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  (a c /2a s ) Z limit =2(a s /a c )A limit Z limit ~ 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. 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 -3 s Typically c/a =2:1

15. Sub-threshold fission resonances

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

18. Fission probability Fission probablity  (N f /(N f +N n +N gamma +N ch.p. ))

19. Multiple chance fission

20. n/fn/f

21. n/fn/f

22. Fission Product Distributions TKE Distribution

23. Fission Mass Distributions

26. Fission Product Charge Distributions

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

28. Prompt Neutrons

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

31. Fission Fragment Angular Distributions