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 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.

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.

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

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.

The one-neutron knockout from ^{52}Ca in inverse kinematics onto a proton target was performed at ∼230  MeV/nucleon combined with prompt γ spectroscopy. Exclusive quasifree scattering cross sections to bound states in ^{51}Ca and the momentum distributions corresponding to the removal of 1f_{7/2} and 2p_{3/2} neutrons were measured. The cross sections, interpreted within the distorted-wave impulse approximation reaction framework, are consistent with a shell closure at the neutron number N=32, found as strong as at N=28 and N=34 in Ca isotopes from the same observables. The analysis of the momentum distributions leads to a difference of the root-mean-square radii of the neutron 1f_{7/2} and 2p_{3/2} orbitals of 0.61(23) fm, in agreement with the modified-shell-model prediction of 0.7 fm suggesting that the large root-mean-square radius of the 2p_{3/2} orbital in neutron-rich Ca isotopes is responsible for the unexpected linear increase of the charge radius with the neutron number.

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 ?? 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.

The nuclear structure of 51Ar, an uncharted territory so far, was studied by the (p,2p) reaction using γ-ray spectroscopy for the bound states and the invariant mass method for the unbound states. Two peaks were detected in the γ-ray spectrum and six peaks were observed in the 50Ar+n relative energy spectrum. Comparing the results to our shell-model calculations, two bound and six unbound states were established. Three of the unbound states could only be placed tentatively due to the low number of counts in the relative energy spectrum of events associated with the decay through the first excited state of 50Ar. The low cross sections populating the two bound states of 51Ar could be interpreted as a clear signature for the presence of significant subshell closures at neutron numbers 32 and 34 in argon isotopes. It was also revealed that due to the two valence holes, unbound collective states coexist with individual-particle states in 51Ar.

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.

The low-lying level structure of V-63 was studied for the first time by the inelastic proton scattering and the proton knock-out reaction in inverse kinematics. The comparison of the newly observed gamma-ray transitions at 696(8) keV and 889(16) keV with our shell-model calculations using the Lenzi-Nowacki-Poves-Sieja interaction established two excited states proposed to be the first 11/2(-) and 9/2(-) levels. The (p, p') excitation cross sections were analyzed by the coupled channel formalism assuming pure quadrupole as well as quadrupole+hexadecapole deformations. This resulted in large deformation parameters placing V-63 in the island of inversion located below Ni-68.

Excited states in the N = 40 isotone Ti-62 were populated via the V-63( p, 2p)Ti-62 reaction at similar to 200MeV/nucleonat the Radioactive Isotope Beam Factory and studied using gamma-ray spectroscopy. The energies of the 2(1)(+) -> 0(gs)(+) and 4(1)(+) -> 2(1)(+) transitions, observed here for the first time, indicate a deformed Ti-62 ground state. These energies are increased compared to the neighboring Cr-64 and Fe-66 isotones, suggesting a small decrease of quadrupole collectivity. The present measurement is well reproduced by large-scale shell-model calculations based on effective interactions, while ab initio and beyond mean-field calculations do not yet reproduce our findings. The shell-model calculations for Ti-62 show a dominant configuration with four neutrons excited across the N = 40 gap. Likewise, they indicate that the N = 40 island of inversion extends down to Z = 20, disfavoring a possible doubly magic character of the elusive Ca-60. (C) 2019 The Authors. Published by Elsevier B.V.

Low-lying excited states in the N = 32 isotope Ar-50 were investigated by in-beam gamma-ray spectroscopy following proton- and neutron-knockout, multinucleon 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(1)(+) state could also correspond to a 2(+) state.

We report on the first gamma-ray spectroscopy of K-51,K-53 produced via the Ca-52,Ca-54(p,2p) reactions at similar to 250 MeV/nucleon. Unambiguous final-state angular-momentum assignments were achieved for beam intensities down to few particles per second by using a new technique based on reaction vertex tracking combined with a thick liquid-hydrogen target. Through gamma-ray spectroscopy and exclusive parallel momentum distribution analysis, 3/(2)+ ground states and 1/2(+) first excited states in K-51,K-53 were established quantifying the natural ordering of the 1d(3/2) and 2s(1/2) proton-hole states that are restored at N = 32 and 34. State-of-the-art ab initio calculations and shell-model calculations with improved phenomenological effective interactions reproduce the present data and predict consistently the increase of the E(1/4)- E(3/4) energy differences towards N = 40. (C) 2020 The Authors. Published by Elsevier B.V.

We report on the first γ-ray spectroscopy of 51,53K produced via the 52,54Ca(p,2p) reactions at ∼250 MeV/nucleon. Unambiguous final-state angular-momentum assignments were achieved for beam intensities down to few particles per second by using a new technique based on reaction vertex tracking combined with a thick liquid-hydrogen target. Through γ-ray spectroscopy and exclusive parallel momentum distribution analysis, 3/2+ ground states and 1/2+ first excited states in 51,53K were established quantifying the natural ordering of the 1d3/2 and 2s1/2 proton-hole states that are restored at N = 32 and 34. State-of-the-art ab initio calculations and shell-model calculations with improved phenomenological effective interactions reproduce the present data and predict consistently the increase of the E(1/21+) - E(3/21+) energy differences towards N = 40.

Within the SEASTAR III campaign at the Radioactive Isotope Beam Factory, at the RIKEN Nishina Center, neutron-rich isotopes in the vicinity of K-53 were produced from the fragmentation of the primary Zn-70 beam on a Be-9 target. After nucleon knockout reactions on the secondary liquid hydrogen MINOS target the known gamma rays of the neutron-rich Sc-55 isotope were observed (shown in this proceedings) and gamma rays from Sc-57,Sc-59 isotopes have been identified for the first time. The evolution of the occupied nucleon orbitals of these nuclei in the ground and excited state is investigated under the prism of the tensor force.

The first γ-ray spectroscopy of ^{52}Ar, with the neutron number N=34, was measured using the ^{53}K(p,2p) one-proton removal reaction at ∼210  MeV/u at the RIBF facility. The 2_{1}^{+} excitation energy is found at 1656(18) keV, the highest among the Ar isotopes with N>20. This result is the first experimental signature of the persistence of the N=34 subshell closure beyond ^{54}Ca, i.e., below the magic proton number Z=20. Shell-model calculations with phenomenological and chiral-effective-field-theory interactions both reproduce the measured 2_{1}^{+} systematics of neutron-rich Ar isotopes, and support a N=34 subshell closure in ^{52}Ar.

Exclusive cross sections and momentum distributions have been measured for quasifree one-neutron knockout reactions from a ^{54}Ca beam striking on a liquid hydrogen target at ∼200  MeV/u. A significantly larger cross section to the p_{3/2} state compared to the f_{5/2} state observed in the excitation of ^{53}Ca provides direct evidence for the nature of the N=34 shell closure. This finding corroborates the arising of a new shell closure in neutron-rich calcium isotopes. The distorted-wave impulse approximation reaction formalism with shell model calculations using the effective GXPF1Bs interaction and ab initio calculations concur our experimental findings. Obtained transverse and parallel momentum distributions demonstrate the sensitivity of quasifree one-neutron knockout in inverse kinematics on a thick liquid hydrogen target with the reaction vertex reconstructed to final state spin-parity assignments.