Developments at the SLOWRI facility at RIKEN: precision optical spectroscopy of 7,9,10,11Be+ ions

Precision optical spectroscopy of radioactive Be isotopes produced in projectile fragmentation has been performed for the first time at the prototype SLOWRI facility of RIKEN RI-Beam Factory. The ground state hyperfine constants of ⁷Be⁺ and ¹¹Be⁺ were determined with relative accuracies of 6 × 10^?7 and 3 × 10^?8, respectively, by laser-microwave double resonance spectroscopy of laser-cooled ions in a trap. The optical transition energies from the ground S-state to the excited P-state of Be isotope ions were also measured to determine the nuclear charge radii from the isotope shifts. Development of the universal slow RI-beam facility??SLOWRI??based on the rf-carpet ion guide technique is progressing at RIKEN RI-beam factory. An additional capability of providing parasitic slow RI-beams from the projectile fragment separator BigRIPS is also discussed.     

Precision laser spectroscopy of Be isotopes and prospects for SLOWRI facility at RIKEN

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.     

Laser spectroscopy of atoms in superfluid helium for the measurement of nuclear spins and electromagnetic moments of radioactive atoms

A new laser spectroscopic method named “OROCHI (Optical RI-atom Observation in Condensed Helium as Ion catcher)” has been developed for deriving the nuclear spins and electromagnetic moments of low-yield exotic nuclei. In this method, we observe atomic Zeeman and hyperfine structures using laser-radio-frequency/microwave double-resonance spectroscopy. In our previous works, double-resonance spectroscopy was performed successfully with laser-sputtered stable atoms including non-alkali Au atoms as well as alkali Rb and Cs atoms. Following these works, measurements with ^84?87Rb energetic ion beams were carried out in the RIKEN projectile fragment separator (RIPS). In this paper, we report the present status of OROCHI and discuss its feasibility, especially for low-yield nuclei such as unstable Au isotopes.     

Polarized proton target for RI Beam experiments

We have constructed a polarized proton solid target system for radioactive nuclear beam experiments at the Center for Nuclear Study, the University of Tokyo. The proton polarization is based on an electron population difference in a photo-excited triplet state of pentacene molecule. The target system was completed in 2003 and applied to a RI beam experiment in 2003 and 2005 by using the projectile fragment separator, RIPS at RIKEN. The maximum polarization reached 20% under the condition of T=100 K and B=0.09 T. Overview of the polarized target and its application in physics experiments at RIPS and RIBF of RIKEN are presented.     

Space-Charge Effects with REXTRAP

The beam quality of radioactive ion beams produced by present target ion source technology is often not sufficient for direct post-acceleration. Furthermore, pulsed beams insure a more efficient use of an accelerator. In the case of REX-ISOLDE, the post accelerator at the CERN ISOLDE facility, a gas-filled Penning trap (REXTRAP) has been chosen for accumulation of the radioactive ions and conversion into cooled bunches. Radial centering of the ions is achieved by applying an rf field with a frequency equal to the cyclotron frequency of the desired ion species. The efficiency achieved in the first tests with different isotopes covering nearly the entire mass range was already >20%. Going to total numbers of >10⁵ stored ions in the trap a shift of the centering frequency could be observed, which is most likely due to space charge effects. Despite this, it was possible to accumulate up to 10⁷ ions and deliver them as cooled bunches. This revised version was published online in July 2006 with corrections to the Cover Date.     

CARIBU: a new facility for the study of neutron-rich isotopes

The Californium Rare Ion Breeder Upgrade (CARIBU) to the ATLAS superconducting linac facility is currently being commissioned. It provides low-energy and re-accelerated beams of neutron-rich isotopes obtained from ²⁵²Cf fission. The fission products from a ²⁵²Cf source are stopped in a large high-intensity gas catcher, thermalized and extracted through an RFQ cooler, accelerated to 50 kV and mass separated in a high-resolution separator before being sent to either an ECR charge breeder for post-acceleration through the ATLAS linac or to a low-energy experimental area. This approach gives access to beams of very neutron-rich isotopes, many of which have not been available at low or Coulomb barrier energies previously. These beams provide unique opportunities for measurements along the r-process path. To take advantage of these unique possibility, the reaccelerated beams from CARIBU will be made available at the experimental stations of ATLAS to serve equipment such as Gammasphere, HELIOS and the reaction spectrometers. In addition, the Canadian Penning Trap (CPT) mass spectrometer has been moved to the CARIBU low-energy experimental area and a new injection line has been built. The new injection line consists of a RFQ buncher sitting on a 50 kV high-voltage platform that will accumulate the mass separated 50 kV radioactive beams, cool and extract them as a pulsed beam of 3 keV. This beam can be sent either to a tape station for diagnostics and tuning, or a cryogenic linear trap for preparation before transfer to the high-precision Penning trap where the mass measurements will take place. Initial CARIBU commissioning is proceeding with a 2 mCi source that will be replaced by a 100 mCi source as the commissioning proceeds. Final operation will use a 1 Ci source and attain yield in excess of 10⁷ ions/sec for the most intense beams at low energy, an order of magnitude less for reaccelerated beams.     

High-accuracy mass measurements in ion traps and storage rings

High-accuracy mass measurements have recently been performed on radioactive isotopes produced by proton-induced spallation at the on-line isotope separator ISOLDE at CERN and by heavy-ion projectile fragmentation at the fragment separator FRS at GSI. At ISOLDE, singly charged ions were injected into the Penning trap mass spectrometer ISOLTRAP and their masses determined by observing their cyclotron frequencies in the homogeneous magnetic field of the ion trap. At GSI, bare, hydrogen, or helium-like ions were injected into the experimental storage ring ESR, electron-cooled to the same velocity, and their masses determined by observing their revolution frequencies in the ESR. With ISOLTRAP and ESR, resolving power in the range of 4 × 10⁵< = m/Δ m(FWHM)< = 10⁷ and an accuracy up to \delta m/m~ 10^-7 were achieved for radioactive isotopes. Mass measurements of highly charged ions of stable isotopes were performed at Stockholm by use of SMILETRAP. In this case, a resolving power of about 10⁸ and an accuracy close to 10^-9 were obtained. This revised version was published online in August 2006 with corrections to the Cover Date.     

Collinear fast atomic-beam laser spectroscopy at riken garis/igisol

We have constructed a fast atomic-beam collinear laser spectroscopy system connected to the on-line isotope separator facility GARIS/IGISOL at RIKEN. This system is potentially powerful in studying refractory elements. To test the system, an off-line collinear experiment was made on stable Hf isotopes produced by means of laser ablation technique. For production of radioactive Hf isotopes, the use of the inverse kinematics of a fusion reaction,⁹Be(¹⁶⁶Er,xn)^175-x Hf, was investigated. The radioactive isotope¹⁶⁹Hf was successfully extracted from GARIS/IGISOL and accelerated up to 30 keV.     

RI beam facility project at RIKEN and physics programs

In the Radioactive-Isotope Beam Factory (RIBF) project in RIKEN, intense primary beams can be provided at the energies E = 350−400MeV over the whole range of atomic number in the cascade-cyclotron acceleration scheme, for which three cyclotrons, fRC, IRC, and SRC, have been newly constructed. The project proceeds through two phases. In the phase-I program, the superconducting in-flight radioactive-isotope beam separator BigRIPS and the following ZeroDegree spectrometer have been installed as well as the three cyclotrons. In the commissioning, after the successful extraction of a ²³⁸U beam from SRC at E = 345 A MeV in the cascade-acceleration scheme, radioactive-isotope beams were produced and isotope-separated with BigRIPS as designed. The RIBF project is fully capitalized in the phase-II program, in which the construction of several experimental key devices has been proposed. The upgrade of the former fragment separator RIPS is also included there. It allow for a scheme to use intense primary beams at the intermediate energy E = 115 A MeV with RIPS. Remarkably, the produced radioactive-isotope beams at this energy can be spin-polarized taking the advantage of the fragmentation-induced spin orientation phenomena. on behalf of the RIBF project     

Technical developments for an upgrade of the LEBIT Penning trap mass spectrometry facility for rare isotopes

The LEBIT (Low Energy Beam and Ion Trap) facility is the only Penning trap mass spectrometry (PTMS) facility to utilize rare isotopes produced via fast-beam fragmentation. This technique allows access to practically all elements lighter than uranium, and in particular enables the production of isotopes that are not available or that are difficult to obtain at isotope separation on-line facilities. The preparation of the high-energy rare-isotope beam produced by projectile fragmentation for low-energy PTMS experiments is achieved by gas stopping to slow down and thermalize the fast-beam ions, along with an rf quadrupole cooler and buncher and rf quadrupole ion guides to deliver the beam to the Penning trap. During its first phase of operation LEBIT has been very successful, and new developments are now underway to access rare isotopes even farther from stability, which requires dealing with extremely short lifetimes and low production rates. These developments aim at increasing delivery efficiency, minimizing delivery and measurement time, and maximizing use of available beam time. They include an upgrade to the gas-stopping station, active magnetic field monitoring and stabilization by employing a miniature Penning trap as a magnetometer, the use of stored waveform inverse Fourier transform (SWIFT) to most effectively remove unwanted ions, and charge breeding.     

A facility for production and utilization of radioactive beams

A radioactive ion beam facility, GIRAFFE, has been built at the CIAE HI-13 Tandem accelerator. The facility makes use of the inverse kinematics. A D-Q-Q type magnetic separation and focusing system was used in the beam line. This simple device is expected to yield some radioactive ion beams(A<20) near the β-stability line with the acceptable intensities(10⁵–10⁶ pps). The ion beams of ⁷Be, ¹¹C and ¹⁷F were delivered, and the ⁷Be beam was applied for two experiments.     

Precision Penning trap mass measurements of rare isotopes produced by projectile fragmentation

The low-energy beam and ion trap facility LEBIT at NSCL/MSU is at present the only facility where precision experiments are performed with stopped rare isotope beams produced by fast-beam fragmentation. LEBIT combines high-pressure-gas stopping with advanced ion manipulation techniques to provide brilliant low-energy beams. So far these beams have mainly been used for mass measurements on short-lived rare isotopes with a 9.4T Penning trap mass spectrometer. Recent examples include ^70m Br , located at the proton dripline, ³²Si and the iron isotopes ^63-65Fe . While the measurement of ³²Si helps to solve a long-standing dispute over the validity of the isobaric multiplet mass equation (IMME) for the A = 32 , T = 2 multiplet, the mass measurements of ^65m,g Fe marked the first time a nuclear isomeric state has been discovered by Penning trap mass spectrometry.     

Spectroscopy on very neutron-rich nuclei beyond N = 20

Recent studies on nuclear structure by using radioactive isotope beams available at the RIKEN projectile-fragment separator (RIPS) are introduced. Special emphasis is given to two selected experiments from recent programs that highlight studies on the magicity loss observed for very neutron-rich nuclei beyond N = 20 in the “island-of-inversion” region; the particle stability of ³¹F, and the low-lying excited states of ³⁴Mg. Received: 1 May 2001 / Accepted: 4 December 2001     

Measurements on diproton emission from the break-up channels of 23Al and 22Mg

Two-proton relative momentum distributions from the break-up channels ²³Al→p+p+²¹Na and ²²Mg→p+p+²⁰Ne at an energy of 60–70 A MeV have been measured together with two-proton opening angles at the projectile fragment separator beamline (RIPS) in the RIKEN Ring Cyclotron Facility. The results demonstrate the existence of diproton emission component from single-step ²He for highly excited ²³Al and ²²Mg.     

The LEBIT facility at MSU

The low-energy beam and ion trap facility LEBIT at the NSCL at MSU has demonstrated that rare isotopes produced by fast-beam fragmentation can be slowed down and prepared such that precision experiments with low-energy beams are possible. For this purpose high-pressure gas-stopping is employed combined with advanced ion manipulation techniques. Penning trap mass measurements on short-lived rare isotopes have been performed with a 9.4 T Penning trap mass spectrometer. Examples include ⁶⁶As, which has a half-live of only 96 ms, and the super-allowed Fermi-emitter ³⁸Ca, for which a mass accuracy of 8 ppb (280 eV) has been achieved. The high accuracy of this new mass value makes ³⁸Ca a new candidate for the test of the conserved vector current hypothesis.      

The β-delayed one- and two-proton emission of 27S

In an experiment performed at the GANIL LISE3 facility, radioactive ²⁷S isotopes have been produced by projectile fragmentation of a 95 AMeV ³⁶Ar primary beam. After selection by means of the LISE3 separator, the isotope of interest was implanted in a silicon-detector telescope where its half-life ( T _1/2 = 15.5(15) ms) and its main decay branches were measured. Received: 19 October 2001 / Accepted: 5 December 2001     

Production of an 8.0 MeV 7Be ion beam at the Naples TTT-3 accelerator

For an improved determination of the ⁷Be(p, γ)⁸B absolute cross section, the reaction is studied in inverse kinematics. The study involves a ⁷Be radioactive ion beam of 8.0 MeV, a windowless H2 gas target, and an efficient recoil separator for the detection of the ⁸B residual nuclides. The techniques used to produce such a beam as well as the first direct observation of ⁸B are reported.     

Forward-angle yields of 2≤Z≤11 isotopes in the reaction of 18O(35A MeV) with 9Be

A systematic investigation of the forward-angle inclusive yields of 2≤Z≤11 isotopes produced in collisions of ¹⁸O projectile nuclei with a ⁹Be target in the Fermi energy region (35A MeV) is performed. The measurements were based on the use of the COMBAS double achromatic kinematical separator in the spectrometry mode at the Flerov Laboratory of Nuclear Reactions at the Joint Institute for Nuclear Research, FLNR (JINR, Dubna). The velocity, isotopic, and element distributions are presented. There is no unique mechanism that would explain the total set of results obtained in this experiment. A dominant role of low-energy reaction mechanisms is observed. The intensity of secondary beams of halolike nuclei ¹¹Li, ¹²Be, and ¹⁴Be is determined.     

Decay study of neutron-rich zirconium isotopes employing a Penning trap as a spectroscopy tool

A new technique to produce isobarically pure ion beams for decay spectroscopy by using a gas-filled Penning trap was commissioned at the ion guide isotope separator on-line facility, IGISOL. β-decays of neutron-rich ¹⁰⁰Zr, ¹⁰²Zr and ¹⁰⁴Zr isotopes were studied with this technique. In addition, the values of ^100,102,104Zr β-decays were determined from the direct mass measurements of zirconium and niobium isotopes performed with a high-precision Penning trap. The mass of ¹⁰⁴Nb was directly measured for the first time and the obtained mass excess value for the longer-living (1⁺) state is -71823±10 keV. For the ground states of ¹⁰⁰Nb and ¹⁰²Nb the obtained mass excess values were -79802±20 keV and -76309±10 keV, respectively. The observed distribution of the β strength supports a prolate deformation assignment for ^100,102,104Zr isotopes.     

Recent radioactive ion beam program at RIKEN and related topics

Recent experimental programs at RIKEN concerning RI beams are reviewed. RIKEN has the ring cyclotron (RRC) with high intense heavy-ion beams and large acceptance fragment separator, RIPS. The complex can provide high intense RI-beams. By using the high intense RI-beams, a variety of experiments have been done. Recently, nuclear structure for unstable nuclei has been paid much attention. In special, disappearance and appearance of magic numbers are discussed experimentally and theoretically. Thus, in this review, related experiments concerning disappearance and appearance of magic numbers are described. Finally, future project in RIKEN, RI-beam factory. is introduced briefly.