Gamma-ray strength functions obtained with the Oslo method

Transcription

1 Gamma-ray strength functions obtained with the Oslo method Ann-Cecilie Larsen July 8, 2008 DEPARTMENT OF PHYSICS UNIVERSITY OF OSLO

2 Collaborators Oslo: A. Bürger, 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

3 Outline Introduction & motivation Oslo experiments Gamma-ray strength functions, mediummass nuclei Gamma-ray strength functions, 116,117 Sn Summary & outlook

4 Decay probability Fermi s Golden Rule: Statistical decay: λ= 2π f ˆ H i λ ρ(e f ) T(E γ ) Gamma-ray strength function: 2 ρ(e f ) f XL =T XL (E γ )/(2π E γ 2L +1 ) i γ f

5 Resonances Giant electric dipole resonance M1 spin-flip, E2 isoscalar M1 scissors mode, E1 skin oscillation 117 Sn(γ,xn) Lepretre et al NPA 219, 39 (1974). QuickTime and a TIFF (Uncompressed) decompressor are needed to see this picture.

6 are needed to see this picture. Experiments at OCL QuickTime and a TIFF (Uncompressed) decompressor Selected reactions: ( 3 He, 3 He γ), ( 3 He,αγ), (p,p γ), (p,tγ)... Low spin and high intrinsic excitation energy CACTUS: 28 5 x5 NaI (~15% eff.) Eight ΔE-E Si particle telescopes

7 Particle - gamma coincidences NaI(Tl) γ α Si ΔE-E telescope 3 He 45 o Target nucleus

8 Coincidence matrices 44 Sc, unfolded 44 Sc, first-generation γ-rays S n S p

9 Extraction of level density and gamma-ray transmission coeff. The first-gen. γ-ray matrix P(E,E γ ) is factorized according to P(E,E γ ) ρ(e-e γ ) T(E γ )

10 Theoretical vs. experimental first-gen. spectra 50 V 44 Sc

11 Normalization 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 Γ γ at S n

12 Gamma-ray strength functions, medium-mass nuclei From transmission coefficient to γ-ray strength function: T XL (E γ ) = 2π E 2L+1 γ f XL f XL = T XL (E γ ) / (2π E 2L+1 γ ) Assuming dipole radiation is dominant: f E1+M1 = T exp (E γ ) / (2π E γ3 )

13 Comparison with models and photoabsorption data, 44 Sc Photoabs. cross section to strength function: E1 strength, Kadmenski, Markushev and Furman model: M1 spin flip and E2 isoscalar (Lorentzians)

14 Is the up-bend structure dependent on excitation energy? 45 Sc, first.gen. matrix

15 Gamma-ray strength functions, 116,117 Sn

16 Comparison with models and microscopic calculations, 116,117 Sn

17 Comparison with photoneutron cross-section data, 116,117 Sn

18 Dependence on excitation energy? 116 Sn, first.gen. matrix

19 E1 pygmy resonance, unstable Sn isotopes 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

20 E1 pygmy resonance, stable Sn isotopes Nuclear resonance fluorescence (γ,γ ) Govaert et al., PRC 57, p (1998) 116Sn: Σ B(E1 ) = 0.204(25) e 2 fm 2 124Sn: Σ B(E1 ) = 0.345(43) e 2 fm % 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

21 Summary & outlook Oslo experiments level density & γ-ray strength function Medium-mass nuclei: Enhanced strength at low γ- 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)?

22 Preliminary data, 46 Ti Level density Gamma-ray strength function