Collective Structures in the Pairing Gap, High Resolution Two Proton Stripping and Connections to Double Beta-Decay, seminar by J. F. Sharpey-Schafer (University of Western Cape, South Africa)

mardi 3 févr. 2015, 10:30 → 11:30 Europe/Paris

seminar room, 2nd floor (bat 27)

It is well known that simple monopole pairing is a pretty crude approximation. It can account for the observations that the ground states of all even-even nuclei have spin-parity 0_1+ and that there is a pairing gap above the ground state in deformed nuclei before particle-hole (p-h) configurations can be excited. As an approximation it is best for proton and neutron mid-shell nuclei where the available single particle Nilsson wavefunctions have large overlaps. However at the beginning of regions of deformation, where high-K orbitals can be bought to the Fermi surface from a lower shell (“Flying Fish” orbitals), simple monopole pairing is inadequate in describing the physics of the observed data. This is because the overlap of the wavefunctions is small for low-K deformation driving prolate orbitals and high-K oblate orbitals extruded from a lower shell. This was initially pointed out by Griffin, Jackson and Volkov [1] and used to account for the back-bending frequencies of bands based on high-K orbitals by Jerry Garrett [2]. More recently, with a considerable increase in the quantity and quality of experimental data available, configuration dependent pairing has been used to account for the properties of low-lying first excited 0_2+ states in N=88 and 90 nuclei at the onset of deformation in the rare earths [3,4]. Hence intrinsic collective excitations in deformed nuclei are γ (Y2,2) and Octupole (Y3,μ) vibrations; β (Y2,0) vibrations are at energies above the pairing gap [2,3] and have not been observed. The underlying microscopic structure of γ-vibrations has, for a long time, been of great interest with descriptions of this collective degree of freedom ranging from traditional strong coupling models to quasiparticle-phonon models to IBA and other symmetry representations. In the past, experimentally rotating deformed intrinsic structures to populate levels at high spin, has been an effective method of revealing details of the underlying microscopic configurations. Not many γ-bands have been extended to high spin due to their distance from the yrast line. However, Coulomb excitation experiments and the use of the very large γ-ray detector arrays have enabled the γ-bands in several nuclei to be extended to much higher spins than previously achieved. High resolution measurements have been made with the (3He,nγ) reaction on targets of 27Al, 59Co, 98Mo, 148Nd and 160Gd using the AFRODITE escape-suppressed γ-ray spectrometer in coincidence with a wall of large scintillator neutron detectors at zero degrees to the beamline. In spite of the geometry strongly favoring only L=0 transitions, strongly populated levels are observed that have the spin change between target and residual nuclei |ΔJ|>0. The data are discussed in terms of the incoming 3He Coulomb and/or inelastically exciting the target nucleus before the single step transfer of the two protons. The inelastic excitation followed by the one-step transfer of two protons opens the opportunity of making (3He,n) measurements on low-lying excited states. I will particularly discuss the two proton transfer to the excited 0_2+ states in 100Ru and 150Sm that have been observed in double beta-decay [5,6]. The questions “do β-vibrations exist at all?” and “are they related to Giant Resonances?” will be addressed [7]. [1] R. E. Griffin, A. D. Jackson and A. B. Volkov, Phys. Lett. 36B, 281 (1971). [2] J D Garrett et al., Phys. Lett. B118, 297 (1982). [3] J F Sharpey-Schafer et al, Eu. Phys. J. A47, 5 (2011). [4] J F Sharpey-Schafer et al, Eu. Phys. J. A47, 6 (2011). [5] M. J. Hornish et al., Phys. Rev. C74, 044314 (2006). [6] A. S. Barabash et al., Phys. Rev. C79, 045501 (2009). [7] D. Zawischa, J. Speth and D. Pal, Nucl. Phys. A311, 445 (1978).