R. Butsch

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 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.

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.

Complete and incomplete fusion in 19 F + 40 Ca and 32 S + 27 Al

Abstract Velocity distributions and excitation functions of fusionlike evaporation residues (ER) were measured for 41–216 MeV 19 F + 40 Ca and 142–393 MeV 32 S + 27 Al . The ER cross sections for 19 F + 40 Ca exceed those of 32 S + 27 Al by up to 44%, mainly due to an earlier onset of incomplete fusion of about 3.7 MeV/nucleon above the interaction barrier in 19 F + 40 Ca , compared to 5.6–7.7 MeV/nucleon in 32 S + 27 Al .

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.

Quasielastic scattering and subbarrier fusion of aligned 23Na with 206Pb

Abstract Based on a local but energy dependent potential for the system 23 Na 206 Pb at sub-and near-barrier energies a consistent description of (quasi) elastic scattering and fusion has been achieved. The local real and imaginary energy dependent potentials were deduced from quasielastic scattering of aligned 23 Na projectiles on 206 Pb. They obey a dispersion relation. In calculations for both quasielactic scattering and fusion, coupling to the first two excited states of the 23 Na projectiles were treated explicitly.

Semiclassical description of analyzing powers in one-nucleon transfer reactions induced by 44MeV polarized 7Li ions on 120Sn

Abstract Analyzing powers of various ranks for the reaction 120 Sn( 7 L , 6Li) at Elab = 44 MeV have been measured as functions of scattering angle and Q-value. The reaction is understood semiclassically as transfer of the neutron from one reaction partner to the other during their relative motion on classical trajectories. An approximate evaluation of transfer amplitudes is presented which takes care of the relative magnitude of amplitudes for various spin quantum numbers and also of the discontinuity of the trajectory angular momentum in the interaction point. Thus simple formulae for analyzing powers are derived. All the characteristic features of the data are reproduced within this model.

Influence of limiting angular momenta on complete and incomplete fusion-evaporation residue cross sections

Abstract Velocity distributions and cross sections of fusion-like evaporation residues were measured for the reactions 260, 287 MeV 23 Na → 40 Ca and 298, 327 MeV 28 Si → 40 Ca. Limiting angular momenta for complete and incomplete fusion reactions were estimated and found to be compatible with fission limits of the corresponding compound nuclei. The implications of such limitations for the dependence of the total evaporation residue cross section on the mass asymmetry in the entrance channel are discussed.

A three dimensionally position sensitive large area detector

Abstract A large area detector consisting of a parallel plate avalanche counter (PPAC) and a trapezohedral ionization chamber (TIC) is described. Its active area is 184 cm 2 . The time resolution of the PPAC is 175 ps. The energy resolution of the TIC is 0.4%, the energy loss resolution 2.8%, the nuclear charge resolution 2.3%. The TIC is position sensitive in three dimensions. The position x is measured via a saw-tooth anode with a resolution of 0.7 mm; the drift time coordinate shows a resolution of δy ≅ mm. The range z is determined by a new technique, a graded density Frisch grid. It enlarges the dynamic range of the charge measurement down to the Bragg maximum at E / A ∼ 1 MeV. The resolution is δZ / Z ≅ 3.5%