Nuclear Structure SnSn P,n p n (  )‏ ( ,Xn)‏ M1E1 p,nn X λ ?E1 ExEx  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 138 Ba A. Tonchev et al., PRL 104, (2010) Fine structureGross structure

124 Xe Present Experimental Activity with N=82 Nuclei 130 Xe Z=54 N= Ba 140 Ce 142 Nd 144 Sm Completed measurements at HIGS p,nn N/Z Sm  Proposed measurements 136 Xe 132 Xe Predicted dependence on N/Z ratio  weak isospin effect 134 Xe B(E1) [10 -3 e 2 fm 2 ]

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, (2008), A.P. Tonchev et al. NIM B 241, (2005); A.P. Tonchev et al. AIP 819, 350 (2006); AIP 1090, 74 (2009); A.P. Tonchev et al. PRL (2010). pn  What we have learned ?

Pygmy Dipole Resonance Impact 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 76 Se and 76 Ge isotopes Testing the nuclear models for stable and extrapolating to exotic nuclei 1998 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 PLB 2003 M. Arnold et al. PR 2005 T. Rauscher NPA solar only GDR GDR + PDR

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

Pygmy Dipole Resonance Impact 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 76 Se and 76 Ge isotopes Testing the nuclear models for stable and extrapolating to exotic nuclei 1998 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 Sm 62 N/Z ↑  ↑ Increased deformation with N/Z Xe 54 N/Z ↑  ↓ Decreased deformation with N/Z F. Dönau et. al. PRC 76, (2007); G. Rusev et al. PRC 73, (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: ⊗ 1-1- Found at sum energy of one-phonon states: ~ 3 MeV Found usually between MeV Proton-Neutron symmetric Existence because of nuclear 2-fluid system ⊗ 2 + FS 2 + MS 1+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: 235 U, 238 U, 232 Th. 2. To be measured: 239 Pu, 237 Np, 233 U. 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 space factor nuclear matrix element neutrino mass 0ν2β:Nuclear Matrix Elements other candidates: 82 Se, 100 Mo, 130 Te, 136 Xe nuclear structure models (Shell Model / QRPA) are needed to derive the nuclear matrix elements => need to fix them by data. Example

Publications G. Rusev et al., Phys. Rev. C 79, (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, (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 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: 136,138 Ba(γ, γ’), 140 Ce(γ, γ’), 142 Nd(γ, γ’), 144 Sm(γ, γ’) from E γ = 4 to 10 MeV 2. Study of the PDR in double-beta-decay related nuclei: 76 Ge(γ, γ’), 76 Se(γ, γ’), 130 Te(γ, γ’) from E γ = 6 to 10 MeV 3. Study of the splitting of the PDR 48 Ca(γ, γ’) from E γ = 6 to 11 MeV