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The so-called “island of inversion” is a region in the nuclear landscape where shell-structure changes are observed and in particular the magic neutron number at N=20 vanishes. For those nuclei at Z=10−12 and around N=20, the shell gap at N=20 quenches and pf-shell intruder configurations become important. We address the question how strong such configurations are for very neutron-rich but Z=9 fluorine isotopes. Such exotic nuclei are produced at the radioactive-ion beam factory (Japan) at beam energies around 250 MeV/u. 29F∗ & 30F are studied in inverse kinematics at the SAMURAI experimental setup by (p,2p) reactions on neon isotopes. The two and one neutron-unbound states, respectively, are investigated in terms of invariant-mass spectroscopy where the decay neutrons are measured explicitly. The resulting excitation-energy spectra are compared to different shell-model based calculations. Moreover, 29F∗ shows a strong two-neutron sequential decay that is also analyzed by means of Jacobi coordinates.
This work is supported by the DFG through grant no. SFB 1245, the BMBF under contract no. 05P15RDFN1, and the GSI-TU Darmstadt cooperation agreement.
The magicity of N=20 in the vicinity of Ne, Na, and Mg isotopes vanishes due to pf-shell intruder configuration, which is called 'island of inversion' [1-3]. In recent years, shell evolution of Ne isotopes in this region is emerging topic of interest [4,5]. Nevertheless, there is no observed state of $^{33}$Ne.
It is only known that $^{33}$Ne is unbound nuclei [6] and the 1n separation energy $S_{n}$ is only predicted to -0.9 MeV [7].
The experiment was carried out at the RIBF in RIKEN. The secondary beam of $^{34}$Na at 264 MeV/nucleon was provided by BigRIPS [8] and impinged on the carbon reaction target. After the one-proton knockout reaction of $^{34}$Na, $^{33}$Ne was produced and immediately decayed into $^{32}$Ne and a neutron. The invariant mass spectrum of $^{32}$Ne + n system was reconstructed by measurement of fragments and neutrons using SAMURAI spectrometer [9]. In this presentation, details of analysis and preliminary results of $^{32}$Ne + n invariant mass spectrum will be discussed.
[1] Z. Elekes et al., Phys. Rev. C 73, 044314 (2006).
[2] P. Doornenbal et al., Phys. Rev. Lett. 103, 032501 (2009).
[3] P. Doornenbal et al., Phys. Rev. C 81, 041305 (2010).
[4] T. Nakamura et al., Phys. Rev. Lett. 112, 142501 (2014).
[5] I. Murray et al., Phys. Rev. C 99, 011302 (2019).
[6] M. Notani et al., Phys. Lett. B 542, 49 (2002).
[7] G. Audi, H. Wapstra, and C. Thibault, Nucl. Phys. A 729, 337 (2003).
[8] T. Kubo, Nucl. Inst. and Meth. B 204, 97 (2003).
[9] T. Kobayashi et al., Nucl. Inst. and Meth. B 317, 294 (2013).
A recent experiment suggests that the ground state of the neutron-unbound nucleus $^\text{26}$O could have a lifetime in the pico-second regime. This would constitute the first case of a radioactive
decay via neutron emission, if this value can be confirmed.
In Dezember 2016, the S20 experiment using a new measurement method to determine the decay lifetime
of the $^\text{26}$O ground state with high sensitivity and precision was performed at SAMURAI. Here, a $^\text{27}$F beam was produced in the fragment separator BigRIPS and impinged on a W/Pt target stack where $^\text{26}$O was produced. According to the lifetime, the decay of $^\text{26}$O happens either in- or outside the target. Thus, the velocity difference between the decay neutrons and the fragment $^\text{24}$O delivers a characteristic spectrum from which the lifetime can be extracted.
The current analysis status will be reported.
Details will be given on the current status of the analysis of the 8He(p, p alpha)4n experiment.
A study of unbound excited states in $^{17}$C through one-neutron knockout of $^{18}$C at the energy of 245 MeV/nucleon on a carbon target was performed using the SAMURAI spectrometer. Relative energy spectrum of unbound $^{17}$C was reconstructed from momentum vectors of $^{16}$C fragments and neutrons. The relative energy spectrum was characterized by six resonances at $E_{\rm rel}$ = 0.54, 0.81, 1.41, 1.92, 2.30, and 3.22 MeV. Three of them at $E_{\rm rel}$ = 0.54, 1.41, and 2.30 MeV were identified to be in coincidence with $^{16}$C(2$^+_1$), while others have no coincidence with that.
Orbital angular momenta of two resonances at $E_{\rm rel}$ = 1.92 and 3.22 MeV were determined as 1 by momentum distributions. The resonance at $E_{\rm rel}$ = 0.81 MeV, assigned as 5/2$^+_2$, was newly observed in the present work. With regard to the resonances having the coincidence with $^{16}$C(2$^+_1$), decay properties of candidate states were examined by branching ratio and shell-model calculations, and spin-parities of them were tentatively assigned. From the present study, it turned out that the YSOX shell-model interaction, involving tensor force for $p$-$sd$ cross-shell part, provides a good account of the observation. In the presentation, the results and detailed interpretation will be shown.
Alpha clustering is the key cornerstone for the complete understanding of the structure of nuclei and fundamental nuclear interactions. So far alpha-particle clustering has dominated cluster states studies among all other possible partitioning. While it is known for long as an important feature of stable N=Z nuclei [1], its existence in exotic nuclei with large imbalance of proton and neutron number is still a question. Neutron-excess Beryllium isotopes 10Be,12Be,14Be are the very appealing candidates of clustering studies as being built on the well-developed alpha-alpha rotor of 8Be (N=4, Z=4). It is predicted by the recent antisymmetrized molecular dynamics model (AMD) that strong degree of alpha clustering in the ground-state remain high from 10Be up to the dripline 14Be [2].
The SAMURAI12 experiment performed at Radioactive Isotope Beam Factory (RIBF) in RIKEN aims to investigate the cluster structure of neutron-rich beryllium isotopes using the cluster quasifree scattering reaction (p,pa) in inverse kinematics. The reactions of interest were induced by beams of 10,12,14Be isotopes at 150MeV/u impinging on a pure large diameter 2 mm thick solid hydrogen target. The detection of Helium residues was performed by using the SAMURAI spectrometer and its standard detectors. ESPRI Recoil Proton Spectrometer (RPS) was implemented for recoil proton detection, covering an angular range of 50°-70°. For detection of alpha clusters, two telescopes composed of Silicon and CsI(Tl) detectors was placed at forward angles to cover the angular range 4°-12°. Their cross sections and momentum distributions allow us to probe the alpha cluster structures directly and quantitatively. In this talk, the status of data analysis will be presented.
I will give a status report on the analysis of the SAMURAI09 experiment.
In July 2017, the SAMURAI37 experiment was performed with the purpose of measuring the multi-neutron decay of $^6$He and $^8$He after heavy-ion-induced electromagnetic excitation in complete kinematics to study the dipole response of these nuclei.
The combination of the neutron detectors NEBULA and NeuLAND at the SAMURAI setup and the high beam intensities available at RIBF made the measurement of the dipole response of $^8$He possible for the first time. The experimental method is based on the measurement of the differential cross section via the invariant-mass method, which allows to extract the dipole strength distribution dB(E1)/dE and the photo-absorption cross section. To induce electromagnetic excitation reactions of $^6$He and $^8$He a lead target was used.
In the talk the status of the ongoing analysis is presented.