Polarizations of spin 32 particles and 7Li58Ni interactions

New representations of polarizations of spin $\text{3}\text{2}$ particles are proposed for elastic scattering. They are useful in investigating spin-dependent interactions. The application to ⁷Li⁵⁸Ni scattering gives weak spin-orbit interactions and rather strong tensor ones of ⁷Li.     

Spin exchange polarization and measurement of the hyperfine splitting of short-lived8Li

Free⁸Li (T _1/2=0.84 sec) atoms were polarized by spin exchange with optically pumped rubidium vapour. The⁸Li nuclei were produced by the reaction⁷Li(d, p)⁸Li in a thin LiF target. They recoiled into helium which was used as a buffer gas for optical pumping as well. Nuclear polarization up to 2% was detected by β-decay asymmetry. Rf transitions between hfs levels of the atomic ground state yielded the hfs separation ΔW=382.543(7) MHz. The nuclear spin assignmentI=2 was confirmed.     

Measurement of the proton polarization in the 3He(d,p)4He Rection between 2.0 and 6.0 MeV

Angular distributions of the polarization of ³He(d, p)⁴He protons have been measured for unpolarized deuterons with laboratory energies of 1.99, 2.81, 3.94 and 6.00 MeV. The polarizations were determined from the left-right asymmetry of elastic scattering through 60° in ⁴He gas using a vaned polarimeter whose analyzing power was computed by trajectory tracing from the known p-α polarizations. A contour diagram of proton polarization for deuteron energies of 1 to 12 MeV is presented. A Legendre polynomial expansion of four or five terms has been fitted to the products p(θ)σ(θ). The coefficients of the expansions of these and other measurements show resonance-like behaviour at ⁵Li excitations of 20.0 and 20.9 MeV. The vector-polarized beam and polarized-target analyzing powers are compared with the proton polarization. The proton polarizations are in good agreement with the ³H(d, n)⁴He neutron polarizations when compared at the same entrance-channel energy but disagree when compared at equal exit-channel energies or compound-nucleus excitations.     

Quadrupole interaction of8Li and9Li in LiNbO3 and the quadrupole moment of9Li

The quadrupole interaction of nuclear spin polarized⁸Li (I=2) and⁹Li (I=3/2) in LiNbO₃ has been studied at room temperature. The polarization was achieved by optical pumping of a fast atomic beam with circularly polarized laser light. The atoms were implanted into a hexagonal LiNbO₃ single crystal and the quadrupole splitting ofβ-NMR spectra was measured. A ratio of ¦Q(⁹Li)/Q(⁸Li)¦=0.88(4) for the nuclear quadrupole moments was deduced, yielding a new value of ¦Q(⁹Li)¦=25.3 (9) mb for the quadrupole moment of⁹Li.     

PRODUCTION OF POLARIZED 7Li ATOMIC BEAM IN STRONG MAGNETIC FIELD BY OPTICAL PUMPING

The production of a polarized ⁷Li atomic beam in a strong magnetic field was experimentally studied by laser optical pumping of a single ground hyperfine F level. It was shown that nearly complete negative and partial positive polarizations of ⁷Li atoms could be realized under appropriate conditions by this pumping scheme, which was in agreement with the rate equation calculations. Based on the analyses of transition probabilities, the maximum polarizations for various transitions and light polarizations were given.     

Polarized recoil nuclei from nuclear reactions induced by polarized lithium ions

The production of vector polarized recoil nuclei was investigated for nuclear reactions initiated by vector polarized⁶Li and⁷Li beams of energies up to 20 MeV. The vector polarization was detected by theβ-asymmetry of the decaying recoil nuclei implanted into various stopper materials. Considerable polarization values were observed for⁸Li produced in the systems⁷Li+⁹Be,⁷Li+⁷Li and⁶Li+⁷Li, and for¹²B in the systems⁷Li+⁷Li and⁶Li+¹¹B. However, no polarization has been observed for heavier nuclei with unknown nuclear moments. The relaxation of the vector polarization of⁸Li implanted into various stopper materials at room temperature was investigated in detail. For a gold stopper it was found to be much larger when compared to earlier results.     

Polarization in the quasielastic scattering of 1-GeV/c protons on light nuclei

Polarization measurements in the A(p, 2p)B reactions on ⁶Li, ⁷Li, and ²⁸Si nuclei at a proton-beam energy of 1 GeV were performed in a kinematically complete experiment. By using a two-arm magnetic spectrometer, two secondary protons were recorded in coincidence at asymmetric scattering angles of θ₁=15°?26° and θ₂=58.6° for residual-nucleus momenta in the range K _B=0–150 MeV/c. Either arm of the spectrometer was equipped with polarimeters based on proportional chambers. The data coming from this experiment are analyzed within the distorted-wave impulse approximation. It is shown that the polarization of recoil protons formed at angle θ₂ in the interaction featuring a proton from the P shell of the ⁷Li nucleus can be described under the assumption of an effective intranuclear-proton polarization by using the single-particle shell-model wave function of the nucleus. Our data on the polarizations of the two protons from the reaction (p, 2p) on a ²⁸Si nucleus also suggest the effective polarization of the protons in the D shell of the ²⁸Si nucleus. It is found that, for high recoil-nucleus momenta of K _B≥90 MeV/c, the effective polarization of the protons in the P shell of the ⁶Li nucleus—this polarization was discovered in studying the polarization of recoil protons in the reaction ⁶Li(p, 2p)⁵He—cannot be described within the shell model assuming LS coupling. As might have been expected, the polarization of recoil protons knocked out from the S shells of the ⁶Li and ⁷Li nuclei comply well with the predictions obtained in the impulse approximation with allowance for the depolarization effect alone.     

Nuclear polarized7Li and14N ions via ion beam surface interaction at grazing incidence

Using singly or doubly reflected 270keV-ions from a Cu-surface at grazing incidence nuclear polarizations in⁷Li ofP _I=(6.8±0.6) % orP _I= (11.5±1.0) % and in¹⁴N ofP _I =(14.8±1.0) % orP _I=(20.0±1.0) % respectively, have been achieved.     

Elastic and inelastic scattering of 6Li and 7Li by 54Fe at E = 36–50 MeV

Detailed angular distributions for the elastic and inelastic scattering of ⁷Li at E = 36, 42, and 48 MeV and of ⁶Li at E = 38, 44, and 50 MeV by ⁵⁴Fe have been measured. It is not possible to describe both sets of data with the same set of optical-model parameters. The ratio of $\text{U}\text{W}$ is 0.8 for ⁷Li and 1.4 for ⁶Li at the strong absorption radius, implying stronger absorption for ⁷Li than for ⁶Li. No energy dependence in the optical-model parameters was necessary for either ⁶Li or ⁷Li. The inelastic scattering from the ⁵⁴Fe 2⁺, 1.41 MeV state was well described by the DWBA and the extracted deformation length (βR = 0.62) was the same for both ⁶Li and ⁷Li scattering. It was not possible to describe the ⁷Li projectile excitation data with collective-model DWBA calculations showing that more detailed calculations for the projectile excitation are necessary.     

STUDY OF OPTICAL PUMPING OF ALKALI ATOMIC BEAM IN STRONG MAGNETIC FIELDS

We have gained a very comprehensive set of results for infrared diffraction (10.6μm wavelength) from fabricated single grooves of reflection with widths in the range of 5μm to 65μm. The experimental results show that the diffraction can still occur when the groove width becomes comparable to or less than the wavelength of the probing light. Variations of the diffraction intensities have a very regular sinusoidal relationship with the polarization angle of the incident light; and the diffraction intensities are the when the incident light is TM polarized. However, the diffraction intensities will be a minimum when the incident light is TE polarized; and the biger the diffraction angle, the higher the rate of change of the diffractive intensity with increasing polarization angle of the incident light.     

Metastability exchange optical pumping of <Superscript>3</Superscript>He at high pressure and high magnetic field for medical applications

Systematic studies of the metastability exchange optical pumping (MEOP) process in ³He gas at high magnetic fields (0.45, 0.9, 1.5, and 2 T) and pressures (32 and 64 mbar) are performed. The impact of experimental parameters such as laser power, beam profile, and shape of the pumping cell is evaluated. By varying the discharge intensity in the cell, the density of metastable state atoms and the plasma-induced nuclear relaxation rate are also controlled, and their effect on the MEOP efficiency can be investigated. Very accurate experimental results are obtained, opening the way to quantitative tests of a recently proposed model of the MEOP process at high magnetic field. We report selected MEOP results with nuclear polarizations exceeding 50 % at 64 mbar and 2 T, which represents a dramatic improvement in performances over MEOP at low magnetic field. The present findings suggest that still higher polarizations can be achieved in higher magnetic fields, and motivate investigations at higher gas pressures. New ways of producing hyperpolarized ³He for magnetic resonance imaging and medical applications can be envisaged, as most clinical whole-body scanners operate at 1.5 T.     

Optical pumping of ions in buffer gases

A method for polarizing thermal ions in buffer gases directly by optical pumping is presented in detail. The production and storage of Sr⁺ (and Ba⁺) ions in noble gases, their diffusion and relevant collision processes are discussed. An arc discharge hollow cathode for the generation of intense ionic resonance lines is described. The Sr⁺ (5² S _1/2) and electron spin polarizations are treated by rate equations. Fitting the solutions to experimental data obtained from transient signals yields an estimate of a few times 10³Å² for the Sr⁺ion-electron spin exchange cross section.     

Biological effects induced by swift heavy ions of lithium on aqueous solution of plasmid pMTa4

Biological samples exposed to swift heavy ions sustain damage on different components. Damage to DNA, a critical component of a living system, has considerable biological implications. In this study aqueous solution of plasmid pMTa4 was exposed to varying fluence of swift ⁷Li ions and its different topological forms were analysed by agarose gel electrophoresis to study the induced damage. To monitor radiation labile nucleotide sequence the ⁷Li ions exposed plasmid was degraded by three different restriction endonucleases and also analysed by agarose gel electrophoresis. The results show that ⁷Li ions predominantly induced double strand breaks in the plasmid DNA in a dose-dependent manner and affected preferentially the GC-rich motifs of the DNA. The results suggest that ⁷Li ions induce premutagenic lesions at an enhanced frequency in segments of the DNA which are rich in CG content as compared to GC-poor segments.     

Quasielastic deuteron and triton knockout from lithium isotopes by intermediate-energy pions

Quasielastic deuteron and triton knockout from ⁶Li and ⁷Li nuclei that was induced by a π ^?-meson beam of momentum p ₀ = 0.72 and 0.88 GeV/c was studied under conditions of full kinematics. The experiment was performed at the Institute of Theoretical and Experimental Physics (ITEP, Moscow) by using a 3-m magnetic spectrometer equipped with spark chambers. The distributions with respect to the momenta of quasideuteron and quasitriton internal motion, the excitation-energy spectra of residual nuclei, and the effective numbers of quasideuterons in ⁶Li and ⁷Li nuclei were obtained.     

Li and K NMR relaxation study of a LiKSO4 single crystal

The ⁷Li and ³⁹K NMR relaxations in a LiKSO₄ single crystal grown by the slow evaporation method were investigated by employing a pulse NMR spectrometer. From the experimental data, the quadrupole coupling constant and asymmetry parameter were determined at the temperatures of 180 and 300 K. The relaxation processes of ⁷Li and ³⁹K were studied for the LiKSO₄ crystal, and the relaxation times for the ⁷Li and ³⁹K nuclei exhibit remarkable changes near T_c2 (=190 K). The activation energies for ⁷Li and ³⁹K were determined in phases I and III. The large change in the activation energy at 190 K indicates that the Li and K ions are significantly affected during this transition. The correlation time of the ⁷Li calculated from the spin-lattice relaxation time and quadrupole parameters was larger than that of the ³⁹K calculated using the same method. The reason for this is that the Li ion undergoes molecular motion as in the LiO₄ groups.     

Projectile breakup and total reaction strengths in Li-induced reactions

Spectra and angular distributions have been measured for the light particles emitted from the reactions induced by 36 MeV ⁶Li and ⁷Li ions on targets of ¹²C, ¹³C and ²⁷Al. Some measurements were also performed at beam energies of 28 and 32 MeV. The spectra are dominated by broad peaks centered at energies corresponding to the beam velocity, a characteristic of projectile breakup. A simple breakup model roughly reproduces the shapes of the measured spectra and angular distributions. The total cross sections observed for projectile breakup average about 600 mb and are almost as large as the fusion cross sections. The ⁷Li breakup yields are somewhat larger than those for ⁶Li, the same trend that was previously observed for the Li-induced fusion cross sections on C. As a result the measured total reaction cross sections induced by ⁷Li ions are about 200 mb larger than those of ⁶Li on the same targets. This difference is not predicted by conventional optical model calculations.     

Strong reaction channels at barrier energies in the system 6Li + 208Pb

Large cross-section reaction channels were measured in the systems ⁶Li( ⁷Li) + ²⁰⁸Pb with high statistical accuracy at 5(3) energies around the Coulomb barrier from 29 to 39 MeV. These channels were assigned (mainly) to the breakup of ⁶Li, very loosely bound, into α + d and to the breakup of ⁵Li, produced by n-transfer to the target, into α + p and to similar processes with ⁷Li beam. The cross-sections with ⁶Li, S _α = 1.475 MeV, are systematically larger than the ⁷Li ones. This reflects, most likely, the higher binding energy of ⁷Li, S _α = 2.468 MeV. Theoretical predictions for the ⁶Li + ²⁰⁸Pb system which include for ⁶Li breakup to continuum states within a continuum discretized coupled-channels approach (CDCC) and resonant breakup plus n-transfer with DWBA reproduce the angular distribution shapes but still underestimate the cross-sections by a factor ∼ 3. Received: 15 January 2001 / Accepted: 3 March 2001     

Laser production of polarized beams and targets

Hyperfine interaction techniques have historically been applied to produce polarized nuclei. The use of optical pumping with tunable lasers is a new hyperfine interaction method which dramatically extends the range of nuclear species which can be polarized, as well as the fluxes and densities which can be achieved and the temperature range in which experiments can be carried out. Lasers are ideally suided for the production of polarized atomic beams, as will be illustrated for the cases of^6,7Li,²³Na and³He. Techniques for polarizing dense atomic vapors will be described with an emphasis on achieving full Doppler coverage by the method of velocity changing collisions. Experiments leading to the production of a⁶Li target for a parity non-conservation study of the nuclear reaction⁶Li(α,γ)¹⁰B will be reviewed and results obtained to date presented.     

Photoassociation of heteronuclear lithium

Photoassociation of ultracold heteronuclear ⁶Li⁷Li molecules is observed inside a combined magneto-optical trap for ⁶Li and ⁷Li. The trapped atomic cloud is illuminated by a tunable single-mode laser and the number of trapped ⁷Li atoms is monitored by absorption spectroscopy. Characteristic hyperfine resolved spectra have been recorded for singlet spin orientation. Interesting saturation effects have been observed. Received: 12 July 2001 / Revised version: 1 October 2001 / Published online: 23 November 2001