IAEA Workshop on NSDD, Trieste, November 2003 The nuclear shell model P. Van Isacker, GANIL, France Context and assumptions of the model Symmetries of the shell model: Racah’s SU(2) pairing model Wigner’s SU(4) symmetry Elliott’s SU(3) model of rotation

IAEA Workshop on NSDD, Trieste, November 2003 Overview of nuclear models Ab initio methods: Description of nuclei starting from the bare nn & nnn interactions. Nuclear shell model: Nuclear average potential + (residual) interaction between nucleons. Mean-field methods: Nuclear average potential with global parametrization (+ correlations). Phenomenological models: Specific nuclei or properties with local parametrization.

IAEA Workshop on NSDD, Trieste, November 2003 Ab initio methods Faddeev-Yakubovsky: A≤4 Coupled-rearrangement-channel Gaussian- basis variational: A≤4 (higher with clusters) Stochastic variational: A≤7 Hyperspherical harmonic variational: Green’s function Monte Carlo: A≤7 No-core shell model: A≤12 Effective interaction hyperspherical: A≤6

IAEA Workshop on NSDD, Trieste, November 2003 Benchmark calculation for A=4 Test calculation with realistic interaction: all methods agree. But E expt = MeV  need for three- nucleon interaction. H. Kamada et al., Phys. Rev. C 63 (2001)

IAEA Workshop on NSDD, Trieste, November 2003 Basic symmetries Non-relativistic Schrödinger equation: Symmetry or invariance under: –Translations  linear momentum P –Rotations  angular momentum J=L+S –Space reflection  parity  –Time reversal

IAEA Workshop on NSDD, Trieste, November 2003 Nuclear shell model Separation in mean field + residual interaction: Independent-particle assumption. Choose V and neglect residual interaction:

IAEA Workshop on NSDD, Trieste, November 2003 Independent-particle shell model Solution for one particle: Solution for many particles:

IAEA Workshop on NSDD, Trieste, November 2003 Independent-particle shell model Antisymmetric solution for many particles (Slater determinant): Example for A=2 particles:

IAEA Workshop on NSDD, Trieste, November 2003 Hartree-Fock approximation Vary  i (ie V) to minize the expectation value of H in a Slater determinant: Application requires choice of H. Many global parametrizations (Skyrme, Gogny,…) have been developed.

IAEA Workshop on NSDD, Trieste, November 2003 Poor man’s Hartree-Fock Choose a simple, analytically solvable V that approximates the microscopic HF potential: Contains –Harmonic oscillator potential with constant . –Spin-orbit term with strength  ls. –Orbit-orbit term with strength  ll. Adjust ,  ls and  ll to best reproduce HF.

IAEA Workshop on NSDD, Trieste, November 2003 Energy levels of harmonic oscillator Typical parameter values: ‘Magic’ numbers at 2, 8, 20, 28, 50, 82, 126, 184,…

IAEA Workshop on NSDD, Trieste, November 2003 Evidence for shell structure Evidence for nuclear shell structure from –Excitation energies in even-even nuclei –Nucleon-separation energies –Nuclear masses –Nuclear level densities –Reaction cross sections Is nuclear shell structure modified away from the line of stability?

IAEA Workshop on NSDD, Trieste, November 2003 Shell structure from E x (2 1 ) High E x (2 1 ) indicates stable shell structure:

IAEA Workshop on NSDD, Trieste, November 2003 Shell structure from S n or S p Change in slope of S n (S p ) indicates neutron (proton) shell closure (constant N-Z plots): A. Ozawa et al., Phys. Rev. Lett. 84 (2000) 5493

IAEA Workshop on NSDD, Trieste, November 2003 Shell structure from masses Deviations from Weizsäcker mass formula:

IAEA Workshop on NSDD, Trieste, November 2003 Shell structure from masses Deviations from improved Weizsäcker mass formula that includes n n  and n +n  terms:

IAEA Workshop on NSDD, Trieste, November 2003 Validity of SM wave functions Example: Elastic electron scattering on 206 Pb and 205 Tl, differing by a 3s proton. Measured ratio agrees with shell-model prediction for 3s orbit with modified occupation. J.M. Cavedon et al., Phys. Rev. Lett. 49 (1982) 978

IAEA Workshop on NSDD, Trieste, November 2003 Nuclear shell model The full shell-model hamiltonian: Valence nucleons: Neutrons or protons that are in excess of the last, completely filled shell. Usual approximation: Consider the residual interaction V RI among valence nucleons only. Sometimes: Include selected core excitations (‘intruder’ states).

IAEA Workshop on NSDD, Trieste, November 2003 The shell-model problem Solve the eigenvalue problem associated with the matrix (n active nucleons): Methods of solution: –Diagonalization (Strasbourg-Madrid): 10 9 –Monte-Carlo (Pasadena-Oak Ridge): –Quantum Monte-Carlo (Tokyo): –Group renormalization (Madrid-Newark):

IAEA Workshop on NSDD, Trieste, November 2003 Residual shell-model interaction Four approaches: –Effective: Derive from free nn interaction taking account of the nuclear medium. –Empirical: Adjust matrix elements of residual interaction to data. Examples: p, sd and pf shells. –Effective-empirical: Effective interaction with some adjusted (monopole) matrix elements. –Schematic: Assume a simple spatial form and calculate its matrix elements in a harmonic- oscillator basis. Example:  interaction.

IAEA Workshop on NSDD, Trieste, November 2003 Schematic short-range interaction Delta interaction in harmonic-oscillator basis. Example of 42 Sc 21 (1 active neutron + 1 active proton):

IAEA Workshop on NSDD, Trieste, November 2003 Symmetries of the shell model Three bench-mark solutions: –No residual interaction  IP shell model. –Pairing (in jj coupling)  Racah’s SU(2). –Quadrupole (in LS coupling)  Elliott’s SU(3). Symmetry triangle:

IAEA Workshop on NSDD, Trieste, November 2003 Racah’s SU(2) pairing model Assume large spin-orbit splitting  ls which implies a jj coupling scheme. Assume pairing interaction in a single-j shell: Spectrum of 210 Pb:

IAEA Workshop on NSDD, Trieste, November 2003 Solution of pairing hamiltonian Analytic solution of pairing hamiltonian for identical nucleons in a single-j shell: Seniority  (number of nucleons not in pairs coupled to J=0) is a good quantum number. Correlated ground-state solution (cfr. super- fluidity in solid-state physics). G. Racah, Phys. Rev. 63 (1943) 367

IAEA Workshop on NSDD, Trieste, November 2003 Pairing and superfluidity Ground states of a pairing hamiltonian have superfluid character: –Even-even nucleus (  =0): –Odd-mass nucleus (  =1): Nuclear superfluidity leads to –Constant energy of first 2 + in even-even nuclei. –Odd-even staggering in masses. –Two-particle (2n or 2p) transfer enhancement.

IAEA Workshop on NSDD, Trieste, November 2003 Superfluidity in semi-magic nuclei Even-even nuclei: –Ground state has  =0. –First-excited state has  =2. –Pairing produces constant energy gap: Example of Sn nuclei:

IAEA Workshop on NSDD, Trieste, November 2003 Two-nucleon separation energies Two-nucleon separation energies S 2n : (a) Shell splitting dominates over interaction. (b) Interaction dominates over shell splitting. (c) S 2n in tin isotopes.

IAEA Workshop on NSDD, Trieste, November 2003 Generalized pairing models Trivial generalization from a single-j shell to several degenerate j shells: Pairing with neutrons and protons: –T=1 pairing: SO(5). –T=0 & T=1 pairing: SO(8). Non-degenerate shells: –Talmi’s generalized seniority. –Richardson’s integrable pairing model.

IAEA Workshop on NSDD, Trieste, November 2003 Pairing with neutrons and protons For neutrons and protons two pairs and hence two pairing interactions are possible: –Isoscalar (S=1,T=0): –Isovector (S=0,T=1):

IAEA Workshop on NSDD, Trieste, November 2003 Superfluidity of N=Z nuclei Ground state of a T=1 pairing hamiltonian for identical nucleons is superfluid, (S + ) n/2  o . Ground state of a T=0 & T=1 pairing hamiltonian with equal number of neutrons and protons has different superfluid character:  Condensate of  ’s (  depends on g 0 /g 1 ). Observations: –Isoscalar component in condensate survives only in N~Z nuclei, if anywhere at all. –Spin-orbit term reduces isoscalar component.

IAEA Workshop on NSDD, Trieste, November 2003 Wigner’s SU(4) symmetry Assume the nuclear hamiltonian is invariant under spin and isospin rotations: Since {S ,T,Y  } form an SU(4) algebra: –H nucl has SU(4) symmetry. –Total spin S, total orbital angular momentum L, total isospin T and SU(4) labels (  ) are conserved quantum numbers. E.P. Wigner, Phys. Rev. 51 (1937) 106 F. Hund, Z. Phys. 105 (1937) 202

IAEA Workshop on NSDD, Trieste, November 2003 Physical origin of SU(4) symmetry SU(4) labels specify the separate spatial and spin-isospin symmetry of the wavefunction: Nuclear interaction is short-range attractive and hence favours maximal spatial symmetry.

IAEA Workshop on NSDD, Trieste, November 2003 Breaking of SU(4) symmetry Breaking of SU(4) symmetry as a consequence of –Spin-orbit term in nuclear mean field. –Coulomb interaction. –Spin-dependence of residual interaction. Evidence for SU(4) symmetry breaking from –Masses: rough estimate of nuclear BE from –  decay: Gamow-Teller operator Y ,  1 is a generator of SU(4)  selection rule in (  ).

IAEA Workshop on NSDD, Trieste, November 2003 SU(4) breaking from masses Double binding energy difference  V np  V np in sd-shell nuclei: P. Van Isacker et al., Phys. Rev. Lett. 74 (1995) 4607

IAEA Workshop on NSDD, Trieste, November 2003 SU(4) breaking from  decay Gamow-Teller decay into odd-odd or even- even N=Z nuclei: P. Halse & B.R. Barrett, Ann. Phys. (NY) 192 (1989) 204

IAEA Workshop on NSDD, Trieste, November 2003 Elliott’s SU(3) model of rotation Harmonic oscillator mean field (no spin-orbit) with residual interaction of quadrupole type: J.P. Elliott, Proc. Roy. Soc. A 245 (1958) 128; 562

IAEA Workshop on NSDD, Trieste, November 2003 Importance and limitations of SU(3) Historical importance: –Bridge between the spherical shell model and the liquid droplet model through mixing of orbits. –Spectrum generating algebra of Wigner’s SU(4) supermultiplet. Limitations: –LS (Russell-Saunders) coupling, not jj coupling (zero spin-orbit splitting)  beginning of sd shell. –Q is the algebraic quadrupole operator  no major-shell mixing.

IAEA Workshop on NSDD, Trieste, November 2003 Generalized SU(3) models How to obtain rotational features in a jj- coupling limit of the nuclear shell model? Several efforts since Elliott: –Pseudo-spin symmetry. –Quasi-SU(3) symmetry (Zuker). –Effective symmetries (Rowe). –FDSM: fermion dynamical symmetry model. –...