| 1. |
- Vandebrouck, M., et al.
(författare)
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Effective proton-neutron interaction near the drip line from unbound states in F-25,F-26
- 2017
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Ingår i: Physical Review C. - 2469-9985 .- 2469-9993. ; 96:5
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Tidskriftsartikel (refereegranskat)abstract
- Background: Odd-odd nuclei, around doubly closed shells, have been extensively used to study proton-neutron interactions. However, the evolution of these interactions as a function of the binding energy, ultimately when nuclei become unbound, is poorly known. The F-26 nucleus, composed of a deeply bound pi 0d(5/2) proton and an unbound v0d(3/2) neutron on top of an O-24 core, is particularly adapted for this purpose. The coupling of this proton and neutron results in a J(pi) = 1(1)(+) - 4(1)(+) multiplet, whose energies must be determined to study the influence of the proximity of the continuum on the corresponding proton-neutron interaction. The J(pi) = 1(1)(+), 2(1)(+), 4(1)(+) bound states have been determined, and only a clear identification of the J(pi) = 3(1)(+) is missing. Purpose: We wish to complete the study of the J(pi) = 1(1)(+) - 4(1)(+) multiplet in F-26, by studying the energy and width of the J(pi) = 3(1)(+) unbound state. The method was first validated by the study of unbound states in F-25, for which resonances were already observed in a previous experiment. Method: Radioactive beams of Ne-26 and Ne-27, produced at about 440AMeV by the fragment separator at the GSI facility were used to populate unbound states in F-25 and F-26 via one-proton knockout reactions on a CH2 target, located at the object focal point of the (RB)-B-3/LAND setup. The detection of emitted. rays and neutrons, added to the reconstruction of the momentum vector of the A - 1 nuclei, allowed the determination of the energy of three unbound states in F-25 and two in F-26. Results: Based on its width and decay properties, the first unbound state in F-25, at the relative energy of 49(9) keV, is proposed to be a J(pi) = 1/ 2(-) arising from a p1/2 proton- hole state. In F-26, the first resonance at 323(33) keV is proposed to be the J(pi) = 3(1)(+) member of the J(pi) = 1(1)(+) - 4(1)(+) multiplet. Energies of observed states in F-25,F-26 have been compared to calculations using the independent-particle shell model, a phenomenological shell model, and the ab initio valence-space in-medium similarity renormalization group method. Conclusions: The deduced effective proton- neutron interaction is weakened by about 30-40% in comparison to the models, pointing to the need for implementing the role of the continuum in theoretical descriptions or to a wrong determination of the atomic mass of F-26.
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| 2. |
- Boillos, J. M., et al.
(författare)
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Isotopic cross sections of fragmentation residues produced by light projectiles on carbon near
- 2022
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Ingår i: Physical Review C. - 2469-9993 .- 2469-9985. ; 105:1
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Tidskriftsartikel (refereegranskat)abstract
- We measured 135 cross sections of residual nuclei produced in fragmentation reactions of C12, N14, and O13−16,20,22 projectiles impinging on a carbon target at kinetic energies of near 400A MeV, most of them for the first time, with the RB3/LAND setup at the GSI facility in Darmstadt (Germany). The use of this state-of-the-art experimental setup in combination with the inverse kinematics technique gave the full identification in atomic and mass numbers of fragmentation residues with a high precision. The cross sections of these residues were determined with uncertainties below 20% for most of the cases. These data are compared to other previous measurements with stable isotopes and are also used to benchmark different model calculations.
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| 3. |
- Atar, L., et al.
(författare)
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Quasifree (p, 2p) Reactions on Oxygen Isotopes: Observation of Isospin Independence of the Reduced Single-Particle Strength
- 2018
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Ingår i: Physical Review Letters. - 1079-7114 .- 0031-9007. ; 120:5
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Tidskriftsartikel (refereegranskat)abstract
- 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 R3B/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. Inclusive cross sections for quasifree knockout reactions of the type OA(p,2p)NA-1 have been determined and compared to calculations based on the eikonal reaction theory. The reduction factors for the single-particle strength with respect to the independent-particle model were obtained and compared to state-of-the-art ab initio predictions. The results do not show any significant dependence on proton-neutron asymmetry.
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| 4. |
- Holl, Matthias, 1986, et al.
(författare)
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Border of the island of inversion: Unbound states in Ne-29
- 2022
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Ingår i: Physical Review C. - 2469-9985 .- 2469-9993. ; 105:3
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Tidskriftsartikel (refereegranskat)abstract
- The nucleus Ne-29 is situated at the border of the island of inversion. Despite significant efforts, no bound low-lying intruder f(7/2) state, which would place Ne-29 firmly inside the island of inversion, has yet been observed. Here, the first investigation of unbound states of Ne-29 is reported. The states were populated in Ne-30(p, pn) and Na-30(p, 2p) reactions at a beam energy of around 230 MeV/nucleon, and analyzed in terms of their resonance properties, partial cross sections, and momentum distributions. The momentum distributions are compared to calculations using the eikonal, direct reaction model, allowing assignments for the observed states. The lowest lying resonance at an excitation energy of 1.48(4) MeV shows clear signs of a significant l = 3 component, giving first evidence for f(7/2) single particle strength in Ne-29. The excitation energies and strengths of the observed states are compared to shell-model calculations using the SDPF-U-MIX interaction.
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| 5. |
- Revel, A., et al.
(författare)
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Extending the Southern Shore of the Island of Inversion to F-28
- 2020
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Ingår i: Physical Review Letters. - 1079-7114 .- 0031-9007. ; 124:15
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Tidskriftsartikel (refereegranskat)abstract
- Detailed spectroscopy of the neutron-unbound nucleus F-28 has been performed for the first time following proton/neutron removal from Ne-29/F-29 beams at energies around 230 MeV=nucleon. The invariant-mass spectra were reconstructed for both the F-27((*)) + n and F-26((*)) + 2n coincidences and revealed a series of well-defined resonances. A near-threshold state was observed in both reactions and is identified as the F-28 ground state, with S-n(F-28) = -199(6) keV, while analysis of the 2n decay channel allowed a considerably improved S-n(F-27) = 1620(60) keV to be deduced. Comparison with shell-model predictions and eikonal-model reaction calculations have allowed spin-parity assignments to be proposed for some of the lower-lying levels of F-28. Importantly, in the case of the ground state, the reconstructed F-27 + n momentum distribution following neutron removal from F-29 indicates that it arises mainly from the 1p(3/2) neutron intruder configuration. This demonstrates that the island of inversion around N = 20 includes F-28, and most probably F-29, and suggests that O-28 is not doubly magic.
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| 6. |
- Revel, A., et al.
(författare)
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Strong Neutron Pairing in core+4n Nuclei
- 2018
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Ingår i: Physical Review Letters. - 1079-7114 .- 0031-9007. ; 120:15
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Tidskriftsartikel (refereegranskat)abstract
- The emission of neutron pairs from the neutron-rich N=12 isotones C18 and O20 has been studied by high-energy nucleon knockout from N19 and O21 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 N19(-1p)C18∗→C16+n+n is clearly dominated by direct pair emission. The two-neutron correlation strength, the largest ever observed, suggests the predominance of a C14 core surrounded by four valence neutrons arranged in strongly correlated pairs. On the other hand, a significant competition of a sequential branch is found in the decay O21(-1n)O20∗→O18+n+n, attributed to its formation through the knockout of a deeply bound neutron that breaks the O16 core and reduces the number of pairs.
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| 7. |
- Wang, H., et al.
(författare)
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Intruder configurations in 29 Ne at the transition into the island of inversion: Detailed structure study of 28 Ne
- 2023
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Ingår i: Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics. - 0370-2693. ; 843
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Tidskriftsartikel (refereegranskat)abstract
- Detailed γ-ray spectroscopy of the exotic neon isotope 28Ne has been performed for the first time using the one-neutron removal reaction from 29Ne on a liquid hydrogen target at 240 MeV/nucleon. Based on an analysis of parallel momentum distributions, a level scheme with spin-parity assignments has been constructed for 28Ne and the negative-parity states are identified for the first time. The measured partial cross sections and momentum distributions reveal a significant intruder p-wave strength providing evidence of the breakdown of the N=20 and N=28 shell gaps. Only a weak, possible f-wave strength was observed to bound final states. Large-scale shell-model calculations with different effective interactions do not reproduce the large p-wave and small f-wave strength observed experimentally, indicating an ongoing challenge for a complete theoretical description of the transition into the island of inversion along the Ne isotopic chain.
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| 8. |
- Boretzky, K., et al.
(författare)
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NeuLAND: The high-resolution neutron time-of-flight spectrometer for R 3 B at FAIR
- 2021
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Ingår i: Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment. - : Elsevier BV. - 0168-9002. ; 1014
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Tidskriftsartikel (refereegranskat)abstract
- NeuLAND (New Large-Area Neutron Detector) is the next-generation neutron detector for the R3B (Reactions with Relativistic Radioactive Beams) experiment at FAIR (Facility for Antiproton and Ion Research). NeuLAND detects neutrons with energies from 100 to 1000 MeV, featuring a high detection efficiency, a high spatial and time resolution, and a large multi-neutron reconstruction efficiency. This is achieved by a highly granular design of organic scintillators: 3000 individual submodules with a size of 5 × 5 × 250 cm3 are arranged in 30 double planes with 100 submodules each, providing an active area of 250 × 250 cm2 and a total depth of 3 m. The spatial resolution due to the granularity together with a time resolution of σt≤ 150 ps ensures high-resolution capabilities. In conjunction with calorimetric properties, a multi-neutron reconstruction efficiency of 50% to 70% for four-neutron events will be achieved, depending on both the emission scenario and the boundary conditions allowed for the reconstruction method. We present in this paper the final design of the detector as well as results from test measurements and simulations on which this design is based.
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| 9. |
- Kondo, Y., et al.
(författare)
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First observation of 28 O
- 2023
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Ingår i: Nature. - 0028-0836 .- 1476-4687. ; 620:7976, s. 965-970
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Tidskriftsartikel (refereegranskat)abstract
- 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 10−21s), provide the most stringent tests of modern nuclear-structure theories. Here we report on the first observation of 28O and 27O through their decay into 24O and four and three neutrons, respectively. The 28O nucleus is of particular interest as, with the Z = 8 and N = 20 magic numbers1,2, it is expected in the standard shell-model picture of nuclear structure to be one of a relatively small number of so-called ‘doubly magic’ nuclei. Both 27O and 28O were found to exist as narrow, low-lying resonances and their decay energies are compared here to the results of sophisticated theoretical modelling, including a large-scale shell-model calculation and a newly developed statistical approach. In both cases, the underlying nuclear interactions were derived from effective field theories of quantum chromodynamics. Finally, it is shown that the cross-section for the production of 28O from a 29F beam is consistent with it not exhibiting a closed N = 20 shell structure.
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| 10. |
- Browne, F., et al.
(författare)
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Pairing Forces Govern Population of Doubly Magic Ca-54 from Direct Reactions
- 2021
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Ingår i: Physical Review Letters. - : American Physical Society (APS). - 0031-9007 .- 1079-7114. ; 126:25
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Tidskriftsartikel (refereegranskat)abstract
- Direct proton-knockout reactions of Sc-55 at similar to 220 MeV/nucleon were studied at the RIKEN Radioactive Isotope Beam Factory. Populated states of Ca-54 were investigated through -ray and invariant-mass spectroscopy. Level energies were calculated from the nuclear shell model employing a phenomenological intemucleon 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 Sc-55 ground state with states in Ca-54. Despite the calculations showing a significant amplitude of excited neutron configurations in the ground-state of Sc-55, valence proton removals populated predominantly the ground state of Ca-54. 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.
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| 11. |
- Storck, Sonja, et al.
(författare)
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Lifetime measurement of the 26 0 g.s. At SAMURAI
- 2020
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Ingår i: Journal of Physics: Conference Series. - : IOP Publishing. - 1742-6588 .- 1742-6596. ; 1643:1
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Konferensbidrag (refereegranskat)abstract
- The ground state of the neutron unbound nucleus O is speculated to have a lifetime in the pico-second regime. In order to determine the decay lifetime of the 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 F beam was produced in the fragment separator BigRIPS and impinged on a W/Pt target stack where O was produced. According to the lifetime, the decay of O happens either in or outside the target. Thus, the velocity difference between the decay neutrons and the fragment O delivers a characteristic spectrum from which the lifetime can be extracted.
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