Physics opportunities with the RI Beam Factory at RIKEN

The RI Beam Factory (RIBF) is presently the top world-class radioactive-isotope (RI) beam facility in the world. Construction of the factory is now in the process of being completed. This facility is based on the in-flight method to produce fast RI beams. High-energy and intense primary beams accelerated by a superconducting ring cyclotron (SRC) are converted, via the projectile fragmentation or fission reaction channels, to RI beams at a new fragment separator called BigRIPS. Construction of major experimental installations is expected to commence in 2007. Physics opportunities with the RIBF are discussed herein.     

Laser spectroscopy of trapped 7Be and 10Be at a prototype slow RI-beam facility of RIKEN

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 ⁸Li 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 ¹⁰Be and ⁷Be 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.     

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.     

Nuclear moments of neutron-rich aluminum isotopes

Ground-state magnetic-dipole moments (μ) of ^30-32Al and electric quadrupole moments (Q) of ^31,32Al have been measured with the β-NMR method using spin-polarized radioactive-isotope beams produced in projectile-fragmentation reactions. Beams of ^30-32Al were obtained by using RIKEN projectile-fragment separator RIPS after the fragmentation of ⁴⁰Ar projectiles at an energy of E = 95A MeV on a ⁹³Nb target. The obtained μ_exp[^30-32Al] and values agree well with shell-model calculations within the sd shell using the USD interaction. Also, Q_exp[³¹Al] was found to be small. Thus, we can conclude that these aluminum isotopes are located outside the island of inversion.     

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.     

Commercial cyclotrons. Part I: Commercial cyclotrons in the energy range 10–30 MeV for isotope production

A survey of commercial cyclotrons for production of medical and industrial isotopes is presented. Compact isochronous cyclotrons which accelerate negative hydrogen ions in the energy range 10–30 MeV have been widely used over the last 25 years for production of medical isotopes and other applications. Different cyclotron models for the energy range 10–12 MeV with moderate beam intensity are used for production of ¹¹C, ¹³N, ¹⁵O, and ¹⁸F isotopes widely applied in positron emission tomography. Commercial cyclotrons with high beam intensity are available on the market for production of most medical and industrial isotopes. In this work, the physical and technical parameters of different models are compared. Possibilities of improving performance and increasing intensity of H^? beams up to 2–3 mA are discussed.     

First Precision Spectroscopy of Pionic Atoms at RI Beam Factory

We conducted an inclusive missing-mass spectroscopy of ¹²²Sn (d, ³He) reaction near the π¯ emission threshold at an incident energy of T _d = 250 MeV/nucleon. The experiment sets its goals to the research and development of the high precision spectroscopy of pionic atoms at the RI beam factory (RIBF) of RIKEN, which precedes a new project, pionic atom factory project (piAF), to observe pionic atoms systematically. Here, we report the preliminary results of the pilot experiment.     

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     

Investigation of the fragmentation of 20Ne and 40Ar ions at the COMBAS setup

Properties of the COMBAS fragment separator are compared with respective properties of similar setups. Results of experiments aimed at obtaining products of one-proton-stripping reactions induced by a beam of ⁴⁰Ar ions with an energy of 35 MeV/A and two-neutron-stripping reactions induced by a beam of ²⁰Ne ions with an energy of 52 MeV/A are presented. A high resolution of the fragment separator in obtaining secondary neutron-rich ³⁹Cl and neutron-deficient ¹⁸Ne ion beams is demonstrated.     

Upgrading the PSI Muon Facility

A new project was initiated at PSI to replace the existing μE4 decay channel with a new beam line delivering surface/cloud beams of highest luminosities. This goal will be accomplished by installing a solenoidal lens system at the main production target E and then transporting the muons with conventional beamline elements of very big apertures to a greatly enlarged experimental floor. Several slit systems and an electrostatic separator will be available to control the beam shape and reduce the electrons and other background. Particle fluxes up to 5 × 10⁸ μ⁺/s and 10⁷ μ⁻/s can be expected at 28 MeV/c beam momentum, using the 600 MeV primary proton beam of 1.7 mA. The operation of the channel will be limited to a maximum momentum of 40 MeV/c. The beam line has been specially designed to provide highest flux to the ultra-slow μ⁺ beam (LEM). This revised version was published online in August 2006 with corrections to the Cover Date.     

Heavy ion beam polarization produced by the multi-tilted-foil interaction

Nuclear polarization of the 7/2^? ground-state of⁵¹V was produced via the Multi-Tilted-Foil (MTF) interaction with a V beam. The induced polarization was determined by measuring the left-right asymmetry of Coulomb excited⁵¹V nuclei and, for a⁵¹V beam at E=100 MeV, was measured to beP _I =0.012(2). The nuclear polarization was also induced atE=50 MeV and, after further acceleration, determined at E=195 MeV to be P_I=0.010(1). These experiments demonstrate the feasibility of polarizing a great variety of heavy-ion beams at arbitrary velocities with subsequent acceleration and passage through magnetic beam-optics elements. Such polarization, albeit small, can be utilized for the determination of electromagnetic moments of exotic beams and separated reaction products.     

兰州重离子加速器冷却储存环

 兰州重离子加速器冷却储存环HIRFL-CSR,是一个多用途、多功能的双冷却储存环同步加速器系统,由主环CSRm和实验环CSRe构成,并以兰州重离子级联回旋加速器HIRFL作注入器。CSR利用高频变谐波的方法,将重离子束的能量从7～25 MeV/u同步加速到２00～1 000 MeV/u,同时利用重离子储存环中空心电子束冷却技术将束流品质提高１个数量级,并通过储存环的快引出及慢引出,提供多种类的重离子束以及放射性次级束(RIBs),以开展范围更广精度更高的物理实验。该装置于2007年投入运行,已取得了重要的运行结果,如实现了剥离注入与多圈注入、空心电子束对重离子束的冷却与累积、变谐波宽能区同步加速、等时性环型谱仪、RIBs的产生收集与ToF高分辨质量测量以及高能重离子束的变能慢引出等。     

Simulation of the required operation modes and analysis of their stability for multipurpose isochronous cyclotrons

A mathematical and computer simulation of the operation modes of multipurpose isochronous cyclotrons is based on a calculation of current values in trim coils correcting the main magnetic field (I _i , i = 1, 2, ..., n) for a certain current level in the main coil (I _mc). This calculation is performed for the given kinetic energy (E _k ) at a certain radius or for the particle orbital frequency (F ₀). The results of this calculation provide a formation with the necessary precision of the required magnetic field in the range from the ion source to the particle extraction system. A number of numerical and physical experiments on calculating the main operation mode of the multipurpose isochronous cyclotron AIC-144 (Institute of Nuclear Physics, Polish Academy of Sciences, Krakow) (p, E _k = 60 MeV, F ₀ = 26.25 MHz) proved the necessity of taking into account the estimate of the stability of the sought solution and the possibility of obtaining accelerated ion beams in the whole range of acceleration radii for small phase losses in the isochronization range of the required magnetic field.     

Measurement of the vector and tensor analyzing powers in dp elastic scattering at the energy of 880 MeV

The preliminary results on the angular distributions of the vector A _y and tensor A _yy , A _xx analyzing powers in dp elastic scattering at T _d = 880MeV are presented. A region of center-of-mass system angles from 60° to 140° is covered. The experimental data are compared with the preliminary calculations obtained within framework of different theoretical approaches. The experiment has been performed at the Internal Target Station at Nuclotron within the framework of the program on the study of light nuclei spin structure proposed at JINR (Russia) and RIBF (Japan).     

The tensor analyzing power Azz AT θ = 0° for the He( , p)He reaction

The tensor analyzing power f_zz has been measured for the ³He( , p)⁴He reaction at 0 = 0° over an incident deuteron energy range E_d = 6.6–15.8 MeV in steps of 0.5 MeV. The present results agree with and extend the previous measurements of Grüebler et al. The present results indicate that this reaction is a very good tensor analyzer for polarized deuteron beams with energies up to 15.8 MeV.     

Energy calibration of a Δ<Emphasis Type="Italic">E</Emphasis>-<Emphasis Type="Italic">E</Emphasis> scintillation telescope in (α, p) reaction

A procedure for and the results from the energy calibration of a ΔE-E scintillation telescope used in experiments performed at the INR to study the nd-breakup reaction are described. The telescope was calibrated using a beam of α particles with an energy of 30 MeV of the U-120 cyclotron (INP). Secondary protons from the ^10,11B(α, p) reaction were recorded by the ΔE-E telescope at several recording angles and with the application of different absorbing foils. The calibration results from the ΔE-E telescope were obtained over the interval E _p = 10–30 MeV, allowing us to measure the energies of secondary protons in the ndbreakup reaction when the energy of primary neutrons is 20–60 MeV.     

Measurement of the beam asymmetry in η photoproduction off the proton

The beam asymmetry, Σ , was measured at ELSA in the reaction p → ηp using linearly polarised tagged photon beams, produced by coherent bremsstrahlung off a diamond. The crystal was oriented to provide polarised photons in the energy range E _γ = 800 to 1400MeV with the maximum polarisation of P _γ = 49 % obtained at 1305MeV. Both dominant decay modes of the η into two photons and 3π⁰ were used to extract the beam asymmetry from the azimuthal modulation of the cross-section. The measurements cover the angular range Θ_cm ≃ 50 -150 degrees. Large asymmetries up to 80% are observed, in agreement with a previous measurement. The eta-MAID model and the Bonn-Gatchina partial wave analysis describe the measurements, but the required partial waves differ significantly.     

Experiments with intense secondary beams of radioactive ions

Electromagnetic mass separation applied on-line to accelerators and nuclear reactors is now a standard technique for producing preselected isotopic beams (A, Z selection) of nuclear reaction products. The development, performance and anatomy of a large on-line isotope separator facility, the CERN-ISOLDE, is discussed. As a result of recent technical developments it is now possible to study individual nuclei of about 40 elements, in many cases out to where the limits of nucleon binding are approached and half-lives become as short as 10 ms. It is shown that the nuclear reaction processes induced with the high-intensity several hundred MeV proton beam can provide secondary radioactive beams in almost all regions of the nuclidic chart with intensities which are not matched by any other method. The intense beams of 10⁸–10¹¹ atoms/s have opened up a number of new experimental possibilities like laser spectroscopy on radioactive atoms, radioactive targets for nuclear reaction spectroscopy, and precision X-ray shift measurements.     

First direct measurement of the total cross-section of 12C(α,γ)16O

The total cross-section of ¹²C(α,γ)¹⁶O was measured for the first time by a direct and ungated detection of the ¹⁶O recoils. This measurement in inverse kinematics using the recoil mass separator ERNA in combination with a windowless He gas target allowed to collect data with high precision in the energy range E = 1.9 to 4.9 MeV. The data represent new information for the determination of the astrophysical S(E) factor.     

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.