arXiv subjects

T. Isobe

Publications and source records attributed to T. Isobe.

At least 55 records · Page 3Linked to original sources

arXiv2020

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. 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 $^{28}$F. Importantly, in the case of the ground state, the reconstructed $^{27}$F$+n$ momentum distribution following neutron removal from $^{29}$F indicates that it arises mainly from the $1p_{3/2}$ neutron intruder configuration. This demonstrates that the island of inversion around $N=20$ includes $^{28}$F, and most probably $^{29}$F, and suggests that $^{28}$O is not doubly magic.

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arXiv2020

$β$-Decay Half-Lives of 55 Neutron-Rich Isotopes beyond the N=82 Shell Gap

The $β$-decay half-lives of 55 neutron-rich nuclei $^{134-139}$Sn, $^{134-142}$Sb, $^{137-144}$Te, $^{140-146}$I, $^{142-148}$Xe, $^{145-151}$Cs, $^{148-153}$Ba, $^{151-155}$La were measured at the Radioactive Isotope Beam Factory (RIBF) employing the projectile fission fragments of $^{238}$U. The nuclear level structure, which relates to deformation, has a large effect on the half-lives. The impact of newly-measured half-lives on modeling the astrophysical origin of the heavy elements is studied in the context of $r$ process nucleosynthesis. For a wide variety of astrophysical conditions, including those in which fission recycling occurs, the half-lives have an important local impact on the second ($A$ $\approx$ 130) peak.

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arXiv2020

Space Charge Effects in the S$π$RIT Time Projection Chamber

Time projection chambers (TPCs) are widely used in nuclear and particle physics. They are particularly useful when measuring reaction products from heavy ion collisions. Most nuclear experiments at low energy are performed in a fixed target configuration, in which the unreacted beam will pass through the detection volume. As the beam intensity increases, the buildup of positive ions created from the ionization of the detector gas by the beam creates the main source of space charge, distorting the nominal electric field of the TPC. This has a profound effect on the accuracy of the measured momenta of the emitted particles. In this paper we will discuss the magnitude of the effects and construct an observable more appropriate for fixed target experiments to study the effects. We also will present an algorithm for correcting the space charge and some of the implications it has on the momentum determination.

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arXiv2020

Charged particle track reconstruction with S$π$RIT Time Projection Chamber

In this paper, we present a software framework, S$π$RITROOT, which is capable of track reconstruction and analysis of heavy-ion collision events recorded with the S$π$RIT time projection chamber. The track-fitting toolkit GENFIT and the vertex reconstruction toolkit RAVE are applied to a box-type detector system. A pattern recognition algorithm which performs helix track finding and handles overlapping pulses is described. The performance of the software is investigated using experimental data obtained at the Radioactive Isotope Beam Facility (RIBF) at RIKEN. This work focuses on data from $^{132}$Sn + $^{124}$Sn collision events with beam energy of 270 AMeV. Particle identification is established using $\left $ and magnetic rigidity, with pions, hydrogen isotopes, and helium isotopes.

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arXiv2019

Shell evolution of $N=40$ isotones towards $^{60}$Ca: First spectroscopy of $^{62}$Ti

Excited states in the $N=40$ isotone $^{62}$Ti were populated via the $^{63}$V$(p,2p)$$^{62}$Ti reaction at $\sim$200~MeV/u at the Radioactive Isotope Beam Factory and studied using $γ$-ray spectroscopy. The energies of the $2^+_1 \rightarrow 0^{+}_{\mathrm{gs}}$ and $4^+_1 \rightarrow 2^+_1$ transitions, observed here for the first time, indicate a deformed $^{62}$Ti ground state. These energies are increased compared to the neighboring $^{64}$Cr and $^{66}$Fe 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 $^{62}$Ti 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 $^{60}$Ca.

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arXiv2019

$^{78}$Ni revealed as a doubly magic stronghold against nuclear deformation

Nuclear magic numbers, which emerge from the strong nuclear force based on quantum chromodynamics, 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. While the sequence of magic numbers is well established for stable nuclei, evidence reveals modifications for nuclei with a large proton-to-neutron asymmetry. Here, we provide the first spectroscopic study of the doubly magic nucleus $^{78}$Ni, fourteen neutrons beyond the last stable nickel isotope. We provide direct evidence for 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. However, our results also provide the first indication of 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 further a rapid transition from spherical to deformed ground states with $^{78}$Ni as turning point.

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arXiv2019

Structure of 13Be probed via quasi-free scattering

We present an investigation of the structure of 13Be obtained via a kinematically complete measurement of the (p; pn) reaction in inverse kinematics at 265 MeV/nucleon. The relative energy spectrum of 13Be is compared to Transfer-to-the-Continuum calculations which use as structure inputs the overlaps of the 14Be ground-state wave function, computed in a three-body model, with the unbound states of the 13Be residual nucleus. The key role of neutron p-wave orbital in the interpretation of the low-relative-energy part of the spectrum is discussed.

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arXiv2019

Study of multi-neutron systems with SAMURAI spectrometer

The tetraneutron has been drawing the attention of the nuclear physics community for decades, but a firm conclusion on its existence and properties is still far from being reached despite many experimental and theoretical efforts. New measurements have recently been performed at RIBF with the SAMURAI spectrometer by applying complementary reaction probes, which will help to pin down the properties of this four-neutron system.

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arXiv2019

First spectroscopy of 61Ti and the transition to the Island of Inversion at N = 40

Isomeric states in 59,61Ti have been populated in the projectile fragmentation of a 345 AMeV 238U beam at the Radioactive Isotope Beam Factory. The decay lifetimes and delayed gamma-ray transitions were measured with the EURICA array. Besides the known isomeric state in 59Ti, two isomeric states in 61Ti are observed for the first time. Based on the measured lifetimes, transition multipolarities as well as tentative spins and parities are assigned. Large-scale shell model calculations based on the modified LNPS interaction show that both 59Ti and 61Ti belong to the Island of Inversion at N=40 with ground state configurations dominated by particle-hole excitations to the g_9/2 and d_5/2 orbits.

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arXiv2019

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 $\sim$210 MeV/u at the RIBF facility. The 2$^{+}_{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.

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arXiv2019

First observation of 20B and 21B

The most neutron-rich boron isotopes 20B and 21B have been observed for the first time following proton removal from 22N and 22C at energies around 230 MeV/nucleon. Both nuclei were found to exist as resonances which were detected through their decay into 19B and one or two neutrons. Two-proton removal from 22N populated a prominent resonance-like structure in 20B at around 2.5 MeV above the one-neutron decay threshold, which is interpreted as arising from the closely spaced 1-,2- ground-state doublet predicted by the shell model. In the case of proton removal from 22C, the 19B plus one- and two-neutron channels were consistent with the population of a resonance in 21B 2.47+-0.19 MeV above the two-neutron decay threshold, which is found to exhibit direct two-neutron decay. The ground-state mass excesses determined for 20,21B are found to be in agreement with mass surface extrapolations derived within the latest atomic-mass evaluations.

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arXiv2018

Commissioning of the BRIKEN detector for the measurement of very exotic beta-delayed neutron emitters

A new detection system has been installed at the RIKEN Nishina Center (Japan) to investigate decay properties of very neutron-rich nuclei. The setup consists of three main parts: a moderated neutron counter, a detection system sensitive to the implantation and decay of radioactive ions, and gamma-ray detectors. We describe here the setup, the commissioning experiment and some selected results demonstrating its performance for the measurement of half-lives and beta-delayed neutron emission probabilities. The methodology followed in the analysis of the data is described in detail. Particular emphasis is placed on the correction of the accidental neutron background.

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arXiv2018

Is Seniority a Partial Dynamic Symmetry in the First $νg_{9/2}$ Shell?

The low-lying structures of the midshell $νg_{9/2}$ Ni isotopes $^{72}$Ni and $^{74}$Ni have been investigated at the RIBF facility in RIKEN within the EURICA collaboration. Previously unobserved low-lying states were accessed for the first time following $β$ decay of the mother nuclei $^{72}$Co and $^{74}$Co. As a result, we provide a complete picture in terms of the seniority scheme up to the first $(8^+)$ levels for both nuclei. The experimental results are compared to shell-model calculations in order to define to what extent the seniority quantum number is preserved in the first neutron $g_{9/2}$ shell. We find that the disappearance of the seniority isomerism in the $(8^+_1)$ states can be explained by a lowering of the seniority-four $(6^+)$ levels as predicted years ago. For $^{74}$Ni, the internal de-excitation pattern of the newly observed $(6^+_2)$ state supports a restoration of the normal seniority ordering up to spin $J=4$. This property, unexplained by the shell-model calculations, is in agreement with a dominance of the single-particle spherical regime near $^{78}$Ni.

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arXiv2018

Spectroscopy of nuclei around $^{100}$Sn populated via two-neutron knockout reactions

We report on the in-beam gamma spectroscopy of $^{102}$Sn and $^{100}$Cd produced via two-neutron removal from carbon and CH$_2$ targets at about 150 MeV/nucleon beam energy. New transitions assigned to the decay of a second 2$^+$ excited state at 2470(60) keV in $^{102}$Sn were observed. Two-neutron removal cross sections from $^{104}$Sn and $^{102}$Cd have been extracted. The enhanced cross section to the 2$^+_2$ in $^{102}$Sn populated via the $(p,p2n)$ reaction is traced back to an increase of shell-model structure overlaps, consistent with the hypothesis that the proton-induced two-deeply-bound-nucleon removal mechanism is of direct nature.

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arXiv2017

Simultaneous investigation of the $\mathbf{T=1~ (J^π=0^+)}$ and $\mathbf{T=0 ~ (J^π=9^+)}$ $β$ decays in $^{70}$Br

The $β$ decay of the odd-odd nucleus $^{70}$Br has been investigated with the BigRIPS and EURICA setups at the Radioactive Ion Beam Factory (RIBF) of the RIKEN Nishina Center. The $T=0$ ($J^π=9^+$) and $T=1$ ($J^π=0^+$) isomers have both been produced in in-flight fragmentation of $^{78}$Kr with ratios of 41.6(8)\% and 58.4(8)\%, respectively. A half-life of $t_{1/2}=2157^{+53}_{-49}$ ms has been measured for the $J^π=9^+$ isomer from $γ$-ray time decay analysis. Based on this result, we provide a new value of the half-life for the $J^π=0^+$ ground state of $^{70}$Br, $t_{1/2}=78.42\pm0.51$ ms, which is slightly more precise, and in excellent agreement, with the best measurement reported hitherto in the literature. For this decay, we provide the first estimate of the total branching fraction decaying through the $2^+_1$ state in the daughter nucleus $^{70}$Se, $R(2^+_1)=1.3\pm1.1\%$. We also report four new low-intensity $γ$-ray transitions at 661, 1103, 1561, and 1749 keV following the $β$ decay of the $J^π=9^+$ isomer. Based on their coincidence relationships, we tentatively propose two new excited states at 3945 and 4752 keV in $^{70}$Se with most probable spins and parities of $J^π=(6^+)$ and $(8^+)$, respectively. The observed structure is interpreted with the help of shell-model calculations, which predict a complex interplay between oblate and prolate configurations at low excitation energies.

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arXiv2017

Single-neutron knockout from $^{20}\textrm{C}$ and the structure of $^{19}\textrm{C}$

The low-lying unbound level structure of the halo nucleus $^{19}\textrm{C}$ has been investigated using single-neutron knockout from $^{20}\textrm{C}$ on a carbon target at 280 MeV/nucleon. The invariant mass spectrum, derived from the momenta of the forward going beam velocity $^{18}\textrm{C}$ fragment and neutrons, was found to be dominated by a very narrow near threshold ($E_\textrm{rel}$ = 0.036(1) MeV) peak. Two less strongly populated resonance-like features were also observed at $E_\textrm{rel}$ = 0.84(4) and 2.31(3) MeV, both of which exhibit characteristics consistent with neutron $p$-shell hole states. Comparisons of the energies, measured cross sections and parallel momentum distributions to the results of shell-model and eikonal reaction calculations lead to spin-parity assignments of $5/2^+_1$ and $1/2^-_1$ for the levels at $E_x$ = 0.62(9) and 2.89(10) MeV with $S_n$ = 0.58(9) MeV. Spectroscopic factors were also deduced and found to be in reasonable accord with shell-model calculations. The valence neutron configuration of the $^{20}\textrm{C}$ ground state is thus seen to include, in addition to the known $1s^2_{1/2}$ component, a significant $0d^2_{5/2}$ contribution. The level scheme of $^{19}\textrm{C}$, including significantly the $1/2^-_1$ cross-shell state, is well accounted for by the YSOX shell-model interaction developed from the monopole-based universal interaction.

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arXiv2017

Pion Production in Rare Isotope Collisions

Pion energy spectra are presented for central collisions of neutron-rich 132Sn+124Sn and neutron-deficient 108Sn+112Sn systems using simulations with Boltzmann-Uehling-Uhlenbeck transport model. These calculations, which incorporate isospin-dependent mean field potentials for relevant baryons and mesons, display a sensitivity to the pion spectra that could allow significant constraints on the density dependence of the symmetry energy and its mean field potential at supra-saturation densities. The predicted sensitivity increases with the isospin asymmetry of the total system and decreases with incident energy.

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arXiv2017

KATANA - a charge-sensitive triggering system for the S$π$RIT experiment

KATANA - the Krakow Array for Triggering with Amplitude discrimiNAtion - has been built and used as a trigger and veto detector for the S$π$RIT TPC at RIKEN. Its construction allows operating in magnetic field and providing fast response for ionizing particles, giving the approximate forward multiplicity and charge information. Depending on this information, trigger and veto signals are generated. The article presents performance of the detector and details of its construction. A simple phenomenological parametrization of the number of emitted scintillation photons in plastic scintillator is proposed. The effect of the light output deterioration in the plastic scintillator due to the in-beam irradiation is discussed.

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