Nuclear-structure and nuclear-astrophysics experiments at the

superconducting electron accelerator ELBE

R. Schwengner1, F. Donau1, M. Erhard1, E. Grosse1,2, A. R. Junghans1, K. Kosev1, C. Nair1, N. Nankov1,3,

G. Rusev1, K.D. Schilling1, A.Wagner1

1 Institut fur Kern- und Hadronenphysik, FZ Rossendorf, 01314 Dresden, Germany2 Institut fur Kern- und Teilchenphysik, Technische Universitat Dresden, 01062 Dresden, Germany

3 Institute for Nuclear Research and Nuclear Energy, BAS, 1784 Sofia, Bulgaria

- The bremsstrahlung facility

- Photon-scattering experiments

- Photoactivation experiments

Supported by the Deutsche Forschungsgemeinschaft

Bremsstrahlung facility

at the ELBE accelerator

Accelerator parameters:

• Maximum electron energy:

20 MeV

• Maximum average current:

1 mA

• Micro-pulse rate:

26 or 260 MHz

• Reduction of 26 MHz

by factors of 2 to 256

• Macro pulse

of 0.1 to 36 ms length

with periods of 40 ms to 1 s

1 m

Be-window

dipole

dipole

dipole

radiator

steerer

quadrupoles

quadrupole

electron-beam dump

quartz-windowbeam-

hardener

collimator

polarisationmonitor

target

PbPE

door

Pb

photon-beam dump

Pb

HPGe detector+ BGO shield

experimentalcave

acceleratorhall

clusterdetector

Electron-beam line

Simulation of the flux of photons passing the collimator

0 100 200 300 400 500

Radiator thickness / (mg/cm2)

103

104

105

Nγ w

ithin

5 m

rad

Eγ > 6 MeV

Ee

kin = 12 MeV

Eγ < 6 MeV

Nb radiator

0 10 20 30 40 50 60 70 80 90 100

θ / mrad

0

107

107

dNγ /

sin

θdθ

1.8 mg/cm2

Ee

kin = 12 MeV

18 mg/cm2

Nb radiator

180 mg/cm2

.

.

2

1

Simulations with GEANT4:

• Number of photons produced

by 109 electrons of Ekine = 12

MeV in a cone with an opening

angle of 5 mrad as a function of

the niobium-radiator thickness.

• Number of photons produced by

109 electrons of E kine

= 12 MeV

in niobium radiators of differ-

ent thicknesses as a function

of the angle between electron

beam and photon.

Radiator

Niobium radiators:

• Six radiator foils of 16 mm diameter

mounted on a water-cooled copper rod

• Thicknesses of 2, 3, 4, 5, 7 and 12.5

µm, corresponding to 2 · 10−4 to 10−3

radiation lengths

• Radiator holder can be moved by a DC

motor drive to select a radiator without

breaking the vacuum

Detector setup

Beam dump

E γ = 12 MeV

Photon-beam dump

Pb

Pb

Pb

Pb

PE

GEANT3 simulation with 1000 trajectories

of 12 MeV photons.

About 0.3 % of the photons are scattered back

towards the detectors.

Background measurements

1000 2000 3000 4000 5000 6000 7000 8000Eγ / keV

100

101

102

103

104

105

Num

ber

of c

ount

s

(a)208

Pb, Ee

kin= 9.0 MeV

40K

208Tl

208Pb

11B

11B

1000 2000 3000 4000 5000 6000 7000 8000Eγ / keV

(b) no target, Ee

kin= 9.0 MeV

40K

208Tl

1000 2000 3000 4000 5000 6000 7000 8000Eγ / keV

(c) room background

40K

208Tl

(a) Radiator thickness 4 µm, average beam current 480 µA, 208Pb target of mass 1 g

(b) Radiator thickness 4 µm, average beam current 480 µA, no target

(c) Room background

• Measuring time 400 min

Measurement of the electron energy via photodisintegration of deuterons

0 1000 2000 3000 4000 5000 6000Ep / keV

100

101

102

103

104

105

Num

ber

of c

ount

s

Si−detector spectra

Ee

kin = 9.0 MeV

CD2 target

Ee

kin = 13.2 MeV

6000 8000 10000 12000 14000Eγ / keV

101

102

Pho

ton

flux

/ a.u

.

Schiff

BH

Np/σdis

Ee

kin = 13.2 MeV

Roche

4000 8000 120000

10

20

σ dis/a

.u.

of the deuteronphotodisintegration

Spectra measured with Si detectors of

300 µm thickness during the irradiation

of a deuterated polyethylene film with

bremsstrahlung.

Spectrum of incident photons recalculated

from the proton spectrum and the cross sec-

tion for the disintegration of the deuteron.

σdis: H. Bethe, C. Longmire, Phys. Rev. 77 (1950) 647

Roche: G. Roche et al., Phys. Rev. A 5 (1972) 2403BH: H. Bethe, W. Heitler, Proc. Roy. Soc. A 146 (1934) 83

Schiff: L.I. Schiff, Phys. Rev. 83 (1951) 252

Production of linearly polarised off-axis bremsstrahlung

detector

collimatorradiator

bremsstrahlung cone

e beam- E

E

E

E

E

detector

steering coils

target

Measurement with polarised photons

4000 6000 8000 10000 12000Eγ / keV

0

0.1

0.2

0.3

Pγ

Ee

kin = 14 MeV

4000 5000 6000 7000 8000Eγ / keV

−0.4

−0.2

0

0.2

0.4

0.6

0.8

Asy

mm

etry

208Pb(γ,γ’)

Ee

kin = 14 MeV E1

M1

E2

Degree of polarisation vs. photon energy

as deduced from proton spectra.

Experimental asymmetries of transitions in208

Pb.

Photon scattering from 208Pb

4000 6000 8000 10000 12000 14000 16000 18000Eγ / keV

101

102

103

104

Num

ber

of c

ount

s

208Pb(γ,γ’)

Ee

kin = 17 MeV

Θ = 127o

11B

12C

74Ge

Sn

Nuclear resonance fluorescence

E γ

0

E

Y YγE

Photon scattering

E, J, π, Γ

γ γ ’0 γ ’f

Experimental needs:

• Continuous photon spectrum of high intensity⇒ bremsstrahlung

• Variable end-point energy⇒ tunable electron energy

Experimental observables:

• Energy of the scattered photons → E

• Intensity of the scattered photons → Γ

• Angular distribution of the scattered photons → J

• Polarisation of the scattered photons → π

⇒ Compton polarimeter (E γ < 5 MeV)⇒ polarised bremsstrahlung for higher γ energies

Photon scattering

Iγ(Eγ, Θ) = σs(Ei) · Φγ(Ei) · ε(Eγ) · Nat ·dΩ

4π· W (Eγ, Θ)

σs(Ei)

σs(EB

i)

=Iγ(Eγ, Θ)

Iγ(EBγ , Θ)

·ε(EB

γ )

ε(Eγ)·W (EB

γ , Θ)

W (Eγ, Θ)·NB

at

Nat

·Φγ(E

Bi)

Φγ(Ei)

σs = g

(

πhc

Ei

)2Γ0Γf

Γ; g =

2Ji + 1

2J0 + 1; Γ =

h

τ

B(E1)↑= g B(E1)↓= 2.866 · 10−3 ·

Γ0/meV

(Eγ/MeV)3e2

fm2

B(M1)↑= g B(M1)↓= 0.2598 ·Γ0/meV

(Eγ/MeV)3µ2

N

B(E2)↑= g B(E2)↓= 6201 ·Γ0/meV

(Eγ/MeV)5e2

fm4

Nuclides under investigation in photon-scattering experiments

Z

42

40

38

36

34

48 50 52 54 56 58 N

nuclide Sn E kine

MeV MeV

92Mo 12.7 6.0, 13.2

98Mo 8.6 13.23.3a, 3.8a

100Mo 8.3 7.8, 13.23.2a, 3.4a, 3.8a

88Sr 11.1 9.0, 13.26.8b

90Zr 12.0 7.0, 9.0

a Dynamitron experiment,

PRL 95 (2005) 062501b S-Dalinac experiment,

PRC 70 (2004) 064307

M1 strength in even-even Mo isotopes up to 4 MeV

2.5 3 3.5 4Ex / MeV

0

0.1

0.2

0.3

0.4

0.5

B(M

1) /

µ N

2

92Mo50

Σ B(M1) = 0.56(4) µN

2

2.5 3 3.5 4Ex / MeV

94Mo52

Σ B(M1) = 0.67(7) µN

2

2.5 3 3.5 4Ex / MeV

96Mo54

Σ B(M1) = 0.47(2) µN

2

2.5 3 3.5 4Ex / MeV

0

0.1

0.2

0.3

0.4

0.5

B(M

1) /

µ N

2

98Mo56

Σ B(M1) = 0.74(3) µN

2

2.5 3 3.5 4Ex / MeV

100Mo58

Σ B(M1) = 0.98(4) µN

294Mo: C. Fransen et al.,

PRC 67 (2003) 02430796Mo: C. Fransen et al.,

PRC 70 (2004) 044317

Deformation of even-even Mo isotopes

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

0

10

20

30

40

50

60

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08ε2

γ

0

10

20

30

40

50

60

0.02 0.03 0.04 0.05 0.06 0.07 0.08ε2

γ

0

10

20

30

40

50

60

0.05 0.06 0.07 0.08 0.09 0.1 0.11 0.12 0.13 0.14ε2

γ

0

10

20

30

40

50

60

0.16 0.18 0.2 0.22 0.24ε2

γ

0

10

20

30

40

50

60

0.16 0.18 0.2 0.22 0.24ε2

γ

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

M1 strength in even-even Mo isotopes up to 4 MeV

2.5 3 3.5 4Ex / MeV

0

0.1

0.2

0.3

0.4

0.5

B(M

1) /

µ N

2

92Mo50

Σ B(M1)EXP = 0.56(4) µN

2

Σ B(M1)RPA = 0.21 µN

2

2.5 3 3.5 4Ex / MeV

94Mo52

Σ B(M1)EXP = 0.67(7) µN

2

Σ B(M1)RPA = 0.19 µN

2

2.5 3 3.5 4Ex / MeV

96Mo54

Σ B(M1)EXP = 0.47(2) µN

2

Σ B(M1)RPA = 0.58 µN

2

2.5 3 3.5 4Ex / MeV

0

0.1

0.2

0.3

0.4

0.5

B(M

1) /

µ N

2

98Mo56

Σ B(M1)EXP = 0.74(3) µN

2

Σ B(M1)RPA = 1.26 µN

2

2.5 3 3.5 4Ex / MeV

100Mo58

Σ B(M1)EXP = 0.98(4) µN

2

Σ B(M1)RPA = 1.45 µN

2 Deformed-RPA calculations

Spin-spin interaction

Suppression of spurious modes

F. Donau, PRL 94 (2005) 092503

Photon scattering from 92Mo, 98Mo and 100Mo

Detection limits

pDL = 3

√

(1+ηP

ηB

)B

R. Jenkins, R.W. Gould, D. Gedcke,

Quantitative X-Ray Spectrometry,

Marcel Dekker, New York, 1981

Angular distributions of transitions in 92Mo, 98Mo and 100Mo

Expected values:

L = 1:Iγ(90)/Iγ(127) = 0.73

L = 2:Iγ(90)/Iγ(127) = 2.28

E1 strength in 92Mo, 98Mo and 100Mo

The summed B(E1) strength decreases with increasing N/Z.

E1 strength in even-even Mo isotopes and in even-even N = 50 isotones

49 50 51 52 53 54 55 56 57 58 59N

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

Σ B

(E1)

/ (

e2 fm2 )

E > 5 MeV

92Mo

98Mo

100Mo

Peaks

RPA

34 35 36 37 38 39 40 41 42 43 44Z

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

Σ B

(E1)

/ (

e2 fm2 )

E > 5 MeV

86Kr50

88Sr50

90Zr50

92Mo50

The summed B(E1) strength decreases

with increasing N/Z.The summed B(E1) strength increases

with increasing N/Z.

Unresolved strength in the continuum

fL(E ) =1

∆

∑

∆

Γ0

E 3

E1 strength in even-even Mo isotopes and in even-even N = 50 isotones

49 50 51 52 53 54 55 56 57 58 59N

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

Σ B

(E1)

/ (

e2 fm2 )

E > 5 MeV

92Mo

98Mo

100Mo

Peaks

Peaks + Cont.

RPA

34 35 36 37 38 39 40 41 42 43 44Z

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

Σ B

(E1)

/ (

e2 fm2 )

E > 5 MeV

86Kr50

88Sr50

90Zr50

92Mo50

The summed B(E1) strength decreases

with increasing N/Z.The summed B(E1) strength increases

with increasing N/Z.

Statistical analysis

Level-spacing distributions for levels with

E > 4 MeV. The drawn lines represent

Wigner distributions.

Distributions of reduced level widths for

levels with E > 4 MeV. The drawn lines

represent Porter-Thomas distributions.

Problem of feeding and branching

0Γ0

Γ1

Ee

E ,x Γ

branchingΓf

feeding

Feeding from high-lying states:

→ Determination of the widths Γ from mea-

surements at various energies

Branching to low-lying states:

→ Correction of the strength distribution f L

by using statistical methods:

The branching ratios Γf /Γ are calculated by

means of Monte Carlo simulations of the de-

cays. Intensities are corrected with calculated

branching ratios.

A level scheme of J = 1 and 2 states is con-

structed with a model using the Wigner level-

spacing distribution and level densities given

by the backshifted Fermi-gas model. The par-

tial decay width is given by:

Γif =∑

X ,L

y 2

XL(Ei − Ef )

2L+1fXL(Ei − Ef )

ρ(Ei , Jπi)

Reconstructed dipole-strength distributions in 92Mo, 98Mo and 100Mo

(γ, n) - experiment:

H. Beil et al.,

NPA 227 (1974) 427

(γ, n) and (γ, p) - theory:

T. Rauscher and

F.-K. Thielemann,

ADNDT 88 (2004) 1

E < Sn : fL(E ) =1

3 (πhc)2∆

∑

i

∫

σcorr

idE

Ei

E > Sn : fL(E ) =1

3 (πhc)2

⟨

σγ,n(E )

E

⟩

Accumulative E1 strength

Photon scattering from 88Sr and 90Zr

4000 5000 6000 7000 8000 9000Eγ / keV

0

100

200

300

400

500

600

700

Num

ber

of c

ount

s

Ee

kin = 9 MeV

θ = 1270

88Sr(γ,γ’)

11B

11B

11B

4000 5000 6000 7000 8000 9000Eγ / keV

0

100

200

300

400

500

600

700

Num

ber

of c

ount

s

Ee

kin = 9 MeV

θ = 1270

90Zr(γ,γ’)

11B

11B

11B

11B

Summary

Dipole-strength distributions of even-even Mo isotopes have been studied up to therespective neutron-separation energies at the photon-scattering facility of the ELBE ac-celerator.

The M1 strength distributions up to Ex < 4 MeV fragment with increasing deformation,which is qualitatively described by deformed-RPA calculations.

The total E1 strength above 5 MeV decreases with increasing N/Z of the Mo isotopes,which is reproduced by deformed-RPA predictions.

The total E1 strength above 5 MeV increases with increasing N/Z for N = 50 isotones.

The high level density above 5 MeV in connection with feeding by high-lying states andbranchings to low-lying states require new techniques in order to reconstruct the originaldipole-strength distributions.

Simulations of the decay of excited levels deliver an estimate of the branchings. Thereconstructed dipole-strength distributions fit the low-energy tails of the giant dipoleresonances.

P-nuclei and the γ-process

• 35 p-nuclei from 74Se to 196Hg cannot be produced in the s- or r-processes.

• They are produced and destructed by thep- or γ-process at temperatures ofT ≈ (2 - 3) · 109 K.

• The γ-process comprises (γ,n), (γ,p)and (γ, α) reactions starting froms- and r-process seed nuclei.

• Abundances of p-nuclei are 10to 1000 times smaller thanthose of their neutron-richisotopes - except forMo and Ru.

Calculation of abundances of p-nuclei

• The abundances of Mo and Ruare underestimated by networkcalculations.

• Are the reaction rates correct?

⇒ Study of the photodissociationof 92Mo

M. Arnould, S. Goriely,

PR 384 (2003) 1

Setup for photoactivation experiments

caveBremsstrahlung

counting setupLow−levelAccelerator

hall

Photoactivationtarget

Electronbeam dump(rotated by 90°)

collimatorAluminium

targetactivationPhoto−

Leadwall

HPGedetectors

housingLead

beam dumpPhoton

PEblocks

castleLead

Photoactivatedtarget

detectorHPGe

Pneumatic delivery

Steel Iron

Electron beam

Graphite

Dewar

Depot

Photoactivation of92Mo

Electron capture (EC) and decay

9142Mo

−652.9 keV1/2

64.6 s

9/2+

15.5 min

0 keV

48.2%

9/2+

680 y

0 keV

−104.5 keV1/2

60.86 d

Nb4191

0.58%

Zr9140

1/2+0.17 ps

1204.8 keV

5/2+stable

0 keV

100% (2.9%) EC 93%

+β( <

0.2

%)

EC 2.9%

EC 4

%

A = 92A = 91Mo42

92

0+stable

0 keV

pS 7456 keV

12672 keVSn

9

3%

+β

+β 5

0%

not t

o sc

ale

Photoactivation process

γ+

transitions

β

Nact(Ee) = Ntar ·

∫Ee

Ethr

σ(γ,x) Φγ(E , Ee) dE

Nact(Ee) = Iγ(Eγ) · ε−1(Eγ) · p

−1(Eγ) · κcorr

Activation yield

100Mo(γ,n)

92Mo(γ,p) + (γ,n)

92Mo(γ, α)

Activation yield of the 197Au(γ, n) re-action. The yield is normalised to thenumber of 197Au atoms and to the ab-solute photon flux at Eγ = 8917 keV.

Activation yields of Mo isotopes normalisedto the activation yield of the 197Au(γ, n) re-action.

Open symbols:

T. Rauscher and F.-K. Thielemann,

ADNDT 88 (2004) 1

Summary

Endpoint energy derived from the photodissociation of 2H.

Photon-flux distribution deduced from photon scattering from states with known widthsin 11B.

Determination of the photon flux in the electron-beam dump by means of gold targets.

Determination of the activation yield of the 92Mo(γ, p)91mNb reaction up to Sn(92Mo)

via the decay 91mNb(EC) → 91Zr (E γ = 1205 keV).

Good agreement of the activation yield with theoretical predictions.

92Mo(γ, α)88Zr observed for the first time.