Photoionization cross-sections of the first excited states of sodium and lithium in a magneto-optical trap

A two element magneto-optical trap (MOT) for Na and ⁷Li or ⁶Li is used to cool and trap each of them separately. A fraction of the cold atoms is maintained in the first ²P_3/2 excited state by the cooling laser. These excited state atoms are ionized by laser light in the near-UV region, giving rise to a smaller number of trapped atoms and to different loading parameters. Photoionization cross-sections were derived out of these data. They are in reasonable agreement with data previously obtained using thermal samples and with theoretical predictions. Received 21 March 2001 and Received in final form 3 August 2001     

Complete and incomplete fusion in reactions of 7Li + 56Fe at E(7Li) = 50 and 68 MeV from analysis of recoil range and light-particle measurements

Excitation functions for the production of eight radioactive products of the reactions of ⁷Li on ⁵⁶Fe have been measured up to E(⁷ Li) = 89 MeV. Recoil range distributions for these products, together with inclusive proton, deuteron, triton and alpha spectra, have been measured at energies of 50 and 68 MeV. The α, t and d spectra show characteristic “break-up” components at forward angles, while the recoil distributions show evidence of complete fusion and incomplete-fusion process ⁵⁶Fe( ⁷Li,α) ⁵⁹Co ^* . A parallel study on ⁵⁵Mn shows some evidences of the ( ⁷Li,t) incomplete-fusion process, but the cross-section for this process is significantly less than for the triton fusion process. The recoil distributions can be reproduced on the assumption that essentially all the observed break-up fragments are in fact associated with incomplete fusion, but uncertainties in normalisation leave open the possibility of a significant contribution of pure break-up. A diffraction model of the ( ⁷Li,α) transfer process reproduces the observed break-up α spectra with some success. Received: 29 March 2001 / Accepted: 12 November 2001     

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     

Impurity nuclear physics

Using a germanium-detector array for hypernuclear γ spectroscopy (Hyperball), we measured B(E2) of the ⁷ _ΛLi hypernucleus and observed a significant shrinkage of the ⁶Li core induced by a Λ-particle. In this way, nuclear properties can be drastically changed by introducing a Λ-particle, which can be investigated by high-resolution hypernuclear γ spectroscopy. In the future neutron-rich hypernuclei will also be studied, where interesting modifications of nuclear structure by a Λ-particle are expected. Received: 1 May 2001 / Accepted: 4 December 2001     

Decay rate measurement of lithium in a magneto-optical trap

A detailed account of various experimental techniques developed during the study on the decay rate coefficient of laser trapped ⁷Li atoms are presented. The frequency of a dye laser is stabilized using a simple sealed-off cell specially designed for Li vapor. The accurate number of trapped atoms are obtained by measuring the fluorescence intensity and the population ratio between the ground and the excited states by absorption coefficient measurement. The absolute value of the collisional lossrate coefficient of trapped ⁷Li atoms is determined by analyzing the temporal change of the fluorescence intensity when the supply of the Li beam is turned off.     

Accurate calculation of the scattering length for the cooling of hydrogen atoms by lithium atoms

An improved ab initio calculation has been performed for the potential for the LiH a ³Σ⁺ state, using two very large basis sets. The Basis Set Superposition Error (BSSE) correction has been determined for both basis sets and the non-Born-Oppenheimer correction estimated to be negligible. The best potential is approximately 10% deeper than the previous estimate. Vibrational energies and scattering lengths have been calculated for ^6,7LiH(D) with both potentials, with and without the BSSE correction, and also with an estimated potential expected to bracket the true potential. The ⁷LiH scattering length is estimated to be (45 ± 4)a₀ and hence the low-energy cross-section in the best a ³Σ⁺ potential is about half that calculated previously. Enhanced cooling by ⁷Li of trapped H atoms remains feasible. Received 30 April 2001     

Low-lying states of 6He studied via the 6Li(t,3He)6He reaction

We present the recent experimental results on the ⁶He structure studied by the ⁶Li(t, ³He)⁶He reaction at 336 MeV. Above the conspicuous peaks for ground and first excited states for ⁶He, we have observed a broad structures at E _x∼ 5 MeV, and E _x∼ 15 MeV. The angular distribution of this structure exhibits the dominance of a ΔL = 1 transition, indicating the existence of intruder dipole states at low excitation energies in ⁶He. A slight admixture of positive-parity states in this structure has been indicated as well. Received: 1 May 2001 / Accepted: 4 December 2001     

ZrO2 films deposited by photo-CVD at low temperatures

Zirconium oxide layers have been successfully deposited by photo-CVD at low temperatures. ZrO₂ growth was observed at temperatures as low as 100 °C. When deposited at 250 °C and above, these films exhibited a polycrystalline structure with a mixture of different crystal phases. Deposition at 300 °C was found to form moisture-free ZrO₂ films with a high refractive index of 2.1, a very low effective density of trapped electrons of ∼8.8×10⁸ cm^-2 and an interface trap density of 6.6×10⁹ cm^-2 eV^-1 being readily obtained. Received: 17 December 2001 / Accepted: 6 January 2002 / Published online: 3 June 2002     

Thermoluminescence and photoluminescence of LiNaSO4:Eu irradiated with 24 and 48 MeV Li ion beam

Thermoluminescence (TL) of LiNaSO₄:Eu phosphor, irradiated with 24 and 48 MeV ⁷Li ions at different fluences in the range 5×10⁹-1×10¹² ion/cm², has been studied. The samples from the same batch were also exposed to γ-rays from a Cs¹³⁷ source for comparative studies. The TL glow curves of the materials, irradiated with ⁷Li ions, have similar structures to that of γ-irradiated sample. They have a simple structure with a prominent peak at 412 K along with small one at around 481 K. The intensity ratios of 412-481 K peaks have been observed to increase with fluence increasing, while that of γ-irradiated sample shows a reverse trend. This could be attributed to the changes in the recombination center populations due to ⁷Li ions, that have been implanted inside the matrix of LiNaSO₄:Eu, during irradiation and might also act as a source for new trapping and luminescent centers. The implantation has been confirmed by TRIM calculations. The penetration depths (where the ion comes to rest) are found to be 145 and 463 μm corresponding to 24 and 48 MeV ion beam energies, respectively, which are less than the thickness of the sample chips (∼800 μm). The efficiencies of LiNaSO₄:Eu to 24 and 48 MeV ⁷Li ions measured relative to γ-rays of Cs¹³⁷ are found to be 0.007 and 0.024, respectively. Theoretical analysis of the glow curves of the samples irradiated by ⁷Li ions and γ-rays were done by glow curve deconvolution method to determine trapping parameters of various peaks. The experimentally observed linearity/sublinearity has been discussed in the frame of track interaction model. Photoluminescence studies in the ⁷Li ions irradiated and un-irradiated samples show that europium ions have incorporated in the host in their divalent (emission at 440 nm) as well as trivalent (emissions at 594, 615 and 700 nm) forms. The intensities of the emission bands of these ions have been observed to increase with fluence increasing.     

Investigation of nuclear-spin couplings in the lithium fluorides as possible candidates for crystal nuclear magnetic resonance quantum computing devices

We have performed ⁷Li and ¹⁹F nuclear magnetic resonance (NMR) in two lithium fluorides BaLiF₃ and YLiF₄ to explore the possibility of a crystal NMR quantum computing device. We find that (1) both the absolute values and the angular dependences of the line widths can primarily be accounted for by the nuclear dipolar fields, and (2) the spin–lattice relaxation times are long enough for quantum computations. These characteristics indicate that these crystals can be possible candidates for quantum computing devices. We also find that, in the perovskite structures like BaLiF₃, magic angles are quite effective to diminish the nuclear dipole fields, which enables us to treat some nuclei as ‘isolated’. We propose using this feature to create low-dimensional nuclear-spin networks in the crystals. Received: 29 January 2001 / Accepted: 6 February 2001 / Published online: 3 April 2001     

Bose–Einstein condensation of lithium

7 Li has been studied in a magnetically trapped gas. Many-body quantum theory predicts that the occupation number of the condensate is limited to about 1400 atoms because of the effectively attractive interactions between ⁷Li atoms. Using a versitile phase-contrast imaging technique, we experimentally observe the condensate number to be consistent with this limit. We discuss our measurements, the current theoretical understanding of BEC in a gas with attractive interactions, and future experiments we hope to perform. Received: 4 June 1997     

Elastic scattering of intermediate-energy weakly bound particles on 12C nuclei

The differential cross-section for elastic scattering of deuterons at 700 MeV, ⁶Li and ⁶He at 2.07 GeV nuclei on ¹²C nucleus is calculated under the assumption of two-cluster (for deuterons and ⁶Li) and three-cluster (for ⁶He) structure of incident particle. For ⁶Li-¹²C and ⁶He-¹²C elastic scattering it is shown that there are quantitative distinctions in the behaviour of the observables calculated in the above approaches. Received: 13 October 1999 / Accepted: 3 January 2001     

Semiclassical approach to the three-body muon-transfer collisions

Muon-transfer rates in collisions of hydrogen-like atoms or with light nuclei t, ³He, ⁴He, ⁶Li or ⁷Li, are calculated in a semiclassical approximation to the Faddeev-Hahn equations. The two nuclei involved are treated classically, while the motion of the muon in their Coulomb field is considered from the quantum mechanical point of view. The experimentally observed strong dependence on the charge of the nuclei is reproduced. Received: 1st November 1997 / Revised: 26 March 1998 / Accepted: 18 August 1998     

Two-neutron elastic transfer 4He(6He,4He)6He at E = 151 MeV

Using the framework of the coupled reaction channels (CRC) the elastic scattering and the elastic transfer in the system ⁶He + ⁴He measured at E = 151 MeV have been analysed. It is shown that the structure observed in the backward range of the angular distributions is influenced by the interference of the elastic 2n-transfer with a two-step process passing through the 2⁺ excitation in ⁶He. The two-neutron transfer mechanism is studied in the microscopic approach and it is found that for the ground-state transition the one step dominates by a factor 10 over the two-step mechanism at this energy. Received: 29 October 2001 / Accepted: 4 December 2001     

Transfer reaction studies to states in 14N and 14C

States in ¹⁴N and ¹⁴C have been populated by the Li-induced reactions ¹⁰B(⁶Li, d), ¹¹B(⁶Li, t/³He), ¹¹B(⁷Li, α), ¹²C(⁶Li, α), and ¹³C(⁷Li, ⁶He/⁶Li) at E_Li = 32 and 34 MeV. Selective population of states is observed with these reactions. Angular distributions were measured for the ¹¹B(⁶Li t/³He) analog reactions. Experimental criteria are developed to allow the identification of analog states up to 18.40 and 16.45 MeV excitation in ¹⁴N and ¹⁴C, respectively.     

The 7Li(γ, N) and 7Li(e,N) reactions at intermediate photon energies

Cross sections for single-photonucleon emissions from ⁷Li have been measured for photon energies in the range 60–120 MeV by detecting the recoiling residual nuclei following excitation with bremsstrahlung radiation of end-point energies 140 and 155 MeV. Measurements of the ⁷Li(e, ⁶Li)e'p and ⁷Li(e, ⁶He)e'n cross sections were also made at the same electron energies. A significant difference between the ratio of electron- and bremsstrahlung-induced yields for proton and neutron emission is observed. The results are compared to a modified quasi-deuteron model and a simple direct-knockout model in which recoil terms are included.     

Compositional characterisation of Zn-diffused lithium niobate waveguides

Rutherford backscattering (RBS) and secondary-ion mass spectrometry techniques have been used to investigate the two-step process involved during waveguide fabrication in LiNbO₃ using Zn-vapour diffusion. Compositional analysis (O, Nb, Li and Zn) in the two steps has been characterised. RBS analysis reveals that the first step, involving a heating of the substrate under a metallic Zn atmosphere, gives rise to a partial exchange between the Nb and Li ions from the crystals and the Zn from the vapour source. The second treatment at higher temperature in an open atmosphere diffuses the Zn deeper into the substrate, thus forming an optical waveguide, while the Nb and Li ions recover their bulk values. Received: 16 May 2001 / Revised version: 7 September 2001 / Published online: 30 October 2001     

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.     

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.     

Collisional damping of giant monopole and quadrupole resonances

Collisional damping widths of giant monopole and quadrupole excitations for ¹²⁰Sn and ²⁰⁸Pb at zero and finite temperatures are calculated within Thomas-Fermi approximation by employing the microscopic in-medium cross-sections of Li and Machleidt and the phenomenological Skyrme and Gogny forces, and are compared with each other. The results for the collisional widths of giant monopole and quadrupole vibrations at zero temperature as a function of the mass number show that the collisional damping of giant monopole vibrations accounts for about 30 - 40% of the observed widths at zero temperature, while for giant quadrupole vibrations it accounts for only 20 - 30% of the observed widths at zero temperature. Received: 9 January 2001 / Accepted: 29 March 2001