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Gamma-ray strength functions

Gamma-ray strength functions. Also called in the literature: radiative strength functions photon strength functions. Presentation OCL group meeting Ann-Cecilie Larsen Tuesday Nov 4, 2008. Some basics about the nucleus.

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Gamma-ray strength functions

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  1. Gamma-ray strength functions Also called in the literature: radiative strength functions photon strength functions Presentation OCL group meeting Ann-Cecilie Larsen Tuesday Nov 4, 2008

  2. Some basics about the nucleus • Protons (p) and neutrons (n) in orbits around the center of mass, kept together by the strong force • Pairs of p and n in time-reversed orbits  nucleon Cooper pairs with speed  0.6c (unpaired nucleons:  0.2c) • Shell structure: some numbers of protons and neutrons give very stable nucleus http://www.jlab.org/news/archive/2003/nucleons.html Nuclear physics group meeting

  3. Excitation of a nucleus • Ground state  excited state • Single-particle excitation • Collective excitation • Pair-breaking • A combination p1/2 p3/2 s1/2 Cooper pair Broken pair Nuclear physics group meeting

  4. Gamma decay • Transition between excited states or to the ground state by emitting a high-energetic photon ( ray) E2 E2-E1 p1/2 E2 E1 E1-0 p3/2 E1 Gr. st. s1/2 Nuclear physics group meeting

  5. Types of  radiation • Electric dipole (E1), quadrupole (E2), ... • Magnetic dipole (M1), quadrupole (M2), ... • Single-particle (Weisskopf) estimates of transition rates ( energy in MeV): NB! Can deviate several orders of magnitude from observed rates! Smaller  little overlap between initial and final Larger  collective modes (more than one nucleon) Nuclear physics group meeting

  6. L If Ii Selection rules • Each photon carries a definite angular momentum L>0 • Dipole: L=1, quadrupole: L=2, ... Ii = L+If: Ei, Ii, i L Ef, If, f Selection rules: |Ii - If|  L  Ii + If  = no: even electric (E2, E4), odd magnetic (M1,M3)  = yes: odd electric (E1,E3), even magnetic (M2,M4) Nuclear physics group meeting

  7. An example: partial decay scheme of 157Gd Nuclear physics group meeting

  8. i  f What happens at high excitation energy? • Lots of levels, impossible to measure all levels and their  transitions • Measure average decay properties • Fermi’s Golden Rule: Transition operator Nuclear physics group meeting

  9. Average decay probability • One state with energy E, lifetime t & width E= (Heisenberg: Et  hbar/2) • Connection with decay rate1: = /hbar • Connection with lifetime:  = 1/t • Average over several states: ,   1From Weisskopf and Wigner, 1930 Nuclear physics group meeting

  10. Decay probability and -ray strength function • Model-independent definition of the -strength function (dipole): f(E) = /(E3D) [MeV-3] • Connection with decay rate: =(fE3D)/hbar [s-1] Neutron resonance spectrum as a function of neutron energy Resonance spacing D Nuclear physics group meeting

  11. protons n n+p n neutrons p How does the -ray strength function look like? Region for Oslo exp. Nuclear physics group meeting

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