Investigation of the Pygmy Dipole Resonance in particle- coincidence experiments

Transcription

1 Investigation of the Pygmy Dipole Resonance in particle- coincidence experiments V. Derya 1*, J. Endres 1, M. N. Harakeh 2,3, D. Savran 4,5, M. Spieker 1*, H. J. Wörtche 2, and A. Zilges 1 1 Institute for Nuclear Physics, University of Cologne, Germany 2 KVI, Rijksuniversiteit Groningen, The Netherlands 3 GANIL, CEA/DSM-CNRS/IN2P3, Caen, France 4 ExtreMe Matter Institute EMMI and Research Division, GSI, Darmstadt, Germany 5 Frankfurt Institute for Advanced Studies FIAS, Frankfurt a.m., Germany 4th Workshop on Nuclear Level Density and Gamma Strength Oslo, May 27 31, 2013 Supported by the DFG (ZI 510/4-2 and SFB 634), by the EU under EURONS Contract No. RII3-CT in the 6th framework programme, and by the Alliance Program of the Helmholtz Association (HA216/EMMI) *Supported by the Bonn-Cologne Graduate School of Physics and Astronomy

2 The PDR in particle- coincidence experiments Introduction The particle- coincidence method Systematic study in (, ) and (, ) First results 140 Ce(p,p ) Summary Outlook

3 Introduction Isovectorial electric Giant Dipole Resonance Low-lying E1 strength 2-Phonon ( ) 1 - Pygmy Dipole Resonance (PDR) (, ) A. Zilges et al., Phys. Lett. B 542 (2002) 43 (,Xn) A. Leprêtre et al., Nucl. Phys. A258 (1976) 350

4 Relevance of the PDR Symmetry energy of the equation of state (EOS) neutron-skin thickness E1 strength Nucleosynthesis neutron-capture rates -ray strength function photo-absorption cross section S. Goriely, Phys. Lett. B 436 (1998) 10

5 PDR studied with (, ) in N=82 isotones Real-photon scattering for stable nuclei below the particle thresholds Strongly fragmented E1 strength Accessible quantities: transition energy E multipole character reduced transition strength B(E1) fragmentation integrated strength A. Zilges et al., Phys. Lett. B 542 (2002) 43 S. Volz et al., Nucl. Phys. A779 (2006) 1 D. Savran et al., Phys. Rev. Lett 100 (2008)

6 Probes for scattering experiments Probe Interaction (dominant) Location of interaction Character of interaction (dominant) photon Electromagnetic whole nucleus isovector particle* hadronic surface isoscalar proton* hadronic surface isoscalar and isovector at medium particle energies ( MeV) Insight into the structure of the (dipole) excitations

7 The particle- coincidence method Reaction: inelastic particle scattering particle particle Coincident detection of particles and rays Performed with particles at Kernfysisch Versneller Instituut in Groningen, The Netherlands beam at E = 136 MeV AGOR cyclotron Cyclotron frequency: 28 MHz Particle current: pna

8 HPGe-detector array Photo by S.G.Pickstone BBS beam 20 cm

9 Big-Bite Spectrometer Photo by S.G. Pickstone 50 cm QQD-type spectrometer Two quadrupole magnets One dipole magnet Maximum solid angle: 9.2 msr

10 Big-Bite Spectrometer Photo by S.G. Pickstone 50 cm EUROSUPERNOVA System for particle detection Focal Plane Detector System 2 Vertical Drift Chambers energy and particle trajectories scintillator layer trigger

11 Excitation spectrum in coincidence 3 1,4,2 S n 94 Mo (, ) 2 1 PDR region Typical energy resolution: 300 kev

12 E [MeV] The - coincidence matrix Intensity 48 Ca(, ) Decay branching Detector response Excited states E X E -E [MeV]

13 E [MeV] The - coincidence matrix Intensity 48 Ca(, ) E X E -E [MeV] Energy spectra through projection Selecting transitions by setting gates

14 Selecting transitions Projected spectra 48 Ca(, ) Gate on E X E Gate on E X E +E + 21

15 - angular correlation from DWBA spin sequence 3.5 beam

16 Experimental method: (, ) Selective excitation Isoscalar probe Mainly low spin from ground state Natural parities Powerful data analysis HPGe detectors with high energy resolution Selection of transitions Cross sections Branching ratios Angular distributions Spin assignments

17 PDR in 140 Ce Splitting: low-energy part (, ) and (, ) high-energy part (, ) only D. Savran et al., Phys. Rev. Lett. 97 (2006)

18 Systematic study in (, ) and (, ) experiments Neutron magic (N=82) isotones 140 Ce and 138 Ba Proton magic (Z=50) isotope 124 Sn D. Savran et al., Phys. Rev. Lett. 97 (2006) J. Endres et al., Phys. Rev. C 80 (2009) J. Endres, E. Litvinova et al., Phys. Rev. Lett. 105 (2010)

19 Systematic study in (, ) and (, ) experiments Neutron magic (N=82) isotones 140 Ce and 138 Ba Proton magic (Z=50) isotope 124 Sn Splitting is a common feature of the low-lying dipole response in semi-magic heavy neutron-rich nuclei D. Savran et al., Phys. Rev. Lett. 97 (2006) J. Endres et al., Phys. Rev. C 80 (2009) J. Endres, E. Litvinova et al., Phys. Rev. Lett. 105 (2010)

20 Interpretation of the splitting Transition densities for two RQTBA states in 124 Sn - In phase - Large neutron contribution at the surface - Slightly out of phase - Enhanced proton contribution Low-lying state: Typical PDR state High-lying state: Transitional region towards the GDR J. Endres et al., Phys. Rev. C 85 (2012) Similar conclusions in N. Tsoneva et al., Phys. Rev. C 77 (2008)

21 (, ) and (, ) in 94 Mo Non-magic (N=52, Z=42) isotope 94 Mo Near to (sub) shell closure V. Derya et al., Nucl. Phys. A906 (2013) 94 C. Romig, private communication

22 Systematic study in (, ) and (, ) experiments Neutron magic (N=82) isotones 140 Ce and 138 Ba Proton magic (Z=50) isotope 124 Sn Splitting is a common feature of the low-lying dipole response in semi-magic heavy neutron-rich nuclei D. Savran et al., Phys. Rev. Lett. 97 (2006) J. Endres et al., Phys. Rev. C 80 (2009) J. Endres, E. Litvinova et al., Phys. Rev. Lett. 105 (2010)

23 Low-lying dipole strength in lighter nuclei Light-mass nuclei: halo nuclei, single-particle character excitations Medium-mass nuclei: development of a more collective electric-dipole excitation mode? Dependence on N/Z ratio in the calcium chain

24 Low-lying dipole strength in lighter nuclei T.D. Poelhekken et al., Phys. Lett. B 278 (1992) 423 T. Hartmann et al., Phys. Rev. C 65 (2002) V. Derya et al., to be published T. Hartmann et al., Phys. Rev. C 65 (2002)

25 J=1 - states in (, ) and (, )

26 J=1 - states in (, ) and (, ) d /d < 0.15 mb/sr? Strongest state in (, ) at 7.3 MeV is missing in (, )

27 J=1 - states in (, ) and (, )? Strongest state in (, ) at 7.3 MeV is missing in (, ) Strongest state in (, ) at 7.6 MeV is weak in (, )

28 Parity Measurement at HI S 2 cm High Intensity -Ray Source (HI S) at the Duke Free Electron Laser Laboratory (DFELL) 100% linear polarized and nearly mono-energetic intense -ray beam (I ~ 10 7 photons/sec) 6 HPGe detectors: One for beam monitoring One at backward angle Four at = 90 in the horizontal and vertical plane Parity assignment direction of polarization Target: 48 Ca Amount: 1 g ( $250,000)

29 The State at MeV -ray beam energy of 7.3 MeV Measured for 1.5 h spectrum of the vertical detectors spectrum of the horizontal detectors DE SE Experimental asymmetry:

30 Parity assignments in 48 Ca 9 dipole excitations were observed The excited dipole states do have negative parity

31 Comparison with theoretical results for 48 Ca QRPA calculations: strong IS LED states present in all even-even Ca isotopes (N=14-40) P. Papakonstantinou et al. PLB 709 (2012) 270 Character changes N proton-skin oscillation pure IS oscillation neutron-skin oscillation Experimental results on 48 Ca: separated strong IS LED state at 7.6 MeV weak in IV channel pure IS oscillation character changes at N > 28 at N=30 (Gogny interaction)

32 Probes for scattering experiments Probe Interaction (dominant) Location of interaction Character of interaction (dominant) photon Electromagnetic whole nucleus isovector particle* hadronic surface isoscalar proton* hadronic surface isoscalar and isovector at medium particle energies ( MeV) Insight into the structure of the (dipole) excitations

33 First results 140 Ce(p,p ) performed at KVI beam energy: 80 MeV central BBS angle: 6 8 HPGe detectors target enrichment: %

34 Excitation spectrum in coincidence

35 spectrum with gate on E x =E

36 Comparison of the probes

37 Comparison of the probes

38 Summary Systematic study of E1 excitations in (, ) and (, ) experiments in stable and spherical nuclei A = N/Z = semi-, doubly-, and non-magic (, ) method is appropriate tool for identifying the contribution of the PDR to the total E1 strength and strength function First results of 140 Ce(p,p ) show qualitatively different behavior for the proton probe

39 Outlook: ithemba LABS K600 spectrometer at 0 Continuation of particle- coincidence experiments at medium particle (p,, ) energies First feasibility test in December 2012 Performed by R. Neveling et al. beam of 160 MeV energy 2 Clover detectors, 1 big NaI detector R. Neveling et al., Progress Report: PR194 (2013)

40 Outlook: &HORUS in Cologne HORUS: spectrometer 14 HPGe detectors 6 BGO shields Photopeak efficiency with SONIC: 2% at 1332 kev Energy resolution with digital DAQ: 2.5 kev at 1332 kev, 6 kcps SONIC: particle spectrometer Silicon Identification Chamber 8 E-E detector tubes Solid angle coverage 4%

41 Outlook: &HORUS in Cologne Combining and particle spectroscopy various probes (p, d,, ) particle- coincidence

42 Outlook: &HORUS in Cologne Combining and particle spectroscopy various probes (p, d,, ) particle- coincidence Excitation-energy spectrum from a very recent experiment: 92 Mo(p,p ) at 10.5 MeV Si detector at 131 E = MeV

43 University of Cologne (Cologne, Germany) V. Derya, J. Endres, A. Hennig, J. Mayer, L. Netterdon, S. G. Pickstone, P. Scholz, M. Spieker, T.-M. Streit, M. Weinert, and A. Zilges Kernfysisch VersnelIer Instituut (Groningen, The Netherlands) S. Bagchi, M. N. Harakeh, N. Kalantar, A. Najafi, C. Rigollet, and H. J. Wörtche ExtreMe Matter Institute (Darmstadt, Germany) E. Fiori, J. Isaak, B. Löher, D. Savran, and J. Silva TU Darmstadt (Darmstadt, Germany) N. Pietralla, C. Romig, and M. Scheck Supported by: