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

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

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

Second minimum in the potential energy surface (macroscopic) liquid drop model (microscopic) shell corrections (Strutinsky, 1967) + deformation 1. minim. 2. 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

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

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

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

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

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

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

Transmission resonance experiments Light-ion induced reactions, e.g.: 233,235U(d,pf)234,236U, 239Pu(d,pf)240Pu Ed ≈ 10-13 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

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

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

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

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

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

‚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

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

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

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

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