Nuclear Structure

Sn

P, n

p n

() (,Xn)

M1 E1

p,n n

Xλ ? E1

Ex

10 2051

1. Study of the pygmy dipole resonance as a function of deformation

2. Study of the scissor mode in actinide nuclei

Fine and Gross Structure of the PDR in 138Ba

A. Tonchev et al., PRL 104, 072501 (2010)

Fine structure Gross structure

124Xe

Present Experimental Activity with N=82 Nuclei

130XeZ=54

N=

82

138Ba

140Ce

142Nd

144Sm

Completed measurements at HIGS

p,n n

N/Z

1.32

1.37

1.41

1.46

154Sm154Sm

2

0.09

0.10

0.09

0.09

Proposed measurements

136Xe132Xe

• Predicted dependence on N/Z ratio weak isospin effect

134Xe

B(E1) [10-3e2fm2]

536

576

600

960

Interpretation of the Pygmy Resonance in QPM calculations

PDR is predominantly E1 mode of excitation

PDR is enhanced strength below the GDR

We unveiled the fine structure of the M1 spin-flip mode

Evidence for surface neutron density oscillations

“Soft dipole mode“ at ~7 MeV is mixture of isoscalar and isovector components

N. Tsoneva, H. Lenske, PRC 77, 024321 (2008), A.P. Tonchev et al. NIM B 241, 51474 (2005); A.P. Tonchev et al. AIP 819, 350 (2006); AIP 1090, 74 (2009); A.P. Tonchev et al. PRL 104 072501 (2010).

p n

What we have learned ?

Pygmy Dipole Resonance Impact

Nuclear astrophysics: r-process 1998 S. Goriely PLB2003 M. Arnold et al. PR2005 T. Rauscher NPA

Neutrino-less double-beta decay physics

2004 J. Bahcall et al. PRD

Extract the γ-ray transition matrix elements for the decay (QRPA)

Study of the structure difference of the initial and final states

Study of the nuclear dipole response in 76Se and 76Ge isotopes

Testing the nuclear models for stable and extrapolating to exotic nuclei1998 S. Goriely, PLB; 2008 G. Rusev ,PRC

Neutron radius: PDR provides experimental constrains on properties of nuclear matter (neutron skin and symmetry energy)

2006 Piekarewicz PRC

Possible influence of the PDR on the r-process:

Nuclear astrophysics: r-process 1998 S. Goriely PLB2003 M. Arnold et al. PR2005 T. Rauscher NPA

solaronly GDRGDR + PDR

Nuclear astrophysics: r-process 1998 S. Goriely PLB2003 M. Arnold et al. PR2005 T. Rauscher NPA

solaronly GDRGDR + PDR

Possible influence of the PDR on the r-process:

Reliable extrapolation to exotic nuclei requires a detailed understanding of the PDR

Pygmy Dipole Resonance Impact

Nuclear astrophysics: r-process 1998 S. Goriely PLB2003 M. Arnold et al. PR2005 T. Rauscher NPA

Neutrino-less double-beta decay physics

2004 J. Bahcall et al. PRD

Extract the γ-ray transition matrix elements for the decay (QRPA)

Study of the structure difference of the initial and final states

Study of the nuclear dipole response in 76Se and 76Ge isotopes

Testing the nuclear models for stable and extrapolating to exotic nuclei1998 S. Goriely, PLB; 2008 G. Rusev, PRC

Neutron radius: PDR provides experimental constrains on properties of nuclear matter (neutron skin and symmetry energy)

2006 Piekarewicz PRC

Proposed Experiment

1. PDR as a function of deformation

144 148 152 154 Sm62

N/Z 1.32 1.39 1.45 1.48 ↑

2 0.09 0.14 0.31 0.34 ↑

Increased deformation with N/Z

128 130 132 134 136 Xe54

N/Z 1.37 1.41 1.44 1.48 1.52 ↑

2 0.18 0.17 0.14 0.12 0.09 ↓

Decreased deformation with N/Z

F. Dönau et. al. PRC 76, 014317 (2007); G. Rusev et al. PRC 73, 044308 (2006); G. Rusev et al., PRC (2009)

QRPA calculations

N

PDR region

Open Questions:

Is the PDR a generic mode for nuclei away from closed shell?

How will the PDR change/fragment with a quadrupole deformation?

What is the interplay between the isospin effect and the deformation?

What is the impact of PDR on the astrophysical reaction rate?

2-phonon excitations:

⊗1-

Found at sum energy of one-phononstates: ~ 3 MeV

Found usuallybetween 2.7-3.6 MeV

Proton-Neutron symmetric Existence because of nuclear 2-fluid system

⊗2+FS

2+MS

1+

Dipole Excitation Below 4 MeV: Scissors Mode

Scissors mode systematics: clearly collective Degree of fragmentation depends on deformation

Dipole Excitation Below 4 MeV

Proposed Experiments

Where we are going: study the scissor mode in actinide nuclei

1. Completed experiments: 235U, 238U, 232Th.

2. To be measured: 239Pu, 237Np, 233U.

3. Working with the theorist to develop a model (RPA-based) to reproduce all experimental observables of the E1 and M1

excitations for actinides nuclei.

Extra Slides

decay rate(work in progress)

phase spacefactor nuclear matrix element

neutrino mass

0ν2β:Nuclear Matrix Elements

other candidates: 82Se, 100Mo, 130Te, 136Xe

nuclear structure models(Shell Model / QRPA) areneeded to derive the nuclearmatrix elements => need tofix them by data.

Example

PublicationsG. Rusev et al., Phys. Rev. C 79, 047601 (2009)G. Rusev et al., AIP 1099, 799 (2009)

A. P. Tonchev et al., AIP 1090, 74 (2009)M. Fritzsche et al., AIP 1090, 591 (2009)N. Pietralla et al., Phys. Lett. B 681, 134 (2009) A. P. Tonchev et al., Phys. Rev. Lett. 104, 072501 (2010)

J. Isaak et al., Accepted in Phys. Rev. C (2010)G. Rusev et al., Prepared for publication (2011)

A. P. Tonchev et al., prepared for publication (2011)

PhD thesisM. Fritzsche, TU of Darmstadt, Germany

R. Massarczyk, Technische Universitaet Dresden, Germany

Diploma thesis Phillip Goddard, University of Surrey, England

REU projectSusan Pratt, University of Rochester

Summary

Summary

What we have learned:

More than 500 new dipole states measured in N = 82 nuclei

PDR is indeed an E1 excitation !

PDR is an enhanced strength below the GDR

We unveiled the character of the PDR as a smooth transition from isoscalar to isovector mode of excitation

What we have completed:

1. Study of the PDR in closed shell nuclei with N = 82:

136,138Ba(γ, γ’), 140Ce(γ, γ’), 142Nd(γ, γ’), 144Sm(γ, γ’) from Eγ = 4 to 10 MeV 2. Study of the PDR in double-beta-decay related nuclei: 76Ge(γ, γ’), 76Se(γ, γ’), 130Te(γ, γ’) from Eγ = 6 to 10 MeV

3. Study of the splitting of the PDR 48Ca(γ, γ’) from Eγ = 6 to 11 MeV