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Ronald Schwengner Institut für Strahlenphysik www.fzd.de Mitglied der Leibniz-Gemeinschaft

GAMMA-RAY STRENGTH-FUNCTION MEASUREMENTS AT ELBE

R. Schwengner1, G. Rusev1,2, N.Tsoneva3, N. Benouaret1,4, R. Beyer1, F.Donau1, M. Erhard1, S. Frauendorf1,5,

E. Grosse1,6, A. R. Junghans1, J. Klug1, K.Kosev1, H. Lenske3, C.Nair1, K.D. Schilling1, A.Wagner1

1 Institut fur Strahlenphysik, Forschungszentrum Dresden-Rossendorf, 01314 Dresden, Germany2 Dept. of Physics, Duke University and Triangle Universities Nuclear Laboratory, Durham, NC 27708

3 Institut fur Theoretische Physik, Universitat Gießen, 35392 Gießen, Germany4 Faculte de Physique, Universite des Sciences et de la Technologie d’Alger, 16111 Bab-Ezzouar, Algerie

5 Department of Physics, University of Notre Dame, Notre Dame, IN 465566 Institut fur Kern- und Teilchenphysik, Technische Universitat Dresden, 01062 Dresden, Germany

- The bremsstrahlung facility

- Photon-scattering experiments

- Data analysis and results

- QRPA and QPM calculations

- Photoactivation experiments

Supported by Deutsche Forschungsgemeinschaft

Ronald Schwengner Institut für Strahlenphysik Forschungszentrum Dresden-Rossendorf www.fzd.de

Dipole strength close to the particle-separation energy

Understanding of astrophysicalprocesses:

– Influence on (γ,n) reactionrates in the p-process.

– Influence on (n,γ) reactionrates in the s-process.

Studies for transmutation:

– Analysis of (n,γ) reactions.

Open problems:

– Precise knowledge of the E1strength on the low-energy tailof the Giant Dipole Resonance.

– Properties of the E1 strengthfunctions at varying proton andneutron numbers: shell effects,deformation etc.

5 6 7 8 9 10 11 12 13 14 15 16 17 18Ex (MeV)

(γ,n)

Open problems:

5 6 7 8 9 10 11 12 13 14 15 16 17 18Ex (MeV)

(γ,n)

Lorentz E0 = 16.8 MeVΓ = 4.0 MeV

Open problems:

5 6 7 8 9 10 11 12 13 14 15 16 17 18Ex (MeV)

(γ,n)

RIPL−2

Open problems:

5 6 7 8 9 10 11 12 13 14 15 16 17 18Ex (MeV)

(γ,n)

RIPL−2

The radiation source ELBE

Electron Linear accelerator of high Brilliance and low Emittance

The bremsstrahlung facility at the electron accelerator ELBE

R.S. et al., NIM A 555 (2005) 211

Accelerator parameters:

• Maximum electron energy:

18 MeV

• Maximum average current:

• Micro-pulse rate:

13 MHz

• Micro-pulse length:

≈ 5 ps1 m

Be-window

dipole

radiator

steerer

quadrupoles

quadrupole

electron-beam dump

quartz-windowbeam-

hardener

collimator

polarisationmonitor

target

photon-beam dump

HPGe detector+ BGO shield

experimentalcave

acceleratorhall

clusterdetector

Detector setup

Nuclides under investigation in photon-scattering experiments

48 50 52 54 56 58 N

nuclide Sn Ekine

MeV (MeV) ELBE

92Mo 12.7 6.0a, 13.2b,c,d

94Mo 9.7 13.2d

96Mo 9.2 13.2d

98Mo 8.6 (3.3, 3.8)a,e, (8.5, 13.2)b,c,d

100Mo 8.3 (3.2, 3.4, 3.8)a, (7.8, 13.2)b,c,d

90Zr 12.0 7.0, 9.0, 13.289Y 11.5 7.0f, 9.5, 13.288Sr 11.1 6.8g, (9.0, 13.2, 16.0)h

87Rb 9.9 4.0i, 13.286Kr 9.9 11.5

a G. Rusev et al., PRC 73 (2006) 044308b R. Schwengner et al., NPA 788 (2007) 331cc G. Rusev et al., PRC 77 (2008) 064321d A. Wagner et al., JPG 35 (2008) 014035e G. Rusev et al., PRL 95 (2005) 062501f J. Reif et al., NPA 620 (1997) 1g L. Kaubler et al., PRC 70 (2004) 064307h R. Schwengner et al., PRC 76 (2007) 034321i L. Kaubler et al., PRC 65 (2002) 054315

Problem of feeding and branching

E ,x Γ

branchingΓf

Measured intensity of a γ transition:

Iγ(Eγ, Θ) = Is(Ex) Φγ(Ex) ǫ(Eγ) Nat W (Θ) ∆Ω

Scattering cross section integral:

σγγ dE =2Jx + 1

2J0 + 1

Absorption cross section:

σγ = σγγ

E1 strength:

B(E1) ∼ Γ0/E 3

E ,x Γ

branchingΓf

feeding

Measured intensity of a γ transition:

Iγ(Eγ, Θ) > Is(Ex) Φγ(Ex) ǫ(Eγ) Nat W (Θ) ∆Ω

Scattering cross section integral:

σγγ dE =2Jx + 1

2J0 + 1

Absorption cross section:

σγ = σγγ

E1 strength:

B(E1) ∼ Γ0/E 3

Absolute detector efficiency and photon flux

0 2000 4000 6000 8000 10000Eγ (keV)

4000 6000 8000 10000 12000Eγ (keV)

Φγ (

kin = 13.2 MeV

Absolute efficiency of two detectors at 127 de-

duced from 22Na, 60Co, 133Ba, (filled circles) and

simulated with GEANT3 (solid line). Relative ef-

ficiencies deduced from 56Co (open circles), 11B

(open triangles) and 16O (open square).

Absolute photon flux deduced from transitions in11B using the calculated efficiency shown in the left

panel and relative photon flux calculated according

to G. Roche et al.

Code by E. Haug, Rad. Phys. Chem. 77 (2008) 207.

Unresolved strength in the continuum

2 3 4 5 6 7 8 9 10 11 12 13 14Eγ (MeV)

spectrum

backgroundatomic

3 4 5 6 7 8 9 10 11 12 13

Ex (MeV)

σ γγ ’ (

peaks + cont.

∆=0.2 MeV

Experimental spectrum of 90Zr (corrected for room

background, detector response, efficiency, measur-

ing time) and simulated spectrum of atomic back-

ground.

Scattering cross sections in 90Zr averaged over en-

ergy bins of 0.2 MeV, not corrected for branch-

ing, derived from the difference of the experimen-

tal spectrum and the atomic background (triangles)

and from the resolved peaks only (circles).

Correction of the strength function by using statistical methods (G. Rusev, Dissertation):

⇒ Monte Carlo simulations of γ-ray cascades from groups of levels in 100 keV bins overthe whole energy range.

⇒ Level scheme of J = 0, 1 and 2 states constructed using: Backshifted Fermi-Gas Model with level-density parameters given in:

T. v. Egidy, D.Bucurescu, PRC 72 (2005) 044311

Wigner level-spacing distributions

⇒ Partial decay widths calculated by using: Photon strength functions approximated by Lorentzian parametrisations.– E1: parameters determined from a fit to (γ,n) data– M1: global parametrisation of M1 spin-flip resonances– E2: global parametrisation of E2 isoscalar resonances

(www-nds.iaea.org/RIPL-2)

Porter-Thomas distributions of decay widths.

⇒ Feeding intensities subtracted and intensities of g.s. transitions corrected with calculatedbranching ratios Γ0/Γ .

Simulations of γ-ray cascades

0 2 4 6 8 10Eγ (MeV)

I γ (ar

Ground−statetransitions

Branching transitions

6 7 8 9 10 11 12Ex (MeV)

b 0∆ (%

Simulated intensity distribution of transitions de-

populating levels in a 100 keV bin around 9 MeV.

⇒ Subtraction of intensities of branching transi-

tions.

Distribution of branching ratios b0 = Γ0/Γ versus

the excitation energy as obtained from the simula-

tions of γ-ray cascades for 90Zr.

⇒ Estimate of Γ0 and σγ.

Test of simulated branching ratios

6 7 8 9 10 11 12Ex (MeV)

b 0∆ (%

Measurement with monochromatic photons at

HIγS (∆E ≈ 200 keV).

Black: Population of 0+

2neglected.

Red: Assumption that b0+

G. Rusev, A.P. Tonchev et al., priv. comm.

Distribution of branching ratios b0 = Γ0/Γ versus

the excitation energy as obtained from the simula-

tions of γ-ray cascades for 90Zr.

Test of E1 strength functions

4 6 8 10 12 14 16100

Lorentzianδ = 0

σ γ (m

Constant

σ γ (m

Ex (MeV)

Lorentzianδ = 1

σ γ (m

σγ(Ex) =2STRK

E2x Γi(Ex)

i− E 2

x )2 + E 2x Γ 2

Γi(Ex) = ΓS · (Ex/Ei)δ; ΓS = 4 MeV;

f1 = σγ/[3(πhc)2Eγ]

STRK =

σγ(E ) dE = 60NZ

AMeV mb

Ei = hω0

1 −2

3ǫ2 cos

γ −

Absorption cross section in 90Zr

5 6 7 8 9 10 11 12 13 14 15 16 17 18Ex (MeV)

σ γ (m

(γ,γ’)uncorr

Present (γ, γ) data

5 6 7 8 9 10 11 12 13 14 15 16 17 18Ex (MeV)

σ γ (m

(γ,γ’)corr

(γ,γ’)uncorr

5 6 7 8 9 10 11 12 13 14 15 16 17 18Ex (MeV)

σ γ (m

(γ,γ’)corr

(γ,p)

(γ, p) calculated

ADNDT 88 (2004) 1

5 6 7 8 9 10 11 12 13 14 15 16 17 18Ex (MeV)

σ γ (m

(γ,γ’)corr

(γ,p) (γ,n)

(γ, p) calculated

ADNDT 88 (2004) 1

(γ, n) data

PR 162 (1967) 1098

5 6 7 8 9 10 11 12 13 14 15 16 17 18Ex (MeV)

σ γ (m

+ (γ, p) + (γ, n) data

5 6 7 8 9 10 11 12 13 14 15 16 17 18Ex (MeV)

σ γ (m

+ (γ, p) + (γ, n) data

Lorentz curve:

E0 = 16.8 MeV

Γ = 4.0 MeVπ

2σ0Γ = 60 NZ

AMeV mb

5 6 7 8 9 10 11 12 13 14 15 16 17 18Ex (MeV)

σ γ (m

+ (γ, p) + (γ, n) data

Lorentz curve:

E0 = 16.8 MeV

Γ = 4.0 MeVπ

2σ0Γ = 60 NZ

AMeV mb

RIPL-2(S. Goriely, E. Khan,NPA 706 (2002) 217)

5 6 7 8 9 10 11 12 13 14 15 16 17 18Ex (MeV)

σ γ (m

+ (γ, p) + (γ, n) data

Lorentz curve:

E0 = 16.8 MeV

Γ = 4.0 MeVπ

2σ0Γ = 60 NZ

AMeV mb

Woods-Saxon basis

Γ = 3.2 MeV

Absorption cross section in 88Sr

6 7 8 9 10 11 12 13 14 15 16 17 18Ex (MeV)

σ γ (m

+ (γ, n) data

Lorentz curve:

E0 = 16.8 MeV

Γ = 4.0 MeVπ

2σ0Γ = 60 NZ

AMeV mb

Woods-Saxon basis:

Γ = 3.2 MeV

Absorption cross section in 89Y

5 6 7 8 9 10 11 12 13 14 15 16 17 18Ex (MeV)

σ γ (m

+ (γ, p) + (γ, n) data

Lorentz curve:

E0 = 16.8 MeV

Γ = 4.0 MeVπ

2σ0Γ = 60 NZ

AMeV mb

Woods-Saxon basis:

Γ = 3.2 MeV

One-phonon QPM calculations for 88Sr and 90Zr

Transition densities of neutrons and protons:

0 5 10−0.15

−0.05

r2 ρ(r)

−1 )

−0.15

−0.05

0 5 10

r (fm)0 5 10

−0.5

0.588Sr

Ex < 9 MeV Ex = 9 − 9.5 MeV Ex = 9 − 22 MeVEnergy range below 9 MeV:

⇒ Oscillations at the surface

by neutrons only

⇒ Indication for a PDR

Absorption cross sections in Mo isotopes

σγ(Ex) =2STRK

E2x Γi(Ex)

i− E 2

x )2 + E 2x Γ 2

Γi(Ex) = ΓS · (Ex/Ei)δ; ΓS = 4 MeV; δ ≈ 0

STRK =

σγ(E ) dE = 60NZ

AMeV mb

Ei = hω0

1 −2

3ǫ2 cos

γ −

Absorption cross sections in Mo isotopes

σγ(Ex) =2STRK

E2x Γi(Ex)

i− E 2

x )2 + E 2x Γ 2

Γi(Ex) = ΓS · (Ex/Ei)δ; ΓS = 4 MeV; δ ≈ 0

STRK =

σγ(E ) dE = 60NZ

AMeV mb

Ei = hω0

1 −2

3ǫ2 cos

γ −

Comparison with other experiments

(γ, n) data

H. Beil et al., NPA 227 (1974) 427

(3He,3He’γ) data

M. Guttormsen et al., PRC 71 (2005) 044307

(n, γ) data

J. Kopecky and M. Uhl, Bologna 1994

QRPA calculations for deformed nuclei

Hamiltonian for 1– states:

- Nilsson or Woods-Saxon mean field plus monopole pairing- isoscalar and isovector dipole-dipole and octupole-octupole interactionsF. Donau, PRL 94 (2005) 092503

F. Donau et al., PRC 76 (2007) 014317

Total energy as a function of the quadrupole deformation ε2 and the triaxiality γ:

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08ε2

0.02 0.03 0.04 0.05 0.06 0.07 0.08ε2

0.05 0.06 0.07 0.08 0.09 0.1 0.11 0.12 0.13 0.14ε2

0.16 0.18 0.2 0.22 0.24ε2

92Mo5094Mo52

96Mo5498Mo56

100Mo58

ε2 = 0.0 ε2 = 0.02 ε2 = 0.10 ε2 = 0.18 ε2 = 0.21

γ = 60 γ = 37 γ = 32

TAC model with shell-correction method:

S. Frauendorf, NPA 557 (1993) 259c, NPA 667 (2000) 115

QRPA calculations in a deformed basis for Mo isotopes

4 5 6 7 8 9 10 11 12 130

92Mo 94Mo 96Mo 98Mo 100Mo

Experiment

QRPA with WS

Ex (MeV)

Σ (Ex) =

Ex∑i

σγ(Ei) ∆E

ΣTRK =

∫∞

σγ(E ) dE = 60NZ

AMeV mb

Summary

Study of dipole-strength distributions at high excitation energy and high level density viaphoton scattering.

Comparison of measured spectrum with calculated atomic background: 30 – 40% of thetotal dipole strength in resolved peaks and 70 – 60% in continuum.

Simulations of statistical γ cascades: Estimate of intensities of inelastic transitions.

Deduced σγ connect smoothly with (γ,n) data.

⇒ (i) Correct determination of σγ up to the (γ,n) threshold.(ii) Information on the photoabsorption cross section over the whole energy range fromlow excitation energy up to the GDR.(iii) Observation of extra strength in the range from 6 to 12 MeV – not described incurrent approximations of dipole-strength functions.

⇒ QPM calculations:(i) Qualitative description of the observed extra strength.(ii) Extra strength below 9 MeV caused by oscillations of the excessive neutrons.(iii) Increase of dipole strength below the neutron-threshold toward the heavier Moisotopes is correlated with increasing deformation.

Photodissociation

Photodissociation reactions:

• (γ, n)

• (γ, p)

• (γ, α)

Method: photoactivation

• (A, Z ) + γ ⇒ (A, Z − 1 ) + p

• Measure decay rate of (A, Z − 1 )

Nact(Ee) = Ntar ·

σ(γ,x) Φγ(E , Ee) dE

Nact(Ee) = Iγ(Eγ) · ε−1(Eγ) · p

−1(Eγ) · κcorr

Photoactivation of92Mo

Electron capture (EC) and decay

9142Mo

−652.9 keV1/2

64.6 s

15.5 min

−104.5 keV1/2

60.86 d

Nb4191

Zr9140

1/2+0.17 ps

1204.8 keV

5/2+stable

100% (2.9%) EC 93%

+β( <

EC 2.9%

A = 92A = 91Mo42

0+stable

pS 7456 keV

12672 keVSn

Photoactivation process

transitions

Nact(Ee) = Ntar ·

σ(γ,x) Φγ(E , Ee) dE

Nact(Ee) = Iγ(Eγ) · ε−1(Eγ) · p

−1(Eγ) · κcorr

Setup for photoactivation experiments

caveBremsstrahlung

counting setupLow−levelAccelerator

Photoactivationtarget

Electronbeam dump(rotated by 90°)

collimatorAluminium

targetactivationPhoto−

Leadwall

HPGedetectors

housingLead

beam dumpPhoton

PEblocks

castleLead

Photoactivatedtarget

detectorHPGe

Pneumatic delivery

Steel Iron

Electron beam

Graphite

Activation yield

Activation yields of Mo isotopes normalised to theactivation yield of the 197Au(γ, n) reaction.

Solid lines: NON-SMOKER,

T. Rauscher and F.-K. Thielemann,

ADNDT 88 (2004) 1

Dashed lines: TALYS,

A. Koning et al.,

AIP Conf. Proc. 769 (2005) 1154

Activation yield

144Sm(γ,n)143Sm

144Sm(γ,n)143mSm

144Sm(γ, α)140Nd

Activation yields of 144Sm normalised to theactivation yield of the 197Au(γ, n) reaction.

Solid lines: NON-SMOKER,

T. Rauscher and F.-K. Thielemann,

ADNDT 88 (2004) 1

Dashed lines: TALYS,

A. Koning et al.,

AIP Conf. Proc. 769 (2005) 1154

Activation yield – low-background measurement

Spectra of the decay of140Nd →

140Pr → 140Cefollowing the144Sm(γ, α) reaction.

Measured in ELBE building

Measured in underground lab“Felsenkeller” in Dresden

Summary

Photodissociation of Mo isotopes and of 144Sm studied via photoactivation at the ELBEaccelerator.

Determination of the photon flux in the electron-beam dump by means of the197Au(γ, n) reaction.

Measurement of weak decay rates in an underground lab.

92Mo(γ, α)88Zr and 144Sm(γ, α)140Nd reactions observed for the first time at astrophys-ically relevant energies.

Rough agreement with predictions of Hauser-Feshbach models for (γ, n) and (γ, p) re-actions. Predictions differ for (γ, α) reactions.

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