RI Beam Factory Research Articles

Precision pionic-atom spectroscopy at RIKEN-RIBF: Test report on primary beam dispersion matching

We performed a test experiment at the RI beam factory (RIBF). In the test experimnt, a 14N beam with an energy of 250 MeV/nucleon was used. We realized the dispersion matching between the beam transfer line and the BigRIPS spectrometer. The position resolution improved by a factor of two after the dispersion matching was applied. The resolving power of the BigRIPS was estimated to be 3860 (FWHM) from the obtained resolution which was 0.83 mm (FWHM). This resolving power corresponds to an experimental resolution of 190 keV (FWHM) for the emitted 3He at 365 MeV from the (d,3He) reaction in the deeply bound-pionic atom spectroscopy.

Measurement of the Isovector Spin Monopole Resonance via the 208Pb,90Zr(t,3He) Reactions at 300 MeV/u

The isovector spin monopole resonances (IVSMR) of the β+ type were observed via the 208Pb and 90Zr(t, 3He) reactions at 300 MeV/u. The experiment was performed at the RI Beam Factory (RIBF) at RIKEN by using the newly-constructed magnetic spectrometer SHARAQ. The double differential cross sections for the 208Pb and 90Zr(t, 3He) reactions were obtained at the excitation energy of 0 ≤ Ex ≤ 70 MeV and the scattering angles of 0° ≤ 9 ≤ 3°. The monopole component was obtained as a difference between the zero-degree and backward-angle spectra. The signatures for the IVSMR were discovered at excitation energies of 12 MeV and 20 MeV in 208Pb and 90Zr, respectively, and they show good correspondences to the theoretical predictions.

Complete Set of Deuteron Analyzing Powers for Deuteron–Proton Elastic Scattering at 250 MeV/nucleon and Three-Nucleon Forces

Measurements of a complete set of deuteron analyzing powers (iT11, T20, T21, and T22) for elastic deuteron-proton scattering at 250 MeV/nucleon have been performed with polarized deuteron beams at RIKEN RI Beam Factory. The obtained data are compared with Faddeev calculations based on the modern nucleon–nucleon forces together with the Tucson- Melbourne'99 and UrbanaIX three-nucleon forces.

Present Status and Perspectives of RIBF

The RIKEN Radioactive Isotope Beam Factory (RIBF) is a top world-class RIB facility, which was constructed to establish new framework of nuclear physics, to elucidate the origin of elements, and to explore new applications with fast RIBs. To achieve these goals, the RIBF facility consists of a high-power accelerator complex, a large-acceptance in-flight separator and unique devices to cover various experimental programs. As illustrated in Fig. 1, the accelerator complex consists of the old facility and a newly constructed one [1]. The old facility has a K540-MeV ring cyclotron (RRC) and two injectors, a variable-frequency heavy-ion linac (RILAC) and a K70-MeV AVF cyclotron. A new highpower heavy-ion booster system consisting of three ring cyclotrons with K = 570 MeV (fixed frequency, fRC), 980 MeV (intermediate stage, IRC), and 2600 MeV (superconducting, SRC), respectively, boost energies of the RRC beams up to 440 A MeV for light ions and 350 A MeV for very heavy ions. The goal of the primary beam intensity is set to be 1 pμA. At the end of 2006, the first primary beam was successfully accelerated at and extracted from SRC. The new in-flight separator BigRIPS [2] is to convert the SRC primary beams into RIBs. Combination of the high-power accelerator complex and the in-flight separator makes it possible to explore the limit of of nuclei and provides high yield rates of exotic nuclei. The RIBF facility has started delivering RI beams since 2007. In 2007, the first commissioning with a 350A MeV U beam successfully produced and identified two new isotopes, Pd, even with a low intensity of 0.03 pnA [3]. In November 2008, the second attempt to search for new isotopes was performed by utilizing the U beam with a 10 times higher intensity of 0.3 pnA and more than 40 of new isotopes were produced

Charge-state distribution measurements using gas charge stripper toward 238U and 136Xe acceleration at RIKEN RIBF

The charge-state distributions of 238U and 136Xe ions at 11MeV/nucleon were measured using a gas charge stripper to test whether the charge states in gaseous media were acceptable for acceleration at the RIKEN RI-Beam Factory (RIBF). A differential pumping system facilitated the increase of the N2 gas thickness to 1.3mg/cm2; this is sufficient for the most probable charge state to attain equilibrium. The charge states of 238U attain equilibrium at 56.0, 56.6, and 55.7 in N2, Ar, and CO2 media with thicknesses of 125, 79, and 126μg/cm2, respectively, whereas those of 136Xe attain equilibrium at 40.5, 40.1, and 40.3 in N2, Ar, CO2 media with thicknesses of 163, 95, and 139μg/cm2, respectively. The equilibrium charge states of 136Xe are acceptable for acceleration by the subsequent cyclotron, whereas those of 238U are not acceptable for acceleration without a major remodeling of the accelerator.

Structural Evolution in the Neutron-Rich NucleiZr106andZr108

The low-lying states in ¹⁰⁶Zr and ¹⁰⁸Zr have been investigated by means of β-γ and isomer spectroscopy at the radioactive isotope beam factory (RIBF), respectively. A new isomer with a half-life of 620 ± 150 ns has been identified in ¹⁰⁸Zr. For the sequence of even-even Zr isotopes, the excitation energies of the first 2⁺ states reach a minimum at N = 64 and gradually increase as the neutron number increases up to N = 68, suggesting a deformed subshell closure at N = 64. The deformed ground state of ¹⁰⁸Zr indicates that a spherical subshell gap predicted at N = 70 is not large enough to change the ground state of ¹⁰⁸Zr to the spherical shape. The possibility of a tetrahedral shape isomer in ¹⁰⁸Zr is also discussed.

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.

Low-Zgas stripper as an alternative to carbon foils for the acceleration of high-power uranium beams

The RIKEN accelerator complex started feeding the next-generation exotic beam facility radioisotope beam factory (RIBF) with heavy-ion beams from 2007 after the successful commissioning of RIBF at the end of 2006. Many improvements made from 2007 to 2010 were instrumental in increasing the intensity of various heavy-ion beams. However, the available beam intensity of very heavy ion beams, especially uranium beams, is far below our goal of $1\text{ }\text{ }\mathrm{p}\ensuremath{\mu}\mathrm{A}$ ($6\ifmmode\times\else\texttimes\fi{}{10}^{12}\text{ }\text{ }\mathrm{particles}/\mathrm{s}$). In order to achieve this goal, upgrade programs are already in progress; the programs include the construction of a new 28-GHz superconducting electron cyclotron resonance ion source and a new injector linac. However, the most serious problem, that of a charge stripper for high-power uranium beams, still remains unsolved, despite extensive research and development work using large foils mounted on a rotating cylinder and a ${\mathrm{N}}_{2}$ gas stripper. A gas stripper is free from problems related to lifetime, though the equilibrium charge state in this stripper is considerably lower than that in a carbon foil, owing to the absence of the density effect. Nevertheless, the merits of gas strippers motivated us to develop a low-$Z$ gas stripper to achieve a higher equilibrium charge state even in gases. We measured the electron-loss and electron-capture cross sections of uranium ions in He gas as a function of their charge state at 11, 14, and $15\text{ }\text{ }\mathrm{MeV}/\mathrm{nucleon}$. The equilibrium charge states extracted from the intersection of the lines of the two cross sections were promisingly higher than those in ${\mathrm{N}}_{2}$ gas by more than 10. Simple simulations of charge development along the stripper thickness were performed by assuming the measured cross sections. The simulation results show that about $1\text{ }\text{ }\mathrm{mg}/{\mathrm{cm}}^{2}$ of He gas should be accumulated to achieve a charge state higher than that of ${\mathrm{N}}_{2}$ gas, notwithstanding the difficulty in accumulation of this helium amount owing to its fast dispersion. However, we now believe that the following two solutions can overcome this difficulty: a gas cell with a very large differential pumping system and a gas cell with a plasma window. Their merits and demerits are discussed in the paper.

Measurement of interaction cross-sections for neutron-rich Na isotopes

The interaction cross-sections (σI) of neutron-rich Na isotopes, 23-35Na, on C target have been measured at 250A MeV using the RI beam factory (RIBF) at RIKEN. Mass dependence of σI for 27-35Na suggests monotonic growth of the skin thickness. The root-mean-square nuclear matter radii (rm) of 23-35Na were deduced from observed σI via a Glauber-type calculation. These rm are in a good agreement with the theoretical prediction by relativistic mean field model (RMF). rm of 33-35Na were determined for the first time.

RIKEN radioactive isotope beam factory project — Present status and perspectives

Programs for studying nuclear reactions and structure of exotic nuclei available at the RIKEN radioactive isotope beam factory project are introduced and discussed by demonstrating recent highlights. Special emphasis is given to the present status and future plans of new devices.

Identification of 45 New Neutron-Rich Isotopes Produced by In-Flight Fission of a 238U Beam at 345 MeV/nucleon

A search for new isotopes using in-flight fission of a 345 MeV/nucleon 238U beam has been carried out at the RI Beam Factory at the RIKEN Nishina Center. Fission fragments were analyzed and identified by using the superconducting in-flight separator BigRIPS. We observed 45 new neutron-rich isotopes: 71Mn, 73,74Fe, 76Co, 79Ni, 81,82Cu, 84,85Zn, 87Ga, 90Ge, 95Se, 98Br, 101Kr, 103Rb, 106,107Sr, 108,109Y, 111,112Zr, 114,115Nb, 115,116,117Mo, 119,120Tc, 121,122,123,124Ru, 123,124,125,126Rh, 127,128Pd, 133Cd, 138Sn, 140Sb, 143Te, 145I, 148Xe, and 152Ba.

New data acquisition system for the RIKEN Radioactive Isotope Beam Factory

A new data-acquisition (DAQ) system has been introduced at the RIKEN Radioactive Isotope Beam Factory (RIBF). This system has functions of network-distributed data processing, hierarchical event building and parallel readout which are achieved with newly developed software and commodity hardware. This system is both versatile and scalable, allowing it to meet the various requirements for RIBF experiments. It has a maximum data processing capability of around 40 MB/s. This paper describes the configuration and the data processing performance of the DAQ system.

Nuclear reaction and structure programs at RIKEN

Programs for studying nuclear reactions and structure of exotic nuclei available at the RIKEN Radioactive Isotope Beam Factory are introduced and discussed through demonstrating recent highlights. Special emphasis is given to present status and future plans of new devices.

RIKEN RI Beam Factory – Recent Results and Perspectives

The new facility of the RIKEN RI Beam Factory (RIBF) is dedicated to provide beams of unstable nuclei very far from the stability valley. It started operation at the end of 2006 after 10 years construction. Three newly-built cyclotrons boost the energy up to 345 MeV/nucleon for various heavy-ion beams accelerated by pre-existed accelerators, a linac (RILAC) and a ring cyclotron (RRC), which have been operated in 20 years. The first experiment in the year 2007, production of new neutron-rich palladium isotopes, was followed by the first experiment of secondary reactions at the end of 2008. The capability in producing nuclei far from the stability is exceeding the one of any other facilities in the world, and will reach the level where about 1000 unknown isotopes can be created. Several new experimental installations are planned or being constructed.

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.

In-beam Moessbauer Spectroscopy and RIKEN RI Beam Factory (RIBF)

In-beam Mossbauer spectroscopy using an unstable nuclear probe or an energetic radioactive isotope (RI) beam is one of the most powerful technique that is used to observe the site occupations, dynamic atomic jump processes, and exotic chemical states of extremely diluted atoms in a material. The on-line Mossbauer technique offers unique information of the probe atoms under non-equilibrium or metastable conditions in the Mossbauer lifetime. Here, the outlines of recent results of 57Mn implantation experiment and the facility of RIKEN RI-beam Factory are explained.

Complete Set of Deuteron Analyzing Powers fordpElastic Scattering at 250 MeV/nucleon and Three Nucleon Forces

Measurements of a complete set of deuteron analyzing powers (iT11, T20, T21, T22) for elastic deuteron–proton scattering at 250 MeV/nucleon have been performed with polarized deuteron beams at RIKEN RI Beam Factory. The obtained data are compared with the Faddeev calculations based on the modern nucleon–nucleon forces together with the Tucson-Melbourne’99, and UrbanaIX three nucleon forces.

Charge strippers for high intensity uranium beams in the RIKEN RI beam factory

We have developed carbon stripper foils, liquid Li strippers, and gas strippers to find the most suitable stripper for high intensity uranium ion beams at the RIKEN RI beam factory (RIBF). The lifetime of newly developed polymer coated carbon foils has been measured online with a beam. A water stripper has been developed offline as a test for a liquid lithium stripper. A windowless gas target has been used as a nitrogen gas stripper and tested offline with the gas injected into the target cell. The maximum pressure of the target cell was 9.25 kPa, which corresponds to a thickness of 1.04 mg/cm 2.

Development of long-life carbon stripper foils for uranium ion beams

Carbon foils (C-foils) are extensively used as charge strippers for heavy-ion beams at the RIKEN RI beam factory. Multi-layer polymer coated C-foils (PCC-foils) of 300 μg/cm 2 carbon thickness were irradiated by a 11 MeV/nucleon 238U 35+ beam and their lifetimes were measured. Using an atomic force microscope (AFM) and an electron probe microanalyzer (EPMA), it was observed that the surface property and cross-sectional image of the PCC-foils had changed after the irradiation. A rotating-cylinder stripper device, which was developed to further increase the lifetime of C-foils, was also tested.

Multi-reflection time-of-flight mass spectrograph for short-lived radioactive ions

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.