RAON, the Rare Isotope Accelerator Complex
in the ISBB
RARE ISOTOPE SCIENCE PROJECT, INSTITUTE FOR BASIC SCIENCE
RAON is a heavy ion accelerator facility of the Rare Isotope Science Project (RISP) launched in 2011 as a part of the International Science Business Belt (ISBB), a large scale project with the vision of "creation of a new growth engine of Korea through revolutionary promotion of basic science." Alongside the establishment of the Institute of Basic Science (IBS) within ISBB, RAON has been promoted to lay a key foundation of a large scale and user oriented research facility for basic science research in Korea.
Fig. 1: Layout of the RAON heavy ion accelerator facility of RISP.
The accelerator system of RAON is based on high-current superconducting linacs (SCLs as shown in Fig. 1) to accelerate both stable and isotope beams up to the power of 400 kW with an energy higher than 200 MeV/u. It is worthwhile mentioning that the rare isotope (RI) production system of RAON is planned to have both Isotope Separation On-Line (ISOL) and In-Flight (IF) fragmentation methods combined to produce rare isotope beams far from the valley of stability. The construction project for the RAON facility, which will be located in the Sindong area near Daejeon-city, is being well progressed with great speed in order to have break-ground work beginning in early 2017. The first beam of low energy stable ions from the post accelerator (SCL3) is planned to be operating by the end of 2019 and the first RI beam by 2020. Thereafter, high energy heavy ion beams through the main linacs (SCL2) and finally on to the IF will follow by 2021 to complete the project.
RAON ACCELERATOR SYSTEM
The RAON consists of three superconducting linacs for which the superconducting cavities are independently phased and operating at three different frequencies . In order to meet the diverse demands, it can deliver various high intensity stable ions from protons to uranium and low intensity rare isotope beams.
The RAON injector system accelerates a heavy ion beam and creates the desired bunch structure for injection into the superconducting linac. The injector system comprises a 28-GHz electron cyclotron resonance ion source (ECR IS), shown in Fig. 2, a low energy beam transport (LEBT), a radio frequency quadrupole (RFQ), and a medium energy beam transport (MEBT) . The ECR adopts a saddle-type sextupole magnet for which one can wind ~20% more superconducting wires for the same space, thus lowering the operating current. The LEBT is designed to transport and to match ion beams from the ECR ion source to the RFQ, The RFQ shown in Fig. 3 is designed to accelerate beams from proton to uranium from 10 to 500 keV/u. One feature is that this RFQ can accelerate two different charge states, for exmaple, of uranium beams (238U33+ and 238U34+ of 12 pμA) simultaneously. The MEBT is designed to transport and match ion beams from the RFQ to the superconducting linac, which consists of three 81.25 MHz rebuncher cavities and eight quadrupoles.
Fig. 2: Photograph of the ECR ion source (ECR-IS) of RAON.
Fig. 3: Drawing and photograph of the RFQ of RAON.
Superconducting Linac (SCL)
The superconducting driver linac accelerates the beam to 200 MeV/u. The driver linac is divided into three different sections: a low energy superconducting linac (SCL3 and SCL1), a charge stripper section (P2DT) and a high-energy superconducting linac (SCL2) . The SCL3 accelerates the beam to 18.5 MeV/u. The SCL3 uses two different families of superconducting resonators, i.e., a quarter wave resonator (QWR) and a half wave resonator (HWR). The P2DT accepts beams at 18.5 MeV/u. The charge stripper strips electrons from the heavy ion beams to enhance the acceleration efficiency in the high-energy linac section. The charge stripping section consists of normal conducting quadrupoles and room temperature 45-degree bending magnets. The quadrupole magnets provide adequate transverse focusing and beam matching to the SCL2, and the bending magnet provides momentum dispersion for charge selection. The SCL2 accepts a beam at 18.5 MeV/u and accelerates it to 200 MeV/u. The SCL2 uses two types of single spoke resonators, i.e., SSR1 and SSR2. Normal conducting quadrupole doublets are employed for transverse focusing, and none of the cryomodules contain superconducting solenoids. The cavity optimum beta and frequency are chosen to maximize acceleration efficiency for each accelerating cavity and to minimize the beam loss. An optimum set of β = [0.047, 0.12, 0.30, 0.51] is obtained. The quarter wave resonator segment is minimally used because of the dipole kick and the asymmetric field of the quarter wave resonator. Unlike the quarter wave resonator, the half wave resonator does not have the β-dependent dipole kick due to the symmetric nature of the electromagnetic fields of the cavity. By adopting the half wave resonator, beam quality control becomes easier especially for high intensity beams. Table 1 lists the optimized parameters for the four types superconducting cavities. Each cavity has been prototyped and tested at cryogenic temperatures . Figure 4 shows the result of measuring the Q-value versus acceleration electric field (Eacc) for the QWR cavity. The cryomodules maintain the operating conditions of the superconducting cavities. A high level of vacuum and thermal insulation are required for the cryomodule to maintain the operating temperature of the superconducting cavities. The cryomodules hosting the QWR and the HWR cavities are box-type while those hosting the SSR1 and the SSR2 cavities are cylindrical . Both QWR and HWR cavities are vertically installed in the cryomodule. Figure 5 shows the QWR crymodule. Fabrication of the RAON accelerator system is progressing for the superconducting linac, which is optimized for the acceleration of both high and low intensity heavy ion beams.
Table 1: Superconducting cavity parameters.
Fig. 4: Measurement of the Q-value of the quarter wave resonator.
Fig. 5: The RAON QWR cryomodule.
RAON RI PRODUCTIOIN SYSTEM
The ultimate goal of RAON is to access unexplored regions of the nuclear landscapes. As the first kind among RI facilities in the world, RAON has a plan to combine the ISOL and IF system. This ISOL and IF combined method expects to produce more exotic RI beams, namely 80% of all isotopes predicted to exist for elements below uranium, and to be studied at various experimental facilities of RAON. The ISOL and IF systems will be described and the advantage of combined ISOL and IF systems will be discussed in the following sections.
ISOL (Isotope Separation On-Line) System
The ISOL system at RAON using proton induced fission on a direct fissile target is being designed and developed for the production and transportation of RI beams at RISP. The goal is to develop a high power target system with a maximum capability of 70 kW, a full remote handling system for target change, high radiation protection, minimization of hazards and etc. A 70 MeV proton cyclotron is used as the ISOL driver. Short-lived neutron-rich isotopes with mostly mass range 80 < A < 160 are produced by fission reactions at a rate of 1013 fission/s in a hot (about 2000°C) target. Figure 6 shows the layout of the ISOL system within its building. Very recently, we have achieved extraction of Sn-isotopes via laser ionization and fabrication of a large LaCx target 50 mm in diameter.
Fig. 6: Layout of the ISOL System.
IF (In-Flight) Fragment System
The IF system is one of the main devices used to produce and separate RI beams using a 400 kW primary beam at RAON. The RAON IF separator is characterized by large acceptances and a two-staged separation. The large acceptances allow RI beams produced by projectile fragmentation as well as U-fission fragmentation. The pre-separator separates the RI beam of interest from unwanted mixed RI beams and the primary beam. The main separator is for identification of the RI beams using the ΔE-TOF-Bρ method. The RI beam separated by the IF separator is delivered to the high energy experimental facility. Figure 7 shows the layout of the IF separator facility and the new blueprint of the building. The major technical challenges in construction of the IF separator are to have a high power production target, high power beam dump for removing the primary beam, a HTS (high temperature superconducting) magnet in a hot cell region, and a large aperture superconducting magnet. The design of the IF system has been completed and the prototyping of each component is being conducted.
Fig. 7: Layout of the IF Separator Facility.
Expected Rare Isotope Yields at RAON
As shown in Fig 8., RAON can provide a wide range of RI beams with various beam energies from a few keV/u to a few hundreds of MeV/u. The combined ISOL and IF system can produce unprecedented neutron-rich RI beams of high N/Z-ratio with considerable intensities.
Fig. 8: Rare Isotope Yields at RAON.
The use of RI beams is essential for exploring the structure of exotic nuclei and the dynamics of heavy ion collisions as well as various questions in nuclear astrophysics. RAON ([6-10]) equipped with both ISOL and IF systems can provide unique opportunities for the study of very exotic nuclei. The experimental systems will facilitate not only nuclear physics research at low and intermediate energies but also research in applied fields including materials science, neutron science, and bio-medical sciences.
KOBRA (Korea Broad acceptance Recoil spectrometer and Apparatus)
A multi-purpose experimental instrument using stable or RI beams, KOBRA, is being designed for studies of various topics in low energy nuclear physics - nuclear structure, nuclear astrophysics, nuclear reaction, etc -at RAON. The stable ions and rare isotopes from the ECR-IS and ISOL facilities are delivered to the KOBRA facility after SCL1 or SCL3 by reaching up to energies of a few tens MeV/n.
Fig. 9: Schematic view of KOBRA stage 1.
The KOBRA facility is designed to include two stages. Stage 1 is utilized mainly for the production of low energy RI beams via multinucleon transfer reactions at about 20 MeV/n or via direct reactions such as (p, n), (d, p) and (3He, n) at a few MeV/n (production mode). Figure 9 represents a schematic view of KOBRA stage 1. Stage 2, which will be placed downstream of stage 1, is employed to separate the fragments following reactions of the RI beam on a reaction target.
LAMPS (Large Acceptance Multi-Purpose Spectrometer)
One of the nuclear science facilities at RAON is LAMPS, for the study of the nuclear equation of state (EoS) via heavy ion collision experiments. Using stable or RI beams allows us to widely vary and control the neutron to proton (N/Z) ratio of a colliding system of interest. With such a controlled experiment it is possible to explore the isospin dependent part of the nuclear matter EoS, known as symmetry energy, which is an important component of understanding neutron stars, supernovas, nuclear synthesis, and exotic nuclei near neutron drip lines [11-15].
Fig. 10: Layout of the LAMPS.
LAMPS will be located at the end of the IF separator in order to enable event reconstruction by detecting all the particles produced in heavy ion collisions within a large acceptance angle to measure particle spectrums, yield, ratio and collective flow of pions, protons, neutrons, and intermediate fragments at the same time. LAMPS consists of a solenoid spectrometer and a forward neutron detector array, as shown in Fig. 10. A Time Projection Chamber (TPC) and a time-of-flight (ToF) detector will be placed inside a cylindrical solenoid magnet of 0.5 T for charged particle tracking and particle identification purposes. The forward neutron array will be made of 8 layers of plastic scintillators for neutron tracking [16-17].
HPMMS (High Precision Mass Measurement System)
The HPMMS is being developed for mass measurement of exotic nuclei under collaborative work with KEK, Japan. This system is going to be installed in the very low energy experimental hall, which is connected to the ISOL facility. Then the mass of various exotic nuclei produced by ISOL can be measured with a high precision (Δm/m < 10-5). The HPMMS consists of an ion trap, transport line and MR-TOF (multi-reflection time-of-flight) system. The MR-TOF method has been proved for high precision mass measurements by means of extremely short measurement times (~ms) in a compact device setup.
Applied Science Systems
Various applied science facilities such as the muon-spin relaxation resonance (μSR) facility for materials science, the nuclear data production system (NDPS) based on fast neutrons for neutron science research, and the uniform beam irradiation system (BIS) for bio-medical science are being developed as part of RISP.
The μSR facility  is considered to be the main facility for materials science research including studies on: spin-related phenomena like magnetically ordered systems, spin-glass systems, frustrated spin systems colossal magnetoresistance, heavy fermion systems, molecular magnetic clusters, weak magnetism, and hyperfine fields in multi-layers. It consists of a muon production system including a target system and a primary beam dump, a beam delivery system for the separation and transport of surface muons, and a spectrometer for beta-radiation measurement as in Fig. 11.
Fig. 11: Layout of the μSR Facility.
In order to design a new fuel cycle for a new types of reactors and advanced accelerator-driven systems for transmutation of actinides, reliable measurements of fission cross sections play an essential role. Until now, it has been reported that differences between the fission cross-section libraries range from 2-3% below 14 MeV neutron energy to over 10% at higher energy. The NDPS at RAON  plans to provide white neutrons up to 53 MeV and mono-energetic neutrons up to 88 MeV. The main components of NDPS consist of the neutron generation target, collimator, neutron monitor, fast ionization chamber for measuring a fission cross-section, and a beam dump as shown in Fig. 12. The fast neutron yield, as shown in the Fig. 12, can be obtained from the d+C reaction with the McDeLi and McDcBe code.
Fig. 12: Layout of the NDPS.
A main device of the biomedical facility is a uniform beam irradiation system (BIS) to use both a stable beam and an RI beam . The passive scanning system (PSS) of RAON BIS, which consists of a magnetic quadratic doublet, wobbling magnet, scatter, and ridge filter is to use stable heavy ion beams as shown in the Fig. 13. The active scanning system of BIS for RI beams can be achieved with modest changes of the PSS. The desired biological dose and beam uniformity was calculated to be 50.2 Gy/min and less than 5% for a 12C beam of 310 MeV/u and 1 nA based on experimental relative biological effect (RBE) data. Figure 13 shows simulated results for the biological dose. Also, the accessible irradiated area is a circle with a diameter larger than 20 cm and the beam uniformity is less than 5% to the parallel and transverse beam direction.
Fig. 13: Passive scanning system (PSS) of RAON BIS.
Rare isotopes sit far from the valley of stability and therefore pose a tough challenge to contemporary nuclear physics. The goal of the theory group at RISP is to achieve a thorough understanding of rare isotopes via various theoretical approaches, which is indispensable for interpreting the results of forthcoming experiments and for planning future experiments at RAON.
Nuclear reactions are essential to describe the production of exotic nuclei and synthesis of super heavy elements. We studied several reactions for the production of exotic nuclei using a di-nuclear system model. We have been developing heavy ion transport codes based on quantum molecular dynamics and the Boltzmann-Uehling-Uhlenbeck approach (DJ-BUU).
One of the central goals of nuclear physics is to understand the structure of nuclei, which will provide eventually an interpretational tool for experimental observations. We work with density functional theory and the random phase approximation to investigate the properties of exotic nuclei. To obtain exact solutions of the quantum many-body problem within controlled approximations, we adopt ab initio approaches and study light and intermediate mass nuclei extending to the vicinity of the drip line.
Recently, we developed an improved nuclear force, dubbed Daejeon16 , by applying phase-equivalent transformations to the similarity renormalization group evolved chiral effective field theory interaction. It turned out that Daejeon16 provides a good description of p-shell nucleus properties such as binding energies, energy spectra, radii, etc. We anticipate that this interaction will be useful for a wide range of applications relating to nuclear structure and reactions.
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Sunchan Jeong is the Director of Rare Isotope Science Project(RISP) at the Institute for Basic Science(IBS) in Republic of Korea. He has received a Ph.D. in physics from University of Tsukuba. He served as a professor at the Department of Physics at Soongsil University from 1992 to 1994, and High Energy Accelerator Research Organization from 1998. Furthermore, over 30 years of expertise in nuclear physics experiments, he participated in constructing, commissioning, and operating rare isotope accelerator in Japan.