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Decay

The Decay of Unstable Nuclei. Decay. Heavy Particle Decay. electrometer. Discovery of a Radioactivity. Heavy nuclides ( Gd , U, Pu ,..) spontaneously emit a particles. Mass systematics  energetically allowed.

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Decay

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  1. The Decay of Unstable Nuclei Decay Heavy Particle Decay

  2. electrometer Discovery of a Radioactivity Heavy nuclides (Gd, U, Pu,..) spontaneously emit a particles.Mass systematics  energetically allowed Marie & Pierre Curie (1897-1904) studied “pitchblende”Ra: powerfulaemitter Alpha Decay a particles energetically preferred (light particles)

  3. Energy Release in a Decay “Q-Value” for a Decay: Q=B(4He)+B(Z-2,A-4)-B(Z,A) Z=82 Shell effect at N=126, Z=82 Odd-even staggering Alpha Decay Geiger-Nuttall Rule:Inverse relation between a-decay half life and decay energy for even-even nuclei

  4. Examples: Alpha Decay Schemes/Spectra Short-range a particles Long-range a particles Alpha Decay Many a emitters: Ea ~6 MeV (short range)Heavy emitters also: Ea~ 8 MeV (long range)

  5. Solution to a Puzzle: Tunneling the Coulomb Barrier Answered Puzzle: If nucleus stable : t1/2 ∞ If nucleus unstable : t1/2 0 Not found in macroscopic nature George Gamov Low stability  high a energy a-Nucleus Coulomb Potential Resolution of Puzzle Quantum meta-stability(if nucleus has intrinsic a structure, Pa=1): Gamov: Intrinsic a wave function “leaks” out Superposition of repulsive Coulomb potential + attractive nuclear potential creates “barrier” which is penetrable for a particle Alpha Decay UaTh= 28 MeV ya Ea= 4MeV RaTh=9 fm Nuclear Potential

  6. Quantal Barrier Penetration General solution of Schrödinger Equ.: Lin. Comb. of exponentials E, U 3 1 2 U E E 0 d x Alpha Decay Particle escape probability l(system decay) depends on barrier height & thickness the number of states

  7. Barriers of Arbitrary Shape Approximate by step function U(r) Ui R1 R2 Alpha Decay Application: Z1=2, Z2=Z-2

  8. The Geiger-Nuttal Rule a half life vs. a energy (years) Alpha Decay What are the Z dependent scaling factors A?

  9. Decay Dynamics Classical considerations/order of magnitude: Pre-existence of a particle in nuclear medium, Pa (Nuclear cluster structure  “Spectroscopic Factor S”) Finite probability for barrier encounters, frequency, fa(velocity, effective mass) Barrier penetration, Ta(geometry of Coulomb barrier) Angle of incidence on barrier, angular momentum, a Great uncertainties in absolute decay rates a-Nucleus Potential Alpha Decay UaTh= 28 MeV Ea=4MeV 0 r RaTh=9 fm Nuclear Potential Ua

  10. Angular Momentum and Parity in a Decay a-Nucleus effective Potential Solve 1-D Schrödinger Equ. For a-daughter system with effective radial potential (Coulomb + centrifugal)  conserved angular momentum Alpha Decay Nuclear Potential Spin/parity selection rule for a transitions:  = 0 most probable a decay Higher  values hindered significantly because of small T Estimate range of -values from Ea and nuclear radii !

  11. -Dependent a Transmission Coefficients  ≠ 0 but not very large: a-daughter effective radial potential (Coulomb + centrifugal)  Linearization at barrier Ea R R r Alpha Decay Absolute values not very good, by orders of magnitude Can decrease T by factors 5-10 for D=+1 Pocket formula Loveland et al., Modern Nuclear Chemistry, Wiley Interscience, 2006

  12. a Decay Patterns Guess some final nuclear spins Ip a Decay of 251Fm a7 479 keV Alpha Decay a1 0 keV From Krane, Introductory Nuclear Physics

  13. Spontaneous Nucleon Decay New: The “Drip” lines: More limits to nuclear stability Sp Sn Alpha Decay Sp Near drip lines: nucleons can be emitted from excited states.Need secondary beams to explore regions far off stability Sn Adapted from NSCL ISF White Paper, 2006

  14. Proton Decay Near proton drip line, nuclei become p-unstable at E* ~ 1 MeV. Coulomb+centrifugal barrier  long lifetime (isomeric state) 0.39 s dN/dEp p-gcoincid. Si singles Alpha Decay I. Mukha et al., PRL 95, 22501 (2005)

  15. Di-Proton Decay Alpha Decay

  16. Spontaneous Neutron Decay Alpha Decay

  17. Spontaneous Cluster Decay Observed for 221Fr – 242Cm: Cluster radioactivity (14C-34Si) Solenoid Magnet Spectrometer Telescope ID(Z, E ) DE-E 223RaSource Baffle energetically possible. Magnetic spectrometer: adjustable acceptance, remove unwanted particles, here: strong a lines Can use very strong sources, 0.2Ci223Ra Alpha Decay Spectrometer Transmission(acceptance) Gales et al., PRL 53 (1984)

  18. Results: 223Ra Decay Products Measured Branching (Telescope in focus): Expected from Gamov factor: Very different preformation factors ? Alpha Decay a E (MeV) Gales et al., PRL 53 (1984)

  19. Structure Effects in Cluster Decay Half Lives Ra Isotopes x.s. x.s. g.s. Alpha Decay g.s. Due to barrier penetrabilities T Unpaired neutron suppresses cluster emission from g.s., not from excited states Hussonnois et al., PRC 42 R495 (1990)

  20. Theoretical Ambiguity Alpha Decay Fission and a approach need different barriers  map barrier shape by inverse (fusion) process at different energies. Barrier for cluster decay A.A. Ogloblin et al., NPA 738, 313(2004)

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