Radioactive Isotope Beam Research Articles

First observation of the π0h11/2⊗ν0h9/2 partner orbital configuration in the odd-odd I138 nucleus

The β-decay scheme of Te138 and the level structure of I138 is reported for the first time. The experiment was performed at the Radioactive Isotope Beam Factory of RIKEN, as one of the EUROBALL-RIKEN Cluster Array campaigns. Secondary radioactive ions, including Te138 and Sb138, were produced by the in-flight fission of a U238 beam with the energy of 345 MeV per nucleon. From the β decay of Te138, the level scheme of I138 was supplemented with new spin and parity assignments, such as the low-lying negative-parity states and a positive-parity 1+ state. This 1+ state can be interpreted as being associated with the π0h11/2⊗ν0h9/2 partner orbital configuration populated by the Gamow-Teller transition between a neutron in the 0h9/2 orbital and a proton in the 0h11/2 orbital. Details of the structure of I138 are discussed in terms of the proton-neutron interactions and Gamow-Teller transition strength within the theoretical context of shell-model calculations.Received 5 December 2021Accepted 16 March 2022DOI:https://doi.org/10.1103/PhysRevC.105.034334©2022 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasBeta decayNuclear structure & decaysProperties90 ≤ A ≤ 149TechniquesRadiation detectorsRadioactive beamsShell modelSpectrometers & spectroscopic techniquesNuclear Physics

Odd-parity Rydberg and autoionizing states of thulium studied by laser resonance ionization spectroscopy

Laser resonance ionization spectroscopy of atomic thulium (Tm) has been performed at TRIUMF's off-line laser ion source test stand with a system of pulsed titanium sapphire (Ti:Sa) lasers. Automated continuous laser frequency scans across Rydberg states and autoionizing (AI) states were carried out using a pulsed, frequency doubled grating-tuned Ti:Sa laser. A combined analysis of the convergence limits resulting from odd-parity Rydberg series and AI Rydberg series improved the value for the ionization potential to 49,880.57(6)stat(4)sys cm−1, one order of magnitude more precise than the currently accepted value of 49,879.8(5) cm−1. An efficient resonance ionization scheme has been deduced from the photoionization spectra, and applied to online radioactive Tm isotope beam delivery at ISAC-TRIUMF. The ionization scheme will be useful for the extraction of Tm isotopes for nuclear medicine. The production yields of 153–165Tm were measured by evaluating the characteristic γ lines of the Tm isotopes at the ISAC yield station.

INCREASE: An in-cell reaction system for multi-nucleon transfer and spontaneous fission at the FRS ion catcher

A complex, long-range experimental program for the production of neutron-rich nuclei via multi-nucleon transfer (MNT) reactions and spontaneous fission (SF) inside the Cryogenic Stopping Cell (CSC) of the FRS Ion Catcher facility at GSI has been suggested and is under development. A key component is a new experimental set-up, consisting of a modified DC cage, a target-wheel, a beam-dump and a space charge suppression system. This setup replaces the standard electrode structure of the CSC, enabling generation of radioactive isotope beams by in-cell MNT reactions inside the CSC and the efficient and fast extraction of ions produced by MNT reactions or spontaneous fission. Furthermore, the system allows to do experiments with beams from the FRS with much higher rates and shorter half-lives. We present the specifications, design and production of this device. We also present the technical solution adopted against the space charge effect and the simulations that validate it.

Mirror energy differences above the 0f7/2 shell: First γ-ray spectroscopy of the Tz = −2 nucleus 56Zn

Excited states in 56Zn were populated following one-neutron removal from a 57Zn beam impinging on a Be target at intermediate energies in an experiment conducted at the Radioactive Isotope Beam Factory at RIKEN. Three γ rays were observed and tentatively assigned to the 6+→4+→2+→0+ yrast sequence. This turns 56Zn into the heaviest Tz=−2 nucleus in which excited states are known. The excitation-energy differences between these levels and the isobaric analogue states in the Tz=+2 mirror partner, 56Fe, are compared with large-scale shell-model calculations considering the full pf valence space and various isospin-breaking contributions. This comparison, together with an analysis of the mirror energy differences in the A=58, Tz=±1 pair 58Zn and 58Ni, provides valuable information with respect to the size of the monopole radial and the isovector multipole isospin-breaking terms in the region above doubly-magic 56Ni.

Investigation of the ground-state spin inversion in the neutron-rich Cl47,49 isotopes

A first gamma-ray study of 47,49Cl spectroscopy was performed at the Radioactive Isotope Beam Factory with 50Ar projectiles at 217 MeV/nucleon, impinging on the liquid hydrogen target of the MINOS device. Prompt de-excitation gamma-rays were measured with the NaI(Tl) array DALI2+. Through the one-proton knockout reaction 50Ar(p,2p), a spin assignment could be determined for the low-lying states of 49Cl from the momentum distribution obtained with the SAMURAI spectrometer. A spin-parity J = 3/2+ is deduced for the ground state of 49Cl, similar to the recently studied N = 32 isotope 51K.

A Novel Method of Analyzing a Reaction Using the Data from Particle and Gamma-ray Detectors

The radioactive isotope beam (RIB) of 25Na at 5MeV/u energy in maximum 3x107 pps intensity was fired on a deuterated polyethylene (CD2) to investigate mainly (d,p gamma) reaction. However, there were other reaction possibilities, which were arisen from the same collision, to be eliminated wisely. As to a particle detector and a gamma-ray detector, highly selective double-sided silicon strip detectors (DSSSD) and high purity germanium detectors (HPGe) were employed to make delicate selections of distinctive energies of experimentally populated excited states of 26Na. The motivation of this work was to show how to combine the information coming out of two different types of detectors in terms of energies to reach out a new unobserved data. This study was a part of the experiment with Silicon Highly-segmented Array for Reactions of Coulex (SHARC) and TRIUMF-ISAC Gamma-ray Escape-Suppressed Spectrometry (TIGRESS) in Canada.

Two-dimensional beam profile monitor for the detection of alpha-emitting radioactive isotope beam

Ions with similar charge-to-mass ratios cannot be separated from existing beam profile monitors (BPMs) in nuclear facilities in which low-energy radioactive ions are produced due to nuclear fusion reactions. In this study, we developed a BPM using a microchannel plate and a charge-coupled device to differentiate the beam profiles of alpha-decaying radioactive isotopes from other ions (reaction products) produced in a nuclear reaction. This BPM was employed to optimize the low-energy radioactive francium ion (Fr+) beam developed at the Cyclotron and Radioisotope Center (CYRIC), Tohoku University, for electron permanent electric dipole moment (e-EDM) search experiments using Fr atoms. We demonstrated the performance of the BPM by separating the Fr+ beam from other reaction products produced during the nuclear fusion reaction of an oxygen (18O) beam and gold (197Au) target. However, as the mass of Au is close to that of Fr, separating the ions of these elements using a mass filter is a challenge, and a dominant number of Au+ renders the Fr+ beam profile invisible when using a typical BPM. Therefore, by employing the new BPM, we could successfully observe the Fr+ beam and other ion beams distinctly by measuring the alpha decay of Fr isotopes. This novel technique to monitor the alpha-emitting radioactive beam covers a broad range of lifetimes, for example, from approximately 1 s to 10 min, and can be implemented for other alpha-emitter beams utilized for medical applications.

Application of machine learning in the determination of impact parameter in the Sn132+Sn124 system

Background: $^{132}$Sn+$^{124}$Sn collisions at the beam energy of 270 MeV$/$nucleon have been performed at the Radioactive Isotope Beam Factory (RIBF) in RIKEN to investigate the nuclear equation of state. Reconstructing impact parameter is one of the important tasks in the experiment as it relates to many observables. Purpose: In this work, we employ three commonly used algorithms in machine learning, the artificial neural network (ANN), the convolutional neural network (CNN) and the light gradient boosting machine (LightGBM), to determine impact parameter by analyzing either the charged particles spectra or several features simulated with events from the ultra-relativistic quantum molecular dynamics (UrQMD) model. Method: To closely imitate experimental data and investigate the generalizability of the trained machine learning algorithms, incompressibility of nuclear equation of state and the in-medium nucleon-nucleon cross sections are varied in the UrQMD model to generate the training data. Results: The mean absolute error $\Delta b$ between the true and the predicted impact parameter is smaller than 0.45 fm if training and testing sets are sampled from the UrQMD model with the same parameter set. However, if training and testing sets are sampled with different parameter sets, $\Delta b$ would increase to 0.8 fm. Conclusion: The generalizability of the trained machine learning algorithms suggests that these machine learning algorithms can be used reliably to reconstruct impact parameter in experiment.

Spectroscopy of Cd98 by two-nucleon removal from In100

Low-lying states of Cd-98 have been populated by the two-nucleon removal reaction (In-100, Cd-98+gamma) and studied using in-beam gamma-ray spectroscopy at the Radioactive Isotope Beam Factory at RIKEN. Two new gamma transitions were identified and assigned as decays from a previously unknown state. This state is suggested to be based on a pi 1g(/9/2)(-1)2p(1/2)(-2) configuration with J(pi) = 5(-). The present observation extends the systematics of the excitation energies of the first 5(-) state in N = 50 isotones toward Sn-100. The determined energy of the 5(- )state in Cd-98 continues a smooth trend along the N = 50 isotones. The systematics are compared with shell-model calculations in different model spaces. Good agreement is achieved when considering a model space consisting of the pi(1f(5/2), 2p(3/2), 2p(1/2), 1g(9/2)) orbitals. The calculations with a smaller model space omitting the orbitals below the Z = 38 subshell could not reproduce the experimental energy difference between the ground and first 5(-) states in N = 50 isotones, because proton excitations across Z = 38 subshell yield a large amount of correlation energy that lowers the ground states. (Less)

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.

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

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.

Experiments on astrophysical reactions with low-energy unstable nuclei beams at CRIB

Studies on nuclear astrophysics, nuclear structure, and other interests have been performed using the radioactive-isotope (RI) beams at the low-energy RI beam separator CRIB, operated by Center for Nuclear Study (CNS), the University of Tokyo. The elastic resonant scattering is a striking tool to study astrophysical reactions and nuclear clusters. In particular, when it is coupled with a thick target and inverse kinematics, the measurement can be very efficient and even feasible with RI beams. By measuring resonant scattering, we can study the properties of resonant states which could play an important role in the astrophysical reaction, or have an exotic nuclear structure. The 10Be+α elastic scattering has been measured at CRIB, and three resonances which are in agreement with the prediction of a linear-chain structure by Suhara-En’yo were observed, giving a strong indication of the existence of such an exotic structure. Measurements based on the indirect technique of the reaction measurement, such as the Trojan horse method, have also been performed at CRIB.

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.

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.

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.