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 8-11 2008, Ohio University, Athens, Ohio

DEPARTMENT OF PHYSICSUNIVERSITY OF OSLO

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

Workshop on Statistical Nuclear Physics and Applications in Astrophysics and Technology, July 8-11 2008, Ohio University, Athens, Ohio

Outline

Introduction & motivation Oslo experiments Gamma-ray strength functions, medium-

mass nuclei Gamma-ray strength functions, 116,117Sn Summary & outlook

Workshop on Statistical Nuclear Physics and Applications in Astrophysics and Technology, July 8-11 2008, 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 8-11 2008, Ohio University, Athens, Ohio

i

f

€

λ=2π

hf ˆ H i

2

ρ(E f )

€

λ∝ρ(E f ) ⋅T(Eγ )

€

fXL =TXL (Eγ ) /(2π Eγ2L +1)

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 8-11 2008, Ohio University, Athens, Ohio

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117Sn(,xn)Lepretre et alNPA 219, 39(1974).

Experiments at OCL

Workshop on Statistical Nuclear Physics and Applications in Astrophysics and Technology, July 8-11 2008, Ohio University, Athens, Ohio

Selected reactions: (3He,3He’), (3He,), (p,p’), (p,t) ... Low spin and high intrinsic excitation energy CACTUS: 28 5”x5” NaI (~15% eff.) Eight E-E Si particle telescopes

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Particle - gamma coincidences

Workshop on Statistical Nuclear Physics and Applications in Astrophysics and Technology, July 8-11 2008, Ohio University, Athens, Ohio

Si E-E telescope

45o

NaI(Tl)

3He

Target nucleus

Coincidence matrices

Workshop on Statistical Nuclear Physics and Applications in Astrophysics and Technology, July 8-11 2008, Ohio University, Athens, Ohio

44Sc, unfolded 44Sc, first-generation -rays

Sn

Sp

Extraction of level density and gamma-ray transmission coeff.

The first-gen. -ray matrix P(E,E)is factorized according to

P(E,E) ρ(E-E)T(E)

Workshop on Statistical Nuclear Physics and Applications in Astrophysics and Technology, July 8-11 2008, Ohio University, Athens, Ohio

Theoretical vs. experimental first-gen. spectra

Workshop on Statistical Nuclear Physics and Applications in Astrophysics and Technology, July 8-11 2008, Ohio University, Athens, Ohio

50V 44Sc

Normalization

Workshop on Statistical Nuclear Physics and Applications in Astrophysics and Technology, July 8-11 2008, Ohio University, Athens, Ohio

Level density: a) Low E: discrete levelsb) At Sn: neutron res. spacing Slope and abs. magnitude

Gamma-ray transm. coeff.: Total, average radiative width at Sn

Gamma-ray strength functions, medium-mass nuclei

Workshop on Statistical Nuclear Physics and Applications in Astrophysics and Technology, July 8-11 2008, Ohio University, Athens, Ohio

TXL(E) = 2 E2L+1 fXL

From transmissioncoefficient to -ray strength function:

fXL = TXL(E) / (2 E

2L+1)

Assuming dipole radiationis dominant:

fE1+M1 = Texp(E) / (2 E3)

Comparison with models and photoabsorption data, 44Sc

Workshop on Statistical Nuclear Physics and Applications in Astrophysics and Technology, July 8-11 2008, 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 8-11 2008, Ohio University, Athens, Ohio

45Sc, first.gen. matrix

Gamma-ray strength functions, 116,117Sn

Workshop on Statistical Nuclear Physics and Applications in Astrophysics and Technology, July 8-11 2008, Ohio University, Athens, Ohio

Comparison with models and microscopic calculations, 116,117Sn

Workshop on Statistical Nuclear Physics and Applications in Astrophysics and Technology, July 8-11 2008, Ohio University, Athens, Ohio

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

Workshop on Statistical Nuclear Physics and Applications in Astrophysics and Technology, July 8-11 2008, Ohio University, Athens, Ohio

Dependence on excitation energy?

Workshop on Statistical Nuclear Physics and Applications in Astrophysics and Technology, July 8-11 2008, Ohio University, Athens, Ohio

116Sn, first.gen. matrix

E1 pygmy resonance, unstable Sn isotopes

Workshop on Statistical Nuclear Physics and Applications in Astrophysics and Technology, July 8-11 2008, Ohio University, Athens, Ohio

LAND group, GSIMeasuring above the neutron separation energy

Adrich et al., PRL 95, 132501 (2005)130Sn: 7(3)% of TRK sum rule132Sn: 4(3)% of TRK sum rule

E1 pygmy resonance, stable Sn isotopes

Workshop on Statistical Nuclear Physics and Applications in Astrophysics and Technology, July 8-11 2008, Ohio University, Athens, Ohio

Nuclear resonance fluorescence (,’)Govaert et al., PRC 57, p. 2229 (1998)116Sn: B(E1) = 0.204(25) e2 fm2

124Sn: B(E1) = 0.345(43) e2 fm2

≈ 0.4–0.6% of TRK sum rule

Oslo measurements:116,117Sn: 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 8-11 2008, Ohio University, Athens, Ohio

Oslo experiments – level density & -ray strength function

Medium-mass nuclei: Enhanced strength at low -ray energies

116,117Sn: functional form of a “pygmy” resonance Future experiments: 90,92Zr, U, Th, Pd... Upgrade of exp. setup: Silicon Ring (SiRi) particle

detection system NaI LaBr3(Ce)?

Preliminary data, 46Ti

Workshop on Statistical Nuclear Physics and Applications in Astrophysics and Technology, July 8-11 2008, Ohio University, Athens, Ohio

Level density Gamma-ray strength function