1. Investigating the Fission Barrier Landscape in Heavy Elements relevant for the r-Process
P.G. Thirolf, D. Habs et al., LMU München
Outline: - introduction to potential energy landscape in actinide region
- present activities of the group:
study of the multiple-humped potential energy landscape
- planned projects within Universe-Cluster:
i) fission barrier systematics from 2nd /3rd minimum in actinides
ii) fission barriers of heavy nuclei from single-particle states
Universe Cluster, Area G Kickoff, 16. Januar 2007

2. G: Heavy Element Enrichment of the Universe
courtesy: R. Diehl
Universe Cluster, Area G Kickoff, 16. Januar 2007

3. r-Process Path in Heavy Element Region
b-delayed fission:
direct access to r-process path out of reach
extrapolations of nuclear models require experimental data
knowledge of fission barriers (and interplay with b decay) is crucial
fission barriers: - determine end of chart of nuclides
- change of shell correction energy by 1 MeV:
fission lifetime changed by 105
Universe Cluster, Area G Kickoff, 16. Januar 2007

4. Second minimum in the potential energy surface
(macroscopic) liquid drop model
(microscopic)
shell corrections (Strutinsky, 1967) +
deformation
1. minim minimum
structures in (prompt)
fission cross section
Double-humped
fission barrier:
spectroscopy of
excited states
shape isomer
Universe Cluster, Area G Kickoff, 16. Januar 2007

5. Second minimum in the potential energy surface
deformed harmonic oscillator:
fluctuation of level density:
- with number of nucleons
- with deformation
-> strong shell gaps: potential minima
Strutinsky method:
- derive shell correction energy from
single-particle level density
fission barriers reflect basic nuclear
properties: Binding energy
Shell correction energy
nucleon number
island of fission isomers:
so far 33 fission isomers are known
- actinide region: Z= 92-97
- lifetimes: ~5ps – 14ms
2. min 3. min
SD HD
experimental challenge: very small isomeric cross sections (~mb)
large background (~105) from prompt fission
Universe Cluster, Area G Kickoff, 16. Januar 2007

6. Multiple-humped fission barrier landscape
Calculated potential energy surface (Moeller + Nix, 1973):
mass asymmetry
~ deformation
Min. 2. Min. 3. Min.
(2:1) (3:1)
- mass asymmetric (hyperdeformed) 3. minimum predicted
- exp. verification/depth of 3. minimum unclear for a long time
- evidence for only shallow 3. minimum in Th-isotopes
Universe Cluster, Area G Kickoff, 16. Januar 2007

7. Calculated potential barriers in light actinide nuclei
deep third minima predicted !
Trend:
- Outer barrier EB:
decrease with increasing Z
- Inner barrier EA:
decrease with decreasing Z
> longterm discrepancy with
exp. findings:
‘Thorium anomaly’
116 ns
195 ns
34ns/
37ps
3.8 ns
6/0.6 ns
calculations: Howard et al, Cwiok et al.
Universe Cluster, Area G Kickoff, 16. Januar 2007

8. Experimental Infrastructure
Tandem accelerator (MLL):
- light/heavy ion particle beams
Q3D: magnet-
spectrograph (MLL):
high-resolution
particle spectroscopy
Radioactive target laboratory (LMU):
- high-quality actinide targets
Universe Cluster, Area G Kickoff, 16. Januar 2007

9. Spectroscopy in the superdeformed 2. Minimum
‘prototype‘ shape isomer: 240fPu (t1/2= 3.8 ns)
Ground state rotational band
Detailed level scheme
Multiphonon excitations
and 2qp states
conversion electrons
(1972, Specht et al.)
g-rays + conversion electrons
(2000, Pansegrau et al.)
(2001, Gassmann et al.)
Transmission resonances
(2001, Hunyadi et al.)
for 240fPu now more spectroscopic information available in 2. minimum
than in first minimum !
Universe Cluster, Area G Kickoff, 16. Januar 2007

10. Transmission Resonance Spectroscopy
- high-lying (multi-phonon) states near barrier top:
(fission)-lifetimes too short for direct spectroscopy:
fs (10-15s) – as (10-18s)
- experimental approach: transmission resonance spectroscopy
- transmission resonances (in prompt fission probability):
compound states in 1. minimum couple to collective
states in 2. (and 3.) minimum
(prompt) fission
probability
-> method to deduce fission barrier parameters
Universe Cluster, Area G Kickoff, 16. Januar 2007

11. Transmission resonance experiments
Light-ion induced reactions, e.g.: 233,235U(d,pf)234,236U, 239Pu(d,pf)240Pu
Ed ≈ MeV
Munich Q3D magnet spectrograph:
Proton detection:
- focal plane detector for
light ions
- high resolution (~5 keV)
Fission fragment detection:
- 2 position sensitive PPACs
- trigger, angular correlation
in collaboration with group of A. Krasznahorkay (Debrecen/Hungary)
Universe Cluster, Area G Kickoff, 16. Januar 2007

12. Spectroscopy in hyperdeformed 3. Minimum
235U(d,pf)236U:
- (rotational-) substructure of transmission resonances resolved
- resonances around 5.3 MeV: known as hyperdeformed from earlier exp.
resonances below 5.2 MeV: previously interpreted as originating from
2. minimum (analog to 240Pu)
Universe Cluster, Area G Kickoff, 16. Januar 2007

13. Analysis of Transmission Resonances: 236U
resonance around 5.1 MeV:
statistical level density
analysis: -> depth of 3. minimum
fit with rotational bands:
free parameters:
band head energy, abs. intensity,
K value
J=5 states
Result: EIII = 2.7(4) MeV
-> deep 3. Minimum (EII=2.81 MeV !):
in agreement with theory
and own results in 234U
consistent fit:
hyperdeformed resonance
Universe Cluster, Area G Kickoff, 16. Januar 2007

14. Triple-Humped Fission Barrier in 236U
Fission barrier parameters:
inner barrier: from HD character of 5.1 MeV resonance:
EA= 5.15(20) MeV
outer barrier: from saturation of prompt fission probability:
EB= 6.1(1) MeV
deep third minimum established
M. Csatlos, PT et al., Phys. Lett. B 615 (2005) 175.
Universe Cluster, Area G Kickoff, 16. Januar 2007

15. Resolving the ‘Thorium Anomaly‘ of Fission Barriers
- outer fission barrier drastically decreases with increasing Z
- older exp. data largely overestimated the inner barrier height (Th, Ra)
- reason: assumption of only a shallow 3. minimum
- new data lead to significantly lowered inner barrier heights
Universe Cluster, Area G Kickoff, 16. Januar 2007

16. Resolving the ‘Thorium Anomaly‘ of Fission Barriers
‚Universe‘ project:
confirm and extent
barrier systematics
- outer fission barrier drastically decreases with increasing Z
- older data largely overestimated the inner barrier height (Th, Ra)
- reason: assumption of only a shallow 3. minimum
- new data lead to significantly lowered inner barrier heights
Universe Cluster, Area G Kickoff, 16. Januar 2007

17. ‚Universe‘ Project: Fission Barrier via Single-Particle Level Density
experimental goal: - first experimental identification of single-particle
states in largely deformed actinides
-> determination of fission barrier from level density
so far single-particle models have not been experimentally tested at
large deformations (shape isomers: b2=0.6)
- no single-particle level in second minimum has been identified so far !
- contradicting theoretical predictions:
237fPu:
Universe Cluster, Area G Kickoff, 16. Januar 2007

18. Spectroscopy of 239fPu 239fPu: double isomer in 2. minimum: 2.6 ns
8 ms
239Pu f
shape isomer
g-spectroscopic study of single-particle structure in 239fPu:
- rotational band (rigid rotor) structure band head spins, parities
- consistent with known 2qp states in 240fPu ?
MINIBALL:
highly efficient, high-resolution
Ge-detector array for g-spectroscopy
derive fission barrier properties from single-particle state level density
but: highly-converted low-energy transitions
> conversion electron data required
Universe Cluster, Area G Kickoff, 16. Januar 2007

19. Conversion Electron Spectroscopy in 239fPu
- advantage in 239fPu: electron data are already known:
238U(a,3n)239Pu (Ea = 33 MeV)
level scheme:
H. Backe, DH et al., PRL 42 (1979) 490
-> basis for conclusive interpretation of g-data is already existing
Universe Cluster, Area G Kickoff, 16. Januar 2007

20. Conclusion longterm experience:
- spectroscopy in the 1st , 2nd and 3rd minimum of actinides
- measuring fission barriers of largely-deformed heavy elements
g-spectroscopy of 239fPu will allow for a first-time identification
of single-particle states in second potential well
important input for improvement of theoretical single-particle
models in 2nd minimum
leading to improved fission barrier predictions (using Strutinsky
method)
Universe Cluster, Area G Kickoff, 16. Januar 2007

21. Collaboration Inst. Nucl. Research Debrecen/Ungarn LMU München
TU München
T. Faestermann
H.-F. Wirth
A. Krasznahorkay
M. Csatlos
L. Csige
J. Gulyas
M. Hunyadi
Z. Mate
D. Habs G. Graw
H.J. Maier R. Hertenberger
T. Morgan O. Schaile
W. Schwerdtfeger J. Szerypo PT
IKP Köln
P. Reiter T. Striepling
B. Bruyneel N. Warr
Universe Cluster, Area G Kickoff, 16. Januar 2007