Polarization in projectile fragmentation andg-factor measurements for neutron-rich nuclei

A method to produce beams of polarized unstable nuclei was developed, which fully exploits the advantageous features of the projectile fragmentation reaction. The method was applied tog-factor measurements of several neutron-rich nuclei. We present experimental results so far obtained and discuss capabilities and limitations the present polarization method has.     

Electromagnetic moments of unstable nuclei studied with polarized projectile fragments

Phenomenon of ejectile spin polarization in the projectile fragmentation reaction was exploited as a tool to produce polarized radioactive nuclei. The polarized unstable nuclei were subjected to the β-detected NMR meaurements, and the magnetic moments and electric quadrupole moments were determined for several neutron-rich light nuclei. The comparison of the experimental results with theoretical predictions shows that the nuclear moments provide useful test of models for nuclear structure and interaction in the region of light-mass unstable nuclei.     

Oxidation states of Fe in LaNi1−x Fe x O3

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.     

The role of d-orbital polarization on rhodium cluster collisions

The effects due to d-orbital polarization in collision processes between a single rhodium atom and a 12 atom rhodium cluster are investigated by means of ab initio molecular dynamics. For the initial configuration of the 12 atom rhodium targets we adopt two different low energy structures. The kinetic energy and impact parameter of the projectile are chosen in such a way that fusion, scattering and fragmentation of the cluster do occur. The collision is treated by means of density functional theory molecular dynamics (DFT-MD). Both spin unpolarized and polarized treatments are implemented in order to clearly distinguish the effects that are due to d-orbital polarization. We find a novel block dynamics, of parts of the cluster, which is due to the directional nature of d-bonds. In addition, the treatment of the collisions by means of high temperature DFT dynamics yields promising minimal energy configurations, which are target dependent but are difficult to obtain otherwise.     

A method to measure neutron polarization using P-even asymmetry of γ-quantum emission in the neutron-nuclear interaction

A new method to measure polarization of cold/thermal neutrons using P-even asymmetry in nuclear reactions induced by polarized neutrons is proposed. A scheme profiting from a large correlation of the neutron spin and the circular ??-quantum polarization in the reaction (n, ??) of polarized neutrons with nuclei is analyzed. This method could be used, for instance, to measure the neutron-beam polarization in experiments with frequently varying configuration. We show that high accuracy and reliability of measurements could be expected.     

Pulsed magnetic field method for measuring polarization of radioactive beams

A new method has been developed for measuring the magnitude of nuclear spin polarization of a secondary, radioactive beam by making a pulsed magnetic field measurement that does not require advance knowledge of the nuclide's magnetic moment. Using a standard β NMR apparatus, a magnetic double ratio is determined from the counting rates in 0° and 180° β detectors for magnetic field on and off conditions. This ratio provides direct information on the induced spin polarization of a radioactive beam. A demonstration of the method was performed using spin polarized ¹²B nuclei produced by fragmentation of an 80 MeV/nucleon ¹⁸O beam in a Nb target. This revised version was published online in August 2006 with corrections to the Cover Date.     

Optimised adiabatic fast passage spin flipping for He neutron spin filters

We describe here a method of performing adiabatic fast passage (AFP) spin flipping of polarized ³He used as a neutron spin filter (NSF) to polarize neutron beams. By reversing the spin states of the ³He nuclei the polarization of a neutron beam can be efficiently reversed allowing for the transmission of a neutron beam polarized in either spin state. Using an amplitude modulated frequency sweep lasting 500 ms we can spin flip a polarized ³He neutron spin filter with only 1.8×10⁻⁵ loss in ³He polarization. The small magnetic fields (10-15 G) used to house neutron spin filters mean the ³He resonant frequencies are low enough to be generated using a computer with a digital I/O card. The versatility of this systems allows AFP to be performed on any beamline or in any laboratory using ³He neutron spin filters and polarization losses can be minimised by adjusting sweep parameters.     

Dynamics of polarization buildup by spin filtering

There has been much recent research into polarizing an antiproton beam, instigated by the recent proposal from the PAX (Polarized Antiproton eXperiment) project at GSI Darmstadt. It plans to polarize an antiproton beam by repeated interaction with a polarized internal target in a storage ring. The method of polarization by spin filtering requires many of the beam particles to remain within the ring after scattering off the polarized internal target via electromagnetic and hadronic interactions. We present and solve sets of differential equations which describe the buildup of polarization by spin filtering in many different scenarios of interest to projects planning to produce high-intensity polarized beams. These scenarios are: 1) spin filtering of a fully stored beam; 2) spin filtering while the beam is being accumulated, i.e. unpolarized particles are continuously being fed into the beam; 3) the particle input rate is equal to the rate at which particles are being lost due to scattering beyond the ring acceptance angle, the beam intensity remaining constant; 4) increasing the initial polarization of a stored beam by spin filtering; 5) the input of particles into the beam is stopped after a certain amount of time, but spin filtering continues. The rate of depolarization of a stored polarized beam on passing through an electron cooler is also shown to be negligible.     

Beta-polarization measurements in solids

Experimental developments recently made in the beta-NMR measurements are reviewed. For production of polarized beta-emitters, the tilted foil technique has been successfully combined with the NMR. The low- and intermediate-energy heavy-ion reactions were recently found to be another sources of polarized beta-emitters. To achieve wide applicability of the NMR technique to nuclei far from stability, the tilted foil technique and the projectile fragmentation process in high-energy heavy-ion collisions were recently combined with the beta-NMR technique in the fragment separator built at the LBL.     

Optical polarization of nuclei and ODNMR in GaAs/AlGaAs quantum wells

Optical orientation of electrons was used to polarize the crystal lattice nuclei in quantum-size heterostructures and to study the effect of the conduction band spin splitting on the spin states of quasi-two-dimensional (2D) electrons drifting in an external electric field. High (～1%) nuclear polarization was registered using polarized luminescence and ODNMR in single GaAs/AlGaAs quantum wells. Measurement was made of the hyperfine interaction fields created by polarized nuclei on electrons and by electrons on nuclei. The spin-lattice relaxation of nuclei on the non-degenerate 2D electron gas was calculated. A comparison of the theoretical and experimental longitudinal relaxation times permitted the conclusion that the localized charge carriers are responsible for nuclear polarization in quantum wells in the temperature range of 2–77 K. A new effect has been studied, i.e. induction of an effective magnetic field acting on 2D electron spins when electrons drift in an external electric field in the quantum well plane. This effective field B_eff is due to the spin splitting of the conduction band of 2D electrons. The paper discusses possible registration of an ODNMR signal when the field B_eff is modulated by an electric current during optical orientation.     

Dynamic Polarization and Relaxation of <Superscript>75</Superscript>As Nuclei in Silicon at High Magnetic Field and Low Temperature

We present the results of experiments on dynamic nuclear polarization and relaxation of ⁷⁵As in silicon crystals. Experiments are performed in strong magnetic fields of 4.6 T and temperatures below 1 K. At these conditions donor electron spins are fully polarized, and the allowed and forbidden electron spin resonance transitions are well resolved. We demonstrate effective nuclear polarization of ⁷⁵As nuclei via the Overhauser effect on the time scale of several hundred seconds. Excitation of the forbidden transitions leads to a polarization through the solid effect. The relaxation rate of donor nuclei has strong temperature dependence characteristic of Orbach process.     

Static Moments of Neutron-Rich Ground States and Isomers Produced by Projectile Fragmentation

New achievements in nuclear moment measurements on nuclei produced and spin-oriented in intermediate energy (50–80 MeV/u) projectile fragmentation reactions are presented. By combining different types of spin-orientation with different experimental techniques, we show that the magnetic dipole moments and electric quadrupole moments of exotic neutron rich nuclei in their ground state as well as in their excited isomeric states can be measured. A selection of recent experiments performed at the LISE spectrometer at GANIL (Caen, France) is described. This revised version was published online in September 2006 with corrections to the Cover Date.     

Alignment and orientation of charge exchange excitation with polarized projectiles

The alignment and orientation of hydrogenlike excited atoms produced by electron capture in collisions of spin polarized bare atomic nuclei with atomic targets is described in terms of Stokes parameters. TheH(2p)^* excited state formed by spin polarized protons making charge exchange collisions on atomic targets is dealt with extensively for the purpose of illustration. The linear polarization and circular polarization of resulting Lyman-α when polarized protons make electron capture collision on atomic hydrogen target are plotted to compare observable aspects of different theories of electron capture.     

Weak interactions in polarized semi-inclusive DIS

We calculate, within the pQCD parton model at leading orders, the expression of the polarization P of spin 1/2 hadrons (typically baryons), produced in polarized semi-inclusive DIS in all possible cases in which weak interactions are involved. We discuss how to gather new information on fragmentation and distribution functions and give numerical estimates in the cases for which data are or will soon be available. Received: 7 June 2001 / Published online: 10 August 2001     

Dynamic nuclear polarization in silicon microparticles

We report record high 29Si spin polarization obtained using dynamic nuclear polarization in microcrystalline silicon powder. Unpaired electrons in this silicon powder are due to dangling bonds in the amorphous region of this intrinsically heterogeneous sample. 29Si nuclei in the amorphous region become polarized by forced electron-nuclear spin flips driven by off-resonant microwave radiation while nuclei in the crystalline region are polarized by spin diffusion across crystalline boundaries. Hyperpolarized silicon microparticles have long T1 relaxation times and could be used as tracers for magnetic resonance imaging.     

Spin transfer from an optically pumped alkali vapor to a solid

We report enhancement of the spin polarization of 133Cs nuclei in CsH salt by spin transfer from an optically pumped cesium vapor. The nuclear polarization was 4.0 times the equilibrium polarization at 9.4 T and 137 degrees C, with larger enhancements at lower fields. This work is the first demonstration of spin transfer from a polarized alkali vapor to the nuclei of a solid, opening up new possibilities for research in hyperpolarized materials.     

Demonstration of electrical spin injection into a semiconductor using a semimagnetic spin aligner

Spin injection into semiconductors has been a field of growing interest during recent years, because of the large possibilities in basic physics and for device applications that a controlled manipulation of the electrons spin would enable. However, it has proven very difficult to realize such a spin injector experimentally. Here we demonstrate electrical spin injection and detection in a GaAs/AlGaAs p-i-n diode using a semimagnetic II–VI semiconductor (Zn_{1 − x − y}Be_xMn_ySe) as a spin aligner. The degree of circular polarization of the electroluminescence from the diode is related to the spin polarization of the conduction electrons. Thus, it may be used as a detector for injected spin-polarized carriers. Our experimental results indicate a spin polarization of the injected electrons of up to 90% and are reproduced for several samples. The degree of optical polarization depends strongly on the Mn concentration and the thickness of the spin aligner. Electroluminescence from a reference sample without spin aligner as well as photoluminescence after unpolarized excitation in the spin aligner sample show only the intrinsic polarization in an external magnetic field due to the GaAs bandstructure. We can thus exclude side effects from Faraday effect or magnetic circular dichroism in the semimagnetic layer as the origin of the observed circularly polarized electroluminescence.     

Longitudinally polarized photoproduction of inclusive hadrons at fixed-target experiments

We present a detailed phenomenological study of spin-dependent single-inclusive high- p_T hadron photoproduction with particular emphasis on the kinematics relevant for the Compass and Hermes fixed-target experiments. We carefully examine the theoretical uncertainties associated with the only moderate transverse momenta accessible in such measurements and analyze the sensitivity of the relevant spin asymmetries to the gluon polarization in the nucleon as well as to the completely unknown parton content of circularly polarized photons. Received: 30 May 2005, Published online: 6 October 2005 PACS: 13.88. + e, 12.38.Bx, 13.85.Ni     

~(129)Xe核自旋极化转移增强分子的核磁共振信号

激光极化的12 9Xe核具有极高的非平衡极化度和长的弛豫时间 ,这一特点使得它能够极化转移增强液体、固体或者固体表面分子中原子核自旋极化。因而 ,提高了它们的核磁共振探测灵敏度和扩展了在材料和表面科学研究中的应用。综述激光极化12 9Xe核与其它分子中原子核之间的极化转移研究与进展 ,介绍相关物理机制和参数的测量。