RIKEN RI Beam Factory Research Articles

Production of a thermal stress resistant mutant Euglena gracilis strain using Fe-ion beam irradiation.

Euglena gracilis is a common phytoplankton species, which also has motile flagellate characteristics. Recent research and development has enabled the industrial use of E. gracilis and selective breeding of this species is expected to further expand its application. However, the production of E. gracilis nuclear mutants is difficult because of the robustness of its genome. To establish an efficient mutation induction procedure for E. gracilis, we employed Fe-ion beam irradiation in the RIKEN RI beam factory. A decrease in the survival rate was observed with the increase in irradiation dose, and the upper limit used for E. gracilis selective breeding was around 50Gy. For a practical trial of Fe-ion irradiation, we conducted a screening to isolate high-temperature-tolerant mutants. The screening yielded mutants that proliferated faster than the wild-type strain at 32 °C. Our results demonstrate the effectiveness of heavy-ion irradiation on E. gracilis selective breeding.

A gas circulation and purification system for gas-cell-based low-energy RI-beam production.

A gas circulation and purification system was developed at the RIKEN Radioactive Isotope Beam Factory that can be used for gas-cell-based low-energy RI-beam production. A high-flow-rate gas cell filled with one atmosphere of buffer gas (argon or helium) is used for the deceleration and thermalization of high-energy RI-beams. The exhausted buffer gas is efficiently collected using a compact dry pump and returned to the gas cell with a recovery efficiency of >97%. The buffer gas is efficiently purified using two gas purifiers as well as collision cleaning, which eliminates impurities in the gas. An impurity level of one part per billion is achieved with this method.

Study of Weakly Bound Nuclei at RIKEN RIBF

Recent highlights of studies on unbound exotic nuclei at the RIKEN RI beam factory (RIBF) are presented. They include spectroscopy of nuetron-rich oxygen isotopes \(^{26}\)O and \(^{28}\)O, search for four-neutron states, and studies of proton unbound states of astrophysical interest.

First test experiment to produce the slowed-down RI beam with the momentum-compression mode at RIBF

The 82Ge beam has been produced by the in-flight fission reaction of the 238U primary beam with 345MeV/u at the RIKEN RI beam factory, and slowed down to about 15MeV/u using the energy degraders. The momentum-compression mode was applied to the second stage of the BigRIPS separator to reduce the momentum spread. The energy was successfully reduced down to 13±2.5MeV/u as expected. The focus was not optimized at the end of the second stage, therefore the beam size was larger than the expectation. The transmission of the second stage was half of the simulated value mainly due to out of focus. The two-stage separation worked very well for the slowed-down beam with the momentum-compression mode.

Nucleus ^{26}O: A Barely Unbound System beyond the Drip Line.

The unbound nucleus ^{26}O has been investigated using invariant-mass spectroscopy following one-proton removal reaction from a ^{27}F beam at 201 MeV/nucleon. The decay products, ^{24}O and two neutrons, were detected in coincidence using the newly commissioned SAMURAI spectrometer at the RIKEN Radioactive Isotope Beam Factory. The ^{26}O ground-state resonance was found to lie only 18±3(stat)±4(syst) keV above threshold. In addition, a higher lying level, which is most likely the first 2^{+} state, was observed for the first time at 1.28_{-0.08}^{+0.11} MeV above threshold. Comparison with theoretical predictions suggests that three-nucleon forces, pf-shell intruder configurations, and the continuum are key elements to understanding the structure of the most neutron-rich oxygen isotopes beyond the drip line.

Spallation reaction study for fission products in nuclear waste: Cross section measurements for 137Cs and 90Sr on proton and deuteron

We have studied spallation reactions for the fission products 137Cs and 90Sr for the purpose of nuclear waste transmutation. The spallation cross sections on the proton and deuteron were obtained in inverse kinematics for the first time using secondary beams of 137Cs and 90Sr at 185 MeV/nucleon at the RIKEN Radioactive Isotope Beam Factory. The target dependence has been investigated systematically, and the cross-section differences between the proton and deuteron are found to be larger for lighter spallation products. The experimental data are compared with the PHITS calculation, which includes cascade and evaporation processes. Our results suggest that both proton- and deuteron-induced spallation reactions are promising mechanisms for the transmutation of radioactive fission products.

Study of Gamow-Teller transitions from132Sn via the (p,n) reaction at 220 MeV/u in inverse kinematics

The charge-exchange (p ,n ) reaction at 220 MeV has been measured to extract the strength distribution of Gamow-Teller transitions from the doubly magic unstable nucleus 132 Sn. A recently developed experimental technique of measuring the (p ,n ) reaction in inverse kinematics has been applied to the study of unstable nuclei in the mass region around A ∼100 for the first time. We have combined the low-energy neutron detector WINDS and the SAMURAI spectrometer at the RIKEN radioactive isotope beam factory (RIBF). The particle identification plot for the reaction residues obtained by the spectrometer provides the clear separation of the CE reaction channel from other background events, enabling us to identify kinematic curves corresponding the (p , n ) reaction. Further analysis to reconstruct the excitation energy spectrum is ongoing.

The SCRIT electron scattering facility project at RIKEN RI beam factory

The self-confining RI target (SCRIT) electron scattering facility has been constructed at RIKEN RI Beam Factory. The commissioning experiment was performed, and the luminosity was achieved to around 1027 cm−2 s−1 with stable ions at a 250 mA electron beam current. For the electron scattering with short-lived unstable nuclei, the radioactive isotope production was started at electron-beam-driven RI separator for SCRIT. Furthermore, new devices, the electron spectrometer window frame spectrometer for electron scattering and the cooler buncher system were constructed. After the setup of new devices, a first electron scattering experiment with short-lived nuclei will be performed soon.

Fast-kicker system for rare-RI ring

We are developing a new fast-kicker system for rare-RI ring at RIKEN RI beam factory. New fast-kicker system enables us to inject a rare particle into the ring individually, and also to extract the rare particle from the ring quickly. It consists of a kicker power supply, which has a fast-response mechanism and a hybrid charging system, and a large acceptance kicker magnet. Propagation time from a trigger signal input to the power supply until the flat-top center of the kicker magnetic field is approximately 465 ns, the result is sufficient to achieve an individual injection with an energy of 200 MeV/nucleon. Reliable operation of the hybrid charging system makes it possible to extract a particle from the ring in 700 μs by using the same kicker magnet. A waveform of the magnetic field is under investigation by using a prototype kicker magnet.

Relativistic Coulomb excitation inMg32near 200 MeV/nucleon with a thick target

Intermediate-energy Coulomb excitation of $^{32}\mathrm{Mg}$ using in-beam $\ensuremath{\gamma}$-ray spectroscopy with a relatively thick lead target was studied at a beam energy of 195 MeV/nucleon at the RIKEN Radioactive Isotope Beam Factory (RIBF). The angular distribution of the inelastic scattering was analyzed with distorted-wave calculations taking into account both the Coulomb and the nuclear excitation contributions. A $B(E2)$ value of 432(51) ${e}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{fm}}^{4}$ was extracted, which agrees with most of the previously reported results obtained at several tens of MeV/nucleon. Our investigation demonstrates that the electromagnetic properties of exotic nuclei can be studied with Coulomb excitation at beam energies of a few hundreds of MeV/nucleon, where a thicker target can be used to increase the excitation yields.

Beam Heat Loads to Superconducting Quadrupoles for BigRIPS In-Flight Separator at RIKEN

The BigRIPS in-flight fragment separator in the RIKEN Radioactive Isotope Beam Factory (RIBF) project was commissioned in 2007 and has been operated for six years. The RIBF accelerator complex has provided a wide range of heavy ion beams from deuteron to uranium with the energy of 250-345 MeV/nucleon. More than 70 experiments of unstable nuclei have been performed using the RI beams produced by the BigRIPS fragment separator. The production target of the RI beams is in close proximity to the first superconducting triplet quadrupoles (STQs) in the BigRIPS fragment separator, so that the STQ is exposed to the high radiation, which causes “beam heat loads” to the cryogenic system. The STQ consists of three air-core NbTi coils with the maximum field gradients of 24 T/m, assembled in the liquid-helium bath cryostat with the warm bore radius of 90 mm. This paper presents measurements of the radiation heat loads to the STQ cryostat for a number of experienced heavy ion beams from oxygen to uranium with the beam power of 1-7 kW. Corresponding radiation transport calculations are presented, and radiation doses and damages to the STQ system are also discussed.

Development of rotating beryllium disk stripper

High-intensity uranium (U) ion beams are successfully provided at the RIKEN RI Beam Factory. We have attempted to extend the lifetime of the charge stripper as the U-beam intensity has increased. An attempt to enlarge the irradiated area using a rotating large carbon disk (C-disk) failed in 2006–2007 because no commercially available C-disk with 0.085 mm thickness met our requirements. In 2012, we tested a beryllium (Be) disk (0.1-mm-thick, diameter = 120 mm) as an alternative to the C-disk. The Be disk proved to be an adequate substitute, and has been successfully used during beam-time operation.

Different mechanism of two-proton emission from proton-rich nuclei 23Al and 22Mg

Two-proton relative momentum (qpp) and opening angle (θpp) distributions from the three-body decay of two excited proton-rich nuclei, namely Al23→p+p+Na21 and Mg22→p+p+Ne20, have been measured with the projectile fragment separator (RIPS) at the RIKEN RI Beam Factory. An evident peak at qpp∼20 MeV/c as well as a peak in θpp around 30° are seen in the two-proton break-up channel from a highly-excited 22Mg. In contrast, such peaks are absent for the 23Al case. It is concluded that the two-proton emission mechanism of excited 22Mg is quite different from the 23Al case, with the former having a favorable diproton emission component at a highly excited state and the latter dominated by the sequential decay process.

Construction of the Rare-RI Ring at RIKEN RI Beam Factory

Construction of the Rare-RI Ring at RIKEN RI Beam Factory

Time-stamping system for nuclear physics experiments at RIKEN RIBF

A time-stamping system for nuclear physics experiments has been introduced at the RIKEN Radioactive Isotope Beam Factory. Individual trigger signals can be applied for separate data acquisition (DAQ) systems. After the measurements are complete, separately taken data are merged based on the time-stamp information. In a typical experiment, coincidence trigger signals are formed from multiple detectors to take desired events only. The time-stamping system allows the use of minimum bias triggers. Since coincidence conditions are given by software, a variety of physics events can be flexibly identified. The live time for a DAQ system is important when attempting to determine reaction cross-sections. However, the combined live time for separate DAQ systems is not clearly known because it depends not only on the DAQ dead time but also on the coincidence conditions. Using the proposed time-stamping system, all trigger timings can be acquired, so that the combined live time can be easily determined. The combined live time is also estimated using Monte Carlo simulations, and the results are compared with the directly measured values in order to assess the accuracy of the simulation.

Electron scattering for exotic nuclei

A brand-new electron scattering facility, the SCRIT Electron Scattering Facility, will soon start its operation at RIKEN RI Beam Factory, Japan. This is the world’s first electron scattering facility dedicated to the structure studies of short-lived nuclei. The goal of this facility is to determine the charge density distributions of short-lived exotic nuclei by elastic electron scattering. The first collision between electrons and exotic nuclei will be observed in the year 2014.

DALI2: A NaI(Tl) detector array for measurements of γ rays from fast nuclei

A NaI(Tl) detector array called DALI2 (Detector Array for Low Intensity radiation 2) has been constructed for in-beam γ-ray spectroscopy experiments with fast radioactive isotope (RI) beams. It consists typically of 186 NaI(Tl) scintillators covering polar angles from ~15° to ~160° with an average angular resolution of 6° in full width at half maximum. Its high granularity (good angular resolution) enables Doppler-shift corrections that result in, for example, 10% energy resolution and 20% full-energy photopeak efficiency for 1-MeV γ rays emitted from fast-moving nuclei (velocities of v/c≃0.6). DALI2 has been employed successfully in numerous experiments using fast RI beams with velocities of v/c=0.3–0.6 provided by the RIKEN RI Beam Factory.

Operational Experience of the Superconducting Ring Cyclotron at RI Beam Factory (RIBF) and Upgrade Plans for RIBF Accelerators

The RIKEN Radioactive Isotope Beam Factory (RIBF) has successfully operated since the first beam was produced at the end of 2006 (more than six years). The aim of this project is to access the unexplored regions on the nuclear chart, those far from β-stability. Continuous effort over the past six years has improved the performance of the RIBF accelerator complex. The world's first superconducting ring cyclotron (SRC) is the final booster in the RIBF accelerator complex; it is able to accelerate heavy ions of any element to a speed of approximately 70% of the speed of light. The ring cyclotron consists of six major superconducting sector magnets, each with a maximum field of 3.8 T. The total stored energy is 240 MJ. The overall size of the cyclotron is 19 m in diameter and 8 m in height with a weight of 8300 tons. Reliable standby power supply systems and accurate control systems are necessary for long-term, stable operation of the cryogenic cooling system. For these purposes, a 6.5-MW co-generation system was installed, and the control system for the cryogenic cooling system was improved on the basis of operational experience. Despite our efforts to achieve long-term, stable operation, we have had some problems such as failure of the He compressors and refrigerators and coil discharge. We learned that any small component could easily disrupt the entire cooling system. The two biggest problems were liquid He leakage thorough a crack in a feedthrough into the thermal insulation vacuum on November 8th, 2005 and oil contamination in the He refrigerator found in February of 2008. Overcoming such big problems has resulted in the present, stable operation of the cryogenic cooling system of the SRC. One option for upgrading the RIBF accelerator includes the construction of another SRC (SRC2) to skip the charge strippers (which decrease the beam intensity by approximately one fifth). Our study shows that the design of the sector magnets of the SRC2 is not much more difficult than that of the first SRC.

Support Structure of Cold-Mass for Superconducting Ring Cyclotron and Electromagnetic Force Measurements With Strain Gauges

The superconducting ring cyclotron (SRC), which is the last-stage accelerator of the RIKEN RI-beam factory, was commissioned in December 2006, and it has since been operated for approximately five months each year. The SRC comprises six superconducting sector magnets, and the maximum magnetic field strength in the beam orbit region and the stored energy are 3.8 T and 240 MJ, respectively. The total weight is 8300 t. The cold-mass in each sector magnet consists of upper and lower main coils connected to each other at liquid helium temperature, and it weighs 24 t. The cold-mass is supported by eight titanium-alloy rods each in the vertical and azimuth directions, and with a stainless steel multifolded pipe in the radial direction to resist large electromagnetic forces. In order to measure the stress and movement in the cold-mass, a total of 208 strain gauges were attached on the main coil vessel, supports, etc., during construction. The strains generated during the excitation of the magnets were almost the same as the calculated values. Furthermore, there were negligible changes in the strains at all positions during the excitation measured in 2006 and 2013. These results verify the design effectiveness of the cold-mass support structure.

Complete Set of Deuteron Analyzing Powers for dp Elastic Scattering at Intermediate Energies and Three Nucleon Forces

With the aim of clarifying roles of the 3NFs in nuclei experimental programs with the polarized deuteron beam at intermediate energies are in progress at RIKEN RI Beam Factory. As the first step, we have measured a complete set of deuteron analyzing powers in deuteron–proton elastic scattering at 250 and 294 MeV/nucleon. 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.