra

  6. 23892U 23490Th + ? T1/2 = 4.5 Gy • alpha • beta + • beta – • e.c. • gamma

  7. alpha beta + beta – e.c. gamma 23490Th 23491Pa + ? T1/2 = 24 days

  8. Why do some things decay, other don’t? Magic Numbers Image from http://pms.iitk.ernet.in/ICT/physics_courses/akj/AKJain_IITR_Ch_2.htm

  9. Why do some things decay, other don’t? Magic Numbers Image from http://pms.iitk.ernet.in/ICT/physics_courses/akj/AKJain_IITR_Ch_2.htm

  10. Small nuclei – N ~Z

  11. Large nuclei – N > Z

  12. Radiometric Dating • Carbon Dating • Cosmic rays release neutrons • 14N + n →14C + 1H • Half life of ~5760 years • Compare carbon-12 to carbon-14 ratio • Limitations • Must be organic • Has the ratio of C-12 to C-14 always been the same? • Calibration • 50,000 years limit (not much C-14 left)

  13. 2 Nephi 2:26 And the Messiah cometh in the fulness of time, that he may redeem the children of men from the fall. And because that they are redeemed from the fall they have become free forever, knowing good from evil; to act for themselves and not to be acted upon, save it be by the punishment of the law at the great and last day, according to the commandments which God hath given.

  14. More Proof

  15. 22688Ra eventually becomes 20682Pb. What is released in this process? • only alphas • only betas • both but more alphas than betas • both but the same number of alphas and betas • both but more betas than alphas

  16. N 145 140 Z 80 85 90

  17. http://en.wikipedia.org/wiki/Decay_chain

  18. Penetration Depth • Alpha– only a few cm through air • Beta—only through air—blocked by paper or sheet of metal • Gamma rays—short wavelength light—penetrate several cm lead or a meter of concrete. Easily penetrates the skin and interacts with human cells—these are the dangerous ones.

  19. What has to be true for a particular type of nuclear decay to happen? • Conservation of . . . • Energy, momentum, angular momentum • Compare rest energy • OK if we go down in energy – turn into kinetic • Lepton number, Baryon number, … • Force to make it happen

  20. For radioactive decay where Nois the number of nuclei you start with and N is the number of nuclei after a time t How do I solve for λ?

  21. Discussion question: Over the course of 3 hours, 15% of a radioactive material decays. What is its half-life? • 4.1 hrs • 12.8 hrs • 24.0 hrs • 68.6 hrs • 84.2 hrs

  22. #decays/sec=λN • 1 Ci = 3.7x1010 decays/sec

  23. Fission: Heavy Elements can reduce energy (i.e. increase binding energy per nucleon) by splitting roughly in half.