Shell evolution and nuclear forces

Transcription

1 1 st Gogny Conference Campus Teratec, Bruyères-le-Châtel Dec (9), 2015 Shell evolution and nuclear forces Takaharu Otsuka University of Tokyo / MSU / KU Leuven Priority Issue project (field 9) by MEXT and JICFuS to be tackled by using Post K Computer

2 Outline 1. tensor-force driven shell evolution and Gogny force 2. Shell evolution and QCD 3. Shape coexistence and fission 4. Summary and perspectives

3 Shell Evolution driven by tensor force Monopole effects on the shell structure from the tensor interaction Type I Shell Evolution : different isotopes : particles Type II Shell Evolution : within the same nucleus : particles : holes

4 Recent example : Appearance of N= 32 and 34 magic structures shell structure for neutrons in Ni isotopes (proton f 7/2 fully occupied) N=34 magic number may appear due to properties of nuclear forces, if proton f 7/2 becomes vacant (Ca) (f 5/2 becomes less bound) f 5/2 34 p 1/2 p 1/2 f 5/2 byproduct 32 p 3/2 p 3/2 f 7/2 28 f 7/2 28 ISOLDE experiment Huck et al., PRC 31, 2226 (1985). Mayer-Jensen Predicted by TO et al, PRL 87, (2001)

5 RIBF à Finally confirmed new RIBF data Steppenbeck et al. Nature, 502, 207 (2013)

6 Our connection and appreciation to Gogny force Readjusted Gogny D1S + AV8 tensor force = GT2 reproduces shell evolution 6

7 Monopole effect of tensor force One-dimensional collision model At collision point: TO, Suzuki et al. PRL 95, (2005) TO, Phys. Scr. T152, (2013) k 2 k 1 k = k 1 k 2, K = k 1 + k 2 k 2 k 1 large relative momentum k strong damping small relative momentum k loose damping wave function of relative coordinate k 2 k 1 k 1 k 2 wave function of relative coordinate

8 Outline 1. tensor-force driven shell evolution and Gogny force 2. Shell evolution and QCD 3. Shape coexistence and fission 4. Summary and perspectives

9 Input from chiral Effective Field Theory (EFT) of QCD N3LO NN interaction V low k treatment of high momentum part Fujita-Miyazawa type 3N interaction Effective NN int. 9

10 Novel method for in-medium correction * Extended KK method ** Divergence problem in multi-shell * ** 10

11 EFT NN int. + Fujita-Miyazawa 3N int. with averaging (to be replaced by EFT N2LO 3N int.) V low k : treatment of high-momentum components EKK : in-medium correction (core polarization) Shell model Hamiltonian Effective single-particle energy (N or Z dependence of effects of monopole int.) Energy levels, electromagnetic matrix elements (diagonalization of Hamiltonian matrix) 11

12 Island of Inversion : neutron effective single-particle energies Present work (EFT + EKK) Meson exchange potential f 7/2 p 3/2 20 d 3/2 Shell evolution arises also from QCD TO et al. Phys. Rev. Lett , (2010)

13 Our connection and appreciation to Gogny force is Readjusted Gogny D1S + AV8 tensor force = GT2 13

14 and other properties obtained by the shell-model diagonalization in the sd + pf shell full 2 + level of N=20 isotones no tensor no 3NF Ground-state energies driplines predicted 14

15 ground-state energy Ca isotopes in the pf + sdg shell The prediction of N=34 magic number (2001) Is consistent with EFT (QCD) + EKK theoretical calculation. TO et al, PRL 87, (2001) Neutron single-particle energies N N=28 N=32 N=34 15

16 Outline 1. tensor-force driven shell evolution and Gogny force 2. Shell evolution and QCD 3. Shape coexistence and fission 4. Summary and perspectives

17 shape coexistence 16 O H. Morinaga (1956) 186 Pb A.N. Andreyev et al., Nature 405, 430 (2000) Island of Inversion (Z=10~12, N=20)

18 Monte Carlo Shell Model (MCSM) calculation on Ni isotopes unoccupied Y. Tsunoda et al pfg9d5 model space 1d 5/2 0g 9/2 1p 1/2, 0f 5/2 1p 3/2 0f 7/2 core: occupied 68 Ni This model space is wide enough to discuss how magic numbers 28, 50 and semi-magic number 40 are visible or smeared out. A3DA is used with minor

19 Energy levels and B(E2) values of Ni isotopes Description by the same Hamiltonian Shape coexistence in 68 Ni exp. 68 Ni calc. calc. exp compilation new data Y. Tsunoda, TO, Shimizu, Honma and Utsuno, PRC 89, (R) (2014)

20 Occupation numbers Effective s.p.e. by actual occupation numbers 0 + state 1st modest correlations 2nd 3rd 2p-2h 6p-6h

21 Underlying mechanism of the appearance of low-lying deformed states : Type II Shell Evolution Monopole effects on the shell structure from the tensor interaction Type I Shell Evolution : different isotopes : particles Type II Shell Evolution : within the same nucleus : particles : holes

22 Type II Shell Evolution in 68 Ni (Z=28, N=40) g 9/2 PES along axially symmetric shape f 5/2 f 7/2 f 5/2 Reset Hamiltonian with Type II SE suppressed Spin-orbit splijng works against quadrupole deforma@on (cf. Ellio) s SU(3) ). weakening of spin-orbit splitting Type II shell evolution stronger deformation of protons à more neutron p-h excitation original Type II shell evolution is suppressed by resetting monopole interactions as πf 7/2 - νg 9/2 = πf 5/2 - νg 9/2 πf 7/2 - νf 5/2 = πf 5/2 - νf 5/2 The local minima become much less pronounced. Shape coexistence is enhanced by type II shell evolution as the same quadrupole interaction works more efficiently.

23 Bohr-model calc. by HFB with Gogny force, Girod, Dessagne, Bernes, Langevin, Pougheon and Roussel, PRC 37,2600 (1988) present

24 Liquid 1 (~constant spherical SPE) Atomic nucleus is a quantum liquid Dual quantum liquids in the same nucleus Certain configurations produce different shell structures owing to (i) tensor force and (ii) proton-neutron compositions Liquid 2 (varying spherical SPE) Landau relevant to normal states in general relevant to specific intruder states core core core core proton neutron proton neutron Note : density -> single-particle energies in other many-body systems

25 Fission and Type II shell evolution There could be a general mechanism to increase fission probability. Specific massive particle-hole excitations driving Type II shell evolution -> lowering of barrier and minimum -> more tunneling in any kind The suppression of this mechanism, e.g., by Pauli principle, hinders fission, leading to the Island of Stability. 25

26 Summary and perspectives Quantum Liquid picture (of Landau) with stable shell structure (a la Mayer-Jensen) has been a good conceptual guidance for most of states of many nuclei near the stability line on the Segre chart. Nuclear forces, including tensor force, produce Type I Shell Evolution in many domains on the nuclear chart particularly away from the stability line, presenting a paradigm shift. Type I Shell Evolution appears in Gogny-GT2 calculation as well as in EFT + EKK calc. etc. There is another new aspect, Type II Shell Evolution, enhancing and stabilizing shape coexistence in a non-linear way. This is a general feature, resulting in Dual Quantum Liquid picture. Could we solve the problem of (spontaneous) fission? Could it be a way to Island of Stability? 26

27 Collaborators Naofumi Tsunoda (CNS-HPCI, Tokyo) K. Takayanagi (Sophia) Morten Hjorth-Jensen (Oslo/MSU) Yusuke Tsunoda (CNS, Tokyo) Noritaka Shimuzu (CNS-HPCI, Tokyo) Yutaka Utsuno (JAEA) Michio Honma (Aizu)

28 Other cases.. just an example Pb Fermi energy of 186 Pb i 13/2 h 9/2 h 9/2 h 11/2 186 Pb Andreyev et al., Nature 405, 430 (2000) proton neutron

29 29

30 Eigenvalues of HO potential 5hω 4hω 3hω 2hω 1hω Magic numbers by Mayer and Jensen (1949) Spin-orbit splitting

31 As N or Z is changed in an open shell, the shell structure is changed (evolved), and the change can be descriobed by Monopole component of the NN interaction Averaged over possible orientations Linearity: Shift n j : # of particles in j This can be substantial change in exotic nuclei. For j = 9/2, the multiplication by a factor of 10! Poves and Zuker made a major contribution in initiating systematic use of the monopole interaction. (Poves and Zuker, Phys. Rep. 70, 235 (1981))

32 The atomic nucleus is a Quantum (Fermi) Liquid (of Landau) described by interplay between single-particle energies and residual interaction - in a way like free particles - ε 8 For most of states, there may have been Ansatz that ε 8 ε 7 ε 6 ε 5 ε 7 ε 6 ε 5 ε 4 ε 3 ε 2 Spherical single particle energies remain basically unchanged. -> spherical part of Nilsson model ε 4 ε 3 ε 2 ε 1 proton ε 1 neutron Correlations originating in nuclear forces (residual interaction) produce various features, including shape evolution and shape coexistence.

33 Evolution of shell structure due to the tensor force Tensor Interaction by pion exchange V T = (τ 1 τ 2 ) ( [σ 1 σ 2 ] (2) Y (2) (Ω) ) Z(r) contributes only to S=1 states relative motion π meson : primary source σ. π σ. ρ meson (~ π+π) : minor (~1/4) cancellation Ref: Osterfeld, Rev. Mod. Phys. 64, 491 (92) Proc. Phys. Math. Soc. Japan 17, 48 (1935)

34 How does the tensor force work? Spin of each nucleon total spin must be S=1 is parallel, because the The potential has the following dependence on the angle θ with respect to the total spin S. V ~ Y 2,0 ~ 1 3 cos 2 θ θ=0 attraction θ=π/2 repulsion S θ relative coordinate nucleons

35 68 Ni 0 + states occupation numbers effective single-particle energies (ESPE) for correlated eigenstate < > monopole part of H < > : by actual occup. numbers ESPE Gaps

36 Effect of the tensor force Present Bohr-model calc. by HFB with Gogny force, Girod, Dessagne, Bernes, Langevin, Pougheon and Roussel, PRC 37,2600 (1988) no (expicit) tensor force

37 Shape evolution and phase transitions 68 Ni 70 Ni 74 Ni critical phenomenon oblate spherical prolate oblate spherical prolate oblate spherical prolate large fluctuation ~6p6h prolate 0 + oblate Ni oblate prolate ~2p2h spherical ó 2 + states in the g 9/2 seniority scheme