Rare Isotope Beams Research Articles

Development and testing of radio frequency modulated electron Gun at VECC, Kolkata

A superconducting electron linac (e-Linac) photo-fission driver is under construction at VECC for the upcoming rare isotope beam facility called ANURIB. The first stage consists of an injector linac based on 1.3 GHz superconducting radio frequency technology designed for accelerating 10 MeV, 2 mA CW electron beam. An rf modulated thermionic electron gun (E-Gun) has been developed and tested for this injector linac. The E-Gun will allow continuous beam operation up to 300 keV with around 3 pC charge per pulse at 650 MHz. Electron beam with RF modulation is extracted from the gun and intensity, energy, size of the DC beam as well as trans-conductance of the electron source are measured. The design of some major components for the electron gun and low energy beam transport line is discussed and results of beam tests are presented.

Neutron rich matter in the laboratory and in the heavens after GW170817

The historic observations of the neutron star merger GW170817 advanced our understanding of r-process nucleosynthesis and the equation of state (EOS) of neutron rich matter. Simple neutrino physics suggests that supernovae are not the site of the main r-process. Instead, the very red color of the kilonova associated with GW170817 shows that neutron star (NS) mergers are an important r-process site. We now need to measure the masses and beta decay half-lives of very neutron rich heavy nuclei so that we can more accurately predict the abundances of heavy elements that are produced. This can be done with new radioactive beam accelerators such as the Facility for Rare Isotope Beams (FRIB). GW170817 provided information on the deformability of NS and the equation of state of dense matter. The PREX II experiment will measure the neutron skin of 208Pb and help constrain the low density EOS. As the sensitivity of gravitational wave detectors improve, we expect to observe many more events. We look forward to exciting advances and surprises!

Investigating the link between proton reaction cross sections and the quenching of proton spectroscopic factors in 48Ca

The nucleon self-energies of 40Ca and 48Ca are determined using a nonlocal dispersive optical model (DOM). By enforcing the dispersion relation connecting the real and imaginary part of the self-energy, scattering and structure data are used to constrain these self-energies. The ability to calculate both bound and scattering states simultaneously puts these self-energies in a unique position to consistently describe exclusive knockout reactions such as (e,e′p). The present analysis reveals the importance of high-energy proton reaction cross-section data in constraining spectroscopic factors required for the description of the (e,e′p) cross sections. In particular, it is imperative that high-energy proton reaction cross-section data are measured for 48Ca in the near future so that the quenching of the spectroscopic factors in the 48Ca(e,e′p)47K reaction can be unambiguously constrained using the DOM. Measurements of proton reaction cross sections in inverse kinematics employing rare isotope beams with large neutron excess will provide corresponding constraints on proton spectroscopic factors for exotic nuclei. Moreover, DOM generated spectral functions indicate that the quenching of spectroscopic factors compared to 40Ca is not only due to long-range correlation, but also partly due to the increase in high-momentum protons in 48Ca on account of the strong neutron-proton interaction. Single-particle momentum distributions of protons and neutrons in 48Ca calculated from these spectral functions confirm that neutron excess causes a higher fraction of high-momentum protons than neutrons.

Intruder dominance in the 02+ state of Mg32 studied with a novel technique for in-flight decays

The development of advanced γ-ray tracking arrays allows for a sensitive new technique to investigate elusive states of exotic nuclei with fast rare-isotope beams. By taking advantage of the excellent energy and position resolution of the Gamma-Ray Energy Tracking In-beam Nuclear Array, we developed a novel technique to identify in-flight isomeric decays of the 0⁺₂ state in 32Mg populated in a two-proton removal reaction. We confirm the 0⁺₂→2+1γ-ray transition of ³²Mg and constrain the 0⁺₂ decay lifetime, suggesting a large collectivity. The small partial cross section populating the 0⁺₂ state in this reaction provides experimental evidence for the reduced occupancy of the normal configuration of the 0⁺₂ state, indicating the intruder dominance of this state.

Electron-beam ion source/trap charge breeders at rare-isotope beam facilities

At accelerator facilities, charge breeders convert ion beams of low charge states (mostly singly charged) into multiply charged ion beams to extend the energy range of beams accelerated and delivered to experiments. A field of application that has grown over the past decades is charge breeding of rare-isotope beams (RIBs). RIBs are of interest in nuclear physics and astrophysics to study nuclear structure and the origin of the chemical elements. Several postaccelerators at RIB facilities in operation and under construction employ electron-beam ion source and trap (EBIS/T) breeders. Compared with other breeding techniques, EBIS/Ts have many advantages: high efficiency, fast and variable breeding times, small beam emittances, and high beam purity. This publication reviews the use of EBIS/T breeders at RIB facilities with a particular emphasis on their use for postacceleration along with advances in related fields.

Probing the role of proton cross-shell excitations in Ni70 using nucleon knockout reactions

The neutron-rich Ni isotopes have attracted attention in recent years due to the occurrence of shape or configuration coexistence. We report on the difference in population of excited final states in 70Ni following gamma-ray tagged one-proton, one-neutron, and two-proton knockout from 71Cu, 71Ni, and 72Zn rare-isotope beams, respectively. Using variations observed in the relative transition intensities, signaling the changed population of specific final states in the different reactions, the role of neutron and proton configurations in excited states of 70Ni is probed schematically, with the goal of identifying those that carry, as leading configuration, proton excitations across the Z = 28 shell closure. Such states are suggested in the literature to form a collective structure associated with prolate deformation. Adding to the body of knowledge for 70Ni, 29 new transitions are reported, of which 15 are placed in its level scheme.

The ORNL Deuterated Spectroscopic Array — ODeSA

Abstract An array consisting of 12 deuterated organic liquid scintillator detectors for fast-neutron spectroscopy was designed and built at Oak Ridge National Laboratory (ORNL). This versatile array is designed for measurements with low reaction yields, such as those performed with rare isotope beams, as well as at high current DC facilities used for underground nuclear astrophysics research. Because some measurements also offer limited, or no, additional timing information, the ORNL Deuterated Spectroscopic Array (ODeSA) was optimized to utilize spectrum unfolding to extract neutron energy spectra . This array was characterized for n/ γ pulse shape discrimination, light response, resolution, and intrinsic efficiency by using the neutron time-of-flight tunnel at the Edwards Accelerator Laboratory at Ohio University. Results and future plans are discussed.

Isotope harvesting at FRIB: additional opportunities for scientific discovery

The upcoming Facility for Rare Isotope Beams (FRIB) at Michigan State University provides a new opportunity to access some of the world’s most specialized scientific resources: radioisotopes. An excess of useful radioisotopes will be formed as FRIB fulfills its basic science mission of providing rare isotope beams. In order for the FRIB beams to reach high-purity, many of the isotopes are discarded and go unused. If harvested, the unused isotopes could enable new research for diverse applications ranging from medical therapy and diagnosis to nuclear security. Given that FRIB will have the capability to create about 80% of all possible atomic nuclei, harvesting at FRIB will provide a fast path for access to a vast array of isotopes of interest in basic and applied science investigations. To fully realize this opportunity, infrastructure investment is required to enable harvesting and purification of otherwise unused isotopes. An investment in isotope harvesting at FRIB will provide a powerful resource for development of crucial isotope applications. In 2010, the United States Department of Energy Office of Science, Nuclear Physics, sponsored the first ‘Workshop on Isotope Harvesting at FRIB’, convening researchers from diverse fields to discuss the scientific impact and technical feasibility of isotope harvesting. Following the initial meeting, a series of biennial workshops was organized. At the fourth workshop, at Michigan State University in 2016, the community elected to prepare a formal document to present their findings. This report is the output of the working group, drawing on contributions and discussions with a broad range of scientific experts.

Beam commissioning in the first superconducting segment of the Facility for Rare Isotope Beams

Beam commissioning in the first superconducting segment of the Facility for Rare Isotope Beams

Test Results for a Superconducting 28-GHz Ion Source Magnet for FRIB

The superconducting ECR source magnet for the Facility for Rare Isotope Beams at Michigan State University was designed and built by the Superconducting Magnet Group at Lawrence Berkeley National Laboratory (LBNL). The 28-GHz NbTi ion source magnet features a sextupole-in-solenoids configuration, which is comparable to the VENUS ECR magnet operated at LBNL. A shell-based support structure using bladders and keys has been incorporated into the design to allow for adjustments of the preload and reversibility of the magnet assembly process. The magnet has been assembled and successfully tested to operational currents at LBNL. This paper describes the basic design of the magnet and the passive quench protection system. The test results are also presented, including the quench behavior of the magnet, the training performance, and the magnetic measurement results.

Mechanical Study of a Superconducting 28-GHz Ion Source Magnet for FRIB

The superconducting electron cyclotron resonance (ECR) source magnet for the facility for rare isotope beams at Michigan State University was designed and built by the Superconducting Magnet Group at Lawrence Berkeley National Laboratory (LBNL) in 2017. The 28 GHz NbTi ion source magnet features a sextupole-in-solenoids configuration which is comparable to the VENUS ECR magnet operated at LBNL. However, the mechanical design of this magnet utilizes a shell-based support structure which allows fine adjustments to the sextupole preload and reversibility of the magnet assembly process. The magnet has been assembled and tested to operational currents at LBNL. This paper describes the mechanical analyses performed to estimate the sextupole's and solenoids' preloads. We will report on the 3-D finite element analysis during room temperature assembly, cool-down, and magnet excitation, and then describe the magnet preload operations. Finally, we will describe the performance of the support structure during the quench training.

High resolution phase space measurements with Allison-type emittance scanners

Allison-type emittance scanners are widely used to measure projected 2D phase space distributions of low energy beams. This paper extends the conventional data analysis model to introduce three significant corrections that commonly arise in the pursuit of high resolution measurements. First, effective longitudinal asymmetry in the E-dipole placement (typically resulting from directional choice of relief cuts in thick slit-plates) causes deviation from the ideal voltage-to-angle conversion relation. Second, finite slit thickness generates variation in weights of data points that should be compensated. Third, when the interval between data points is smaller than the device resolution (ordinary in the angular data accumulation), a detailed account of the phase space region contributing to each data point can be used to resolve the beam distribution more accurately. These findings are illustrated by simulations with numerically generated phase space distributions. The improved model is applied to experimental measurements of an Ar ion beam with an Allison scanner operating at the front-end of the Facility for Rare Isotope Beams (FRIB) at Michigan State University. Results show that the improved model obtains better agreement among a set of measurements and modifies beam moments significantly (can be $\ensuremath{\sim}10%$ relative to conventional methods, with larger deviations at increasing angular divergence), thus rendering the corrections important for accurate high resolution phase-space characterizations. Python code tools that implement the improved analysis described are made available. These tools are readily applicable to any Allison scanner given a specification of the device geometry and scan ranges associated with each measurement.

Constraining spectroscopic factors near the r -process path using combined measurements: Kr86 (d, p)87Kr

Background: Modern nuclear structure models suggest that the shell structure near the valley of stability, with well-established shell closures at $N=50$, for example, changes in very neutron-rich nuclei far from stability. Single-particle properties of nuclei away from stability can be probed in single-neutron $(d,p)$ transfer reactions with beams of rare isotopes. The interpretation of these data requires reaction theories with various effective interactions. Often, approximations made to the final bound-state potential introduce a large uncertainty in the extracted single-particle properties, in particular the spectroscopic factor.Purpose: Mitigate this uncertainty using a combined measurement method to constrain the shape of the bound-state potential and to reliably extract the spectroscopic factor.Methods: The $^{2}\mathrm{H}(^{86}\mathrm{Kr},p)^{87}\mathrm{Kr}$ reaction was measured at 33 MeV/u at the National Superconducting Cyclotron Laboratory (NSCL) as a test of the combined method. The reaction protons were detected with the Oak Ridge Rutgers University Barrel Array (ORRUBA) of position sensitive silicon strip detectors, the first implementation of ORRUBA coupled to the S800 spectrograph with fast beams at NSCL.Results: These measurements at 33 MeV/u are combined with previous studies of the $^{86}\mathrm{Kr}(d,p)$ reaction at 5.5 MeV/u to demonstrate a successful case of the combined method to constrain the shape of the single-particle potential and deduce asymptotic normalization coefficients and spectroscopic factors. In particular, the single-particle asymptotic normalization coefficient for the ground state of $^{87}\mathrm{Kr}$ was constrained to ${b}_{d5/2}=6.{46}_{\ensuremath{-}0.57}^{+1.12}\phantom{\rule{4pt}{0ex}}{\mathrm{fm}}^{\ensuremath{-}1/2}$, and therefore the deduced spectroscopic factor is $S=0.{44}_{\ensuremath{-}0.13}^{+0.09}$ with uncertainties dominated by experimental statistics.Conclusions: By combining measurements at two very different beam energies, single-particle asymptotic normalization coefficients, at least for low angular momentum transfers, can be constrained. Therefore, spectroscopic factors can be deduced with uncertainties dominated by experimental uncertainties, rather than limited knowledge of bound-state potential parameters.

Studies of systematic effects of the AstroBox2 detector in online conditions

After commissioning the AstroBox2 detector has been used already for several physics experiments. During some of the experiments systematic effects in the measured decay spectra have been observed and related to gain shifts due to the environment, the chemical nature of the isotope under investigation, and to the beam implantation rate. In this article we report the observations and influence of these systematic effects that may well be of interest to other similar experiments with stopped rare isotope beams and detectors relying on gas amplification. Full characterization and understanding of these effects call for further systematic studies.

Status of the CANREB high-resolution separator at TRIUMF

A new ISOL rare isotope beam production facility, ARIEL, is under construction at TRIUMF. ARIEL aims at increasing the delivery of radioactive beams three fold with respect to the present capability of the ISAC facility. Part of ARIEL is the new CANREB equipment that can be described by the two main functionalities: a charge breeding system that includes RFQ cooler, EBIS and Nier separator, and a high-resolution mass separator system. The latter is designed to achieve a resolving power of twenty thousand for a transmitted emittance of three micrometer. The separator optics has been designed with symmetry in order to minimize high-order aberrations. The dispersion of the system is created by two identical ninety degree magnetic dipoles with a field flatness of one part in one hundred thousand. The dipoles are tested and the magnetic field characterized before being installed on line for operation. High-order aberrations can also be corrected by an electrostatic multipole; this features a novel design as well as a new tuning technique. In this paper we will present the latest results from the field characterization and discuss the high level application to tune the multipole.

Simultaneous acceleration of two kinds of ion beams in the RISP

The Rare Isotope Science Project (RISP) is a research complex consisting of a heavy-ion accelerator, which contains a front-end system, a super-conducting linear accelerator, an isotope separator online (ISOL) system, and an in-flight system. The original purpose of the post-linear-accelerator (post-linac) section was to accelerate either a stable driver beam derived from an electron cyclotron resonance ion source, or an unstable rare-isotope beam from an ISOL system. The post-linac lattice has now been redesigned using a novel and improved acceleration concept that allows the simultaneous acceleration of both a stable driver beam and a radioisotope beam. To achieve this, the post-linac lattice is set for a mass-to-charge ratio (A/q) that is the average of the two beams. The performance of this simultaneous two-beam acceleration is here assessed using two ion beams: \(^{58}{\mathbf{Ni}}^{8+}\) and \(^{132}{} {\mathbf{Sn}}^{20+}\). A beam dynamics simulation was performed using the TRACK and TraceWin codes. The resultant beam dynamics for the new RISP post-linac lattice design are examined. We also estimate the effects of machine errors and their correction on the post-linac lattice.

Online tests of the Advanced Cryogenic Gas Stopper at NSCL

Linear gas stoppers filled with helium have become a common tool to convert high energy rare isotope beams into low-energy beams. The National Superconducting Cyclotron Laboratory (NSCL) has designed and fabricated a new cryogenic gas stopper to maximize efficiency and beam rate capability in order to increase scientific reach at the facility. The Advanced Cryogenic Gas Stopper (ACGS) will increase extraction efficiency, reduce transport time, reduce molecular contamination of the isotope of interest, and minimize space charge effects. A novel 4-phase Radio Frequency wire-carpet generates a traveling electrical wave for fast ion transport, cryogenic cooling of the helium gas chamber reduces unwanted molecular formation, and the new planar geometry with the wire-carpet in the mid-plane of stopper alleviates space charge effects. Prototype testing of ACGS components have shown wire-carpet transport efficiencies greater than 95% and transport speeds up to 100 m/s. This presentation will show the first online tests with radioactive beams and report the efficiencies of the ACGS.

Performance validation of the iron-dominated, normal-conducting 45°dipole magnets in the ReA3 HEBT line at NSCL, MSU

Abstract The Re-Accelerator (ReA3) facility at the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University (MSU) delivers rare isotope and stable beams ranging from 0.3 MeV/u to 6 MeV/u to experimental end stations. The High Energy Beam Transport (HEBT) line into the ReA3 experimental hall houses several quadrupole magnets, dipole magnets, and combined function horizontal and vertical corrector magnets. All of these are iron-dominated, resistive type magnets. This paper describes the magnetic performance of 45°dipole magnets installed in both the vertical and horizontal section of the ReA3 beamline. The computed field map and the derived data provide a reference for the operational setting of the magnet and input for the beam optics study. The field measured by both Hall and NMR probes agrees well with the calculations suggesting that the magnet performance meets the design specification.

Heavy ion beam acceleration in the first three cryomodules at the Facility for Rare Isotope Beams at Michigan State University

The Facility for Rare Isotope Beams (FRIB) being constructed at Michigan State University [J. Wei et al., The FRIB superconducting linac---status and plans, LINAC'16, Lansing, MI, p. 1, http://accelconf.web.cern.ch/AccelConf/linac2016/papers/mo1a01.pdf] is based on a cw superconducting linear accelerator which is designed to deliver unprecedented 400 kW heavy ion beam power to the fragmentation target. The installation of the accelerator equipment is approaching completion and multistage beam commissioning activities started in the summer of 2017 with expected completion in 2021. A room-temperature test electron cyclotron resonance ion source, ARTEMIS, provided argon and krypton beams for the commissioning of the low energy beam transport, a radio frequency quadrupole (RFQ), the medium energy beam transport (MEBT) and the first three accelerating cryomodules. The commissioning of the first linac segment (LS1), composed of 15 cryomodules, is planned in the spring of 2019. This paper describes the first results of experimental beam dynamics studies in the LEBT, RFQ, MEBT and the first three cryomodules with comparison to the numerical simulations.

New ion-optical operating modes of the BigRIPS and ZeroDegree spectrometer for the production and separation of high-quality rare isotopes beams and high-resolution spectrometer experiments

BigRIPS is a powerful two-stage in-flight separator for the research with exotic nuclei studied in frontier experiments since more than a decade. The ion-optical system is very versatile due to the multi-stage structure of BigRIPS combined with the ZeroDegree spectrometer or the Superconducting Ring Cyclotron (SRC). Various optical modes can be flexibly realized according to the purpose of experiments. Two categories of developments are presented here. One is a new operating mode of BigRIPS aiming at higher ion-optical resolving power. BigRIPS itself has a two-stage structure. Spatial isotope separation is made at both the first and second stages. In the standard operating mode of BigRIPS, at the second stage the two spatial separations with energy degraders are subtractive in their resolving powers. Here, we present the additive mode. With the resulting increased spatial separation power, the isotopic background can be substantially reduced. Higher ion-optical resolving powers of the first and second BigRIPS degrader stages are also investigated with the goals to reduce further the background and to yield access to new isotopes of heavier elements. The other development is a dispersion-matched system with BigRIPS for high-resolution spectrometer experiments. The BigRIPS and ZeroDegree spectrometer are presently two independent, coupled achromatic systems. A new dispersion-matched mode of BigRIPS and ZeroDegree will enable novel experiments. For high-resolution spectroscopy experiments with high-intensity light projectiles, SRC and BigRIPS can be operated as a dispersion-matched system. The described different ion-optical developments are a base for a new category of experiments exploring exotic nuclei and mesic atoms. Characteristic future experiments with these new ion-optical developments are exemplified in this report.