G. Tungate

The source of polarized heavy ions (PSI) at the Heidelberg MP tandem

Abstract The new polarized source at the Heidelberg MP tandem is described. The source produces beams of 6 Li , 7 Li and 23 Na ions, which are preaccelerated to 150–200 keV and injected into the MP tandem. The results of the first test runs are presented. A 44 MeV 7 Li beam has been produced and the polarization was detected in a nuclear reaction. A Na beam will be accelerated during one of the next few runs. Improvements of the polarization by laser optical pumping are discussed.

The interaction of aligned 7Li with 58Ni

Abstract Elastic and inelastic scattering of aligned 7 Li on 58 Ni was studied experimentally for several energies between 14 and 22 MeV. Complete sets of second-rank tensor analyzing powers were measured for two energies, 14.2 and 22.0 MeV. The shape-effect model has been used for a qualitative interpretation of the data. The success of this model indicates the spectroscopic deformation of 7 Li to be the main source of the observed second-rank tensor analyzing powers. The result enters the formulation of the diffraction as well as the optical model both of which are used to improve the analysis of the data. Besides the parameters which determine the central interaction the spectroscopic deformation was the only adjustable parameter. The deformation was found to be negative, in agreement with the oblate shape of 7 Li. However the extracted value of the deformation varies within a factor of two when comparing different experiments and/or analyses with different models. The second-rank tensor analyzing powers for projectile excitation are roughly explained by the assumption that the angular momentum transfer is perpendicular to the momentum transfer.

Nuclear spin polarized alkali beams (Li and Na): Production and acceleration

Abstract Recent improvements of the Heidelberg source for polarized heavy ions (PSI) are described. By means of optical pumping in combination with the existing multipole separation magnet the beam figure of merit (polarization 2 × intensity) was doubled. 7 Li and 23 Na atomic beams can now be produced in pure hyperfine magnetic substrates. Fast switching of the polarization is achieved by an adiabatic medium field transition. The hyperfine magnetic substate population is determined by laser-induced fluorescence spectroscopy. In routine operation atomic beams with nuclear polarization p α ≧ 0.85 ( α = z, zz) are obtained. The acceleration of polarized 23 na − ions by a 12 MV tandem accelerator introduces a new problem: the energy at the terminal stripper foil is not sufficient to produce a usable yield of naked ions. For partially stripped ions hyperfine interaction of the remaining electrons with the nuclear spin reduces the polarization. Using in addition the Heidelberg postaccelerator 23 Na 9+ beams of energies between 49 and 184 MeV were obtained with an alignment on target of P zz ≊ 0.45 . 7 Li beams have also been accelerated up to 45 MeV with an alignment of P zz = 0.69.

Elastic and inelastic scattering of polarised 7Li from 120Sn

The differential cross section and analysing powers iT11, TT20, T20, T21 and TT30 have been measured for both the elastic scattering of 44 MeV polarised 7Li from 120Sn and for projectile excitation to the 1/2 state at 478 keV. The data have bee fitted with coupled-channels calculations which include ground-state reorientation of the 7Li and coupling to excited states of 7Li. The sensitivity of the different observables to a number of coupling schemes is discussed. The potentials were derived from a double folding model which explicitly includes the 7Li deformation.

Elastic scattering of vector polarized 7Li on 58Ni

The elastic scattering of 6Li from 4He has been subject of recent investigations1,2). In all cases the observed angular distributions of the differential cross section show pronounced structures at intermediate and backward angles. These structures have been interpreted1,2) as due to the elastic transfer of a deuteron between the two 4He-cores. In order to obtain a more detailed information on the reaction mechanism a study of the vector analyzing power for this reaction had been started using the polarized 6Li beam of the Heidelberg EN-tandem3). The experiment has been performed at 22.8 MeV beam energy and scattering angles of 35 < θcm < 150°. As 4He target a gas cell was used which was specifically constructed to allow for measurements of the left-right asymmetry. The entrance and exit windows consisted of Havar-foils of 2.1 mg/cm2. The target has been operated at pressures around 500 torr 4He corresponding to an effective target thickness between 30 and 80 μg/cm2. Taking into account the energy loss in the entrance foil and in the gas target the beam energy at the active volume of the gas cell is Elab ≈ 21.3 MeV. The scattered 6Li- and the rescattered 4He-nuclei have been detected with ΔE-E telescopes. Relative and absolute normalization of the data has been obtained by measuring the Rutherford scattering of 6Li from a 3.6% Kr-admixture to the 4He-gas.

Dependence of deformed heavy ion (7Li) scattering on the orientation of the alignment axis

Abstract The scattering of aligned 7 Li on 58 Ni at E Li = 14.22 MeV was investigated for different spin alignment axes of the mass quadrupole moment of 7 Li. For a fixed scattering angle the change in cross section due to the alignment of the quadrupole moment turns out to be proportional to the change in distance between the 7 Li and 58 Ni surfaces at the distance of closest approach.

Elastic and inelastic scattering of polarized 7Li ions from a 26Mg target

Abstract Differential cross sections and analyzing powers T T 10 , T T 20 , T 20 , T 21 and T T 30 for elastic scattering of 44 MeV polarized 7Li from 26Mg, for projectile excitation to the 1− 2 state at 0.48 MeV, target excitation to the 2+ state at 1.81 MeV and for the double excitation of both these states have been measured. The mechanism of these processes is studied within a coupled channel formalism. A strong influence of the deformation and excitation of the projectile on all measured observables is found. Further the results of the coupled channel calculations underline the importance of the third rank (TT30) analyzing powers in the analysis of polarized 7Li scattering on 26Mg.

The interaction of polarized 7Li with 12C at 21.1 MeV

Abstract Angular distributions of the vector analyzing power iT 11 and of the second-rank tensor analyzing powers T T 20 , T 20 , T 21 , T 22 have been measured for the elastic scattering of 7 Li from 12 C at 21.1 MeV. The data are discussed within the shape-effect model and analyzed with an optical model including a complex tensor and spin-orbit interaction. Additionally, angular distributions of the differential cross section and of a complete set of tensor analyzing powers were determined for the one-neutron transfer reaction 12 C( 7 Li, 6 Li) 13 C g.s. .

Study of the one-neutron transfer reactions induced by polarized 7Li ON 26Mg and 120Sn

Abstract Angular distributions of the cross sections and analyzing powers up to third rank have been measured for the one-neutron transfer reactions 26Mg(7Li,6Li)27Mg, 120Sn(7Li,6Li)121Sn and 120Sn(7Li, 8Li)129Sn initiated by polarized 7Li ions of 44 MeV. Third-rank analyzing power has been measured for the first time in transfer reactions. Coupled-channels calculations in which the ground and first excited states of 7Li are coupled together by collective interactions and one-neutron transfers are calculated in exact finite range explain the experimental data for low-lying states in final nuclei. Extracted spectroscopic factors are in good agreement with those obtained from other one-neutron transfer reactions on the same targets.

Tidal symmetry in scattering of polarised 7Li projectiles

Tidal symmetry is a recently proposed concept which introduces considerable simplifications into the theoretical analysis of heavy-ion scattering and fusion. It also facilitates the understanding of features of nuclear reactions induced by polarised heavy ions. Key pointers to this symmetry are presented and compared with new experimental results and published data for 7Li elastic scattering. The discussion gives an indication of the usefulness of the tidal symmetry concept.