The electric dipole strength above the one-neutron separation energy has been measured in the neutron-rich nucleus 52Ca using the Coulomb excitation at 223 MeV/nucleon in inverse kinematics. The gamma-ray detector array CATANA, the neutron detector NeuLAND demonstrator, and the SAMURAI spectrometer at RIKEN Nishina Center were combined to reconstruct the excitation energy of 52Ca. A observed sharp peak at the very low neutron energy in the 52Ca -> 51Ca +n channel indicates that 52Ca has a sizable amount of dipole strength just above the one neutron threshold.

The N = 34 isotope Sc-55 has been investigated using in-beam gamma-ray spectroscopy at the RIKEN Radioactive Isotope Beam Factory. Spectra from the direct (p, pn) reaction as well as indirect reaction channels have been investigated. gamma rays with energies 496(10), 570(12), 682(14), 1510(30), 1780(36), 2345(57) and 2470(50) keV have been observed. A level scheme was constructed based on gamma gamma coincidence analysis and relative intensities. The results have been compared to the level scheme already reported in literature, as well as to large-scale shell model calculations in the sd - pf model space. A new level at 1510keV, decaying directly to the ground state, has been proposed and spin-parity J(pi) = 7/2(-) was tentatively assigned. The effect of including the nu g(9/2) orbital is discussed. It can be concluded that the main low-energy properties of Sc-55 seem to be included in the original sd - pf model space.

The shell closure at N = 32 has been investigated by a first spectroscopy of the N = 31 nucleus 49Ar at the Radioactive Isotope Beam Factory. Using the 50Ar(p, pn) reaction channel in inverse kinematics, 50Ar projectiles at 217 MeV/nucleon impinged on a 150 mm long liquid hydrogen target, part of the MINOS device. Prompt deexcitation gamma rays were measured with the NaI(Tl) array DALI2+. Reaction products were analyzed with the SAMURAI spectrometer, which allowed the measurement of the momentum distributions and angular momentum transfer. Data were compared to state-of-the-art theoretical predictions, including shell -model, energy -density functional, and ab initio calculations. An onset of collectivity is suggested besides the spherical configuration typical of a closed shell nucleus, such as for 52Ca.

The calcium isotopes are an ideal system to investigate the evolution of shell structure and magic numbers. Although the properties of surface nucleons in calcium have been well studied, probing the structure of deeply bound nucleons remains a challenge. Here, we report on the first measurement of unbound states in 53Ca and 55Ca, populated from 54,56Ca(p,pn) reactions at a beam energy of around 216 MeV/nucleon at the RIKEN Radioactive Isotopes Beam Factory. The resonance properties, partial cross sections, and momentum distributions of these unbound states were analyzed. Orbital angular momentum l assignments were extracted from momentum distributions based on calculations using the distorted wave impulse approximation (DWIA) reaction model. The resonances at excitation energies of 5516(41)keV in 53Ca and 6000(250)keV in 55Ca indicate a significant l =3 component, providing the first experimental evidence for the ν0f7/2 single-particle strength of unbound hole states in the neutron-rich Ca isotopes. The observed excitation energies and cross-sections point towards extremely localized and well separated strength distributions, with some fragmentation for the ν0f7/2 orbital in 55Ca. These results are in good agreement with predictions from shell-model calculations using the effective GXPF1Bs interaction and ab initio calculations and diverge markedly from the experimental distributions in the nickel isotones at Z=28.

Gamow-Teller (GT) transitions in an exotic neutron-rich nucleus 11Li have been measured via the 11Li(p, n)11Be reaction at 182 MeV/u in inverse kinematics at RI Beam Factory (RIBF) of RIKEN Nishina Center. The neutron detector array PANDORA and the SAMURAI spectrometer were used to detect recoil neutrons and decay products produced in the (p, n) reaction, respectively. Preliminary results of kinematic correlation of recoil neutrons, corresponding to GT transitions, are presented for several decay channels of 11Be reaction product. Fourteen decay channels with light-particle emission from 11Be excited states, including seven new channels, are identified, in addition to the non-particle-emission channel from 11Be ground or low-lying states.

The low-energy states of the proton-rich nucleus Ge-62 were studied by the multinucleon knock-out reaction Se-67(C-12,X)Ge-62 using a C-12 target. The analysis of the Doppler-corrected singles spectrum of the gamma rays showed two transitions at 744(20) keV and 948(17) keV, which were found to be in coincidence with each other, forming a cascade and establishing two states at 948(17) keV and 1692(26) keV. The 744 keV transition was detected for the first time, and based on a comparison of the experimental data to shell-model and symmetry-conserving-configuration-mixing-model calculations, it connects the second and first 2(+) levels. The beyond-mean-field model suggests that these states belong to two different bands with triaxial features and similar deformation.

The structure and decay of the most neutron-rich beryllium isotope, ^16Be, has been investigated following proton knockout from a high-energy ^17B beam. Two relatively narrow resonances were observed for the first time, with energies of 0.84(3) and 2.15(5) MeV above the two-neutron decay threshold and widths of 0.32(8) and 0.95(15) MeV respectively. These were assigned to be the ground (J^π=0^+) and first excited (2^+) state, with E_x=1.31(6) MeV. The mass excess of ^16Be was thus deduced to be 56.93(13) MeV, some 0.5 MeV more bound than the only previous measurement. Both states were observed to decay by direct two-neutron emission. Calculations incorporating the evolution of the wavefunction during the decay as a genuine three-body process reproduced the principal characteristics of the neutron-neutron energy spectra for both levels, indicating that the ground state exhibits a strong spatially compact dineutron component, while the 2^+ level presents a far more diffuse neutron-neutron distribution.

Direct reaction experiments in inverse kinematics are one of the best experimental tools to study a wide range of nuclear properties, providing a great probe into the nuclear structure of exotic nuclei and enabling the measurement of reactions relevant to many astrophysical scenarios. In order to fully exploit the next generation of radioactive ion beam facilities, a large amount of effort was devoted to developing nuclear detectors specially designed for direct reaction experiments. An instrumental part of these detector devices is the Micron X6 position-sensitive double sided silicon strip detector. This custom-made detector is segmented in 4 strips on its ohmic side and 8 resistive charge-splitting strips on its junction side, providing excellent position measurement of charged particles with a much smaller number of signals than traditional DSSSD with similar position resolution.

The characteristics of a Silicon Photo Multiplier (SiPM)-based scintillation detector system were studied. The influence of the bias voltage and the number of SiPMs on the energy resolution was identified. The single SiPM energy spectra in the multi-SiPM system showed their dependency on the position of the 60Co radiation source. The application of geometric mean on the multi-SiPM system was also considered to bypass single channel calibrations. Preliminary results on the performances of developing scintillation detectors with a CsI(Tl) crystal or a plastic scintillator are demonstrated. The capability of charged particle measurement for CsI(Tl) crystal and sub-nanosecond timing resolution of plastic scintillator were identified.

The β-delayed proton decay of ^{13}O has previously been studied, but the direct observation of β-delayed 3αp decay has not been reported. Rare 3αp events from the decay of excited states in ^{13}N^{⋆} provide a sensitive probe of cluster configurations in ^{13}N. To measure the low-energy products following β-delayed 3αp decay, the Texas Active Target (TexAT) time projection chamber was employed using the one-at-a-time β-delayed charged-particle spectroscopy technique at the Cyclotron Institute, Texas A&M University. A total of 1.9×10^{5} ^{13}O implantations were made inside the TexAT time projection chamber. A total of 149 3αp events were observed, yielding a β-delayed 3αp branching ratio of 0.078(6)%. Four previously unknown α-decaying excited states were observed in ^{13}N at 11.3, 12.4, 13.1, and 13.7 MeV decaying via the 3α+p channel.

Silicon Telescope Array for Reaction studies in inverse Kinematics, STARK, is under development at the Center for Exotic Nuclear Studies to perform nuclear reaction experiments including elastic scattering and neutron transfer reaction. The array consists of 40 double-sided, resistive silicon strip detectors and 12 single-sided, non-resistive strip detectors, and they form three rings covering a large angular range. The expected angular resolution is less than 1°, and the angular coverage of polar angles is 43–78° and 105–150° in the lab frame. The GET (General Electronics for Time projection chamber project) electronics system is used to handle about 1000 channels from detectors. Several elastic scattering experiments are considered as the commissioning of the STARK at KoBRA in the early stage of RAON.

The Wien filter is one of the key components in ion optics to improve the mass separation performance. The KoBRA Wien filter will be installed at the low-energy beamline KoBRA of RAON in Korea. The specifications of the KoBRA Wien filter were determined based on the ion beams expected in the KoBRA beamline, especially, beam energies less than about 5 MeV/nucleon suitable for nuclear astrophysics experiments. The Wien filter is designed to have the maximum field intensities of 0.2 T for the magnetic field and 2.0 kV/mm for the electric field in the ±75(H)×±50(V)×2500(L)mm3 good-field region. Performance of the Wien filter was estimated by the ion optics calculations of the KoBRA beamline for 40Ar beams at 18.5 MeV/nucleon and 14O beams at 2.5 MeV/nucleon. The mass resolving powers are 42.65 and 517, respectively. Currently, the KoBRA Wien filter is being manufactured, and will perform a factory acceptance test.

Gamma decays were observed in ^56Ca and ^58Ca following quasi-free one-proton knockout reactions from ^57,59Sc beams at ≈ 200 MeV/nucleon. For ^56Ca, a γ ray transition was measured to be 1456(12) keV, while for ^58Ca an indication for a transition was observed at 1115(34) keV. Both transitions were tentatively assigned as the 2^+_1 → 0^+_gs decays, and were compared to results from ab initio and conventional shell-model approaches. A shell-model calculation in a wide model space with a marginally modified effective nucleon-nucleon interaction depicts excellent agreement with experiment for 2^+_1 level energies, two-neutron separation energies, and reaction cross sections, corroborating the formation of a new nuclear shell above the N = 34 shell. Its constituents, the 0f_5/2 and 0g_9/2 orbitals, are almost degenerate. This degeneracy precludes the possibility for a doubly magic ^60Ca and potentially drives the dripline of Ca isotopes to ^70Ca or even beyond.

A direct cross-section measurement of the 14O(α,p)17F reaction is important to understand the light curves of x-ray bursts. The measurement will be performed using the Texas Active Target TPC version 2 (TexAT_v2). The TexAT_v2 aims at measuring lower energy protons from the reaction than the original TexAT. Newly developed silicon and CsI(Tl) detector arrays are added at the left, right and bottom of a modified field cage to increase its detection efficiency. This paper describes the overall specifications and two commissioning experiments performed at Texas A&M University.

KoBRA of RAON has been prepared for various low energy nuclear physics studies such as nuclear structure, reactions, and astrophysics. An α -particle transport test was performed using a standard α -source of 241 Am so as to examine the design parameters. The position distribution of the α -particles was measured with a PPAC at the dispersive and achromatic focal planes, and compared with that of a lise ++ Monte Carlo calculation. The results are consistent with each other, confirming a few design parameters. We report on the preliminary results of the α -particle transport test for KoBRA.

The nuclear structure of 66Se, nucleus beyond the N=Z line on the proton-rich side of the valley of stability, was investigated by the neutron knock-out reaction 67Se(12C,X)66Se using a 12C target. The analysis of the singles spectrum of the γ-rays emitted during the de-excitation of the populated low-lying excited states revealed two previously detected (927(4) keV, 1460(32) keV) and three new (744(6) keV, 1210(17) keV, 1661(23) keV) transitions. The 744-keV, the 1210-keV, and the 1460-keV transitions were found to be in coincidence with the one at 927 keV. The spectrum coincident with the 927-keV transition showed a further possible transition at 299(35) keV, which was obscured by significant atomic background in the singles spectrum. This transition might correspond to a peak previously reported at 273(5) keV that could not be assigned to 66Se unambiguously. Based on a comparison of the experimental data to theoretical calculations, four new excited states are proposed which suggest that 66Se exhibits shape coexistence.

The low-lying level structure of 59V and 61V was investigated for the first time. The neutron knockout reaction and inelastic proton scattering were applied for 61V while the neutron knock-out reaction provided the data for 59V. Four and five new transitions were determined for 59V and 61V, respectively. Based on the comparison to our shell-model calculations using the Lenzi-Nowacki-Poves-Sieja (LNPS) interaction, three of the observed γ rays for each isotope could be placed in the level scheme and assigned to the decay of the first 11/2− and 9/2− levels. The (p,p′) excitation cross sections for 61V were analyzed by the coupled-channels formalism assuming quadrupole plus hexadecapole deformations. Due to the role of the hexadecapole deformation, 61V could not be unambiguously placed on the island of inversion.Received 26 August 2022Accepted 9 December 2022DOI:https://doi.org/10.1103/PhysRevC.106.064321©2022 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasElectromagnetic transitionsEnergy levelsNuclear reactionsNuclear structure & decaysNuclear Physics

A long-standing question in nuclear physics is whether chargeless nuclear systems can exist. To our knowledge, only neutron stars represent near-pure neutron systems, where neutrons are squeezed together by the gravitational force to very high densities. The experimental search for isolated multi-neutron systems has been an ongoing quest for several decades 1 , with a particular focus on the four-neutron system called the tetraneutron, resulting in only a few indications of its existence so far 2 – 4 , leaving the tetraneutron an elusive nuclear system for six decades. Here we report on the observation of a resonance-like structure near threshold in the four-neutron system that is consistent with a quasi-bound tetraneutron state existing for a very short time. The measured energy and width of this state provide a key benchmark for our understanding of the nuclear force. The use of an experimental approach based on a knockout reaction at large momentum transfer with a radioactive high-energy 8 He beam was key.

States in the N=35 and 37 isotopes 55,57Ca have been populated by direct proton-induced nucleon removal reactions from 56,58Sc and 56Ca beams at the RIBF. In addition, the (p,2p) quasi-free single-proton removal reaction from 56Ca was studied. Excited states in 55K, 55Ca, and 57Ca were established for the first time via in-beam γ-ray spectroscopy. Results for the proton and neutron removal reactions from 56Ca to states in 55K and 55Ca for the level energies, excited state lifetimes, and exclusive cross sections agree well with state-of-the-art theoretical calculations using different approaches. The observation of a short-lived state in 57Ca suggests a transition in the calcium isotopic chain from single-particle dominated states at N=35 to collective excitations at N=37.