Gamma-ray strength functions obtained with the Oslo method Ann-Cecilie Larsen July 8, 2008 Workshop on Statistical Nuclear Physics and Applications in Astrophysics and Technology, July , Ohio University, Athens, Ohio DEPARTMENT OF PHYSICS UNIVERSITY OF OSLO

Collaborators  Oslo: A. B ü rger, M. Guttormsen, S. Messelt, F. Ingebretsen, H. T. Nyhus, J. Rekstad, S. Siem, N.U.H. Syed, and H. K. Toft  Livermore National Lab.: U. Agvaanluvsan, L. Bernstein  North Carolina State University/TUNL: R. Chankova, G. E. Mitchell  Åbo Akademi University, Finland: T. Lönnroth  Ohio University: A. Schiller A. Voinov Workshop on Statistical Nuclear Physics and Applications in Astrophysics and Technology, July , Ohio University, Athens, Ohio

Outline  Introduction & motivation  Oslo experiments  Gamma-ray strength functions, medium- mass nuclei  Gamma-ray strength functions, 116,117 Sn  Summary & outlook Workshop on Statistical Nuclear Physics and Applications in Astrophysics and Technology, July , Ohio University, Athens, Ohio

Decay probability  Fermi’s Golden Rule:  Statistical decay:  Gamma-ray strength function: Workshop on Statistical Nuclear Physics and Applications in Astrophysics and Technology, July , Ohio University, Athens, Ohio i f 

Resonances  Giant electric dipole resonance  M1 spin-flip, E2 isoscalar  M1 scissors mode, E1 skin oscillation Workshop on Statistical Nuclear Physics and Applications in Astrophysics and Technology, July , Ohio University, Athens, Ohio 117 Sn( ,xn) Lepretre et al NPA 219, 39 (1974).

Experiments at OCL Workshop on Statistical Nuclear Physics and Applications in Astrophysics and Technology, July , Ohio University, Athens, Ohio  Selected reactions: ( 3 He, 3 He’  ), ( 3 He,  ), (p,p’  ), (p,t  )...  Low spin and high intrinsic excitation energy  CACTUS: 28 5”x5” NaI (~15% eff.)  Eight  E-E Si particle telescopes

Particle - gamma coincidences Workshop on Statistical Nuclear Physics and Applications in Astrophysics and Technology, July , Ohio University, Athens, Ohio Si  E-E telescope 45 o NaI(Tl) 3 He Target nucleus  

Coincidence matrices Workshop on Statistical Nuclear Physics and Applications in Astrophysics and Technology, July , Ohio University, Athens, Ohio 44 Sc, unfolded 44 Sc, first-generation  -rays SnSn SpSp

Extraction of level density and gamma-ray transmission coeff. The first-gen.  -ray matrix P(E,E  ) is factorized according to P(E,E  )   (E-E  )  T (E  ) Workshop on Statistical Nuclear Physics and Applications in Astrophysics and Technology, July , Ohio University, Athens, Ohio

Theoretical vs. experimental first-gen. spectra Workshop on Statistical Nuclear Physics and Applications in Astrophysics and Technology, July , Ohio University, Athens, Ohio 50 V 44 Sc

Normalization Workshop on Statistical Nuclear Physics and Applications in Astrophysics and Technology, July , Ohio University, Athens, Ohio Level density: a)Low E: discrete levels b)At S n : neutron res. spacing  Slope and abs. magnitude Gamma-ray transm. coeff.: Total, average radiative width    at S n

Gamma-ray strength functions, medium-mass nuclei Workshop on Statistical Nuclear Physics and Applications in Astrophysics and Technology, July , Ohio University, Athens, Ohio T XL (E  ) = 2  E  2L+1 f XL From transmission coefficient to  -ray strength function:  f XL = T XL (E  ) / (2  E  2L+1 ) Assuming dipole radiation is dominant: f E1+M1 = T exp (E  ) / (2  E  3 )

Comparison with models and photoabsorption data, 44 Sc Workshop on Statistical Nuclear Physics and Applications in Astrophysics and Technology, July , Ohio University, Athens, Ohio Photoabs. cross section to strength function: E1 strength, Kadmenski, Markushev and Furman model: M1 spin flip and E2 isoscalar (Lorentzians)

Is the up-bend structure dependent on excitation energy? Workshop on Statistical Nuclear Physics and Applications in Astrophysics and Technology, July , Ohio University, Athens, Ohio 45 Sc, first.gen. matrix

Gamma-ray strength functions, 116,117 Sn Workshop on Statistical Nuclear Physics and Applications in Astrophysics and Technology, July , Ohio University, Athens, Ohio

Comparison with models and microscopic calculations, 116,117 Sn Workshop on Statistical Nuclear Physics and Applications in Astrophysics and Technology, July , Ohio University, Athens, Ohio

Comparison with photoneutron cross-section data, 116,117 Sn Workshop on Statistical Nuclear Physics and Applications in Astrophysics and Technology, July , Ohio University, Athens, Ohio

Dependence on excitation energy? Workshop on Statistical Nuclear Physics and Applications in Astrophysics and Technology, July , Ohio University, Athens, Ohio 116 Sn, first.gen. matrix

E1 pygmy resonance, unstable Sn isotopes Workshop on Statistical Nuclear Physics and Applications in Astrophysics and Technology, July , Ohio University, Athens, Ohio LAND group, GSI Measuring above the neutron separation energy Adrich et al., PRL 95, (2005) 130 Sn: 7(3)% of TRK sum rule 132 Sn: 4(3)% of TRK sum rule

E1 pygmy resonance, stable Sn isotopes Workshop on Statistical Nuclear Physics and Applications in Astrophysics and Technology, July , Ohio University, Athens, Ohio Nuclear resonance fluorescence ( ,  ’) Govaert et al., PRC 57, p (1998) 116Sn:  B(E1  ) = 0.204(25) e 2 fm 2 124Sn:  B(E1  ) = 0.345(43) e 2 fm 2  ≈ 0.4–0.6% of TRK sum rule Oslo measurements: 116,117 Sn: 16(7) MeV mb (QRPA) 17(7) MeV mb (GLO)  ≈ 1.0(4)% of TRK sum rule

Summary & outlook Workshop on Statistical Nuclear Physics and Applications in Astrophysics and Technology, July , Ohio University, Athens, Ohio  Oslo experiments – level density &  -ray strength function  Medium-mass nuclei: Enhanced strength at low  - ray energies  116,117 Sn: functional form of a “pygmy” resonance  Future experiments: 90,92 Zr, U, Th, Pd...  Upgrade of exp. setup: Silicon Ring (SiRi) particle detection system  NaI  LaBr 3 (Ce)?

Preliminary data, 46 Ti Workshop on Statistical Nuclear Physics and Applications in Astrophysics and Technology, July , Ohio University, Athens, Ohio Level densityGamma-ray strength function