OUTLINEOUTLINE• Motivation: fission Motivation: fission isomersisomers• Spectroscopy of Spectroscopy of 237f237fPu:Pu:

• measurementmeasurement• resultsresults

• Outlook Outlook

Isolde Workshop 14Isolde Workshop 14thth of February of February 20072007

experimental goal:experimental goal: - - first experimental identification of first experimental identification of single-particle states in largely deformed single-particle states in largely deformed actinidesactinides -> determination of fission barrier from level -> determination of fission barrier from level densitydensity

T. Morgan LMU MunichT. Morgan LMU Munich

g-g-Spectroscopy of Spectroscopy of superdeformed superdeformed

237237PuPu

prompt fission

2:12:1

direct access to r-process path out of reachdirect access to r-process path out of reach extrapolations of nuclear models require experimental dataextrapolations of nuclear models require experimental data knowledge of fission barriers is crucialknowledge of fission barriers is crucial fission barriers:fission barriers: - determine end of chart of nuclides - determine end of chart of nuclides - change of shell correction energy by 1 MeV:- change of shell correction energy by 1 MeV: fission lifetime changed by 10fission lifetime changed by 1055

T. MorganT. Morgan

no experimental datano experimental data

-> contradicting theoretical -> contradicting theoretical predictionspredictions::

r-Process Path in Heavy r-Process Path in Heavy Element RegionElement Region

240f240fPu Pu (even-(even-eveneven))

(3.8 ns)(3.8 ns)

237f237fPu Pu (odd-even)(odd-even)

(110 ns)(110 ns)

238238U(U(aa, 2n) 24 , 2n) 24 MeVMeV

235235U(U(aa, 2n) 24 , 2n) 24 MeVMeV

ssdelay delay = 10 = 10 mmbb ssdelay delay ~1 - 2 ~1 - 2 mmbb

T. MorganT. Morgan

(Future)

• • Present Present KnowledgeKnowledge

• D. Pansegrau et al. Phys. Lett. B 484 (2000) 1

• D. Gaßmann et al. Phys. Lett B. 497 (2001) 181

in spite of low cross section in spite of low cross section

comparable yields expectedcomparable yields expected

Spectroscopy of the first Spectroscopy of the first Odd-N fission isomer Odd-N fission isomer

237f237fPu: rigid rotorPu: rigid rotor

=> regular rotational band structure=> regular rotational band structure

prompt

delay

1.21.2×10×10-4-4

prompt

delay

1.21.2×10×10-5-5

• • ReactionReaction: : 235235U(U(aa,2n),2n)237f237fPu; Pu;

• • BeamBeam: E: Eaa = 24 MeV: = 24 MeV: pulsed, pulsed, DDt = 400ns, width ~ 4 ns, t = 400ns, width ~ 4 ns,

Cologne TandemCologne Tandem• • TargetTarget: thick (rolled) : thick (rolled) 235235U; U; 3.7 mg/cm 3.7 mg/cm22 →→ gg-emission at rest-emission at rest

metallic: low reaction background from e.g. oxygen and carbonmetallic: low reaction background from e.g. oxygen and carbon but: highly oxidising, all handling under vacuum or Ar atmospherebut: highly oxidising, all handling under vacuum or Ar atmosphere

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Detectors:Detectors:

• fission fragments:fission fragments: compact PPAC array - 8 trapezoidal modulescompact PPAC array - 8 trapezoidal modules

- 13 fold segmented anode- 13 fold segmented anode

- position sensitive for time of flight- position sensitive for time of flight

- large solid angle (73 %)- large solid angle (73 %)

• gg-rays:-rays: MINIBALLMINIBALL

- 8 triple cryostats, distance to target ~10 cm- 8 triple cryostats, distance to target ~10 cm

- high resolution 2.3 keV (1.3 MeV)- high resolution 2.3 keV (1.3 MeV)

- high efficiency - high efficiency eephph ~ 9% (1.3 MeV) ~ 9% (1.3 MeV)

- trigger: fission fragment & - trigger: fission fragment & gg-ray-ray

- 3 weeks beamtime- 3 weeks beamtime

~ 2~ 2×10×104 4 delayed fission events (Ndelayed fission events (Ngg = 1 ~ 90 %; N = 1 ~ 90 %; Ngg = 2 ~10 = 2 ~10 %)%)

Experimental Experimental ProcedureProcedure

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• • theoretical theoretical expectation:expectation:

• • experimental experimental finding:finding:

• • relative isomeric relative isomeric populationpopulation

=> => consistent with consistent with literatureliterature

Isomeric LifetimeIsomeric Lifetime

5102.1 prompt

isomer

5102.1 prompt

isomer

~ 2mb

1

2~

long

short

Time [ns]Time [ns]

Cou

nts

/[n

s]C

ou

nts

/[n

s]

nsisomer 165nsshort 115ns

long1120

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110 ns

Disentanglement Disentanglement of isomeric of isomeric gg-rays-rays

115 ± 15 ns

1210 ± 25 ns

Eg = 363.1 keV

Eg = 363.1 keV

Eg = 213.1 keV

Eg = 213.1 keV

1120 ns

170 170 gg lines in lines in spectrumspectrum

115 115 ××115 ns 115 ns gg transitionstransitions55 55 ××1120 ns 1120 ns gg transitionstransitions

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→ → No obvious correlationsNo obvious correlations

→ → automatic search via peak automatic search via peak correlationcorrelation

Identification Identification of rotational of rotational bandsbands

237f237fPu: rigid rotorPu: rigid rotor

=> regular rotational band structure=> regular rotational band structure 2

)1(2

JJE

gg--gg correlation (2000 N correlation (2000 Ngg = 2 events): = 2 events):

(SD axis ratio 2:1(SD axis ratio 2:1 ~ 3.3 keV)~ 3.3 keV)

Variation of rotational parameter 2

2

=> result:

keVkeV]20[28.32

2

• • 9 rotational bands9 rotational bands

••

test case for peak correlation algorithm:test case for peak correlation algorithm:

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Isomeric rotational Isomeric rotational bandsbands

115 ns115 ns

1120 ns1120 ns

3/2 and 5/23/2 and 5/2

9/2 and 9/2 and 11/211/2

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Construction of Construction of isomeric level isomeric level scheme(s)scheme(s)

→ → Ritz combinatorial search program usedRitz combinatorial search program used (courtesy of T. v. Egidy, E18 TU München):

ground state bands + 170 ground state bands + 170 gg transitions transitions

keVkeV]20[28.32

2

2)1(2

2

1

JEE JJ

89

53

23

0- 15

115 ns :

7/2

5/2

9/2

3/2

11/2

2)1(

2 JJEJ

15/2

0 9/2

11/2

13/2

36

77

125ground state rotational bands:ground state rotational bands:

1120 ns:

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Rotational level scheme Rotational level scheme for short lived isomer for short lived isomer ((115 ns115 ns) )

237f237fPu:Pu:

connecting inter-band transitions identifiedconnecting inter-band transitions identified-> consistent picture -> consistent picture

T. Morgan

Rotational level scheme Rotational level scheme for long lived isomer for long lived isomer ((1120 ns1120 ns) )

237f237fPuPu

work in progress:

Summary:Summary:

•• first high resolution first high resolution gg-spectroscopy in Odd-N fission isomers -spectroscopy in Odd-N fission isomers •• regular rigid rotor pattern allows to interpret low-statistics spectraregular rigid rotor pattern allows to interpret low-statistics spectra

•• 9 rotational bands identified with SD moment of inertia9 rotational bands identified with SD moment of inertia • • gg-spectra disentangled into contributions from 2 fission isomers-spectra disentangled into contributions from 2 fission isomers

•• level schemes constructed with Ritz combination (gs-band and interband level schemes constructed with Ritz combination (gs-band and interband transitions)transitions)

Outlook:Outlook:

•• finalise analysisfinalise analysis •• ccomplementary conversion electron spectroscopyomplementary conversion electron spectroscopy (Mini Oranges) (Mini Oranges)

=> => bb-vibration, E0 transitions-vibration, E0 transitions ● ● gg-spectroscopy of -spectroscopy of 239f239fPuPu– conversion electron data already availableconversion electron data already available– identification of Nilsson orbitalsidentification of Nilsson orbitals– localisation of r-process path via fission barriers (theory)localisation of r-process path via fission barriers (theory)

T. Morgan

Summary and Summary and OutlookOutlook

T. MorganT. Morgan11, A. Blazhev, A. Blazhev22, S. Becker, S. Becker11, B. Bruyneel, B. Bruyneel22, L. , L. CsigeCsige33, ,

F. FinkeF. Finke22, D. Habs, D. Habs11, H. Hess, H. Hess22, A. Holler, A. Holler22, H. H, H. Hüübelbel44, A. , A. ImigImig22, ,

M. KalkühlerM. Kalkühler22, R. Lutter, R. Lutter11, H. J. Maier, H. J. Maier11, P. Reiter, P. Reiter22, , O. SchaileO. Schaile11, C. Sch, C. Schüürmannrmann11, W. Schwerdtfeger, W. Schwerdtfeger11, M. , M.

SeidlitzSeidlitz22, , T. KotthausT. Kotthaus22, P. G. Thirolf, P. G. Thirolf11, N. Warr, N. Warr22, A. Wiens, A. Wiens22, K. , K.

WimmerWimmer11 and the MINIBALL Collaboration and the MINIBALL Collaboration 1. Ludwig Maximilians Universit1. Ludwig Maximilians Universitäät Mt Müünchennchen

and MLL Garchingand MLL Garching

2. Universit2. Universitäät zu Kt zu Köölnln3. Inst. of Nucl. Research of the Hungarian 3. Inst. of Nucl. Research of the Hungarian Academy of Sciences (Atomki) DebrecenAcademy of Sciences (Atomki) Debrecen

4. Rheinische Friedrich Wilhelms Universität Bonn4. Rheinische Friedrich Wilhelms Universität Bonn

T. Morgan

CollaborationCollaboration