Polarization detection of trapped electrons via interaction with polarized atoms

Electrons were trapped in an electrostatic quadrupole trap with superimposed homogeneous magnetic field. The electrons were polarized by spin exchange with a polarized atomic beam. The free trapped electron polarization was converted to a change in the electron translational energy via spin-dependent inelastic collisions with the atomic beam, and the electron translational temperature was monitored. Discussed are the development of this variation of the measurement technique, characteristics of electron storage, and the electron-polarized atom inelastic interaction as a function of electron temperature and time. The method has been applied to the detection of the (g-2) resonance of free, stored electrons.     

PHENIX spin program,recent results

Acceleration of polarized protons in Relativistic Heavy Ion Collider (RHIC) provides unique tool to study the spin structure of the nucleon. We give a brief overview of the PHENIX program to investigate poorly known gluon and flavor decomposed see quark polarization in the proton, utilizing polarized proton collisions at RHIC. We report PHENIX first results on transverse single-spin asymmetry inπ ⁰ and charged hadron production and longitudinal double-spin asymmetry inπ ⁰ production at mid-rapidity.     

RHIC polarized proton status and plan

The Relativistic Heavy Ion Collider (RHIC) as the first high energy polarized proton collider has been providing collisions at a beam energy of 100 GeV since 2001. Equipped with two full Siberian snakes in each ring, polarization is preserved during the acceleration from injection to 100 GeV with careful control of the betatron tunes and the vertical orbit distortions. In the latest RHIC polarized proton run in 2006, a peak luminosity of 28 × 10³⁰cm⁻² s⁻¹ with 60% average polarization at store was achieved. During the run, RHIC also demonstrated its capability in providing a combination of polarized proton collisions with longitudinal polarization and radial polarization were provided to the STAR experiment and PHENIX experiment with the local spin rotators installed on either side of the STAR detector and PHENIX detector. Polarized protons were also first accelerated to 250 GeV at the end of RHIC 2006 run with a 46% polarization measured at this new store energy in one of the RHIC accelerators. Currently, the luminosity in RHIC is limited by the beam-beam effect. The plan is to triple the luminosity. Plans to achieve polarized proton collision at 250 GeV are also reported.     

Induced electron capture in field-assisted energetic atomic collisions

We present model considerations for the process of the electron capture in energetic nonrelativstic collisions of light atomic particles in the presence of a relatively weak low-frequency external electromagnetic field. The field is treated as an elliptically polarized quantum single-mode field. Establishing validity of the dipole approximation to the electron transfer (where the total momentum of all emitted or absorbed photons can be well above the typical inneratomic momenta of an electron in the initial and final states) and neglecting the Doppler and aberration effects, we give a fully nonrelativistic treatment for the field-assisted collisions and show that the capture cross section is invariant with respect to the Galilean transformations. The model consideration suggests that the field can substantially influence the capture dynamics and considerably change the capture cross section compared to the field-free collisions. This is especially the case if the “resonance” conditions nω≈±v ²/2 are satisfied, with nω being the energy transferred to or absorbed from the electromagnetic field and v the collision velocity.     

Spin polarization and structure of molecular oxygen adsorbed on Fe(1 1 0)

We investigated the structural and magnetic properties of Fe(1 1 0) for molecular oxygen adsorption at room temperature. For the c(2 × 2) and c(3 × 1) superstructures, spin-polarized secondary electron emission (SPSEE) induced by protons and electrons reveals a nearly unchanged polarization compared to clean Fe(1 1 0). An appreciable decrease in polarization is found for the disordered layer of oxygen at a coverage Θ ≈ 1. This decrease is more pronounced for the spin polarization determined by electron capture (EC) to excited atomic He levels after grazing scattering. From a comparison of data obtained by proton-induced SPSEE and spin-polarized EC we conclude that the polarization at the vacuum boundary vanishes for an oxygen coverage Θ ≈ 1 while the polarization in the underlying Fe substrate layers is hardly changed.     

Measurement of excited state number densities of oxygen and nitrogen in a non-equilibrium plasma

Number densities of several excited states of atomic oxygen and nitrogen have been measured in the decaying non-thermal plasma of a θ-pinch afterglow. The spatial variation of the electron density and temperature as functions of time after initiation of main bank discharge have also been measured to facilitate a comparison of the excited state number densities with model calculations. Measurements of the atomic oxygen excited states indicate that quintet to triplet spin exchange collisions and doubly excited states must be included in the model. The measured populations of the excited atomic nitrogen states agree well with those calculated at high density (N_e≈ 10¹⁴ cm^?3), but disagree badly at lower densities (N_e ≈ 10¹² cm^?3). The discrepancies seem to be real since they are larger than expected measurement uncertainty.     

Coherent Polarization Transfer Effects Are Crucial for Interpreting Low-Field CIDNP Data

In this work we demonstrate that low-field chemically induced dynamic nuclear polarization (CIDNP) is strongly affected by re-distribution of polarization, which is formed in the course of spin evolution in transient radical pairs, in diamagnetic reaction products. This phenomenon is of importance when the spins of the reaction product are coupled strongly meaning that spin–spin interactions between them are comparable to the differences in their Zeeman interactions with the external magnetic field. In this case, polarization transfer relies on a coherent mechanism; as a consequence, spins can acquire significant polarization even when they have no hyperfine coupling to the electron spins in the radical pairs, i.e., cannot be polarized directly by CIDNP. This is demonstrated by taking CIDNP of n-butylamine as an example: in this case only the α-CH₂ protons are polarized directly, which is confirmed by high-field CIDNP, whereas the β-CH₂, γ-CH₂ and δ-CH₃ protons get polarized only indirectly due to the transfer of polarization from the α-CH₂ protons. These results show that low-field CIDNP data should be interpreted with care to discriminate between the effects of spin evolution in transient radical pairs and in diamagnetic reaction products.     

Formation of doubly excited He atoms during scattering of He ions from a clean and oxygen covered Ni(1 1 0) surface

The formation of doubly excited states of He atoms during collisions of He²⁺ ions with energies between 60 eV and 1 keV with a Ni(1 1 0) surface is studied via Auger electron spectroscopy. We observe that the electron spectra from autoionization of doubly excited states of 2s², 2s2p, 2p² configurations show a pronounced dependence on the coverage of the target surface with oxygen. For a controlled O₂ adsorption on the Ni(1 1 0) surface we can explain the resulting changes in the electron spectra by the modification of the work function of the target surface. Thermal desorption and dissolution into the bulk of surface contaminations at elevated temperatures provides an alternative interpretation of recent work where the local electron spin polarization of a Ni(1 1 0) surface was deduced from changes in the electron spectra as function of target temperature.     

Spin-exchange-induced circularly polarized molecular fluorescence

We have measured the circular polarization of light emitted from both atomic H and molecular H2 after bombarding H2 with longitudinally polarized electrons. For both atomic and molecular fluorescence near threshold we observe a circular polarization as great as 10% of the electron polarization. This represents the first direct observation of spin transfer in electron-molecule collisions.     

Spin correlation in pp elastic scattering and in pion production near threshold

An internal polarized gas target in conjunction with a beam of polarized protons stored in the IUCF Cooler storage ring has been used to measure analyzing power and spin correlation parameters between 200 MeV and 450 MeV over a wide range of angles. A consistent absolute beam and target polarization calibration was established by ramping the stored protons up and down in energy. The analyzing power A _y and the spin correlations A _xx , A _yy and A _xz were measured with transverse beam polarization, while determination of A _zz required the development of a longitudinally polarized beam. First experiments on the spin dependence on π₀-production have recently been carried out.     

Beitrag zur Kernspektroskopie an36Cl,90Y und40K durch Messung der Polarisation von γ-Strahlung nach Neutroneneinfang

The circular polarization ofγ-rays following the capture of polarized thermal neutrons in nuclei was measured. The degree of polarization confirmed the spin 2⁺ for the 0.777 MeV level of⁹⁰Y and the spin 0^? or 3^? for the 0.029 MeV level of⁴⁰K. Further theE2/M1 mixing ratio of the 8.58 MeV decay to the ground state of³⁶Cl was measured.     

Effects of adsorbates on the formation of doubly excited He atoms during impact of He2+ ions on Fe(110) and Ni(110) surfaces

The formation of doubly excited states of He atoms during collisions of He²⁺ ions with projectile energies between 74 eV and 124 eV with an Fe(110) and a Ni(110) surface is studied via Auger electron spectroscopy. We observe that the electron spectra from autoionization of doubly excited states of 2s², 2s2p, 2p² configurations show a pronounced dependence on the coverage with oxygen for both surfaces. For a controlled O₂ adsorption on the Fe(110) and Ni(110) surfaces we can explain the resulting changes in the electron spectra by the modification of the measured work functions of the target surfaces. In terms of thermal desorption and dissolution into the bulk of surface contaminations at elevated temperatures, we present an alternative interpretation of similar previous studies by another group, where the local electron spin polarization of Fe(110) and Ni(110) surfaces was deduced from changes in the electron spectra as function of target temperature.     

Intense pulses of polarized electrons produced by Fano effect

A pulsed polarized electron source using the Fano effect on cesium is described in detail. A frequency doubled dye laser producing 2 mJ/pulse at 305 nm is used as the source of circularly polarized light. The light beam interacts with an array of 20 atomic beams. The atomic beam oven operates in a closed cycle thereby increasing running time by a factor of 30. Intensities of 3×10⁹ e ^?/0.5 μs with a polarization of 90% have been routinely obtained.     

One-electron capture in collisions of fast ions with atoms

The properties of one-electron charge transfer between ions and atomsB ^Z++A →B ^(Z?1)+ A ⁺ are studied at relative velocities of the colliding particles higher than target electron velocities. Calculations of partial and total cross sections in collisions of protons and multiply-charged ions with neutral atoms are performed and compared with experimental data. The universal curve for the capture of the targetK- andL-electrons is given. In all cases at sufficiently high collision energies the electron capture from outer shells decreases and the capture of electrons from inner shells of the target atom becomes predominant.     

Collisional alignment and orientation of atomic outer shells. III. Spin-resolved excitation

The review generalizes the formalism of the first part of this series [N. Andersen, J.W. Gallagher and I.V. Hertel, Phys. Rep. 165 (1988) 1–188] on electron-atom collisions to cases where the spin polarization of the electron is investigated at least once, i.e., before and/or after the collision. In addition, the target atom may be spin polarized. The preparation of initially polarized beams and/or the analysis of photon and spin polarization in the final state significantly increase the number of cases where a “perfect scattering experiment” can be performed. The connection between the scattering amplitudes and the generalized Stokes and STU parameters is analyzed. Favorable scattering geometries for perfect experiments and possibilities for consistency checks are pointed out. Recommendations are made about directions of future work.     

Slow cross-symmetry phase relaxation in complex collisions

We discuss the effect of slow phase relaxation and the spin off-diagonal S-matrix correlations on the cross-section energy oscillations and the time evolution of the highly excited intermediate systems formed in complex collisions. Such deformed intermediate complexes with strongly overlapping resonances can be formed in heavy-ion collisions, bimolecular chemical reactions, and atomic cluster collisions. The effects of quasiperiodic energy dependence of the cross sections, coherent rotation of the hyperdeformed ?(3 : 1) intermediate complex, Schrödinger cat states, and quantum-classical transition are studied for ²⁴Mg + ²⁸Si heavy-ion scattering.     

The RHIC polarized source upgrade

The RHIC polarized H^? ion source is being upgraded to higher intensity and polarization for use in the RHIC polarization physics program at enhanced luminosity RHIC operation. The higher beam intensity will allow reduction of the longitudinal transverse beam emittance at injection to AGS to reduce polarization losses in AGS. There is also a planned RHIC luminosity upgrade by using the electron beam lens to compensate the beam-beam interaction at collision points. This upgrade is also essential for future BNL plans for a high-luminosity electron-proton (ion) Collider eRHIC. The basic limitations on the high-intensity H^? ion beam production in charge-exchange collisions of the neutral atomic hydrogen beam in the Na-vapor jet ionizer cell were experimentally studied.