Isotope Beams Research Articles

Pairing Forces Govern Population of Doubly Magic ^{54}Ca from Direct Reactions.

Direct proton-knockout reactions of ^{55}Sc at ∼220 MeV/nucleon were studied at the RIKEN Radioactive Isotope Beam Factory. Populated states of ^{54}Ca were investigated through γ-ray and invariant-mass spectroscopy. Level energies were calculated from the nuclear shell model employing a phenomenological internucleon interaction. Theoretical cross sections to states were calculated from distorted-wave impulse approximation estimates multiplied by the shell model spectroscopic factors, which describe the wave function overlap of the ^{55}Sc ground state with states in ^{54}Ca. Despite the calculations showing a significant amplitude of excited neutron configurations in the ground-state of ^{55}Sc, valence proton removals populated predominantly the ground state of ^{54}Ca. This counterintuitive result is attributed to pairing effects leading to a dominance of the ground-state spectroscopic factor. Owing to the ubiquity of the pairing interaction, this argument should be generally applicable to direct knockout reactions from odd-even to even-even nuclei.

A liquid hydrogen target for radioactive beam experiments using the missing mass method

We developed a liquid hydrogen target dedicated to missing mass spectroscopy using radioactive isotope beams. The target has a circular aperture of 20 mm and a maximum thickness of 1.5 mm at the center. A test experiment with a 12C beam at 54.4 MeV/u was performed at GANIL. The 12C(p,d)11C reaction was measured with the charged-particle telescopes MUST2. An excitation energy resolution of 500-keV r.m.s. was obtained for the observed ground state. The target thickness was confirmed from the comparison of the measured differential cross sections to previous measurements.

The SπRIT time projection chamber.

The Superconducting Analyzer for MUlti-particles from RAdioIsotope (SAMURAI) Pion-Reconstruction and Ion-Tracker Time Projection Chamber (SπRIT TPC) was designed to enable measurements of heavy ion collisions with the SAMURAI spectrometer at the RIKEN radioactive isotope beam factory and provides constraints on the equation of state of neutron-rich nuclear matter. The SπRIT TPC has a 50.5cm drift length and an 86.4 × 134.4cm2 pad plane with 12 096 pads that are equipped with the generic electronics for TPCs. The SπRIT TPC allows for an excellent reconstruction of particles and provides isotopic resolution for pions and other light charged particles across a wide range of energy losses and momenta. The details of the SπRIT TPC are presented, along with discussion of the TPC performance based on cosmic rays and charged particles emitted in heavy ion collisions.

The beams at the edge of physics

Creating a new cutting-edge accelerator isn’t cheap or easy. But as Kit Chapman discovers, the upcoming Facility for Rare Isotope Beams (FRIB) in Michigan promises great things for nuclear physicists especially those with applications in mind

UCGretina geant4 simulation of the GRETINA Gamma-Ray Energy Tracking Array

UCGretina, a geant4 simulation of the GRETINA gamma-ray tracking array of highly-segmented high-purity germanium detectors is described. We have developed a model of the array, in particular of the Quad Module and the capsules, that gives good agreement between simulated and measured photopeak efficiencies over a broad range of gamma-ray energies and reproduces the shape of the measured Compton continuum. Both of these features are needed in order to accurately extract gamma-ray yields from spectra collected in in-beam gamma-ray spectroscopy measurements with beams traveling at v∕c≳0.3 at the National Superconducting Cyclotron Laboratory and the Facility for Rare Isotope Beams. In the process of developing the model, we determined that millimeter-scale layers of passive germanium surrounding the active volumes of the simulated crystals must be included in order to reproduce measured photopeak efficiencies. We adopted a simple model of effective passive layers and developed heuristic methods of determining passive-layer thicknesses by comparison of simulations and measurements for a single crystal and for the full array. Prospects for future development of the model are discussed.

Beam energy determination via collinear laser spectroscopy

An approach to determine the kinetic beam energy at the $10^{-5}$ level is presented, which corresponds to an improvement by more than one order of magnitude compared to conventional methods. Particularly, collinear fluorescence and resonance ionization spectroscopy measurements on rare isotope beams, where the beam energy is a major contribution to the uncertainty, can benefit from this method. The approach is based on collinear spectroscopy and requires no special equipment besides a wavelength meter, which is commonly available. Its advent is demonstrated in a proof-of-principle experiment on a Ni beam. In preparation for the energy measurement, the rest-frame transition frequencies of the $3d^9 4s\;^3\mathrm{D}_3 \rightarrow 3d^9 4p\;^3\mathrm{P}_2$ transitions in neutral nickel isotopes have been identified to be $\nu_0(^{58}\mathrm{Ni})=850\,343\,678\,(20)$ MHz and $\nu_0(^{60}\mathrm{Ni})=850\,344\,183\,(20)$ MHz.

Design and Test of an 8T Focusing Superconducting Solenoid of FRIB Driver Linac

The Facility for Rare Isotope Beams (FRIB) was funded by the U.S. Department of Energy, which is the world’s most powerful isotope beam device. We have developed an 8T superconducting solenoid magnet which is used to focus and steer the heavy ion beams for FRIB Driver Linac. The solenoid consists of a main focusing coil, two shielding coils and two pairs of corrector dipoles, fabricated by NbTi wire only. A helium vessel was designed and fabricated to accommodate the coils with an entire length of 652 mm with end flange. The magnet in an operating current of 89.6 A achieved the maximum magnetic field of 8T in 40-mm cold bore without quench. In this paper, the overall design of the whole magnet was introduced, including the magnetic design, mechanical design, quench simulation and the protection circuit. Moreover, the winding process and the test results were showed.

Aqueous harvesting ofZr88at a radioactive-ion-beam facility for cross-section measurements

Isotope harvesting is a method of collecting the long-lived radioisotopes that build up during the operation of ion-beam facilities in a way that is useful for subsequent research. As a demonstration of this method for the collection of a group IV metal at a fragmentation facility, the high-energy $^{88}\mathrm{Zr}$ secondary beam produced from a 140-MeV/u $^{92}\mathrm{Mo}$ primary beam at the National Superconducting Cyclotron Laboratory (NSCL) was stopped in a water target. The setup aimed to mimic the aqueous beam dump that will be implemented at the Facility for Rare Isotope Beams (FRIB). The collected $^{88}\mathrm{Zr}$ and accompanying $^{88}\mathrm{Y}$ decay daughter were radiochemically extracted from the solution and made into target samples suitable for neutron-capture cross-section measurements. These samples were then irradiated at two reactor facilities, and the $^{88}\mathrm{Zr}$ average thermal-neutron-capture cross section (${\ensuremath{\sigma}}_{T}$) and resonance integral ($I$) were determined to be ${\ensuremath{\sigma}}_{T}=(8.04\ifmmode\pm\else\textpm\fi{}0.63)\ifmmode\times\else\texttimes\fi{}{10}^{5}$ b and $I=(2.53\ifmmode\pm\else\textpm\fi{}0.28)\ifmmode\times\else\texttimes\fi{}{10}^{6}$ b. The ${\ensuremath{\sigma}}_{T}$ value agrees well with previous results and $I$, determined for the first time here, was found to be the largest measured resonance integral by two orders of magnitude. The $^{88}\mathrm{Y}$ thermal-neutron-capture cross section was determined to be less than $1.8\ifmmode\times\else\texttimes\fi{}{10}^{4}$ b. This work demonstrates the steps needed to make cross-section measurements with samples produced via aqueous isotope harvesting.

Observation of new neutron-rich isotopes in the vicinity of Zr110

Neutron-rich radioactive isotope (RI) beams in the vicinity of Zr110 were produced using the in-flight fission of a 345-MeV/nucleon U238 beam at the RIKEN Radioactive Isotope Beam Factory. The RI beams were separated in flight by the large-acceptance two-stage fragment separator BigRIPS. Isotopes were clearly identified in a low-background particle-identification plot, which was obtained from the measurements of time of flight, magnetic rigidity, and energy loss using beam-line detectors placed in the BigRIPS separator and the ZeroDegree spectrometer. Nine new isotopes, Br101, Kr102, Rb105,106, Sr108, Y110,111, Zr114, and Nb117, are reported for the first time, with their measured production cross sections compared with the extrapolations of lighter isotopes down to the femtobarn regime.Received 6 July 2020Accepted 13 January 2021DOI:https://doi.org/10.1103/PhysRevC.103.014614©2021 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasFissionLow & intermediate energy heavy-ion reactionsNuclear structure & decaysRare & new isotopesUnstable nuclei induced nuclear reactionsr processProperties90 ≤ A ≤ 149Nuclear Physics

Physics with Radioactive Isotope Beams and Its Global Expansions

Physics with Radioactive Isotope Beams and Its Global Expansions

Multi-Charged Heavy Ion Sources for Radioactive Isotope Beam Factory

Multi-Charged Heavy Ion Sources for Radioactive Isotope Beam Factory

Simple and Scalable Streaming: The GRETA Data Pipeline

The Gamma Ray Energy Tracking Array (GRETA) is a state of the art gamma-ray spectrometer being built at Lawrence Berkeley National Laboratory to be first sited at the Facility for Rare Isotope Beams (FRIB) at Michigan State University. A key design requirement for the spectrometer is to perform gamma-ray tracking in near real time. To meet this requirement we have used an inline, streaming approach to signal processing in the GRETA data acquisition system, using a GPU-equipped computing cluster. The data stream will reach 480 thousand events per second at an aggregate data rate of 4 gigabytes per second at full design capacity. We have been able to simplify the architecture of the streaming system greatly by interfacing the FPGA-based detector electronics with the computing cluster using standard network technology. A set of highperformance software components to implement queuing, flow control, event processing and event building have been developed, all in a streaming environment which matches detector performance. Prototypes of all high-performance components have been completed and meet design specifications.

N=32 shell closure below calcium: Low-lying structure of Ar50

Low-lying excited states in the $N=32$ isotope $^{50}$Ar were investigated by in-beam $\gamma$-ray spectroscopy following proton- and neutron-knockout, multi-nucleon removal, and proton inelastic scattering at the RIKEN Radioactive Isotope Beam Factory. The energies of the two previously reported transitions have been confirmed, and five additional states are presented for the first time, including a candidate for a 3$^-$ state. The level scheme built using $\gamma\gamma$ coincidences was compared to shell-model calculations in the $sd-pf$ model space, and to ab initio predictions based on chiral two- and three-nucleon interactions. Theoretical proton- and neutron-knockout cross sections suggest that two of the new transitions correspond to $2^+$ states, while the previously proposed $4^+$ state could also correspond to a $2^+$ state.

Lifetime measurement of the 260 g.s. at SAMURAI

The ground state of the neutron unbound nucleus 26O is speculated to have a lifetime in the pico-second regime. In order to determine the decay lifetime of the 26 O ground state with high sensitivity and precision, a new method has been applied. The experiment was performed in December 2016 at the Superconducting Analyzer for MUlti-particle from Radio Isotope Beams (SAMURAI) at the Radioactive Isotope Beam Factory (RIBF) at RIKEN. A 27F beam was produced in the fragment separator BigRIPS and impinged on a W/Pt target 26 O 26 O outside the target. Thus, the velocity difference between the decay neutrons and the fragment 1 O delivers a characteristic spectrum from which the lifetime can be extracted.

A High-Resolution Study of the 110Tc → 110Ru → 110Rh → 110Pd Decay Chain with the GRETINA Array

Spectroscopic data, such as precise γ-ray branching and E2/M1 multipole-mixing ratios, provide vital constraints when performing multi-dimensional Coulomb-excitation analyses. Consequently, as part of our new Coulomb-excitation campaign aimed at investigating the role of exotic non-axial (triaxial) deformations in the unstable refractory Ru-Mo isotopes, additional beta-decay data was obtained. These measurements make use of ANL’s CARIBU facility, which provides intense beams of radioactive refractory isotopes along with the excellent efficiency and angular resolution of the GRETINA γ-ray tracking array. In this article, we report on the analysis of the A = 110 decay chain, focussing on the identification of previously unreported states in 110Ru following the decay of 110Tc.

Heavy ion beam physics at Facility for Rare Isotope Beams

The Facility for Rare Isotope Beams (FRIB) will be the world's premier rare-isotope beam facility. Experiments with the majority (∼80%) of the isotope predicted to exist will become available. The FRIB facility is based on a superconducting (SC) heavy ion linac with output energy above 200 MeV/u for any ions at beam power of 400 kW. FRIB includes a target facility for in-flight production of rare isotopes. A three-stage fragment separator will be used to prepare fast rare isotope beams with high-purity for nuclear physics experiments. The installation work of the accelerator and experimental systems is approaching completion and multi-stage beam commissioning activities started in summer 2017 with expected project completion in early 2022. The commencement of operation for users' experiments is planned immediately following the project completion.

RF deflecting cavity for fast radioactive ion beams

The Facility for Rare Isotope Beams (FRIB) will be a new scientific user facility that produces rare-isotope beams for experiments from the fragmentation of heavy ions at energies of 100–200 MeV/u. During the projectile fragmentation, the rare isotope of interest is produced along with many contaminants that need to be removed before the beam reaches detectors. At FRIB, this is accomplished with a magnetic projectile fragment separator. However, to achieve higher beam purity, in particular for proton-rich rare isotopes, additional purification is necessary. RadiaBeam in collaboration with Michigan State University (MSU) has designed a 20.125 MHz radiofrequency (RF) fragment separator capable of producing a 4 MV kick with 18 cm aperture in order to remove contaminant isotopes based on their time of flight. In this paper, we will discuss the RF and engineering design considerations of this separator cavity.

RIB studies for explosive scenarios and future opportunities at FRIB

An accreting compact star in a binary system can generate periodic thermonuclear runaways on its surface. In the case of a white dwarf star, the result is a classical nova, which enriches the interstellar medium with newly synthesized nuclides. In the case of a neutron star, a detectable burst of X-rays is emitted. Nucleosynthesis and energy generation in these events depends on resonant thermonuclear reaction rates, which are especially challenging to measure directly in the laboratory when they involve radioactive reactants. Fortunately, the resonances can be discovered and their relevant properties can be constructed using nuclear structure experiments. We will describe a program of beta decay experiments the National Superconducting Cyclotron Laboratory to this end. In particular, the Gaseous Detector with Germanium Tagging (GADGET) system, developed to measure very weak low-energy beta delayed proton emission branches and gamma rays, is now operational. Recent results and future opportunities at the Facility for Rare Isotope Beams will be discussed.

Determination of beta-delayed neutron emission probability limits of rhodium isotopes by gamma-ray spectroscopy

The decay of five neutron-heavy rhodium isotopes were studied at the Radioactive Isotope Beam Factory (RIBF) Facility at the RIKEN Nishina Center after relativistic fission of 238U beam on a thick beryllium target. Previously unknown associated gamma-ray decay energies are reported for each nuclide, and through evaluating the intensity of the 2+ → 0+ E2 transition in the even-even palladium daughter nuclei, 120,122,124Pd, from the beta-tagged gamma-ray spectra an upper or lower limit of beta-delayed neutron emission is deduced for each nuclei. A general, expected trend of increasing Pn is observed in the direction of the neutron drip line.

Performance of neutron spectrum unfolding using deuterated liquid scintillator

The performance of the maximum likelihood expectation maximization method for unfolding neutron energy spectra using deuterated liquid scintillator is evaluated for future utilization with rare isotope beams. High-resolution neutron energy spectra as well as the detector response matrix were measured at the Edwards Accelerator Laboratory at Ohio University. Maximum-likelihood expectation maximization (MLEM) unfolded neutron spectra are compared with spectra from neutron time-of-flight. The effects of the MLEM stopping criteria and spectrum statistics are also investigated.