D. Kassen

Elastic Scattering of Vector Polarized 6Li

Using the vector polarized 6Li-beam of the Heidelberg EN-Tandem1) the elastic scattering of 6Li on 12C, 16O, 28Si and 58Ni has been investigated at a bombarding energy of ELi = 22.8 MeV. The structure of the observed angular distributions of σ/σR depends in heavy ion scattering critically on E/Ecoul. Therefore the observed angular distributions of σ/σR display for 12C (E/Ecoul≈ 3.5) an oscillating pattern (fig.1) which looks like Frauenhofer diffraction scattering. The angular distribution of σ/σR for 58Ni (E/Ecoul≈ 1.4) is without any structure (fig.1) and looks like Fresnel diffraction scattering. The analyzing power had been obtained by measuring successively with a polarized and unpolarized beam (high frequency transition of the source1) on and off respectively). The polarization of the beam is known to be between 0.52 and 0.6. The lower bound is deduced from the highest up to now with this source observed asymmetry in the 6Li-4He elastic scattering2). The higher bound is estimated from the source parameters itself1). The polarization was monitored after some couple of runs either with the 1H(6Li,4He)3He reaction or, in a later stage of the experiment using the elastic scattering of 6Li on 12C. Around θcm = 44.5° and ELi = 22.8 MeV a flat maximum of the asymmetry connected with a fairly high cross section had been observed (fig.1).

The source for a vector polarized lithium-6 beam at the Heidelberg en-tandem

Abstract The source for negative polarized lithium ions at the Heidelberg EN-tandem is described. The polarized beam is produced following the atomic beam method. A nearly 100% efficient surface ionizer is used. Accelerated beams of 6 Li 3+ -ions are up to 150 nA on the target. The vector polarization can be estimated to be 0.57≤| P |≤0.65. The production and acceleration of vector and tensor polarized beams of 6 Li, 7 Li and 23 Na are discussed.

Shape effects in heavy ion (Li) elastic scattering

Abstract The elastic scattering of aligned 6 Li and 7 Li on 58 Ni at E cm = 12.7 MeV displays an “isotopic” effect which can be traced to the very different (mass) quadrupole moments of both isotopes. The elastic scattering of aligned 6 Li and 7 Li on 12,13 C at E Li = 9 MeV displays large effects, but the angular dependence is much more complicated compared to the scattering on 58 Ni.

Vector analyzing power in the elastic scattering of 6Li by 4He

Abstract The elastic scattering of vector polarized 6 Li by 4 He has been investigated in the energy range of 15.1 to 22.7 MeV. The observed large analyzing powers indicate a strong spin-orbit potential for the 6 Li 4 He interaction. Optical model calculations using a simple L · S -potential without exchange do not reproduce the data.

Transfer reactions initiated on 12C by vector polarized 6Li

Abstract With a vector polarized 6 Li beam the 12 C( 6 Li, d) and 12 C( 6 Li,α) reactions have been investigated at E Li =20.0 MeV. The observed analyzing powers turn out to be large. The four-nucleon transfer to the ground state of 16 O is well described by an optical potential (central and spin-orbit) which was obtained from the elastic scattering of vector polarized 6 Li on 12 C.

Production and calibration of a tensor polarized 6Li-beam

Abstract The installation of strong field transitions makes it possible to produce tensor polarized 6 Li-beams at the Heidelberg EN-Tandem. The tensor polarisation of the accelerated 6 Li-beam determined can be determined absolutely. With the present arrangement it was found to be up to 95% of its maximum value, depending on the adjustment of the two-level transitions.

Elastic scattering of aligned lithium ions from nickel

Aligned lithium 6 and 7 beams were elastically scattered from nickel-58 at 12.7 MeV. The measured tensor analyzing power was converted into a defined cross section ratio for the fully aligned and the upolarized beams. The angular distributions for the two beams coincide within experimental errors. 3 references. (JFP)

The Source for Polarized 6-Lithium Ions at the Heidelberg En-Tandem

The source was developed at the University of Hamburgl,2,3) and installed at the Heidelberg EN-Tandem4). The principle is that of an atomic beam source, but the usually used electron bombardement ionizer is replaced by a surface ionizier with an efficiency close to one. The outline of the source is displayed in fig.1. An intense atomic beam is formed by evaporating 6Li from an oven through a Laval nozzle (0.5 mm o). After that the core of the beam is skimmed by a conical and a plane heated orifice (1 and 1.5 mm o respectively). The beam is polarized with respect to the elctron spin by passing a permanent 6-pole magnet. Than it enters a weak field transition. The transition unit, which had an efficiency of 90% 4) has been redesigned in some parts and reaches now full efficiency. The surface ionizer consists of a 10×25 mm2, 0.1 thick tungsten strip heated to about 1000° C. A monolayer of oxygen which increases the work function of tungsten to values higher than the ionization energy of lithium5) is provided by an oxygen leak near the tungsten strip. In this way a nearly 100% efficiency of the ionizer is achieved. Since the surface absorption time is known to be 20 to 100 ms for temperatures around 1000°C 6) a strong magnetic field (B/B c=4) has to be provided for the ionization region in order to avoid depolarization effects and to define the direction of polarization.

Elastic Scattering of Vector Polarized 6 Li on 4 He

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.

Population of an Atomic Substate by the Weak Field Transition and Vector Polarization

The Heidelberg source for polarized lithium ionsl) offers the possibility of measuring the population number N4 of the 4th hfs component (F = 3/2, mF = −3/2). This can be done by analyzing the electronic polarization of the atomic beam by a second 6-pole magnet2). A small portion of the atomic beam passes the ionizer strip and reaches the Langmuir-Taylor detector (see ref.1,Fig.1; ion current about 10 nA). Therefore the measurement of N4 is possible during the normal operation of the source. The proper operation of the weak field transition, which is assumed to transfer mF into -mF components, is indicated by a fall-off of the detector current to 2/3, when switching on the weak field transition. This means that the population number N4 was changed by the transition from zero to 1/3.