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Lesson 7

Lesson 7. Alpha Decay. Alpha decay ( ). Decay by the emission of doubly charged helium nuclei 4 He 2+ . 238 U 234 Th + 4 He Z = -2, N=-2, A=-4 All nuclei with Z≥83 decay by -decay as do some rare earth nuclei.Alpha decay is also known in the 100 Sn region. Alpha Decay.

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Lesson 7

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  1. Lesson 7 Alpha Decay

  2. Alpha decay () • Decay by the emission of doubly charged helium nuclei 4He2+. • 238U234Th + 4He • Z = -2, N=-2, A=-4 • All nuclei with Z≥83 decay by -decay as do some rare earth nuclei.Alpha decay is also known in the 100Sn region.

  3. Alpha Decay • The emitted -particles are monoenergetic,ranging in energy from 1.8-11.6 MeV (typically 4-9 MeV). • They can be stopped by a piece of paper and are thus an internal radiation hazard rather than an external hazard. • The naturally occurring -emitters form long series of nuclei that decay to one another. Some of these naturally occurring decays series involve isotopes of Rn, a gas.

  4. Important Features of Alpha Decay • Generally energy of decay increases with increasing Z, but in any case the energy of the emitted -particle is less than the Coulomb barrier for the -nucleus interaction. • For e-e nuclei, decay leads to gs of daughter. For odd A nuclei, decay is not to the gs but a low-lying excited state.

  5. Energetics of Alpha Decay • Qalpha

  6. Energetics of Alpha Decay (cont.) • Qalpha --Generally increases with increasing Z, but is subject to shell effects. Z=82, N=126 N=152-154

  7. Q, T Difference between detecting emitted alpha particle and detecting alpha from implanted atom.

  8. Closed decay cycles Used to measure unknown masses or Q values for beta decay

  9. Understanding natural alpha-decay chains • In the U natural decay series, see pattern of mixed alpha and beta decays. Why?

  10. Understanding Alpha Decay • The problem:

  11. Understanding Alpha Decay (cont.) • The Geiger Nuttall Law

  12. Theory of Alpha Decay • One of the first successes of quantum mechanics

  13. After some algebra and simplifying for the nuclear case Then This is called the “one-body” theory of alpha decay

  14. How well does one-body theory work? Role of “pre-formation factor”

  15. Effect of angular momentum Selection Rules

  16. Which states of daughter are populated in alpha decay? • Dominant effect is Q • Angular momentum effect Angular momentum = Parity = (-1)

  17. Hindrance Factors Hindrance Factor=t1/2(meas)/t1/2(one-body)

  18. Proton Decay • Same theory as alpha decay, except no pre-formation factor for protons • Where do you expect to see proton decay? (Sp=0) proton dripline

  19. Proton Decay(cont.) • Proton energies are low and transmission factors are small.

  20. Proton Decay(cont.) • Alpha decay complicates measurements

  21. Heavy Particle Decay • What about emitting particles other than alphas or protons? Q12C

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