Rare Isotope Beams Research Articles

Design study of Beam Loss Monitoring system for the Rare Isotope Science Project in Korea

A new heavy ion accelerator facility called RAON is being constructed in Daejeon, Korea to produce rare isotope beams of various energies for the Rare Isotope Science Project (RISP). This facility is designed to use both the In-flight Fragmentation (IF) and Isotope Separation On-Line (ISOL) systems to provide a wide range of rare isotope beams to be utilized in many fundamental physics experiments and in various applications. The RAON can use both stable heavy ion beams and rare isotope beams with energies up to a few hundreds of MeV/u with 400 kW of beam power. One of the greatest challenges in operating such a high beam power facility (∼400 kW) is to accurately monitor the beam loss and trigger the Machine Protection System (MPS) reasonably quickly. In this paper, we report the conceptual design of the RAON beam loss monitoring system. Monte Carlo simulations using the MCNPX code were performed to understand beam loss-induced neutron and gamma radiation. The types of detectors were determined based on radiation simulations while considering the sensitivities of the detectors and response time requirements.

SCL3 cryogenic plant process design for RAON

RAON, Rare isotope Accelerator complex for ON-line experiments, is a facility for rare isotope beams in S. Korea. It has three superconducting linear accelerators, SCL1, SCL2, and SCL3. They could serve a broad power range of stable and unstable ion beams. The third superconducting linac, SCL3, which could accelerate rare isotope beams made by an ISOL (Isotope Separation On-Line) facility, consists of three types of cryomodules with two kinds of SC cavities, QWR (Quarter Wave Resonator, 81.25 MHz) and HWR (Half Wave Resonator, 162.5 MHz). A cryogenic plant for SCL3 will cover three cryogenic circuits, bath cooling for QWR at 4.5 K, bath cooling for HWR at 2.05 K, and forced cooling for thermal shields of all cryomodules at 35 – 55 K. ALAT who was a winner in this bidding and has studied our requirements and process designs will supply the cryogenic plant with a helium management system. This paper shows results of the process designs and current status including final heat loads from SCL3, specifications of equipment, a layout, and exergy efficiencies at operating modes.

Dynamic modeling and control of the SPIRAL2 cryomodules

SPIRAL 2 (Caen, France) facility aims at delivering high intensity of rare isotope beams. The linear accelerator (LINAC) of the SPIRAL 2 facility is composed of 26 superconducting accelerating cavities distributed into 19 cryomodules cooled down with liquid helium at 4.4 K. A dynamic model of the cryomodules and their associated shields and valves thermodynamic behavior is proposed. This dynamic model is validated through comparisons between simulation and experimental data. Since the model is developed with the Simcryogenics library for MATLAB/Simulink environment, It is convenient for control loops design. Using the model, advanced control algorithms have been developed in order to achieve the cryomodule’s pressure stability required for beam acceleration. This paper presents the dynamic model, experimental versus simulation results as well as the control outcome.

A new concept for searching for time-reversal symmetry violation using Pa-229 ions trapped in optical crystals

Certain pear-shaped nuclei are expected to have enhanced sensitivity to time-reversal and parity-violating interactions originating within the nuclear medium. In particular, Protactinium-229 is thought to be about 100,000 times more sensitive than Mercury-199 which currently sets some of the most stringent limits for these types of interactions. Several challenges would first have to be addressed in order to take advantage of this discovery potential. First, there is not currently a significant source of Pa-229 (1.5 day half-life); however, there are plans to harvest Pa-229 at the Facility for Rare Isotope Beams at Michigan State University. Second, the spin-5/2 nucleus of Pa-229 limits its coherence time while also making it sensitive to systematic effects related to local electric field gradients. On the other hand, this also give Pa-229 an additional source of signal in the form of a magnetic quadrupole moment (MQM) which violates the same symmetries as an EDM but is not observable in spin-1/2 systems. Third, in order to compensate for the small atom numbers and short coherence times, the Pa-229 atoms would have to be probed with exceptionally large electric and magnetic fields that may be possible if Pa-229 ions are embedded inside an optical crystal. We will describe some aspects of this concept using the stable Praseodymium-141 isotope as a surrogate which has the same nuclear spin and similar atomic structure of Pa-229.

Study of slowing down and thermalization of externally injected ion beams in electron cyclotron resonance ion source plasmas

Electron cyclotron resonance ion source based charge breeder is a promising way to produce rare isotope beams with a high charge state, based on capture and thermalisation of the primary ion beam by the ECR plasma. A simulation technique based on a Monte Carlo collision operator is developed to study the capture of externally injected ion beams in ion source plasmas. To illustrate the utility of the method, we have studied the dynamics of Ar1+ ions in oxygen plasma. Evolution of statistically averaged quantities such as the beam size, average directional velocity, and rms velocity is studied for various beam parameters. It is observed that the number of undamped particles and the steady state beam size increase with the increase in beam emittance. The simulation result indicates that the capture and thermalisation of the ion beam are affected by the kinetic energy and emittance of the beam.Electron cyclotron resonance ion source based charge breeder is a promising way to produce rare isotope beams with a high charge state, based on capture and thermalisation of the primary ion beam by the ECR plasma. A simulation technique based on a Monte Carlo collision operator is developed to study the capture of externally injected ion beams in ion source plasmas. To illustrate the utility of the method, we have studied the dynamics of Ar1+ ions in oxygen plasma. Evolution of statistically averaged quantities such as the beam size, average directional velocity, and rms velocity is studied for various beam parameters. It is observed that the number of undamped particles and the steady state beam size increase with the increase in beam emittance. The simulation result indicates that the capture and thermalisation of the ion beam are affected by the kinetic energy and emittance of the beam.

Advances of the FRIB project

The Facility for Rare Isotope Beams (FRIB) Project has entered the phase of beam commissioning starting from the room-temperature front end and the superconducting linac segment of first 15 cryomodules. With the newly commissioned helium refrigeration system supplying 4.5[Formula: see text]K liquid helium to the quarter-wave resonators and solenoids, the FRIB accelerator team achieved the sectional key performance parameters as designed ahead of schedule accelerating heavy ion beams above 20[Formula: see text]MeV/u energy. Thus, FRIB accelerator becomes world’s highest-energy heavy ion linear accelerator. We also validated machine protection and personnel protection systems that will be crucial to the next phase of commissioning. FRIB is on track towards a national user facility at the power frontier with a beam power two orders of magnitude higher than operating heavy-ion facilities. This paper summarizes the status of accelerator design, technology development, construction, commissioning as well as path to operations and upgrades.

Interplay between nuclear shell evolution and shape deformation revealed by the magnetic moment of 75Cu

Exotic nuclei are characterized by a number of neutrons (or protons) in excess relative to stable nuclei. Their shell structure, which represents single-particle motion in a nucleus, may vary due to nuclear force and excess neutrons, in a phenomenon called shell evolution. This effect could be counterbalanced by collective modes causing deformations of the nuclear surface. Here, we study the interplay between shell evolution and shape deformation by focusing on the magnetic moment of an isomeric state of the neutron-rich nucleus 75Cu. We measure the magnetic moment using highly spin-controlled rare-isotope beams and achieving large spin alignment via a two-step reaction scheme that incorporates an angular-momentum-selecting nucleon removal. By combining our experiments with numerical simulations of many-fermion correlations, we find that the low-lying states in 75Cu are, to a large extent, of single-particle nature on top of a correlated 74Ni core. We elucidate the crucial role of shell evolution even in the presence of the collective mode, and within the same framework, we consider whether and how the double magicity of the 78Ni nucleus is restored, which is also of keen interest from the perspective of nucleosynthesis in explosive stellar processes.

Direct reactions as quantum probes for nuclei

Direct reactions are important and useful tools for obtaining information on nuclear properties. Basic quantum scattering theories, their features and underlying pictures are presented and discussed. New aspects using rare-isotope beams are also introduced.

Status of the JENSA gas-jet target for experiments with rare isotope beams

The JENSA gas-jet target was designed for experiments with radioactive beams provided by the rare isotope re-accelerator ReA3 at the National Superconducting Cyclotron Laboratory. The gas jet will be the main target for the Separator for Capture Reactions SECAR at the Facility for Rare Isotope Beams on the campus of Michigan State University, USA. In this work, we describe the advantages of a gas-jet target, detail the current recirculating gas system, and report recent measurements of helium jet thicknesses of up to about 1019 atoms/cm2. Finally a comparison with other supersonic gas-jet targets is presented.

Attenuation curves of neutrons from 400 to 550 Mev/u for Ca, Kr, Sn, and U ions in concrete on a graphite target for the design of shielding for the RAON in-flight fragment facility in Korea

Abstract Rare isotope beam facilities require shielding data in early stage of their design. There is much less shielding data on neutrons from the reactions between heavy ion beams and matter than the data on neutrons produced by protons. The purpose of the present work is to produce and thus increase the amount of shielding data on neutrons generated by high-energy heavy ion beams based on the RAON in-flight fragment facility. Calculations were performed with the computational Monte Carlo codes PHITS and MCNPX. The secondary neutron source terms were evaluated at 550 MeV/u for Ca, Kr, and Sn and at 400 MeV/u for U ions on a graphite target. Source terms and attenuation lengths were obtained by fitting the ambient dose equivalent inside an ordinary concrete shield.

Commissioning of the FRIB RFQ

The radio-frequency quadrupole (RFQ) at the Facility for Rare Isotope Beams (FRIB) is a 4-vane type cavity designed to accelerate heavy ion beams with charge states Q/A between 1/7 and 1/3 from 12 keV/u to 0.5 MeV/u. The RFQ was assembled in the FRIB tunnel in November 2016. Bead-pull measurements and tuning were performed with low RF power. The RFQ has been conditioned to 59 kW in August 2017, which is sufficient to accelerate the Key Performance Parameter (KPP) beams, Argon and Krypton. The RFQ has been successfully commissioned with KPP beams in CW regime in October 2017. 40Ar9+ and 86Kr17+ beams were accelerated by the FRIB RFQ in the CW regime to the designed energy of 0.5 MeV/u. With the multi-harmonic buncher operational, the FRIB RFQ commissioning has been completed with bunched beam in February 2018. The beam transmission efficiency through the RFQ was in good agreement with PARMTEQ simulation results. The detailed results from the FRIB RFQ tuning, high power conditioning and beam commissioning will be presented in this paper.

Localizing the Shape Transition in Neutron-Deficient Selenium.

Neutron-deficient selenium isotopes are thought to undergo a rapid shape change from a prolate deformation near the line of beta stability towards oblate deformation around the line of N=Z. The point at which this shape change occurs is unknown, with inconsistent predictions from available theoretical models. A common feature in the models is the delicate nature of the point of transition, with the introduction of even a modest spin to the system sufficient to change the ordering of the prolate and oblate configurations. We present a measurement of the quadrupole moment of the first-excited state in radioactive ^{72}Se-a potential point of transition-by safe Coulomb excitation. This is the first low-energy Coulomb excitation to be performed with a rare-isotope beam at the reaccelerated beam facility at the National Superconducting Cyclotron Laboratory. By demonstrating a negative spectroscopic quadrupole moment for the first-excited 2^{+} state, it is found that any low-spin shape change in neutron-deficient selenium does not occur until ^{70}Se.

Lifetime Measurements and Triple Coexisting Band Structure in ^{43}S.

Lifetime measurements of excited states in the neutron-rich nucleus ^{43}S were performed by applying the recoil-distance method on fast rare-isotope beams in conjunction with the Gamma-Ray Energy Tracking In-beam Nuclear Array. The new data based on γγ coincidences and lifetime measurements resolve a doublet of (3/2^{-}) and (5/2^{-}) states at low excitation energies. Results were compared to the π(sd)-ν(pf) shell model and antisymmetrized molecular dynamics calculations. The consistency with the theoretical calculations identifies a possible appearance of three coexisting bands near the ground state of ^{43}S: the K^{π}=1/2^{-} band built on a prolate-deformed ground state, a band built on an isomer with a 1f_{7/2}^{-1} character, and a suggested excited band built on a newly discovered doublet state. The latter further confirms the collapse of the N=28 shell closure in the neutron-rich region.

Activation Levels, Handling, and Storage of Activated Components in the Target Hall at FRIB

The Facility for Rare Isotope Beams (FRIB) is a major new scientific user facility under construction in the United States for nuclear science research with beams of rare isotopes. 400 kW beam operations with heavy ions ranging from oxygen to uranium will create a high radiation environment for many components, particularly for the beam line components located in the target hall, where approximately 100 kW of beam power are dissipated in the target and another 300 kW are dissipated in the beam dump. Detailed studies of the component activation, their remote handling, storage, and transport, have been performed to ensure safe operation levels in this environment. Levels of activation are calculated for the beam line components within the FRIB target hall.

Analysis of residual activity at the FRIB linear accelerator

The Facility for Rare Isotope Beams (FRIB) is an accelerator facility being established at Michigan State University (MSU). The facility will utilize a broad range of primary ion beams from 16O to 238U with a beam power of up to 400 kW and energy of 200 MeV/u for 238U in its baseline configuration to produce rare isotopes. A possible facility upgrade will include an increase of the beam energy up to 400 MeV/u for 238U and addition of new light ion beams down to 3He and protons for Isotope Separation Online (ISOL) operations. The FRIB double-folded linear accelerator will accommodate several high-rate beam loss devices. The residual activity of these devices were analysed and results are presented in this work.

First two operational years of the electron-beam ion trap charge breeder at the National Superconducting Cyclotron Laboratory

The electron-beam ion trap (EBIT) charge breeder of the ReA post-accelerator, located at the National Superconducting Cyclotron Laboratory (Michigan State University), started on-line operation in September 2015. Since then, the EBIT has delivered many pilot beams of stable isotopes and several rare-isotope beams. An operating aspect of the ReA EBIT is the breeding of high charge states to reach high reaccelerated beam energies. Efficiencies in single charge states of more than 20% were measured with ${^{39}\mathrm{K}}^{15+}$, ${^{85}\mathrm{Rb}}^{27+}$, ${^{47}\mathrm{K}}^{17+}$, and ${^{34}\mathrm{Ar}}^{15+}$. Producing high charge states demands long breeding times. This reduces the ejection frequency and, hence, increases the number of ions ejected per pulse. Another operating aspect is the ability to spread the distribution in time of the ejected ion pulses to lower the instantaneous rate delivered to experiments. Pulse widths were stretched from a natural $25\text{ }\text{ }\ensuremath{\mu}\mathrm{s}$ up to $\ensuremath{\sim}70\text{ }\text{ }\mathrm{ms}$. This publication reviews the progress of the ReA EBIT system over the years and presents the results of charge-breeding efficiency measurements and pulse-stretching tests obtained with stable- and rare-isotope beams. Studies performed with high sensitivity to identify and quantify stable-isotope contaminants from the EBIT are also presented, along with a novel method for purifying beams.

Commissioning of the ECR ion source of the high intensity proton injector of the Facility for Antiproton and Ion Research (FAIR).

The CEA at Saclay is in charge of developing and building the ion source and the low energy line of the proton linac of the FAIR (Facility for Antiproton and Ion Research) accelerator complex located at GSI (Darmstadt) in Germany. The FAIR facility will deliver stable and rare isotope beams covering a huge range of intensities and beam energies for experiments in the fields of atomic physics, plasma physics, nuclear physics, hadron physics, nuclear matter physics, material physics, and biophysics. A significant part of the experimental program at FAIR is dedicated to antiproton physics that requires an ultimate number 7 × 1010 cooled pbar/h. The high-intensity proton beam that is necessary for antiproton production will be delivered by a dedicated 75 mA/70 MeV proton linac. A 2.45 GHz microwave ion source will deliver a 100 mA H+ beam pulsed at 4 Hz with an energy of 95 keV. A 2 solenoids low energy beam transport line allows the injection of the proton beam into the radio frequency quadrupole (RFQ) within an acceptance of 0.3π mm mrad (norm. rms). An electrostatic chopper system located between the second solenoid and the RFQ is used to cut the beam macro-pulse from the source to inject 36 μs long beam pulses into the RFQ. At present time, a Ladder-RFQ is under construction at the University of Frankfurt. This article reports the first beam measurements obtained since mid of 2016. Proton beams have been extracted from the ECR ion source and analyzed just after the extraction column on a dedicated diagnostic chamber. Emittance measurements as well as extracted current and species proportion analysis have been performed in different configurations of ion source parameters, such as magnetic field profile, radio frequency power, gas injection, and puller electrode voltage.

An electron beam ion trap and source for re-acceleration of rare-isotope ion beams at TRIUMF.

Electron beam driven ionization can produce highly charged ions (HCIs) in a few well-defined charge states. Ideal conditions for this are maximally focused electron beams and an extremely clean vacuum environment. A cryogenic electron beam ion trap fulfills these prerequisites and delivers very pure HCI beams. The Canadian rare isotope facility with electron beam ion source-electron beam ion sources developed at the Max-Planck-Institut für Kernphysik (MPIK) reaches already for a 5 keV electron beam and a current of 1 A with a density in excess of 5000 A/cm2 by means of a 6 T axial magnetic field. Within the trap, the beam quickly generates a dense HCI population, tightly confined by a space-charge potential of the order of 1 keV times the ionic charge state. Emitting HCI bunches of ≈107 ions at up to 100 Hz repetition rate, the device will charge-breed rare-isotope beams with the mass-over-charge ratio required for re-acceleration at the Advanced Rare IsotopE Laboratory (ARIEL) facility at TRIUMF. We present here its design and results from commissioning runs at MPIK, including X-ray diagnostics of the electron beam and charge-breeding process, as well as ion injection and HCI-extraction measurements.

Excitation of the Isovector Spin Monopole Resonance via the Exothermic ^{90}Zr(^{12}N,^{12}C) Reaction at 175 MeV/u.

The (^{12}N, ^{12}C) charge-exchange reaction at 175 MeV/u was developed as a novel probe for studying the isovector spin giant monopole resonance (IVSMR), whose properties are important for better understanding the bulk properties of nuclei and asymmetric nuclear matter. This probe, now available through the production of ^{12}N as a secondary rare-isotope beam, is exothermic, is strongly absorbed at the surface of the target nucleus, and provides selectivity for spin-transfer excitations. All three properties enhance the excitation of the IVSMR compared to other, primarily light-ion, probes, which have been used to study the IVSMR thus far. The ^{90}Zr(^{12}N,^{12}C) reaction was measured and the excitation energy spectra up to about 70MeV for both the spin-transfer and non-spin-transfer channels were deduced separately by tagging the decay by γ emission from the ^{12}C ejectile. Besides the well-known Gamow-Teller and isobaric analog transitions, a clear signature of the IVSMR was identified. By comparing with the results from light-ion reactions on the same target nucleus and theoretical predictions, the suitability of this new probe for studying the IVSMR was confirmed.

Construction and Testing of Curved ReBCO Coils

In many applications dipole magnets with coils having significant curvature are needed. This is particularly challenging for high temperature superconductors (HTSs) as they are brittle. One possible application for curved HTS coils was the fragment separator dipole magnets for the Facility for Rare Isotope Beams (FRIB). For this application these magnets would operate in a high radiation environment and would be subject to a high heat load. Removal of heat generated in magnets in this environment using conventional Ni–Ti and Nb3Sn superconductors, which generally operate at ∼4.5 K, is difficult. However, an HTS conductor can be used to permit operation at 40 K where heat removal is significantly more efficient. As these coils are curved, one side of the coils has a reverse curvature requiring the development of special technology to wind the coils. As part of an STTR grant to develop and demonstrate a super-ferric design for a 2.2 T magnet, two curved coils were fabricated with a 12-mm-wide SuperPower ReBCO conductor and first tested in liquid N2 at 77 K. Afterwards the coils were installed into a cryostat and cooled to the design temperature of 48 K with cryocoolers. This paper presents the construction details and test results for these coils.