p.g. thirolf, lmu münchenuniverse cluster, area g kickoff, 16. januar 2007 investigating the...

Universe Cluster, Area G Kickoff, 16. Januar 2007

P.G. Thirolf, LMU München

Investigating the Fission Barrier Investigating the Fission Barrier Landscape in Heavy Elements Landscape in Heavy Elements

relevant for the r-Processrelevant for the r-ProcessP.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



G: Heavy Element Enrichment of the G: Heavy Element Enrichment of the UniverseUniverse

courtesy: R. Diehl



r-Process Path in Heavy Element r-Process Path in Heavy Element RegionRegion

direct access to r-process path out of reach extrapolations of nuclear models require experimental data knowledge of fission barriers (and interplay with decay) is crucial fission barriers: - determine end of chart of nuclides - change of shell correction energy by 1 MeV: fission lifetime changed by 105

-delayed fission:



Second minimum in the potential energy Second minimum in the potential energy surfacesurface

Double-humpedfission barrier:

(macroscopic) liquid drop model (microscopic)shell corrections (Strutinsky, 1967)

++deformation

structures in (prompt)fission cross section

spectroscopy ofexcited states

shape isomer

1. minim. 2. minimum



Second minimum in the potential energy Second minimum in the potential energy surfacesurface

fluctuation of level density: - with number of nucleons - with deformation -> strong shell gaps: potential minima

deformed harmonic oscillator:

Strutinsky method: - derive shell correction energy from single-particle level density- fission barriers reflect basic nuclear properties: Binding energy Shell correction energy

experimental challenge: very small isomeric cross sections (~b) large background (~105) from prompt fission

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

nu

cle

on

nu

mb

er



Multiple-humpedMultiple-humped fission barrier fission barrier landscapelandscape

1. Min. 2. Min. 3. Min. (2:1) (3:1)

Calculated potential energy surface (Moeller + Nix, 1973):

- 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

mass a

sym

metr

y

~ deformation



Calculated potential barriers in light Calculated potential barriers in light actinide nucleiactinide nuclei

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

3.8 ns6/0.6 ns34ns/37ps

calculations: Howard et al, Cwiok et al.

deep third minima predicted !



Experimental InfrastructureExperimental 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



Spectroscopy in the superdeformed 2. Spectroscopy in the superdeformed 2. MinimumMinimum

for 240fPu now more spectroscopic information available in 2. minimumthan in first minimum !

‘prototype‘ shape isomer: 240fPu (t1/2= 3.8 ns)

Ground state rotational band

conversion electrons (1972, Specht et al.)

Detailed level scheme

-rays + conversion electrons (2000, Pansegrau et al.) (2001, Gassmann et al.)

Multiphonon excitations and 2qp states

Transmission resonances (2001, Hunyadi et al.)



Transmission Resonance Transmission Resonance SpectroscopySpectroscopy

- 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

-> method to deduce fission barrier parameters

(prompt) fission probability



Transmission resonance Transmission resonance experimentsexperiments

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)



Spectroscopy in hyperdeformed 3. Spectroscopy in hyperdeformed 3. MinimumMinimum

- (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)

235U(d,pf)236U:



Analysis of Transmission Analysis of Transmission Resonances: Resonances: 236236U U

fit with rotational bands:

- consistent fit: hyperdeformed resonance

free parameters: band head energy, abs. intensity, K value

resonance around 5.1 MeV: statistical level density analysis: -> depth of 3. minimum

Result: EIII = 2.7(4) MeV

-> deep 3. Minimum (EII=2.81 MeV !): in agreement with theory and own results in 234U

J=5 states



Triple-Humped Fission Barrier in Triple-Humped Fission Barrier in 236236UU

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

M. Csatlos, PT et al., Phys. Lett. B 615 (2005) 175.deep third minimum established



Resolving the ‘Thorium Anomaly‘ of Fission Resolving the ‘Thorium Anomaly‘ of Fission BarriersBarriers

- 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



Resolving the ‘Thorium Anomaly‘ of Fission Resolving the ‘Thorium Anomaly‘ of Fission BarriersBarriers

- 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‘ project:confirm and extentbarrier systematics



‚‚Universe‘ Project: Universe‘ Project: Fission Barrier via Single-Particle Level Fission Barrier via Single-Particle Level

DensityDensity 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: 2=0.6)- no single-particle level in second minimum has been identified so far !- contradicting theoretical predictions:

237fPu:



Spectroscopy of Spectroscopy of 239f239fPuPu

-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-resolutionGe-detector array for -spectroscopy

- derive fission barrier properties from single-particle state level density- but: highly-converted low-energy transitions-> conversion electron data required

2.6 ns

8 s239Pu f

239fPu: double isomer in 2. minimum:

shape isomer



Conversion Electron Spectroscopy in Conversion Electron Spectroscopy in 239f239fPuPu

6,)7(17)(

)( ,240

239

f

konvf

iso

fiso

Pu

Pu

238U(,3n)239Pu (E = 33 MeV)

- advantage in 239fPu: electron data are already known:

H. Backe, DH et al., PRL 42 (1979) 490

level scheme:

-> basis for conclusive interpretation of -data is already existing



ConclusionConclusion

longterm experience: - spectroscopy in the 1st , 2nd and 3rd minimum of actinides - measuring fission barriers of largely-deformed heavy elements

-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)



CollaborationCollaboration

Inst. Nucl. ResearchDebrecen/Ungarn

LMU München TU München

T. FaestermannH.-F. WirthA. Krasznahorkay

M. CsatlosL. CsigeJ. GulyasM. HunyadiZ. Mate

D. Habs G. GrawH.J. Maier R. HertenbergerT. Morgan O. SchaileW. Schwerdtfeger J. SzerypoPT

P. Reiter T. StrieplingB. Bruyneel N. Warr

IKP Köln

p.g. thirolf, lmu münchenuniverse cluster, area g kickoff, 16. januar 2007 investigating the...

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