Magicity versus Superfluidity around ^{28}O viewed from the Study of ^{30}F.

The neutron-rich unbound fluorine isotope ^{30}F_{21} has been observed for the first time by measuring its neutron decay at the SAMURAI spectrometer (RIBF, RIKEN) in the quasifree proton knockout reaction of ^{31}Ne nuclei at 235  MeV/nucleon. The mass and thus one-neutron-separation energy of ^{30}F has been determined to be S_{n}=-472±58(stat)±33(sys)  keV from the measurement of its invariant-mass spectrum. The absence of a sharp drop in S_{n}(^{30}F) shows that the "magic" N=20 shell gap is not restored close to ^{28}O, which is in agreement with our shell-model calculations that predict a near degeneracy between the neutron d and fp orbitals, with the 1p_{3/2} and 1p_{1/2} orbitals becoming more bound than the 0f_{7/2} one.

First observation of O.

Subjecting a physical system to extreme conditions is one of the means often used to obtain a better understanding and deeper insight into its organization and structure. In the case of the atomic nucleus, one such approach is to investigate isotopes that have very different neutron-to-proton (N/Z) ratios than in stable nuclei. Light, neutron-rich isotopes exhibit the most asymmetric N/Z ratios and those lying beyond the limits of binding, which undergo spontaneous neutron emission and exist only as very short-lived resonances (about 10 s), provide the most stringent tests of modern nuclear-structure theories.

Observation of a correlated free four-neutron system.

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, with a particular focus on the four-neutron system called the tetraneutron, resulting in only a few indications of its existence so far, leaving the tetraneutron an elusive nuclear system for six decades.

Probing the Symmetry Energy with the Spectral Pion Ratio.

Many neutron star properties, such as the proton fraction, reflect the symmetry energy contributions to the equation of state that dominate when neutron and proton densities differ strongly. To constrain these contributions at suprasaturation densities, we measure the spectra of charged pions produced by colliding rare isotope tin (Sn) beams with isotopically enriched Sn targets. Using ratios of the charged pion spectra measured at high transverse momenta, we deduce the slope of the symmetry energy to be 42 View Article and Find Full Text PDF

Formation of α clusters in dilute neutron-rich matter.

The surface of neutron-rich heavy nuclei, with a neutron skin created by excess neutrons, provides an important terrestrial model system to study dilute neutron-rich matter. By using quasi-free α cluster-knockout reactions, we obtained direct experimental evidence for the formation of α clusters at the surface of neutron-rich tin isotopes. The observed monotonous decrease of the reaction cross sections with increasing mass number, in excellent agreement with the theoretical prediction, implies a tight interplay between α-cluster formation and the neutron skin.

Extending the Southern Shore of the Island of Inversion to ^{28}F.

Detailed spectroscopy of the neutron-unbound nucleus ^{28}F has been performed for the first time following proton/neutron removal from ^{29}Ne/^{29}F beams at energies around 230  MeV/nucleon. The invariant-mass spectra were reconstructed for both the ^{27}F^{(*)}+n and ^{26}F^{(*)}+2n coincidences and revealed a series of well-defined resonances. A near-threshold state was observed in both reactions and is identified as the ^{28}F ground state, with S_{n}(^{28}F)=-199(6)  keV, while analysis of the 2n decay channel allowed a considerably improved S_{n}(^{27}F)=1620(60)  keV to be deduced.

Fragmentation of Single-Particle Strength around the Doubly Magic Nucleus ^{132}Sn and the Position of the 0f_{5/2} Proton-Hole State in ^{131}In.

Spectroscopic factors of neutron-hole and proton-hole states in ^{131}Sn and ^{131}In, respectively, were measured using one-nucleon removal reactions from doubly magic ^{132}Sn at relativistic energies. For ^{131}In, a 2910(50)-keV γ ray was observed for the first time and tentatively assigned to a decay from a 5/2^{-} state at 3275(50) keV to the known 1/2^{-} level at 365 keV. The spectroscopic factors determined for this new excited state and three other single-hole states provide first evidence for a strong fragmentation of single-hole strength in ^{131}Sn and ^{131}In.

Strong Neutron Pairing in core+4n Nuclei.

The emission of neutron pairs from the neutron-rich N=12 isotones ^{18}C and ^{20}O has been studied by high-energy nucleon knockout from ^{19}N and ^{21}O secondary beams, populating unbound states of the two isotones up to 15 MeV above their two-neutron emission thresholds. The analysis of triple fragment-n-n correlations shows that the decay ^{19}N(-1p)^{18}C^{*}→^{16}C+n+n is clearly dominated by direct pair emission. The two-neutron correlation strength, the largest ever observed, suggests the predominance of a ^{14}C core surrounded by four valence neutrons arranged in strongly correlated pairs.

Quasifree (p, 2p) Reactions on Oxygen Isotopes: Observation of Isospin Independence of the Reduced Single-Particle Strength.

Quasifree one-proton knockout reactions have been employed in inverse kinematics for a systematic study of the structure of stable and exotic oxygen isotopes at the R^{3}B/LAND setup with incident beam energies in the range of 300-450  MeV/u. The oxygen isotopic chain offers a large variation of separation energies that allows for a quantitative understanding of single-particle strength with changing isospin asymmetry. Quasifree knockout reactions provide a complementary approach to intermediate-energy one-nucleon removal reactions.

Observation of the competitive double-gamma nuclear decay.

The double-gamma (γγ)-decay of a quantum system in an excited state is a fundamental second-order process of quantum electrodynamics. In contrast to the well-known single-gamma (γ)-decay, the γγ-decay is characterized by the simultaneous emission of two γ quanta, each with a continuous energy spectrum. In nuclear physics, this exotic decay mode has only been observed for transitions between states with spin-parity quantum numbers J(π) = 0(+) (refs 1-3).

Measurement of the dipole polarizability of the unstable neutron-rich nucleus 68Ni.

The E1 strength distribution in 68Ni has been investigated using Coulomb excitation in inverse kinematics at the R3B-LAND setup and by measuring the invariant mass in the one- and two-neutron decay channels. The giant dipole resonance and a low-lying peak (pygmy dipole resonance) have been observed at 17.1(2) and 9.

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.

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.

Structural evolution in the neutron-rich nuclei ¹⁰⁶Zr and ¹⁰⁸Zr.

The low-lying states in ¹⁰⁶Zr and ¹⁰⁸Zr have been investigated by means of β-γ and isomer spectroscopy at the radioactive isotope beam factory (RIBF), respectively. A new isomer with a half-life of 620 ± 150 ns has been identified in ¹⁰⁸Zr. For the sequence of even-even Zr isotopes, the excitation energies of the first 2⁺ states reach a minimum at N = 64 and gradually increase as the neutron number increases up to N = 68, suggesting a deformed subshell closure at N = 64.

β-decay half-lives of very neutron-rich Kr to Tc isotopes on the boundary of the r-process path: an indication of fast r-matter flow.

The β-decay half-lives of 38 neutron-rich isotopes from (36)Kr to (43)Tc have been measured; the half-lives of (100)Kr, (103-105)Sr, (106-108)Y, (108-110)Zr, (111,112)Nb, (112-115)Mo, and (116,117)Tc are reported here. The results when compared with previous standard models indicate an overestimation in the predicted half-lives by a factor of 2 or more in the A≈110 region. A revised model based on the second generation gross theory of β decay better predicts the measured half-lives and suggests a more rapid flow of the rapid neutron-capture process (r-matter flow) through this region than previously predicted.

Spectroscopy of 32Ne and the "Island of Inversion".

We report on the first spectroscopic study of the N=22 nucleus 32Ne at the newly completed RIKEN Radioactive Ion Beam Factory. A single gamma-ray line with an energy of 722(9) keV was observed in both inelastic scattering of a 226 MeV/u 32Ne beam on a carbon target and proton removal from 33Na at 245 MeV/u. This transition is assigned to the deexcitation of the first Jpi=2+ state in 32Ne to the 0+ ground state.

0(gs)+ -->2(1)+ transition strengths in 106Sn and 108Sn.

The reduced transition probabilities, B(E2; 0(gs)+ -->2(1)+), have been measured in the radioactive isotopes (108,106)Sn using subbarrier Coulomb excitation at the REX-ISOLDE facility at CERN. Deexcitation gamma rays were detected by the highly segmented MINIBALL Ge-detector array. The results, B(E2;0(gs)+ -->2(1)+)=0.

Coulomb excitation of neutron-rich Zn isotopes: first observation of the 2(1)+ state in 80Zn.

Neutron-rich, radioactive Zn isotopes were investigated at the Radioactive Ion Beam facility REX-ISOLDE (CERN) using low-energy Coulomb excitation. The energy of the 2(1)+ state in 78Zn could be firmly established and for the first time the 2+ --> 0(1)+ transition in 80Zn was observed at 1492(1) keV. B(E2,2(1)+ --> 0(1)+) values were extracted for (74,76,78,80)Zn and compared to large scale shell model calculations.

Selective population and neutron decay of an excited state of 23O.

We have observed a resonance in neutron-fragment coincidence measurements that is presumably the first excited state of 23O at 2.8(1) MeV excitation energy which decays into the ground state of 22O. This interpretation is consistent with theory.

Bremsstrahlung in alpha decay reexamined.

A high-statistics measurement of bremsstrahlung emitted in the alpha decay of (210)Po has been performed, which allows us to follow the photon spectra up to energies of approximately 500 keV. The measured differential emission probability is in good agreement with our theoretical results obtained within the quasiclassical approximation as well as with the exact quantum mechanical calculation. It is shown that, due to the small effective electric dipole charge of the radiating system, a significant interference between the electric dipole and quadrupole contributions occurs, which is altering substantially the angular correlation between the alpha particle and the emitted photon.

Coulomb excitation of 68,70Cu: first use of postaccelerated isomeric beams.

We report on the first low-energy Coulomb excitation measurements with radioactive Ipi=6- beams of odd-odd nuclei 68,70Cu. The beams were produced at ISOLDE, CERN and were post-accelerated by REX-ISOLDE to 2.83 MeV/nucleon.

Measurement of the sign of the spectroscopic quadrupole moment for the 2(1)+ state in 70Se: no evidence for oblate shape.

Using a method whereby molecular and atomic ions are independently selected, an isobarically pure beam of 70Se ions was postaccelerated to an energy of 206 MeV using REX-ISOLDE. Coulomb-excitation yields for states in the beam and target nuclei were deduced by recording deexcitation gamma rays in the highly segmented MINIBALL gamma-ray spectrometer in coincidence with scattered particles in a silicon detector. At these energies, the Coulomb-excitation yield for the first 2+ state is expected to be strongly sensitive to the sign of the spectroscopic quadrupole moment through the nuclear reorientation effect.

"Safe" Coulomb excitation of 30Mg.

We report on the first radioactive beam experiment performed at the recently commissioned REX-ISOLDE facility at CERN in conjunction with the highly efficient gamma spectrometer MINIBALL. Using 30Mg ions accelerated to an energy of 2.25 MeV/u together with a thin (nat)Ni target, Coulomb excitation of the first excited 2+ states of the projectile and target nuclei well below the Coulomb barrier was observed.

Shell structure of the near-dripline nucleus 23O.

Breakup reactions were used to study the ground-state configuration of the neutron-rich isotope 23O. The 22O fragments produced in one-nucleon removal from 23O at 938 MeV/nucleon in a carbon target were detected in coincidence with deexciting gamma rays. The widths of the longitudinal momentum distributions of the 22O fragments and the one-neutron removal cross sections were interpreted in the framework of a simple theoretical model which favors the assignment of Ipi = 1/2+ to the 23O ground state.