Probing the evolution of shell structure with in-beam spectroscopy

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1 Probing the evolution of shell structure with in-beam spectroscopy Alexandra Gade National Superconducting Cyclotron Laboratory and Department of Physics and Astronomy at Michigan State University, East Lansing, Michigan 48824

2 Outline One-nucleon exchange (HIinduced) and N=30 in neutron-rich Ar and S nuclei or the structure beyond N=28 Two-proton knockout towards N=40 or mounting evidence for intruder configurations near N=40 Nuclear structure at N=50 news on the collectivity of Se and Ge

3 Experimental task: Quantify changes encountered in rare isotopes and measure observables that are calculable and can serve to discriminate between theories - -ray spectroscopy Identify the final state Tag the inelastic process Experimental approach In-beam -ray spectroscopy v/c= exotic beam target -Particle spectroscopy Identification of the reaction residues Momentum distributions Scattering angle reacted beam Focal plane Target position

4 Reaching nuclei with more neutrons than the primary and secondary beams Inverse-kinematics, HI-induced nucleonexchange reactions with fast beams: 9 Be( 48 K, 48 Ar+ )X and 9 Be( 46 Cl, 46 S+ )X 48 Ca primary beam 110 x K per second secondary projectile beam (pure) 6 x Cl per second secondary projectile beam (purity exceeding 98%) 376 mg/cm 2 9 Be reaction target Inclusive cross sections: 48 Ar: =0.13(1)mb (~1 out of 310,000) 46 S: =0.057(6)mb (~1 out of 700,000) True needle in haystack problem A. Gade et al., Phys. Rev. Lett. 102, (2009)

5 Reaching closer to the driplines Inverse-kinematics, HI-induced nucleon exchange reactions with fast beams: 9 Be( 48 K, 48 Ar+ )X A. Gade et al., Phys. Rev. Lett. 102, (2009) Narrow momentum distribution in agreement with 2-body reaction and observations by G.A. Souliotis et al., PRC 46, 1383 (1992)

6 Gamma-ray spectroscopy closer to the neutron dripline Inverse-kinematics, HI-induced nucleon exchange reactions with fast beams A. Gade et al., Phys. Rev. Lett. 102, (2009) Nuclei with more neutrons than the beam can be produced at cross sections sufficient for -ray spectroscopy 48 Ar: transitions in agreement with A. Navin et al., PRL 101, (2008) Deep-inelastic scattering at GANIL

7 Region around the key nucleus 42 Si - The role of N=30 Status Sizeable Z=14 sub-shell gap from cross sections in one- and two-proton knockout at NSCL J. Fridmann et al., Nature 435, 922 (2005); PRC 74, (2006) First in-beam -ray spectroscopy of 42 Si at GANIL revealed low-lying 2 + state B. Bastin, S. Grevy et al., PRL 99, (2007) New effective interaction in the sd-fp valence space, SDPF-U, one version for Z>14 and one version for Z 14 (essential for the description of 42 Si (N=28): monopole shift (fp shell gap) and pairing modifications) F. Nowacki and A. Poves, PRC 79, (2009) N=20 N=22 N=24 N=28 Question: What is the influence of the monopole shift and pairing modifications on N=30?

8 Systematics of states in comparison to shell model SDPF-NR interaction: S. Nummela et al., PRC 63, (2001) SDPF-NR 2 + of 40,42 Si too high by 400 and 700 kev, respectively, 46 S too high relative to 44 S Implement pairing and shell gap modifications stepby-step with linear Z-dependent interpolation

9 Shell model pairing modification SDPF-NR2 2 + of 42 Si still too high 44 S too low SDPF-NR2 SDPF-NR for Ca (Z=20) J=0 + fp-shell matrix elements V nn (pairing) reduced by 15% for Z=14; Z-dependent, linear interpolation for Z=16,18

10 Shell model fp shell gap SDPF-NR3 2 + of 40,42 Si good 44 S much too low SDPF-NR3 SDPF-NR for Ca (Z=20), SDPF-NR2 for Z=14,16,18 Lowering the neutron p 1/2 and p 3/2 orbits by 1 MeV for Z=14; Z-dependent, linear interpolation for Z=16,18

11 Shell model conclusion Outcome for the systematic of states The modifications needed to describe 42 Si SDPF-NR3 fail for 44 S and 46 Ar (linear interpolation does not work, no smooth change in the effective interaction, rather a sudden change in agreement with the findings by Nowacki and Poves) The description of the N=30 isotones improves, but the N=30 isotones are much less impacted by the changes that drive structure 42 Si While SDPF-NR overpredicts the 2 + energy of 48 Ar and 46 S by 136 kev and 282 kev, respectively, SDPF-NR3 moderately lowers these energies and gives better agreement (now within 51 kev and 86 kev) A single effective interaction valid in the sd-pf shell is still a challenge

12 Motivation Another Island of Inversion around N=40? Beta decay to 64,66 Fe [M. Hannawald et al., PRL 82, 1391 (1999)] Beta decay to Cr [O. Sorlin et al., EPJ A 16, 55 (2003)] Rotational band built on 9/2 + in 55,57 Cr [A. Deacon et al., PLB 622, 151 (2005)] Low-lying 9/2 + isomer in 59 Cr at 503 kev with possibly oblate deformation [S. J. Freeman et al., PRC 69, (2005)] Large deformation of 62 Cr [N. Aoi et al., J. Phys.: Conf. Ser. 49, 190 (2006), PRL 102, (2009)] T. Otsuka et al., PRL (2005) The attractive monopole part of the tensor force ( f 7/2 - f 5/2 ) weakens as protons are removed from f 7/2. The f 5/2 orbit shifts up in energy and the gap between the f 5/2 and g 9/2 orbits is reduced, allowing neutron occupancy of the intruder g 9/2 orbit already for nuclei with fewer than 40 neutrons

13 The experiment P. Adrich et al., PRC 77, (2008) Projectile beam energies between MeV/u One-proton knockout from 67 Co and 69 Co 66 Fe and 68 Fe Two-proton knockout from 66 Fe, 68 Ni and 70 Ni 64 Cr, 66 Fe and 68 Fe Inclusive cross section for the production of 64 Cr from 66 Fe -ray spectroscopy of 66 Fe and 68 Fe

14 Iron isotopes N=40 P. Adrich et al., PRC 77, (2008) and transitions in agreement with the measurement of M. Hannawald et al., PRL 82, 1391 (1999) In a single-particle picture, the removal of 2 protons from f 7/2 can populate states with spins from 0 +, 2 +, 4 + and 6 +

15 Iron isotopes N=42 P. Adrich et al., PRC 77, (2008) Observation of excited states in 68 Fe

16 P. Adrich et al., PRC 77, (2008) A structural change between Ni and Fe at N=40 The direct two-proton knockout probes the overlap of the wave functions of the projectile ground state and the residues final state. Small cross section structural change (reduced overlap) between the ground state and final state configurations in 66 Fe and 64 Cr

17 A possible explanation thanks to K. Sieja and F. Nowacki

18 Intermediate-energy Coulomb excitation of 82 Ge and 84 Se Se and Ge isotopes have rich nuclear structure Shape coexistence rules from A ~ 70 to N=Z Most neutron-rich Se and Ge accessible for experiments are around N=50 north of doublemagic 78 Ni

19 Collectivity at N=50 Shell Model Work in progress N=50 Se and Ge isotones emerge as very useful to guide effective interactions! Work in progress, B.A. Brown with input from M. Honma et al: Where are the very different predictions for the B(E2) values for the N=50 Ge and Se originating from in the different effective interactions? JUN45: M. Honma, T. Otsuka et al., PRC 80, (2009) jj4xyz: Interactions from B.A. Brown, used in D. Verney et al., PRC 76, (2007)

20 CHFB-5DCH: J.-P. Delaroche, M. Girod, et al., PRC 81, (2010) HFB-17: S. Goriely et al., PRL 102, (2009) Collectivity in the Se and Ge isotopic chains Mean field CHFB-5DCH describes well the trend beyond A=74 but overpredicts the B(E2) values towards the N = Z line HFB-17 approximately reproduces the trend for Ge heavier than A=70 but overpredicts the collectivity towards N=Z as well. The Se chain is not well described.

21 Excitation of 4 + states in the inelastic scattering off 9 Be?! why are Se and Ge so different? Inelastic and proton scattering on the stable selenium isotopes Se revealed that the states are excited the strongest, followed by the first 3 and, at markedly less cross section, higher-lying 2 + states and the level. One might expect 9 Be-induced scattering to yield a similar population pattern, however, it seem that in 84 Se+ 9 Be the state is more strongly excited than the 3 state.

22 An old discussion - hexadacapole degrees of freedom in Se isotopes From inelastic scattering of polarized protons on Se Matsuki et al. present indications for a static or dynamic hexadecapole shape transition that occurs between the light ( 74,76,78 Se) and heavier ( 80,82 Se) selenium isotopes and point out that the hexadecapole degree of freedom plays an important role in the selenium isotopes. Ogino et al. find the hexadecapole strength fragmented strongly for Se.The transition strength to the state was found to be weak except for the case of 82 Se where a transition strength of almost 3 spu was measured for the first 4 + state. S. Matsuki et al., Phys. Rev. Lett. 51, 1741 (1983). K. Ogino, Phys. Rev. C 33, 71 (1989).

23 To be finished soon but for now Work in Progress Origin of the difference in the SM effective interactions at N=50 for Se and Ge (B. A. Brown with input from M. Honma et al.) Why are the 4 + states excited with very different cross sections for the Se and Ge N=50 isotones? In general, population of excited states beyond the first 2 + in inelastic scattering induced by a 9 Be target (collaboration with J. A. Tostevin, Surrey)

24 Summary and outlook The role of N=30 in the region around 42 Si? N=30 isotones are much less impacted by the changes that drive structure 42 Si Effective interaction changes suddenly (Z-dependent interpolation failed) In-beam -ray spectroscopy following HI-induced nucleon exchange reactions may provide access to nuclei previously thought out of reach Evidence for a significant structural change between 66 Fe and 64 Cr (N=40) Sensitivity of two-nucleon knockout to wave-function overlaps Nuclear structure at N=50, the role of Ge and Se Shell-model interactions disagree and puzzling excitation of a 4 + state work in progress stay tuned.

25 A collaborative effort P. Adrich T. Baugher D. Bazin J. M. Cook C. Aa Diget A. Gade T. Glasmacher G.F. Grinyer S. McDaniel A. Ratkiewicz K. P. Siwek K. Walsh D. Weisshaar B. A. Brown J. A. Tostevin T. Otsuka M. Honma

26 A collaborative effort P. Adrich T. Baugher D. Bazin J. M. Cook C. Aa Diget A. Gade T. Glasmacher G.F. Grinyer S. McDaniel A. Ratkiewicz K. P. Siwek K. Walsh D. Weisshaar B. A. Brown J. A. Tostevin T. Otsuka M. Honma