Okada Kunihiro

We describe a simple and fast method for simulating observed images of ion Coulomb crystals. In doing so, cold elastic collisions between Coulomb crystals and virtual very light atoms are implemented in a molecular dynamics ( MD) simulation code. Such an approach reproduces the observed images of Coulomb crystals by obtaining density plots of the statistics of existence of each ion. The simple method has the advantage of short computing time in comparison with previous calculation methods. As a demonstration of the simulation, the formation of a planar Coulomb crystal with a small number of ions has been investigated in detail in a linear ion trap both experimentally and by simulation. However, also large Coulomb crystals including up to 1400 ions have been photographed and simulated to extract the secular temperature and the number of ions. For medium-sized crystals, a comparison between experiments and calculations has been performed. Moreover, an MD simulation of the sympathetic cooling of small molecular ions was performed in order to test the possibility of extracting the temperature and the number of refrigerated molecular ions from crystal images of laser-cooled ions. Such information is basic to studying ultracold ion-molecule reactions using ion Coulomb crystals including sympathetically cooled molecular ions.

Development of a universal slow RI-beam facility based on rf-carpet ion guide technique is being progressing at RIKEN RI-beam factory. With a prototype setup, precision optical spectroscopy experiments were performed for beryllium isotopes. An additional capability of providing parasitic slow RI-beams from the projectile fragment separator BigRIPS is also discussed.

RIKEN's new RI-Beam Factory (RIBF) will provide unprecedented access to neutron-rich nuclei of importance to r-process nucleosynthesis. We are constructing an advanced multi-reflection time-of-flight mass spectrograph to perform precision mass measurements of these nuclei. We discuss the device and compare its performance to that of the well-known Penning trap mass spectrometer.

We have performed precision atomic spectroscopy of trapped radioactive Be isotopes aiming at studies of the charge and magnetization radii of these nuclei especially for a single-neutron halo nucleus Be-11. Some experimental results and the status of the analysis are discussed.

Ion Coulomb crystals of laser-cooled Ca+ ions have been observed in a cryogenic linear octupole rf ion trap for the first time. Due to the almost field-free central region, large ion crystals containing over 104 ions of millimeter size were successfully produced. The shapes of the crystals can be manipulated by changing the static voltages applied to the octupole rods and the endcap electrodes. Quasi-two-dimensional Coulomb crystals consisting of a small number of ions were also observed with single-ion sensitivity. When a large number of ions is crystallized in the octupole trap, a new kind crystal image with a dip along the axial direction was obtained. The results show that a linear octupole trap and multipole traps are available for producing large, cold, and novel ion Coulomb crystals. The planar ring-type crystal which has not been observed before may lead to future interesting applications. © 2009 The American Physical Society.

Precision atomic spectroscopy experiments for Be isotopes have been carried out at the prototype universal slow RI-beam (SLOWRI) setup at RIKEN. Radioactive Be ions produced at 1 GeV were decelerated and thermlized in an RF-carpet ion guide. The thermalized ions were transferred to an ion trap where laser cooling was used to reduce the ion energy to the order of 1 μeV. Laser microwave double resonance spectroscopy was performed for the hyperfine structure measurements of trapped and laser cooled 7Be+ and 11Be+ ions. Measurements of the S1/2 → P1/2,P3/2 transition frequencies of 7,9,10,11Be+ ions are also in progress. These results are briefly discussed. Future prospects for expanding the capability of SLOWRI is also discussed. © 2009 American Institute of Physics.

A fragment separator at heavy ion accelerator facilities is a versatile instrument to provide wide variety of radioactive isotope (RI) beams. However, more than 99.99% of precious RI-ions are simply dumped in the slits or elsewhere in the fragment separator. A novel concept to restore such RI-ions for parasitic slow RI-beams is proposed. Installation of a laser ionization gas catcher in the vicinity of the first or second focal point of the fragment separator enables to collect dead isotopes in the slits. The design concept and expected performance are discussed.

A universal slow RI-beam facility (SLOWRI) for precision atomic spectroscopy is being built at the RIKEN RI-beam factory. The facility will provide a wide variety of low-energy nuclear ions of all elements produced by projectile fragmentation of high-energy heavy-ion beams and thermalized by an RF-carpet ion guide. At prototype SLOWRI, radioactive Be isotope ions produced at 1 GeV were decelerated and cooled in an ion trap down to 1 mu eV by employing laser cooling. The ground state hyperfine structures of Be-7(+) and Be-11(+) were measured accurately by laser microwave double resonance spectroscopy. Measurements of the S-1/2 -> P-1/2, P-3/2 transition frequencies of Be-7,9,10,11(+) ions are also in progress aiming at the study of the nuclear charge radii. Other possible experiment at SLOWRI, such as mass spectroscopy, are also discussed.

Absolute measurements of spectral lines of Ar+ ions using collinear and anticollinear geometries were performed. To provide a precise reference for the laser wavelength, iodine saturation spectroscopy was applied. The precision of this reference is effected by observing the beat node between the spectroscopy laser and the corresponding mode of a femtosecond laser frequency comb. Laser-induced fluorescence allowed to perform precision frequency measurements of an Ar+ transition in collinear and anticollinear geometries simultaneously; then an exact relativistic formula for the absolute transition frequency nu(0) = root nu(c)nu(a) was used. In this geometry the influence of ion source instabilities due to pressure and anode voltage fluctuations was minimized. The result is nu(0)=485,573,619.7(3) MHz, which corresponds to Delta nu/nu=6*10(-10). This represents an improvement of two orders of magnitude over the previous NIST published value.

The ground state hyperfine splitting of (7)Be(+) has been measured by laser-microwave double-resonance spectroscopy in the online rf trap of RIKEN's slow RI-beam facility. Be ions produced by projectile fragmentation of (13)C at approximate to 1 GeV were thermalized in a rf ion guide gas cell and subsequently laser cooled in the ion trap to approximate to 1 mu eV. This 10(15)-fold reduction of the kinetic energy allows precision spectroscopy of these ions. A magnetic hfs constant of A=-742.772 28(43) MHz was measured for (7)Be(+), from which a nuclear magnetic moment of mu(I)=-1.399 28(2)mu(N) was deduced.

A laser-cooling experiment with Ca+ ions trapped in a linear rf octupole ion trap is presented. The phase transition of the laser-cooled Ca+ ions from the cloud to the crystal state is observed by an abrupt dip of the laser-induced fluorescence spectrum and indicates that mK temperatures are obtained. We have also performed molecular dynamics simulations under various conditions to confirm this property by deducing axially symmetric structures of Coulomb crystals and by evaluating the translational temperatures of the laser-cooled ions. The simulation results show that for small numbers of ions novel ring-shaped crystals are produced. As the number of ions is increased, cylindrical layers in the ring crystal are sequentially formed. For more than 100 ions, also hexagonal and spiral structures emerge in parts of the large-size ion crystal, which has a length on the order of millimeters for the present geometrical arrangement and voltages. An advantage of the linear rf octupole trap is its large almost-field-free region in the middle of the trap, where the micromotion amplitude is small for trapped ions. These results demonstrate that such a multipole trap has attractive features for quantum computing and ultracold ion-atom collision studies. © 2007 The American Physical Society.

Apparent generalized oscillator strengths (apparent GOS's) have been measured for three types of optically forbidden transitions in rare-gas atoms as functions of the squared momentum transfer K-2 at small K-2 range (<= 0.4 a.u.). The apparent GOS's were deduced from the differential cross sections for excitation, which were measured by means of the electron energy-loss spectroscopy. Electron impact energies were 100, 300, and 500 eV, and the scattering angles were from 0.8 degrees to 10 degrees. In the case where the first Born approximation does not hold, the apparent GOS as a function of K-2 (the apparent GOS function) shows characteristic dependence on the electron collision energy according to the character of the transition. In the present observation, for the np(6) S-1(0)-> np(5)(n+1)p(')[1/2](0) transitions, the specific behavior has been observed in the apparent GOS functions characteristic of that for the S-1(0)-> S-1(0) type transition, in which the term symbols of the initial and the final states do not change. For the np(6) S-1(0)-> np(5)(n+1)p[5/2](2,3); [3/2](1,2) transitions, a certain new type of deviations from the first Born approximation, which is interpreted to be characteristic of the S-1(0)-> D-1(2) type transition, have been observed in the apparent GOS functions with some modifications depending on respective atomic species. For the 5p(6) S-1(0)-> 5p(5)5d [7/2](3); [5/2](3) transitions in Xe, it is observed that the apparent GOS curves have no impact energy dependence for impact energies from 100 eV to 500 eV, which suggests that the first Born approximation is valid for such low impact energies and the curves agree with the Bethe-GOS. It is found that the GOS's varies in proportional to K-4 at small K-2 region (<= 0.1 a.u.), which suggests that the octupole moment is dominant in these transitions. We interpret that these features of the behavior are specific for the S-1(0)-> F-1(3) type transition. On the whole, it is found that the behavior of the apparent GOS function can be well understood by considering the main contribution from a certain LS-coupling state when the excited state is represented by the intermediate coupling scheme which is expressed as a linear combination of the LS-coupled states.

Radioactive beryllium isotope ions (Be-7(+) and Be-10(+)) that are provided by a projectile fragment separator with approximate to 1 GeV beams, as well as stable isotope ions (Be-9(+)) are stored and laser cooled in an online ion trap. Their absolute transition energies of the 2s S-2(1/2)-> 2p P-2(3/2) transition were measured with an accuracy of similar to 10(-8). In this way isotope shifts of beryllium ions were obtained and the differential mass polarization parameter kappa=-0.286 41(70) a.u. as well as the 2s S-2 -> 2p P-2 transition energy of an infinitely heavy beryllium ion h nu(infinity)=0.145 524 290(42) a.u. were determined for the first time.

A next-generation slow radioactive nuclear ion beam facility (SLOWRI) which provides slow, high-purity and small emittance ion beams of all elements is being build as one of the principal facilities at the RIKEN RI-beam factory (RIBF). High energy radioactive ion beams from the projectile fragment separator BigRIPS are thermalized in a large gas catcher cell. The thermalized ions in the gas cell are guided and extracted to a vacuum environment by a combination of dc electric fields and inhomogeneous rf fields (rf carpet ion guide). From there the slow ion beam is delivered via a mass separator and a switchyard to various devices: such as an ion trap, a collinear fast beam apparatus, and a multi-reflection time of flight mass spectrometer. In the R&D works at the present RIKEN facility, an overall efficiency of 5% for a 100A MeV Li-8 ion beam from the present projectile fragment separator RIPS was achieved and the dependence of the efficiency on the ion beam intensity was investigated. Recently our first spectroscopy experiment at the prototype SLOWI was performed on Be isotopes. Energetic ions of Be-10 and Be-7 from the RIPS were trapped and laser cooled in a linear rf trap and the specific mass shifts of these isotopes were measured for the first time.

The space-charge shift in secular motion resonance of Ca+ induced by simultaneously trapped molecular ions was observed through the laser-induced fluorescence of buffer-gas-cooled Ca+ ions in a linear Paul trap. The shift in the resonance was measured as a function of the production time of molecular ions. A simple pseudopotential model was developed to explain the space-charge shift and the number of trapped molecular ions was estimated using the model. The method can be used to monitor the total amount of invisible ions in principle, as long as the weak coupling condition is satisfied between the observable ions and the invisible ions.

The photodissociation of trapped CaOH+ produced by Ca+*(3d D-2(J)) + H2O reaction was performed in order to regenerate Ca+ in a linear Paul trap. Most Ca+ ions were successfully regenerated from molecular ions by deep UV light irradiation. The photon energy threshold in the CaOH+ + hv -> Ca+ + OH reaction was estimated to be similar to 4.9 eV. The dissociation energies of XH+ -> X+ + H and XOH+ -> X+ + OH (X = Be, Mg, Ca) were calculated using standard quantum chemistry codes to search for the possibility of regenerating light alkaline earth ions from molecular ions. The present experiment suggests that the photodissociation method is applicable to the regeneration of alkaline earth ions lost by chemical reactions with typical residual molecules in an ion trap experiment.

A next-generation slow radioactive nuclear ion beam facility (SLOWRI) which provides slow, high-purity and small emittance ion beams of all elements has been proposed as one of the pricipal experimental facilities at the RIKEN RI-beam factory (RIBF). High energy radioactive ion beams from the projectile fragment separator BigRIPS are thermalized in a large gas catcher cell. The thermal ions in the gas cell are guided and extracted to vacuum by a combination of DC electric fields and inhomogeneous rf fields in the cell (rf ion guide). In the R&D works at the present RIKEN facility, an overall efficiency of approximate to 5% for approximate to 100A MeV Li-8 ion beam from the present projectile fragment separator RIPS was achieved and the dependence of the efficiency on the beam intensity was investigated. A first spectroscopy experiment at the prototype SLOWI was performed on Be isotopes.

Slow radioactive ion beams have been produced with an overall efficiency of 4% by thermalizing energetic ions produced by a projectile fragment separator in a He-gas cell and guiding them to a vacuum vessel by dc and rf fields. Space charge was observed to have a limiting effect. Since the ionization of He atoms by energetic ions creates a region of high space charge, many thermalized ions of interest are pushed toward the walls of the gas cell. Such losses have been investigated for different incoming ion intensities. (c) 2005 American Institute of Physics.

The projectile fragment separator provides a wide variety of short-lived RI-ions with less restrictions on their chemical property or lifetime limit. The beam energy and quality is, however, not adequate for low-energy beam experiments, in particular for trapping experiments. Recently, one has proposed to obtain a low-energy beam from an energetic RI-beam leaving a projectile fragment separator by using a large gas-catcher and an rf ion-guide system. In off-line and in on-line test experiments, the principle of the rf ion-guide was proven. An overall efficiency of 0.2% for 70 MeV/u Li-8 from the RIKEN projectile fragment separator (RIPS) was obtained so far. (C) 2003 Elsevier Science B.V. All rights reserved.

The reactivity of laser-excited Ca+* ions in 3 D-2(J) (J = 3/2, 5/2) states with neutral H2O molecules was experimentally investigated. The reaction-product ions were sympathetically cooled by laser-cooled Ca-40(+) ions and detected by laser-induced fluorescence mass spectroscopy. The reaction rate coefficient of Ca+*(D-J) + H2O products was found to be 1.8(1) x 10(3) s(-1) Pa-1 at an ion temperature of T 3.1(1) x 10(3) K. The ion temperature dependence of the reaction rate coefficient was also measured for buffer-gas-cooled Ca+ ions.