Spin-polarized Beam Research Articles

Measurement of Parity Nonconservation and an Anapole Moment in Cesium

The amplitude of the parity-nonconserving transition between the 6S and 7S states of cesium was precisely measured with the use of a spin-polarized atomic beam. This measurement gives Im(E1pnc)/beta = -1.5935(56) millivolts per centimeter and provides an improved test of the standard model at low energy, including a value for the S parameter of -1.3(3)exp (11)theory. The nuclear spin-dependent contribution was 0.077(11) millivolts per centimeter; this contribution is a manifestation of parity violation in atomic nuclei and is a measurement of the long-sought anapole moment.

Precision measurement of the ratio of scalar to tensor transition polarizabilities for the cesium 6S-7S transition

The ratio of scalar to tensor transition polarizabilities (\ensuremath{\alpha}/\ensuremath{\mathrm{B}}) for the cesium ${6\mathrm{S}}_{1\mathrm{/}2}$F=3 to ${7\mathrm{S}}_{1\mathrm{/}2}$F\ensuremath{'}=3 transition has been measured using a spin-polarized atomic beam and an interference technique. We use an electric- and magnetic-field configuration where the Stark-induced electric dipole transition amplitudes arising from the scalar and tensor polarizabilities interfere, giving rise to large changes in the 6S-7S transition rate. The measured ratio is \ensuremath{\alpha}/\ensuremath{\mathrm{B}}=-9.905\ifmmode\pm\else\textpm\fi{}0.011. This result represents a tenfold improvement in precision, and agrees well with previous measurements and calculations. The ratio \ensuremath{\alpha}/\ensuremath{\mathrm{B}} is of critical importance for cesium atomic parity violation experiments.

A New Spin Filter: The Magnetic Schottky Diode

ABSTRACTWe observe spin-dependent transmission of hot electrons through the ferromagnetic metallic layer of a Schottky diode. A 25% spin-polarized electron beam is produced in vacuum by a GaAs photocathode under illumination with circularly polarized light. The photoemitted electrons are then injected from the vacuum into the sample: a 3.5 nm-thick Fe layer deposited on a n-doped GaAs substrate. We measure a transmission coefficient of the order of a few 10-4. Using an incident energy above the Fermi level equal to 5 eV, a 5% asymmetry in the transmitted current is observed by changing the spin-polarization of the incident electrons (i.e. the light polarization) and/or reversing the Fe-layer magnetization. This corresponds to a transmission spin-dependence of 20% for a 100% spin-polarized incident beam.

Inelastic scattering of polarized electrons from nonmagnetic metals via electron-hole excitation

Inelastic scattering of low-energy electrons from nonmagnetic metals due to electron-hole pair excitation is treated in a distorted-wave first Born approximation including exchange. Screened Coulomb interaction matrix elements between bulk Bloch states and plane waves (including elastic scattering in a simple approximation) are evaluated to yield, for a spin-polarized primary beam, the intensities of electrons scattered with and without spin flip. Some basic insight is obtained by calculations for a homogeneous electron gas. Numerical results for Cu(001) are found to agree fairly well with experimental data and provide clear evidence for the importance of elastic scattering processes in electron energy loss spectroscopy.

Installation of an on-line Lamb–shift spin–filter polarimeter in the Triangle Universities Nuclear Laboratory atomic beam polarized ion source (abstract)a)

An atomic beam polarized ion source, used heavily since 1989 for producing polarized H± and D± beams for experiments between 25 keV and 20 MeV, have been modified to accept a Lamb–shift, spin–filter polarimeter. In this source, polarized ground-state H or D atoms enter an electron cyclotron resonance ionizer where they are stripped to produce an outgoing positive polarized ion beam. When negative ions are desired, cesium vapor is introduced into a downstream charge-exchange canal. The polarimeter, based on an atomic physics concept first developed to produce nuclear–spin-polarized beams at Los Alamos, is designed to monitor the polarization of 2S1/2 metastable H or D atoms emerging from this cesium canal. Metastable 2S1/2 atoms created by electron pickup in a collision with cesium are ‘‘filtered’’ by the polarimeter according to a magnetic substate, as the magnetic field imposed on the polarimeter cavity is tuned between 53 and 61 mT. Photons produced by subsequent quenching of these filtered atoms to their ground state are monitored downstream by a phototube to reveal the magnetic substate population of the incident positive beam. To install the polarimeter cavity and phototube assembly, the existing polarized ion source was lengthened by 30 cm. Installation is complete, its calibration has been checked, and initial impressions gained from its use are presented.

Observation of a Second-Order Spin-Depolarizing Resonance.

A recent experiment using a 160 MeV stored spin-polarized proton beam found clear evidence for a second-order intrinsic depolarizing resonance. With a 20% partial Siberian snake, we studied a first-order intrinsic depolarizing resonance; we then discovered and studied the second-order depolarizing resonance by changing both the horizontal and vertical betatron tunes. As expected, this second-order resonance is much weaker than the first-order resonance; it is probably due to sextupole fields plus coupling between the horizontal and vertical betatron oscillations caused by the 20% snake.

A model of quantum measurement in Josephson junctions

A model for the quantum measurement of the electronic current in a Josephson junction is presented and analyzed. The model is similar to a Stern-Gerlach apparatus, relying on the deflection of a spin-polarized particle beam by the magnetic field created by the Josephson current. The aim is (1) to explore, with the help of a simple model, some general ideas about the nature of the information which can be obtained by measurements upon a quantum system and (2) to find new approaches for obtaining information about the nature of the states of a macroscopic quantum system. In the case of sufficiently strong coupling between the system and the apparatus, we find that the model provides in principle a standard ideal measurement of the value of the instantaneous Josephson current. In the case of weak coupling, where the measurement is not ideal, we show that the scattering of neutrons from a junction can in principle be used to measure the average value of the Josephson current, thereby allowing an experimental distinction to be made between an eigenstate of relative phase and one of relative Cooper pair number. The possibility of the latter type of measurement suggests an experimental approach to answer a question of fundamental interest, namely whether two isolated superconductors (or superfluids) possess a definite relative phase or a definite relative number of superconducting (or super/lowing) particles.

Spin-polarized radioactive beams and β-NMR experiments

We discuss a new method of producing spin-polarized radioactive nuclear probes which capitalizes characteristic features of projectile fragmentation reaction at intermediate energies. Experimental results are given for the behavior of polarization as a function of the fragment momentum, target mass number, and beam energy. As an application of this method, we also report on results ofg-factor measurements for several neutron-rich nuclei. A full account of theg-factor measurement will be presented elsewhere.

Gluon bremsstrahlung from massive quarks in high energy collisions of polarized electrons and positrons.

The gluon bremsstrahlung cross section ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{\ensuremath{-}}}$\ensuremath{\rightarrow}qq\ifmmode\bar\else\textasciimacron\fi{}g, including effects of finite quark and antiquark masses, is calculated for arbitrarily spin-polarized electron-positron beams. Mass effects and polarization effects are given and are shown to have a sizable influence on the cross section. It is shown, however, that for the left-right asymmetry ${\mathit{A}}_{\mathit{L}\mathit{R}}$ the mass corrections and radiative corrections vanish at the ${\mathit{Z}}^{0}$ pole. The use of longitudinal polarized electrons in measurements of the forward-backward asymmetry ${\mathit{A}}_{\mathit{F}\mathit{B}}$ may give sizable enhancements.

First test of a partial Siberian snake during polarized beam acceleration.

We recently studied the first acceleration of a spin-polarized beam through a depolarizing resonance using a partial Siberian snake. We accelerated polarized protons from 95 to 140 MeV while ramping a 10% partial Siberian snake along with the acceleration cycle. The 10% partial snake suppressed all observable depolarization due to the $G\ensuremath{\gamma}=2$ imperfection depolarizing resonance which occurred near 108 MeV during acceleration. However, 20% and 30% partial Siberian snakes apparently moved the $G\ensuremath{\gamma}=7\ensuremath{-}{\ensuremath{\nu}}_{y}$ intrinsic depolarizing resonance from near 177 MeV into our energy range; this caused some interesting but not-yet-fully understood depolarization.

High-brightness, high-purity spin-polarized cesium beam.

We describe a cesium atomic beam (${10}^{14}$ atoms ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$, 6\ifmmode\times\else\texttimes\fi{}${10}^{16}$ atoms ${\mathrm{sr}}^{\mathrm{\ensuremath{-}}1}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$) spin polarized using light from two diode lasers. Of the atoms, 95% may be placed into either the 6${\mathit{S}}_{1/2}$(F,${\mathit{m}}_{\mathit{F}}$)=(3,3) or (4,4) state, with less than 2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$ of the atoms left in the depleted hyperfine level. The latter fraction rises linearly with atomic beam intensity because of reabsorption of light scattered during the optical pumping process. This and other effects limiting complete hyperfine pumping are discussed.

The RIKEN radioactive beam facility

A radioactive beam facility based on projectile fragmentation, RIPS, has been constructed at RIKEN to enable radioactive-beam experiments. The facility is characterized by the production of high-intensity beams and spin-polarized beams. Since the beginning of 1990, experiments on exotic nuclei have been extensively performed exploiting these useful features. The characteristics and the present status of the RIPS are described.

Spin-exchange optical pumping as a source of spin-polarized atomic deuterium.

Optical pumping of potassium atoms followed by spin-exchange scattering with deuterium atoms in a high magnetic field was found to produce an intense, highly spin-polarized beam of atomic deuterium. In particular, the atomic polarization of deuterium was determined to be (73\ifmmode\pm\else\textpm\fi{}3)% at an intensity of 2.1\ifmmode\times\else\texttimes\fi{}${10}^{17}$ atoms/s.

Production and diagnosis of a highly spin-polarized Na beam

We describe optically pumping a beam of sodium atoms to >96% mS and >92% mS, mI state selection. (We have accurately measured the population of every mS, mI state in the optically pumped beam.) For the optical pumping both ground hyperfine states are pumped, using single-mode cw dye-laser radiation tuned to the 3S1/2–3P1/2 transition that is phase modulated in a LiTaO3 crystal to produce first-order sidebands at approximately the 1772-MHz hyperfine splitting of the ground state. The z-directed optical pumping is performed in a z-directed magnetic field of ~5 G. The state-selected atoms then move, in ~1 cm, into an ~200 G, z-directed field. The downstream probe laser beam is scanned through the 3S1/2 (mS, mI) → 3P3/2 (mS′, mI) transitions, which are spectroscopically resolved at 200 G, and the fluorescence intensities portray the residual populations in each of the eight 3S1/2 states.

Proposal of Metastable Spin-Polarized He as a Probe of Antiferromagnetic Transition Metal Oxide Surfaces

ABSTRACTWe suggest a new application for elastic scattering of a metastable spin-polarized atomic helium beam at thermal energy. We demonstrate how angle-resolved measurements of the Bragg peaks of scattered surviving metastable atoms can give information about the spin-ordering of an antiferromagnetic (AF) transition metal oxide surface. In this paper, we discuss the feasibility of such measurements for NiO(100) and MnO(100), based on available information about their electronic structure and the properties of spin-polarized metastable helium. On impact with a surface, the survival probability of metastables is generally very low (<10-2). There are two possible decay mechanisms for metastables, a resonance ionization followed by auger neutralization, or an auger de-excitation process. For AF surfaces which fulfill certain requirements on their electronic structure, spin-selection rules will inhibit the decay of the metastable atoms from a favourably aligned magnetic sublattice. The survival probability will then be dramatically enhanced from the chosen sublattice, and the coherently scattered surviving metastables will reflect the periodicity of that magnetic sublattice. In contrast to other methods currently applied to magnetic systems, this method does not rely on difference spectra. Consequently, reversal of spinorientation is not necessary for the observation of magnetic ordering.

Sources of spin-polarized beams (invited)

Sources producing beams of spin-polarized atoms and ions are now widely used in nuclear physics. Modern systems are increasingly expensive and complex. They provide a variety of species and beam polarizations with intensities often sufficient for experimental needs. The laboratory seeking such a device for its program must evaluate a variety of options and match them to its accelerator, experimental program, and expected resources. The major systems and techniques used are surveyed and criteria dictating the choice of source for particular experimental applications are discussed briefly.

Polarized light emission after grazing ion-surface scattering due to capture of spin-polarized electrons.

We have observed the capture of polarized electrons into excited terms of atoms after the interaction of fast ions with a magnetized Fe(110) surface at grazing incidence. The spin polarization of captured electrons results in a modified circular polarization fraction of fluorescence light. This experiment has considerable potential as a new analytical tool for investigating surface magnetism with extreme surface sensitivity, as a method for detailed studies of ion-surface interaction, and as a means to produce nuclear spin-polarized beams.

Spin-polarized electron microscopy

A current scientific challenge with many ramifications for magnetic technology is to image magnetic microstructure with the highest possible spatial resolution in order to observe magnetic domains or even spin configurations within a domain wall. Ultimately one can envision imaging the magnetic moment of individual atoms which would also make possible the observation of antiferromagnetic structures. The measurement of the spin polarization of secondary electrons generated by a finely focussed (unpolarized) scanning electron microscope (SEM) beam to obtain high-resolution magnetization images is presented. An alternative measurement, using a spin-polarized incident beam in an SEM, has many difficulties which are discussed. To measure spin configurations with higher spatial resolution, the possibility of introducing electron spin polarization in scanning field-emission and tunneling microscopy is considered. The measurement of the spin polarization of secondary electrons generated by a specially prepared single-atom scanning field-emission tip looks promising. High-resolution imaging of spin configurations in scanning tunneling microscopy appears possible if the tip itself is a source of spin-polarized electrons. The potential advantages and unsolved problems involved in using a ferromagnetic tip or an optically pumped semiconductor tip are described.

Collisional Pumping for the Production of Intense Spin-Polarized Neutral Beams: Target Considerations

Neutral-beam heating and/or fueling offers the possibility of establishing deuteron and triton spins oriented parallel to the magnetic field (or antiparallel to the field). It is believed that the polarizations can be maintained by guide fields until the particles are within the reactor. A beam suitable for this magnetic fusion-energy application would require five to six orders of magnitude increase in polarized beam intensity from the limits imposed by present technology, which is on the order of 10-100 ..mu..A. We have recently proposed that ampere-sized beams of nuclear-spin-polarized ions and atoms can be produced by multiple atomic collisions in an electron-spin-polarized medium. By analogy to optical pumping, we have called the process ''collisional pumping.'' Such pumping will occur when an ion beam passes through a thick electron-spinpolarized target in a low magnetic field. As an ion in the beam undergoes a succession of electron-capture and -loss collisions, polarization is transferred from the electron to the nucleus through the hyperfine interaction. After a sufficient number of charge-changing collisions, both the electron and nuclear polarization of the beam will be pumped to the electron polarization of the target. We have analyzed and theoretically demonstrated several examples of collisional pumping; the case thatmore » lends itself to earliest testing is that of a low-energy (0.1 - 10 keV/u) hydrogen or deuterium beam passing through a thick electron-spin-polarized alkalivapor target in a low magnetic field.« less

New Aspect in Application of Spin-Polarized Beam to Nuclear Reactions

New Aspect in Application of Spin-Polarized Beam to Nuclear Reactions