Rare-RI Ring Research Articles

Recent achievements at the Rare-RI Ring, a unique mass spectrometer at the RIBF/RIKEN

Recent achievements at the Rare-RI Ring, a unique mass spectrometer at the RIBF/RIKEN

First Application of Mass Measurements with the Rare-RI Ring Reveals the Solar r-Process Abundance Trend at A=122 and A=123.

The Rare-RI Ring (R3) is a recently commissioned cyclotronlike storage ring mass spectrometer dedicated to mass measurements of exotic nuclei far from stability at Radioactive Isotope Beam Factory (RIBF) in RIKEN. The first application of mass measurement using the R3 mass spectrometer at RIBF is reported. Rare isotopes produced at RIBF-^{127}Sn, ^{126}In, ^{125}Cd, ^{124}Ag, ^{123}Pd-were injected in R3. Masses of ^{126}In, ^{125}Cd, and ^{123}Pd were measured whereby the mass uncertainty of ^{123}Pd was improved. This is the first reported measurement with a new storage ring mass spectrometry technique realized at a heavy-ion cyclotron and employing individual injection of the preidentified rare nuclei. The latter is essential for the future mass measurements of the rarest isotopes produced at RIBF. The impact of the new ^{123}Pd result on the solar r-process abundances in a neutron star merger event is investigated by performing reaction network calculations of 20 trajectories with varying electron fraction Y_{e}. It is found that the neutron capture cross section on ^{123}Pd increases by a factor of 2.2 and β-delayed neutron emission probability, P_{1 n}, of ^{123}Rh increases by 14%. The neutron capture cross section on ^{122}Pd decreases by a factor of 2.6 leading to pileup of material at A=122, thus reproducing the trend of the solar r-process abundances. The trend of the two-neutron separation energies (S_{2n}) was investigated for the Pd isotopic chain. The new mass measurement with improved uncertainty excludes large changes of the S_{2n} value at N=77. Such large increase of the S_{2n} values before N=82 was proposed as an alternative to the quenching of the N=82 shell gap to reproduce r-process abundances in the mass region of A=112-124.

Experimental Challenges of the First Mass Measurement Campaign at the Rare-RI Ring

With the recent commissioning of the Rare-RI Ring (R3), nuclear mass measurement of rare isotopes (RIs) produced at the RI Beam Factory (RIBF) at RIKEN has become possible. The R3 spectrometer is based on the Isochronous Mass Spectrometry technique that allows for reaching a mass measurement precision of 10−6 within less than 1 ms. With the newly established self-triggered individual injection method, R3 specialized in mass measurements of extremely short-lived nuclei with low production yields.In this paper, we report the first mass measurement campaign conducted at the R3 addressing nuclei in the vicinity of N=50 and N=82 neutron magic numbers, with a particular focus on the challenges of this new facility.

Development and operation of an electrostatic time-of-flight detector for the Rare RI storage Ring

An electrostatic time-of-flight detector named E-MCP has been developed for quick diagnostics of circulating beam and timing measurement in mass spectrometry at the Rare-RI Ring in RIKEN. The E-MCP detector consists of a conversion foil, potential grids, and a microchannel plate. Secondary electrons are released from the surface of the foil when a heavy ion hits it. The electrons are accelerated and deflected by 90∘ toward the microchannel plate by electrostatic potentials. A thin carbon foil and a thin aluminum-coated mylar foil were used as conversion foils. We obtained time resolutions of 69(1) ps and 43(1) ps (standard deviation) for a 84Kr beam at an energy of 170 MeV/u when using the carbon and the aluminum-coated mylar foils, respectively. A detection efficiency of approximately 90% was obtained for both foils. The E-MCP detector equipped with the carbon foil was installed inside the Rare-RI Ring to confirm particle circulation within a demonstration experiment on mass measurements of nuclei around 78Ge produced by in-flight fission of uranium beam at the RI Beam Factory in RIKEN. Periodic time signals from circulating ions were clearly observed. Revolution times for 78Ge, 77Ga, and 76Zn were obtained. The results confirmed successful circulation of the short-lived nuclei inside the Rare-RI Ring.

Efficiency and timing performance of time-of-flight detector utilizing thin foils and crossed static electric and magnetic fields for mass measurements with Rare-RI Ring facility

We developed a time-of-flight (TOF) detector for mass measurements of rare radioactive isotopes (RIs) with a storage ring, called the Rare-RI Ring, in RIKEN. For successful mass measurements, a time resolution of less than 100 ps and a detection efficiency close to 100% are required. Additionally, the change of ion velocity in the detector should be as small as possible (<10−4). To satisfy these requirements, the TOF detector utilizes ion-induced secondary electrons emitted from a thin foil and the crossed static electric and magnetic fields to transport the electrons isochronously to the microchannel plate detectors. The TOF detector was tested in both offline test with an alpha source and online test with heavy ions. In the online test with 84Kr ions of 200 MeV/nucleon, a time resolution of 38.6(2) ps in sigma and a position-averaged detection efficiency of 95.2(2)% were achieved in the entire area of 45-mm-diameter aluminum-coated Mylar foil. This good performance is attributed to the electromagnetic field achieved, which is the strongest thus far for a detector with this design.

Scheme of high-resolution identification and selection of secondary ions for mass measurements with the Rare-RI Ring

To achieve high-resolution selection and separation of mono-isotopic beams through projectile fragmentation or in-flight fission and to efficiently transport the secondary beams to the Rare-RI Ring (R3) for mass determination, the BigRIPS separator combined with the High-resolution Achromatic (HA) beam-line of SHARAQ is served as a two-stage separator. The first stage from F0 to F2 is used for separation of the nuclei of interest through a Bρ-ΔE-Bρ method and the second stage from F3 to S0 is employed for identification of the beam with a Bρ-ΔE-TOF method. With this scheme, all the secondary ions can be well separated on an event by event basis and be utilized for mass measurements by two complementary methods: the conventional magnetic-rigidity-time-of-flight (Bρ-TOF) method employing the BigRIPS-HA beam-line to measure exotic nuclides within 1 μ s and the newly established isochronous mass spectrometry (IMS) approach via the R3 with a measurement duration less than 1 ms. This report presents the technique advances of the BigRIPS-HA-SHARAQ-R3 as a separator/spectrometer and give a brief description of its application to the complementary TOF methods for mass measurements at RIBF.

Development of a new in-ring beam monitor in the Rare-RI Ring

A beam monitor is developed at the Rare-RI Ring storage ring facility at the RI beam factory at RIKEN. Because the Rare-RI Ring stores only a single exotic ion according to the operation principle, conventional beam instruments cannot be employed. In the present study, we design a simple monitoring system to confirm particle storage. The system comprises a thin foil and scintillators with photon sensors. When an ion passes through the foil, secondary electrons, including δ-rays, are emitted and are detected by a set of scintillation detectors without any guiding field. A beam test using 78Ge with 175 MeV/nucleon successfully demonstrates the periodic revolution of the stored ions.

Development of Large area position sensitive MCP detector at Rare RI Ring

Development of Large area position sensitive MCP detector at Rare RI Ring

Isochronous field study of the Rare-RI Ring

Construction of the Rare-RI Ring to measure masses of short-lived rare-RI with a relative precision of 10−6 is in progress at RIKEN. The Rare-RI Ring consists of six sectors where each sector consists of four dipole magnets. Since the mass measurement is done by the isochronous mass spectrometry in the Rare-RI Ring, creating isochronous magnetic field is one of the important issues in mass measurements with the Rare-RI Ring. In order to make an isochronous field, we installed ten trim coils in the two outer dipoles among the four dipoles in each sector magnet. The isochronism of the magnetic field have been confirmed by measuring time-of-flight (TOF) of alpha particles from an alpha-source (241Am). We measured TOF of alpha particles while changing the radial gradient of the magnetic field by trim coils and evaluated the isochronism from standard deviation of the TOF spectrum. The TOF width is minimum for a radial gradient of magnetic field (/)/B0 = 0.205 m−1, which is in good agreement with the simulated value.

Present status of Rare-RI Ring facility at RIBF

A new storage ring facility called the Rare-RI Ring is currently under preparation at the RI Beam Factory (RIBF) in RIKEN. The storage ring is dedicated to the single-ion precision mass spectrometry of neutron-rich exotic nuclei. The masses are essential to elucidate the evolution of the nuclear shell structure far from the β stability and to determine the pathway of astrophysical nucleosynthesis. Such exotic nuclei are provided by the large-acceptance superconducting fragment separator, BigRIPS, at the RIBF accelerator complex. The experimental principle of the Rare-RI Ring mass measurements is based on isochronous mass spectrometry combined with the individual injection technique. This novel technique enables exotic species of interest to be produced randomly, in time to be sequentially stored in the storage ring. The Rare-RI Ring facility realizes the most efficient measurements for rare isotopes. An overview of the project is presented, along with its present status.

A resonant Schottky pick-up for Rare-RI Ring at RIKEN

The Rare-RI Ring project has been launched at RIKEN. The Rare-RI Ring is a storage ring specially designed for the isochronous mass spectrometry of unstable nuclei. Precise mass measurements () are necessary to reveal the r-process path and, therefore, the ion-optical conditions must be tuned to yield isochronicity of order 10−6. For this purpose, we employ a highly sensitive resonant Schottky cavity as a probe for single-ion detection. Here, we first explain this technique theoretically and derive the necessary equations. Then, based on the results of off-line tests, we determine the sensitivity of the Schottky pick-up and estimate the intensities of the signals induced inside the cavity.

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.

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

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

Time-of-flight detector applied to mass measurements in Rare-RI Ring

A large time-of-flight (TOF) detector has been developed for the Rare-RI Ring. This detector consists of a Multi Channel Plate (MCP) and a carbon foil. Secondary electrons from the carbon foil are transported to the MCP by crossed electric and magnetic fields. In order to cover the beam size of the ring, a large and thin carbon foil (100mm×50mm2 and 60μg/cm2) is used as a sensitive material. The time resolution of σ≈130ps, the detection efficiency about 56% and a position dependence of the TOF about 1ns are obtained. A calculated position dependence of TOF adopting experimental (inhomogeneous) electric field and a homogeneous magnetic field is in agreement with the experimental one. These results suggest that the homogeneity of electric field is important to improve the time resolution in the large size detector.

Design study of a resonant Schottky pick-up for the Rare-RI Ring project

The new heavy-ion storage ring project Rare-RI Ring at the RIKEN RI Beam Factory launched in 2012 aims at precision isochronous mass spectrometry for single ions of exotic nuclides. To precisely tune the storage-ring ion optical conditions to the isochronous field setting, Schottky noise pick-up technique will be employed. Pillbox-type resonant cavity, similar to the Schottky pick-up system successfully operating at the experimental storage ring ESR at GSI, is adopted for the pick-up probe. To determine the design values of the cavity, we systematically performed a simulation study of three-dimensional electromagnetic field calculations in the high frequency region. High shunt impedance obtained in the simulation will enable us to detect stored single ions with charge down to q≃15.

Construction of rare-RI ring at RIKEN RI Beam Factory

An isochronous mass spectroscopy system using a newly constructed storage ring named the “rare-RI ring” is expected to be implemented at the RIKEN Nishina Center to determine the masses of short-lived rare nuclei including those in the r-process region with a relative precision of the order of 10-6 even for only one particle. In an isochronous storage ring, the mass is determined by measuring the revolution time of each nucleus. Our rare-RI ring consists of six magnetic sectors, and each sector consists of four bending magnets. To precisely optimize the isochronous conditions of the circulating particles for large acceptance, we install ten trim coils to half of the bending magnets. A fast-response and fast-charging kicker system enables selective and efficient injection of the produced rare nuclei into the ring one by one, along with facilitating efficient extraction of the circulating particles for time-of-flight measurement. Construction of the rare-RI ring was begun in the middle of the fiscal year 2012 at the RIKEN RI Beam Factory, and the ring is expected to be fully functional by 2015.

The rare-RI ring

We describe the rare-RI (radioactive isotope) ring at the RI Beam Factory (RIBF). The main purpose of the rare-RI ring is to measure the mass of very neutron-rich nuclei, the production rates of which are very small (hence ‘rare RI’) and the lifetimes of which are predicted to be very short. In the rare-RI ring, there are two innovative pieces of apparatus: individual injection, which can realize the injection of 200 A MeV rare RIs one by one, and a cyclotron-like storage ring, which allows high isochronous magnetic fields with large angular and momentum acceptances. With these devices, we will achieve a 10−6 mass resolution, and will be able to access rare RIs, the production rate of which is down to 1 event/day/pnA. Construction of the rare-RI ring started in fiscal year 2012.

PRESENT STATUS OF RARE-RI RING PROJECT IN RIKEN RIBF

We are going to construct a precise mass-measurement apparatus; "Rare-RI Ring" at RIKEN RI Beam Factory and measure masses of exotic neutron-rich nuclei far from the stability line with the assistance of high intensity primary beam from the cyclotrons at RIBF. The present status of our project and the progress of some parts of Rare-RI Ring will be presented.

BEAM OPTICS SIMULATION OF RARE-RI RING AT RI BEAM FACTORY IN RIKEN

The cyclotron-like storage ring dedicated to Rare-RI Ring project consists of 6 magnetic sectors and 6 straight sections, having a circumference of 56.13 m. The magnetic sector works for both bending and focusing. The total circulation is assumed to be 1,000 turns. Over the momentum range from -1% to +1% in ∆p/p, the required isochronisity is 10-6 while the beam emittance is sevaral tens of π mm-mrad. To examine the design of cyclotron-like storage ring and fix its parameters, we have developed a high precision beam optics simulation. To achieve the precision as high as possible within a feasible computational time, we have adopted a geometrical tracking assuming a circular orbit for a small spatial segment. For that purpose, it is enough that the magnetic sector is divided into 150 sub-sectors in calculation. In each sub-sector, the magnetic field is given as a function of radial position but uniform around the vicinity of beam trajectory. The beam trajectory is evaluated in 4th order Runge-Kutta algorithm. Finally, we have achieved a precision of 10-9 in ∆T/T and a computational time of 1.8 sec on a typical PC server for ray tracing of single particle undergoing a circulation of 1,000 turns.

Rare-RI ring project at RIKEN RI beam factory

A new apparatus named “Rare-RI Ring”, for precisely measuring the masses of short-lived rare nuclei including the r-process region is proposed. The Rare-RI Ring is one of the major experimental installations at the RIKEN RI beam factory. It consists of three main parts: a long injection beam line, a fast kicker system and a cyclotron-like storage ring. The combination of a long injection beam line and a fast kicker system enables us to inject short-lived rare nuclei into the ring one by one. The revolution time of each nucleus in the ring is measured with an accuracy of better than 10-6. Overall, the masses of short-lived rare nuclei including the r-process region can be determined with an order of 10-6 precision.