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

2. Study of the scissor mode in actinide nuclei

M1

E1

Xλ ?

E1

n

p

p,n

P,

n

n

Sn

1

10

20

5

Ex

()

(,Xn)

Fine and Gross Structure of the PDR in 138Ba

Fine structure

Gross structure

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

Present Experimental Activity with N=82 Nuclei

2

0.09

0.10

0.09

0.09

B(E1) [10-3e2fm2]

536

576

600

960

N/Z

1.32

1.37

1.41

1.46

144Sm

154Sm

154Sm

142Nd

140Ce

138Ba

124Xe

130Xe

Z=54

132Xe

136Xe

134Xe

N=82

Completed measurements at HIGS

Proposed measurements

p,n

n

- Predicted dependence on N/Z ratio weak isospin effect

Interpretation of the Pygmy Resonance in QPM calculations

p

n

What we have learned ?

- 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).

Nuclear astrophysics: r-process

1998 S. Goriely PLB

2003 M. Arnold et al. PR

2005 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

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

2006 Piekarewicz PRC

Testing the nuclear models for stable and extrapolating to exotic nuclei

- 1998 S. Goriely, PLB; 2008 G. Rusev ,PRC

Possible influence of the PDR on the r-process:

Nuclear astrophysics: r-process

1998 S. Goriely PLB

2003 M. Arnold et al. PR

2005 T. Rauscher NPA

solar

only GDR

GDR + PDR

Possible influence of the PDR on the r-process:

Nuclear astrophysics: r-process

1998 S. Goriely PLB

2003 M. Arnold et al. PR

2005 T. Rauscher NPA

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

solar

only GDR

GDR + PDR

Nuclear astrophysics: r-process

1998 S. Goriely PLB

2003 M. Arnold et al. PR

2005 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

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

2006 Piekarewicz PRC

Testing the nuclear models for stable and extrapolating to exotic nuclei

- 1998 S. Goriely, PLB; 2008 G. Rusev, PRC

QRPA calculations

PDR as a function of deformation

144 148 152 154Sm62

N/Z 1.32 1.39 1.45 1.48 ↑

20.09 0.14 0.31 0.34 ↑

Increased deformation with N/Z

128 130 132 134 136Xe54

N/Z 1.37 1.41 1.44 1.48 1.52 ↑

20.18 0.17 0.14 0.12 0.09 ↓

Decreased deformation with N/Z

N

PDR region

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

- 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+FS

2+MS

1+

Dipole Excitation Below 4 MeV: Scissors Mode

2-phonon excitations:

⊗

1-

Found at sum energy of one-phonon

states: ~ 3 MeV

Found usually

between 2.7-3.6 MeV

Existence because of nuclear 2-fluid system

Proton-Neutron symmetric

Scissors mode systematics: clearly collective

Degree of fragmentation depends on deformation

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.

phase space

factor

decay rate

(work in progress)

nuclear matrix element

neutrino mass

other candidates:

82Se, 100Mo,

130Te, 136Xe

Example

nuclear structure models

(Shell Model / QRPA) are

needed to derive the nuclear

matrix elements => need to

fix them by data.

Publications

G. 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 thesis

M. Fritzsche, TU of Darmstadt, Germany

R. Massarczyk, Technische Universitaet Dresden, Germany

Diploma thesis

Phillip Goddard, University of Surrey, England

REU project

Susan Pratt, University of Rochester

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

- 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

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