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Understanding Compound Nuclear Reactions: Characteristics and Mechanisms

This lesson explores compound nuclear reactions, focusing on the formation and characteristics of the compound nucleus (CN), a long-lived intermediate stage arising from the fusion of projectile and target nuclei. It discusses how energy is distributed among nucleons, the measurable lifetime of the CN, and the independence of its decay mode from its formation. The impact of conservation laws, statistical equilibrium assumptions, and experimental evidence from techniques like angular distribution measurements are covered. Different reaction types, excitation functions, and recent developments in high-energy nuclear reactions are also highlighted.

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Understanding Compound Nuclear Reactions: Characteristics and Mechanisms

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  1. Lesson 11 Nuclear Reactions --Part 2

  2. Compound Nuclear Reactions • The compound nucleus is a long-lived reaction intermediate that is formed by a complex set of interactions between the projectile and the target. • The projectile and target nuclei fuse and the energy of the projectile is shared among all the nucleons of the composite system. • The lifetime of the CN is ~ 10-18-10-16 s. This lifetime is directly measurable by crystal-blocking techniques.

  3. Independence Hypothesis or “Amnesia Assumption” • The mode of decay of the CN is independent of its mode of formation. • Caveat: Conservation laws apply. • Experimental evidence: the Ghoshal expt. • Angular distributions symmetric about 90 in CN frame.

  4. 62Cu>63Zn>62Ni ang. mom effects

  5. Cross Sections--General

  6. Cross Sections--Energy Dependence

  7. /D << 1 For reaction a + A Cb + B Applying this to (n,) reactions

  8. /D >>1 For reaction a + A Cb + B ab = C PC(b)

  9. /D >>1 • Assumption is that of statistical equilibrium • For reaction a + A Cb + B ab = C PC(b) • So the problem is to calculate the probability that C will decay to B + b.

  10. Level densities

  11. If emitted particles are neutrons

  12. If emitted particles are charged particles

  13. Excitation functions

  14. Photonuclear Reactions

  15. Photonuclear Reactions • GDR-- a giant oscillation of the nuclear protons vs the nuclear neutrons • dipole sum rule

  16. Heavy Ion Reactions • Classical motion • Dominated by high angular momentum • Aproj > 4

  17. Mechanisms

  18. Mechanisms

  19. Elastic scattering

  20. Fusion

  21. Deep Inelastic Scattering

  22. High Energy Reactions • Low energy (< 10 MeV/A) • Intermediate Energy ( 20-250 MeV/A) • High Energy (>250 MeV/A)

  23. Spallation

  24. New high energy mechanisms • Spallation • IMF formation • Cascade processes • Participant-Spectator Picture

  25. Cascades nucleon-nucleon collisions

  26. Participant-Spectator Physics

  27. Multifragmentation • “Multifragmentation” refers to central collisions where several IMFs are emitted. • Caloric curve

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