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Lesson 12

Lesson 12. Fission. Importance of Fission. Technological importance (reactors, bombs) Socio-political importance Role of chemists Very difficult problem. Overview of fission. Probability of Fission.

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Lesson 12

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  1. Lesson 12 Fission

  2. Importance of Fission • Technological importance (reactors, bombs) • Socio-political importance • Role of chemists • Very difficult problem

  3. Overview of fission

  4. Probability of Fission • Divide study of fission into two parts, the gssaddle point (probability of fission) and the saddlescission point (distribution of fission products) • Use liquid drop model to study gssaddle point

  5. Liquid drop model

  6. Limits on the Periodic Table • Notice that x (ac/2as) • Zlimit=2(as/ac)Alimit • Zlimit ~ 125 • For all stable nuclei, x < 1 • As nucleus deforms, pot. energy increases by • Eventually Coulomb energy will cause deformation energy • to decrease, ie, get fission barrier.

  7. Fission Barriers

  8. Shell Effects

  9. Consequences of Double Humped Fission Barriers • Spontaneously fissioning isomers • Superdeformed nuclei • Subthreshold resonances

  10. Spontaneous fission

  11. Understanding spontaneous fission lifetimes

  12. Spontaneously fissioning isomers

  13. Spontaneously fissioning isomers • Spntaneously fissioning isomers are nuclei caught in states in the second minimum of the fission potential energy surface. Their sf decay is enhanced relative to gs sf. • Lifetimes are 10-9 - 10-3s • Typically c/a =2:1

  14. Sub-threshold fission resonances

  15. “Normal fission”--the fission transition state nucleus • Has the same role as the transition state in chemical reactions. • prob. of fission =Aexp(-Bf/T) • Bn > Bf (235U); Bn < Bf (238U) • Big Three (233U, 235U, 239Pu)

  16. Fission probability • Fission probablity (Nf/(Nf+Nn+Ngamma+Nch.p.))

  17. Multiple chance fission

  18. n/f

  19. n/f

  20. Fission Product Distributions TKE Distribution

  21. Fission Mass Distributions

  22. Fission Mass Distributions

  23. Fission Mass Distributions

  24. Fission Product Charge Distributions

  25. Energetics of Fission • Q value ~ 200 MeV • TKE ~172 MeV • Neutrons ~18 MeV • Gammas ~ 7.5 MeV • , etc ~2.5 MeV

  26. Prompt Neutrons

  27. Prompt Neutrons

  28. Prompt Neutron Spectra • Average neutron energy ~ 2 MeV • Spectrum: frame of moving fragment; Maxwellian P (E)=Enexp (-En/T) lab frame; Watt spectrum

  29. Fission Fragment Angular Distributions

  30. Fission Fragment Angular Distributions

  31. Fission Fragment Angular Distributions

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