Collapse of the N=28 shell closure in (42)Si

Nuclear Structure of Dripline Nuclei Elucidated through Precision Mass Measurements of ^{23}Si, ^{26}P, ^{27,28}S, and ^{31}Ar

Using the Bρ-defined isochronous mass spectrometry technique, we report the first determination of the ^{23}Si, ^{26}P, ^{27}S, and ^{31}Ar masses and improve the precision of the ^{28}S mass by a factor of 11. Our measurements confirm that these isotopes are bound and fix the location of the proton dripline in P, S, and Ar. We find that the mirror energy differences of the mirror-nuclei pairs ^{26}P-^{26}Na, ^{27}P-^{27}Mg, ^{27}S-^{27}Na, ^{28}S-^{28}Mg, and ^{31}Ar-^{31}Al deviate significantly from the values predicted assuming mirror symmetry. In addition, we observe similar anomalies in the excited states, but not in the ground states, of the mirror-nuclei pairs ^{22}Al-^{22}F and ^{23}Al-^{23}Ne. Using ab initio VS-IMSRG and mean field calculations, we show that such a mirror-symmetry breaking phenomenon can be explained by the extended charge distributions of weakly bound, proton-rich nuclei. When observed, this phenomenon serves as a unique signature that can be valuable for identifying proton-halo candidates.

First Exploration of Monopole-Driven Shell Evolution above the N=126 Shell Closure: New Millisecond Isomers in ^{213}Tl and ^{215}Tl

Isomer spectroscopy of heavy neutron-rich nuclei beyond the N=126 closed shell has been performed for the first time at the Radioactive Isotope Beam Factory of the RIKEN Nishina Center. New millisecond isomers have been identified at low excitation energies, 985.3(19) keV in ^{213}Tl and 874(5) keV in ^{215}Tl. The measured half-lives of 1.34(5) ms in ^{213}Tl and 3.0(3) ms in ^{215}Tl suggest spins and parities 11/2^{-} with the single proton-hole configuration πh_{11/2} as leading component. They are populated via E1 transitions by the decay of higher-lying isomeric states with proposed spin and parity 17/2^{+}, interpreted as arising from a single πs_{1/2} proton hole coupled to the 8^{+} seniority isomer in the ^{A+1}Pb cores. The lowering of the 11/2^{-} states is ascribed to an increase of the πh_{11/2} proton effective single-particle energy as the second νg_{9/2} orbital is filled by neutrons, owing to a significant reduction of the proton-neutron monopole interaction between the πh_{11/2} and νg_{9/2} orbitals. The new ms isomers provide the first experimental observation of shell evolution in the almost unexplored N>126 nuclear region below doubly magic ^{208}Pb.

Nuclear Charge Radii of Silicon Isotopes

The nuclear charge radius of ^{32}Si was determined using collinear laser spectroscopy. The experimental result was confronted with ab initio nuclear lattice effective field theory, valence-space in-medium similarity renormalization group, and mean field calculations, highlighting important achievements and challenges of modern many-body methods. The charge radius of ^{32}Si completes the radii of the mirror pair ^{32}Ar-^{32}Si, whose difference was correlated to the slope L of the symmetry energy in the nuclear equation of state. Our result suggests L≤60  MeV, which agrees with complementary observables.

Proton Shell Gaps in N=28 Nuclei from the First Complete Spectroscopy Study with FRIB Decay Station Initiator

The first complete measurement of the β-decay strength distribution of _{17}^{45}Cl_{28} was performed at the Facility for Rare Isotope Beams (FRIB) with the FRIB Decay Station Initiator during the second FRIB experiment. The measurement involved the detection of neutrons and γ rays in two focal planes of the FRIB Decay Station Initiator in a single experiment for the first time. This enabled an analytical consistency in extracting the β-decay strength distribution over the large range of excitation energies, including neutron unbound states. We observe a rapid increase in the β-decay strength distribution above the neutron separation energy in _{18}^{45}Ar_{27}. This was interpreted to be caused by the transitioning of neutrons into protons excited across the Z=20 shell gap. The SDPF-MU interaction with reduced shell gap best reproduced the data. The measurement demonstrates a new approach that is sensitive to the proton shell gap in neutron rich nuclei according to SDPF-MU calculations.

N=16 Magicity Revealed at the Proton Drip Line through the Study of ^{35}Ca

The last proton bound calcium isotope ^{35}Ca has been studied for the first time, using the ^{37}Ca(p,t)^{35}Ca two neutron transfer reaction. The radioactive ^{37}Ca nuclei, produced by the LISE spectrometer at GANIL, interacted with the protons of the liquid hydrogen target CRYPTA, to produce tritons t that were detected in the MUST2 detector array, in coincidence with the heavy residues Ca or Ar. The atomic mass of ^{35}Ca and the energy of its first 3/2^{+} state are reported. A large N=16 gap of 4.61(11) MeV is deduced from the mass measurement, which together with other measured properties, makes ^{36}Ca a doubly magic nucleus. The N=16 shell gaps in ^{36}Ca and ^{24}O are of similar amplitude, at both edges of the valley of stability. This feature is discussed in terms of nuclear forces involved, within state-of-the-art shell model calculations. Even though the global agreement with data is quite convincing, the calculations underestimate the size of the N=16 gap in ^{36}Ca by 840 keV.

Extended p_{3/2} Neutron Orbital and the N=32 Shell Closure in ^{52}Ca

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.

Crossing N=28 Toward the Neutron Drip Line: First Measurement of Half-Lives at FRIB

New half-lives for exotic isotopes approaching the neutron drip-line in the vicinity of N∼28 for Z=12-15 were measured at the Facility for Rare Isotope Beams (FRIB) with the FRIB decay station initiator. The first experimental results are compared to the latest quasiparticle random phase approximation and shell-model calculations. Overall, the measured half-lives are consistent with the available theoretical descriptions and suggest a well-developed region of deformation below ^{48}Ca in the N=28 isotones. The erosion of the Z=14 subshell closure in Si is experimentally confirmed at N=28, and a reduction in the ^{38}Mg half-life is observed as compared with its isotopic neighbors, which does not seem to be predicted well based on the decay energy and deformation trends. This highlights the need for both additional data in this very exotic region, and for more advanced theoretical efforts.

Discovery of ^{39}Na

The new isotope ^{39}Na, the most neutron-rich sodium nucleus observed so far, was discovered at the RIKEN Nishina Center Radioactive Isotope Beam Factory using the projectile fragmentation of an intense ^{48}Ca beam at 345  MeV/nucleon on a beryllium target. Projectile fragments were separated and identified in flight with the large-acceptance two-stage separator BigRIPS. Nine ^{39}Na events have been unambiguously observed in this work and clearly establish the particle stability of ^{39}Na. Furthermore, the lack of observation of ^{35,36}Ne isotopes in this experiment significantly improves the overall confidence that ^{34}Ne is the neutron dripline nucleus of neon. These results provide new key information to understand nuclear binding and nuclear structure under extremely neutron-rich conditions. The newly established stability of ^{39}Na has a significant impact on nuclear models and theories predicting the neutron dripline and also provides a key to understanding the nuclear shell property of ^{39}Na at the neutron number N=28, which is normally a magic number.

Shape Changes in the N=28 Island of Inversion: Collective Structures Built on Configuration-Coexisting States in ^{43}S

The neutron-rich nuclei in the N=28 island of inversion have attracted considerable experimental and theoretical attention, providing great insight into the evolution of shell structure and nuclear shape in exotic nuclei. In this work, for the first time, quadrupole collectivity is assessed simultaneously on top of the 3/2^{-} ground state and the 7/2^{-} shape-coexisting isomer of ^{43}S, putting the unique interpretation of shape and configuration coexistence at N=27 and 28 in the sulfur isotopic chain to the test. From an analysis of the electromagnetic transition strengths and quadrupole moments predicted within the shell model, it is shown that the onset of shape coexistence and the emergence of a simple collective structure appear suddenly in ^{43}S with no indication of such patterns in the N=27 isotone ^{45}Ar.

Mapping of a New Deformation Region around ^{62}Ti

We performed the first direct mass measurements of neutron-rich scandium, titanium, and vanadium isotopes around the neutron number 40 at the RIKEN RI Beam Factory using the time-of-flight magnetic-rigidity technique. The atomic mass excesses of ^{58-60}Sc, ^{60-62}Ti, and ^{62-64}V were measured for the first time. The experimental results show that the two-neutron separation energies in the vicinity of ^{62}Ti increase compared to neighboring nuclei. This shows that the masses of Ti isotopes near N=40 are affected by the Jahn-Teller effect. Therefore, a development of Jahn-Teller stabilization appears below the Cr isotopes, and the systematics in Sc, Ti, and V isotopes suggest that ^{62}Ti is located close to the peak of the Jahn-Teller effect.

Quasifree Neutron Knockout from ^{54}Ca Corroborates Arising N=34 Neutron Magic Number

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.

Is the Structure of ^{42}Si Understood?

A more detailed test of the implementation of nuclear forces that drive shell evolution in the pivotal nucleus ^{42}Si-going beyond earlier comparisons of excited-state energies-is important. The two leading shell-model effective interactions, SDPF-MU and SDPF-U-Si, both of which reproduce the low-lying ^{42}Si(2_{1}^{+}) energy, but whose predictions for other observables differ significantly, are interrogated by the population of states in neutron-rich ^{42}Si with a one-proton removal reaction from ^{43}P projectiles at 81  MeV/nucleon. The measured cross sections to the individual ^{42}Si final states are compared to calculations that combine eikonal reaction dynamics with these shell-model nuclear structure overlaps. The differences in the two shell-model descriptions are examined and linked to predicted low-lying excited 0^{+} states and shape coexistence. Based on the present data, which are in better agreement with the SDPF-MU calculations, the state observed at 2150(13) keV in ^{42}Si is proposed to be the (0_{2}^{+}) level.

78Ni revealed as a doubly magic stronghold against nuclear deformation

Nuclear magic numbers correspond to fully occupied energy shells of protons or neutrons inside atomic nuclei. Doubly magic nuclei, with magic numbers for both protons and neutrons, are spherical and extremely rare across the nuclear landscape. Although the sequence of magic numbers is well established for stable nuclei, experimental evidence has revealed modifications for nuclei with a large asymmetry between proton and neutron numbers. Here we provide a spectroscopic study of the doubly magic nucleus ⁷⁸Ni, which contains fourteen neutrons more than the heaviest stable nickel isotope. We provide direct evidence of its doubly magic nature, which is also predicted by ab initio calculations based on chiral effective-field theory interactions and the quasi-particle random-phase approximation. Our results also indicate the breakdown of the neutron magic number 50 and proton magic number 28 beyond this stronghold, caused by a competing deformed structure. State-of-the-art phenomenological shell-model calculations reproduce this shape coexistence, predicting a rapid transition from spherical to deformed ground states, with ⁷⁸Ni as the turning point.

How Robust is the N=34 Subshell Closure? First Spectroscopy of ^{52}Ar

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.

Probing the nuclear structure in the vicinity of 78Ni

Theoretical and experimental studies of neutron-rich nuclei have shown that the general concept of shell structure is not as robust and universal as earlier thought, but can exhibit significant changes as a function of neutron excess. New magic numbers appear and some other conventional ones disappear mainly because of a different ordering of the single-particle orbitals. In the present contribution, recent experimental studies of neutron-rich Cu isotopes, performed at RIKEN using β decay and one-proton knockout reactions, will be discussed. Neutron-rich nuclei near 78Ni were populated through in-flight fission of 238U on thick 9Be targets in both experiments. In the β-decay study, 75,77Ni nuclei were implanted into the WAS3ABi silicon array, while γ rays from excited states in 75,77Cu emitted after β decay of the implanted ions were detected with the EURICA Ge detector array that was surrounding the active stopper. In a second experiment within the SEASTAR campaign at RIKEN, the same 75,77Cu nuclei were produced in (p,2p) knockout reactions from 76,78Zn beam particles at around 250 MeV/nucleon impinging onto the MINOS liquid hydrogen target. In the latter experiment the DALI2 NaI array was used to detect de-excitation γ rays measured in coincidence with Cu nuclei identified in the Zero Degree Spectrometer. Both studies are complimentary and greatly contribute to our understanding on the nuclear structure in the 78Ni region.

Magic Nature of Neutrons in ^{54}Ca: First Mass Measurements of ^{55-57}Ca

We perform the first direct mass measurements of neutron-rich calcium isotopes beyond neutron number 34 at the RIKEN Radioactive Isotope Beam Factory by using the time-of-flight magnetic-rigidity technique. The atomic mass excesses of ^{55-57}Ca are determined for the first time to be -18650(160), -13510(250), and -7370(990)  keV, respectively. We examine the emergence of neutron magicity at N=34 based on the new atomic masses. The new masses provide experimental evidence for the appearance of a sizable energy gap between the neutron 2p_{1/2} and 1f_{5/2} orbitals in ^{54}Ca, comparable to the gap between the neutron 2p_{3/2} and 2p_{1/2} orbitals in ^{52}Ca. For the ^{56}Ca nucleus, an open-shell property in neutrons is suggested.

Lifetime Measurements and Triple Coexisting Band Structure in ^{43}S

Lifetime measurements of excited states in the neutron-rich nucleus ^{43}S were performed by applying the recoil-distance method on fast rare-isotope beams in conjunction with the Gamma-Ray Energy Tracking In-beam Nuclear Array. The new data based on γγ coincidences and lifetime measurements resolve a doublet of (3/2^{-}) and (5/2^{-}) states at low excitation energies. Results were compared to the π(sd)-ν(pf) shell model and antisymmetrized molecular dynamics calculations. The consistency with the theoretical calculations identifies a possible appearance of three coexisting bands near the ground state of ^{43}S: the K^{π}=1/2^{-} band built on a prolate-deformed ground state, a band built on an isomer with a 1f_{7/2}^{-1} character, and a suggested excited band built on a newly discovered doublet state. The latter further confirms the collapse of the N=28 shell closure in the neutron-rich region.

Persistence of the Z=28 Shell Gap Around ^{78}Ni: First Spectroscopy of ^{79}Cu

In-beam γ-ray spectroscopy of ^{79}Cu is performed at the Radioactive Isotope Beam Factory of RIKEN. The nucleus of interest is produced through proton knockout from a ^{80}Zn beam at 270  MeV/nucleon. The level scheme up to 4.6 MeV is established for the first time and the results are compared to Monte Carlo shell-model calculations. We do not observe significant knockout feeding to the excited states below 2.2 MeV, which indicates that the Z=28 gap at N=50 remains large. The results show that the ^{79}Cu nucleus can be described in terms of a valence proton outside a ^{78}Ni core, implying the magic character of the latter.

Are There Signatures of Harmonic Oscillator Shells Far from Stability? First Spectroscopy of ^{110}Zr

The first measurement of the low-lying states of the neutron-rich ^{110}Zr and ^{112}Mo was performed via in-beam γ-ray spectroscopy after one proton removal on hydrogen at ∼200  MeV/nucleon. The 2_{1}^{+} excitation energies were found at 185(11) keV in ^{110}Zr, and 235(7) keV in ^{112}Mo, while the R_{42}=E(4_{1}^{+})/E(2_{1}^{+}) ratios are 3.1(2), close to the rigid rotor value, and 2.7(1), respectively. These results are compared to modern energy density functional based configuration mixing models using Gogny and Skyrme effective interactions. We conclude that first levels of ^{110}Zr exhibit a rotational behavior, in agreement with previous observations of lighter zirconium isotopes as well as with the most advanced Monte Carlo shell model predictions. The data, therefore, do not support a harmonic oscillator shell stabilization scenario at Z=40 and N=70. The present data also invalidate predictions for a tetrahedral ground state symmetry in ^{110}Zr.

Mass measurements demonstrate a strong N=28 shell gap in argon

We present results from recent time-of-flight nuclear mass measurements at the National Superconducting Cyclotron Laboratory at Michigan State University. We report the first mass measurements of ^{48}Ar and ^{49}Ar and find atomic mass excesses of -22.28(31)  MeV and -17.8(1.1)  MeV, respectively. These masses provide strong evidence for the closed shell nature of neutron number N=28 in argon, which is therefore the lowest even-Z element exhibiting the N=28 closed shell. The resulting trend in binding-energy differences, which probes the strength of the N=28 shell, compares favorably with shell-model calculations in the sd-pf shell using SDPF-U and SDPF-MU Hamiltonians.

β-Decay half-lives of 76,77Co, 79,80Ni, and 81Cu: experimental indication of a doubly magic 78Ni

The half-lives of 20 neutron-rich nuclei with Z=27-30 have been measured at the RIBF, including five new half-lives of (76)Co(21.7(-4.9)(+6.5) ms), (77)Co(13.0(-4.3)(+7.2) ms), (79)Ni(43.0(-7.5)(+8.6) ms), (80)Ni(23.9(-17.2)(+26.0) ms), and (81)Cu(73.2 ± 6.8 ms). In addition, the half-lives of (73-75)Co, (74-78)Ni, (78-80)Cu, and (80-82)Zn were determined with higher precision than previous works. Based on these new results, a systematic study of the β-decay half-lives has been carried out, which suggests a sizable magicity for both the proton number Z = 28 and the neutron number N=50 in (78)Ni.

Evidence for a new nuclear 'magic number' from the level structure of 54Ca

Atomic nuclei are finite quantum systems composed of two distinct types of fermion--protons and neutrons. In a manner similar to that of electrons orbiting in an atom, protons and neutrons in a nucleus form shell structures. In the case of stable, naturally occurring nuclei, large energy gaps exist between shells that fill completely when the proton or neutron number is equal to 2, 8, 20, 28, 50, 82 or 126 (ref. 1). Away from stability, however, these so-called 'magic numbers' are known to evolve in systems with a large imbalance of protons and neutrons. Although some of the standard shell closures can disappear, new ones are known to appear. Studies aiming to identify and understand such behaviour are of major importance in the field of experimental and theoretical nuclear physics. Here we report a spectroscopic study of the neutron-rich nucleus (54)Ca (a bound system composed of 20 protons and 34 neutrons) using proton knockout reactions involving fast radioactive projectiles. The results highlight the doubly magic nature of (54)Ca and provide direct experimental evidence for the onset of a sizable subshell closure at neutron number 34 in isotopes far from stability.

In-beam γ-ray spectroscopy of ^34,36,38Mg: merging the N=20 and N=28 shell quenching

Neutron-rich N=22, 24, 26 magnesium isotopes were studied via in-beam γ-ray spectroscopy at the RIKEN Radioactive Isotope Beam Factory following secondary fragmentation reactions on a carbon target at ≈200 MeV/nucleon. In the one- and two-proton removal channels from 39Al and 40Si beams, two distinct γ-ray transitions were observed in 38Mg, while in the one-proton removal reaction from 37Al a new transition was observed in addition to the known 2(1)(+)→0(g.s.)(+) decay. From the experimental systematics and comparison to theoretical predictions it is concluded that the transitions belong to the 2(1)(+)→0(g.s.)(+) and 4(1)(+)→2(1)(+) decays in 36Mg and 38Mg, respectively. For 34Mg, previously reported 2(1)(+) and 4(1)(+) level energies were remeasured. The deduced E(4(1)(+))/E(2(1)(+)) ratios for 34,36,38Mg of 3.14(5), 3.07(5), and 3.07(5) are almost identical and suggest the emergence of a large area of deformation extending from the N=20 to the N=28 shell quenching.

Isomers in 128Pd and 126Pd: evidence for a robust shell closure at the neutron magic number 82 in exotic palladium isotopes

The level structures of the very neutron-rich nuclei 128Pd and 126Pd have been investigated for the first time. In the r-process waiting-point nucleus 128Pd, a new isomer with a half-life of 5.8(8) μs is proposed to have a spin and parity of 8(+) and is associated with a maximally aligned configuration arising from the g(9/2) proton subshell with seniority υ=2. For 126Pd, two new isomers have been identified with half-lives of 0.33(4) and 0.44(3) μs. The yrast 2(+) energy is much higher in 128Pd than in 126Pd, while the level sequence below the 8(+) isomer in 128Pd is similar to that in the N=82 isotone 130Cd. The electric quadrupole transition that depopulates the 8(+) isomer in 128Pd is more hindered than the corresponding transition in 130Cd, as expected in the seniority scheme for a semimagic, spherical nucleus. These experimental findings indicate that the shell closure at the neutron number N=82 is fairly robust in the neutron-rich Pd isotopes.

Spins and magnetic moments of 49K and 51K: establishing the 1/2+ and 3/2+ level ordering beyond N = 28

The ground-state spins and magnetic moments of (49,51)K have been measured using bunched-beam high-resolution collinear laser spectroscopy at ISOLDE CERN. For 49K a ground-state spin I = 1/2 was firmly established. The observed hyperfine structure of 51K requires a spin I > 1/2 and strongly suggests I = 3/2. From its magnetic moment μ(51K) = +0.5129(22)μ(N) a spin-parity I(π) = 3/2+ with a dominant π1d(3/2)(-1) hole configuration was deduced. This establishes for the first time the reinversion of the single-particle levels and illustrates the prominent role of the residual monopole interaction for single-particle levels and shell evolution.

Correlations in intermediate energy two-proton removal reactions

We report final-state-exclusive measurements of the light charged fragments in coincidence with (26)Ne residual nuclei following the direct two-proton removal from a neutron-rich (28)Mg secondary beam. A Dalitz-plot analysis and comparisons with simulations show that a majority of the triple-coincidence events with two protons display phase-space correlations consistent with the (two-body) kinematics of a spatially correlated pair-removal mechanism. The fraction of such correlated events, 56(12)%, is consistent with the fraction of the calculated cross section, 64%, arising from spin S=0 two-proton configurations in the entrance-channel (shell-model) (28)Mg ground state wave function. This result promises access to an additional and more specific probe of the spin and spatial correlations of valence nucleon pairs in exotic nuclei produced as fast secondary beams.

Well developed deformation in 42Si

Excited states in (38,40,42) Si nuclei have been studied via in-beam γ-ray spectroscopy with multinucleon removal reactions. Intense radioactive beams of ^{40}S and (44)S provided at the new facility of the RIKEN Radioactive Isotope Beam Factory enabled γ-γ coincidence measurements. A prominent γ line observed with an energy of 742(8) keV in (42) Si confirms the 2(+) state reported in an earlier study. Among the γ lines observed in coincidence with the 2^{+} → 0+ transition, the most probable candidate for the transition from the yrast 4(+) state was identified, leading to a 4(1)+) energy of 2173(14) keV. The energy ratio of 2.93(5) between the 2(1)+ and 4(1)(+) states indicates well-developed deformation in (42) Si at N = 28 and Z = 14. Also for 38,40)Si energy ratios with values of 2.09(5) and 2.56(5) were obtained. Together with the ratio for (42)Si, the results show a rapid deformation development of Si isotopes from N = 24 to N = 28.

N = 16 spherical shell closure in 24O

The unbound excited states of the neutron drip-line isotope 24O have been investigated via the 24O(p,p')23O + n reaction in inverse kinematics at a beam energy of 62  MeV/nucleon. The decay energy spectrum of 24O* was reconstructed from the momenta of 23O and the neutron. The spin parity of the first excited state, observed at E(x) = 4.65±0.14  MeV, was determined to be J(π) = 2+ from the angular distribution of the cross section. Higher-lying states were also observed. The quadrupole transition parameter β2 of the 2(1)+ state was deduced, for the first time, to be 0.15±0.04. The relatively high excitation energy and small β2 value are indicative of the N = 16 shell closure in 24O.

Hindered proton collectivity in 16(28)S12: possible magic number at Z=16

The reduced transition probability B(E2;0(gs)(+)→2(1)(+)) for (28)S was obtained experimentally using Coulomb excitation at 53 MeV/nucleon. The resultant B(E2) value 181(31) e(2)fm(4) is smaller than the expectation based on empirical B(E2) systematics. The double ratio |M(n)/M(p)|/(N/Z) of the 0(gs)(+)→2(1)(+) transition in (28)S was determined to be 1.9(2) by evaluating the M(n) value from the known B(E2) value of the mirror nucleus (28)Mg, showing the hindrance of proton collectivity relative to that of neutrons. These results indicate the emergence of the magic number Z=16 in the |T(z)|=2 nucleus (28)S.

Quadrupole collectivity beyond N = 28: intermediate-energy Coulomb excitation of (47,48)Ar

We report on the first experimental study of quadrupole collectivity in the very neutron-rich nuclei (47,48)Ar using intermediate-energy Coulomb excitation. These nuclei are located along the path from doubly magic Ca to collective S and Si isotopes, a critical region of shell evolution and structural change. The deduced B(E2) transition strengths are confronted with large-scale shell-model calculations in the sdpf shell using the state-of-the-art SDPF-Uand EPQQM effective interactions. The comparison between experiment and theory indicates that a shell-model description of Ar isotopes around N=28 remains a challenge.

Is the 7/2(1)- isomer state of 43S spherical?

We report on the spectroscopic quadrupole moment measurement of the 7/2(1)(-) isomeric state in (16)(43)S(27) [E*=320.5(5) keV, T(1/2)=415(3) ns], using the time dependent perturbed angular distribution technique at the RIKEN RIBF facility. Our value, |Q(s)|=23(3) efm(2), is larger than that expected for a single-particle state. Shell model calculations using the modern SDPF-U interaction for this mass region reproduce remarkably well the measured |Q(s)|, and show that non-negligible correlations drive the isomeric state away from a purely spherical shape.

Prolate-spherical shape coexistence at N=28 in 44S

The structure of 44S has been studied by using delayed γ and electron spectroscopy. The decay rates of the 02+ isomeric state to the 2(1)+ and 0(1)+ states, measured for the first time, lead to a reduced transition probability B(E2: 2(1)+→0(2)+)=8.4(26) e(2) fm4 and a monopole strength ρ2(E0: 0(2)+→0(1)+)=8.7(7)×10(-3). Comparisons to shell model calculations point towards prolate-spherical shape coexistence, and a two-level mixing model is used to extract a weak mixing between the two configurations.

Magnetic dipole moment of (57,59)Cu measured by in-gas-cell laser spectroscopy

For the first time, in-gas-cell laser spectroscopy study of the (57,59,63,65)Cu isotopes has been performed using the 244.164 nm optical transition from the atomic ground state of copper. The nuclear magnetic dipole moments for (57,59,65)Cu relative to that of (63)Cu have been extracted. The new value for (57)Cu of mu((57)Cu) = +2.582(7)mu(N) is in strong disagreement with the previous literature value but in good agreement with recent theoretical and systematic predictions.

Lifetime measurements of the neutron-rich N = 30 isotones 50Ca and 51Sc: orbital dependence of effective charges in the fp shell

The lifetimes of the first excited states of the N = 30 isotones (50)Ca and (51)Sc have been determined using the Recoil Distance Doppler Shift method in combination with the CLARA-PRISMA spectrometers. This is the first time such a method is applied to measure lifetimes of neutron-rich nuclei populated via a multinucleon transfer reaction. This extends the lifetime knowledge beyond the f_{7/2} shell closure and allows us to derive the effective proton and neutron charges in the fp shell near the doubly magic nucleus (48)Ca, using large-scale, shell-model calculations. These results indicate an orbital dependence of the core polarization along the fp shell.

In-beam gamma-ray spectroscopy of very neutron-rich nuclei: excited states in 46S and 48Ar

We report on the first in-beam gamma-ray spectroscopy study of the very neutron-rich nucleus 46S. The N=30 isotones 46S and 48Ar were produced in a novel way in two steps that both necessarily involve nucleon exchange and neutron pickup reactions 9Be(48Ca,48K)X followed by 9Be(48K,48Ar+gamma)X at 85.7 MeV/u midtarget energy and 9Be(48Ca,46Cl)X followed by 9Be(46Cl,46S+gamma)X at 87.0 MeV/u midtarget energy, respectively. The results are compared to large-scale shell-model calculations in the sd-pf shell using the SDPF-NR effective interaction and Z-dependent modifications.

Structure of neutron-rich Ar isotopes beyond N = 28

Results from the gamma-ray spectroscopy of {47,48}Ar exemplifying new limits of sensitivity for characterizing neutron-rich nuclei at energies around the Coulomb barrier are presented. The present results, along with interacting shell model calculations, highlight the role of cross-shell excitations and indicate the presence of a nonaxial deformation in 48Ar.