Nuclear Structure

1. Nuclear Structure 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)‏

2. Fine and Gross Structure of the PDR in 138Ba Fine structure Gross structure A. Tonchev et al., PRL 104, 072501 (2010)

3. 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

4. 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).

5. 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 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

6. 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

7. 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

8. 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 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

9. Proposed Experiment 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)

10. 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?

11. ⊗ 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

12. Dipole Excitation Below 4 MeV Scissors mode systematics: clearly collective Degree of fragmentation depends on deformation

13. 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.

14. Extra Slides

15. 0ν2β:Nuclear Matrix Elements 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.

16. Summary 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

17. Summary 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