arXiv subjects

N. Kalantar-Nayestanaki

Publications and source records attributed to N. Kalantar-Nayestanaki.

At least 19 recordsLinked to original sources

arXiv2026

Probing high-momentum components in $^{16}$O via zero-degree $(p,d)$ cross-section ratios

The $^{16}$O$(p,d)^{15}$O reaction was measured at zero degrees for incident-proton energies of 403, 604, 907 and 1209 MeV. Cross-section ratios for the 6.18-MeV (3/2$^-$) and 5.24-MeV (5/2$^+$) excited-state transitions, normalized to the ground-state transition, were previously reported in Wang et al., Phys. Lett. B 879, 140563 (2026), doi:10.1016/j.physletb.2026.140563, together with their implications for high-momentum neutron components in $^{16}$O. The present work extends that study by providing additional experimental and data-analysis details, and by reporting for the first time the ratio $R_{7.3}$, defined as the cross section populating the 7.28-MeV (7/2$^+$) state relative to that for the ground state. The positive-parity state ratios exhibit a pronounced momentum dependence, consistent with representative variational Monte Carlo trends.

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arXiv2026

The dipole strength distribution of $^8$He and decay characteristics

The weak binding and spatially extended neutron densities characteristic of drip-line nuclei give rise to a distinctive low-energy dipole response. The drip-line nucleus $^8$He is the most neutron-rich bound nucleus with a mass-to-charge ratio of $A/Z=4$. We measure the dipole response of $^8$He, including for the first time the four-neutron decay channel. A total dipole strength of $\sum B(E1)(E^*<15$~MeV$)=0.95(16)~e^2$fm$^2$ and a dipole polarizability of $α_D = 0.61(1)$~fm$^3$ are extracted from the differential Coulomb-excitation cross section and compared to state-of-the-art theoretical calculations employing coupled cluster and three-body approaches. We find that the dipole continuum is dominated, even at high excitation energies well above the $4n$ decay threshold, by two-neutron emission, pointing to a $^6$He$+2n$ structure of the excited dipole mode. No indication was found for a $4n$ final-state correlation, while pronounced $nn$ and $^6$He-$n$ final-state correlations are apparent.

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arXiv2026

Direct observation of three-neutron emission from $^7$He$^*$ and the search for the trineutron

Three-neutron emission from $^7$He has been directly measured for the first time, following neutron knockout from a $^8$He beam at 156 MeV/nucleon. A resonance-like structure at $2.08(4)$ MeV above the $^4$He+$3n$ threshold [$E_x=2.68(4)$ MeV] with a width of $3.9(2)$ MeV was observed and deduced to arise predominately from the predicted $J^π=3/2^{-}_2$ level. The three-neutron invariant-mass spectrum was reconstructed and found to peak at around 1 MeV and could, through complete simulations incorporating neutron-neutron correlations, be very well described by the sequential decay of $^7$He$^*$ via the $2_1^+$ excited state of $^6$He. No evidence was found for any significant three-neutron correlations beyond those expected from well-established two-body interactions, including a trineutron resonance.

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arXiv2025

Isoscalar Giant Resonances in Highly-Deformed $^{172}$Yb

To study the isoscalar giant resonances in a deformed case, background-free $α$-particle inelastic scattering measurements using a 386 MeV $α$ beam were performed on the highly-deformed $^{172}$Yb nucleus using the Grand Raiden spectrometer at the Research Center for Nuclear Physics (RCNP) at very forward angles, including $0^\circ$. The strength distributions for the isoscalar giant resonances up to $L \leq 3$ were obtained using multipole decomposition analysis. The isoscalar giant monopole resonance (ISGMR) strength exhibits a splitting into two components, interpreted as the coupling of the ISGMR with the $K=0$ component of the isoscalar giant quadrupole resonance (ISGQR). A \textit{bimodal} structure is observed in the strength distribution of the isoscalar giant dipole resonance. The ISGQR strength shows an enhancement near 25 MeV, attributed to the excitation of an overtone mode, while the broadening of the main-tone peak is associated with nuclear deformation. The experimental results are well reproduced by theoretical strength distributions calculated using the quasiparticle finite amplitude method for $L \leq 3$.

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arXiv2025

Excitation Spectra of the ${}^{12}{\rm C}(p,d)$ Reaction near the $η'$-Meson Emission Threshold Measured in Coincidence with High-Momentum Protons

The missing mass of the ${}^{12}{\rm C}(p,d)$ reaction has been measured near the $η'$-meson emission threshold in coincidence with a high-momentum proton to selectively collect $η'$-${}^{11}{\rm C}$ mesic nucleus formation events at GSI, Germany. A 2.5 GeV proton excites a carbon nucleus to form an $η'$-mesic nucleus emitting a deuteron forward with an energy of $\sim 1.6$ GeV. The deuteron is momentum-analyzed by the Fragment Separator used as a high-resolution spectrometer to deduce the excitation energy of the residual system. The large-acceptance detector WASA surrounding the target identifies high-momentum protons emitted in the decay of the $η'$-mesic nucleus. The measured semi-exclusive spectrum exhibits structures below the threshold though the statistical significance is limited. The spectrum is fitted by theoretically calculated spectra varying optical-potential parameters of the $η'$-nucleus interaction. The analysis results indicate $η'$-mesic nuclei formation for the real potential depth of $\sim -61$ MeV with a local statistical significance of $3.5 σ$ and, taking into account the look-elsewhere effect, a global significance of $2.1 σ$.

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arXiv2025

Observation of Tensor-Driven High-Momentum Neutrons in ${}^{16}$O via ($p,d$) Reactions and Zero-Degree Deuteron Momentum Spectroscopy

The $^{16}\mathrm{O}(p,d)^{15}\mathrm{O}$ reaction has been studied at $0^{\circ}$ using 403-, 604-, 907- and 1209-MeV protons, comparing cross sections populating positive- and negative-parity states in $^{15}\mathrm{O}$. Transitions to positive-parity states exhibit strong sensitivity to high-momentum neutrons, while negative-parity transitions show much smaller effects. The cross-section ratio between positive- and negative-parity states rises sharply with momentum transfer, matching theoretical predictions that include tensor interactions, particularly the peak near $2~\mathrm{fm}^{-1}$ for the $5/2^{+}$ to ground-state ratio. These results highlight $0^{\circ}$ neutron-pickup reactions as a sensitive probe for tensor-driven high-momentum components, paving the way for studies in exotic nuclei via radioactive beams.

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arXiv2025

Performance of newly constructed plastic scintillator barrel in the WASA-FRS experiments and evaluation of radiation damage effects on multi-pixel photon counter

A barrel-shaped plastic scintillation counter with Multi-Pixel Photon Counter (MPPC) readout has been developed and operated in the first WASA-FRS experimental campaign at GSI. The detector was used to measure charged particles emitted from reactions induced by a 2.5 GeV proton beam incident on a carbon target, providing particle identification in combination with momentum reconstruction in a 1 T magnetic field. The performance of this detector, particularly its response to energy deposition and time resolution, was systematically investigated as a function of count rate and total number of irradiating protons. A time resolution of 45-75 ps ($σ$), depending on the energy deposition, was achieved. Stable performance was maintained under high-rate conditions up to 1.35 MHz per single counter, with no significant degradation in either signal amplitude or timing response. Radiation-induced damage to the MPPCs was observed primarily as a reduction in signal amplitude, with approximately $35\%$ decrease at an estimated 1 MeV neutron-equivalent fluence of $2.4 \times 10^{10}$ cm$^{-2}$.

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arXiv2025

Investigation of Direct Nuclear Reactions in a Storage Ring Using In-Ring Detection

\textbf{Background:} Experiments involving nuclear reactions in a storage ring offer exceptional possibilities for precise measurements in inverse kinematics. These experiments provide excellent angular and energy resolution by particle spectroscopy, in addition to high luminosities. However, the extremely low-pressure environment maintained in the storage rings poses significant difficulties for experiments employing detectors or any outgassing material in the ring. \textbf{Purpose:} To investigate nuclear reactions in inverse kinematics using the storage-ring technique. The reactions were induced by scattering of a ${}^{20}\mathrm{Ne}$ beam off a hydrogen target at an energy of 50~MeV/u. \textbf{Method:} A beam of fully stripped ${}^{20}$Ne ions was injected into the ESR storage ring at an energy of 50 MeV/u. The beam interacted with an internal hydrogen gas-jet target. An ultra-high vacuum compatible detector setup was installed around the gas jet inside the ring to measure the recoiling particles generated by nuclear reactions. \textbf{Results:} Multiple reaction channels were observed during the experiment. In particular, we present the results from studies on elastic and inelastic scattering, as well as the neutron transfer reaction ${}^{20}\mathrm{Ne}(p,d){}^{19}\mathrm{Ne}^*$. The experimental data were compared to calculations that took into account the most significant excited states, using a coupled-reaction channel approach. A very good agreement with the experimental data was achieved. \textbf{Conclusions:} The present results are the first demonstration of the investigation transfer reactions using detectors directly installed in the ring. This provides an important proof-of-principle for prospective studies with far-from-stability radioactive beams in the future.

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arXiv2024

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 quasi-free 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 $S_n = -472\pm 58 \mathrm{(stat.)} \pm 33 \mathrm{(sys.)}$ 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. This degeneracy and reordering of orbitals has two potential consequences: $^{28}$O behaves like a strongly superfluid nucleus with neutron pairs scattering across shells, and both $^{29,31}$F appear to be good two-neutron halo-nucleus candidates.

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arXiv2024

Effect of ground-state deformation on the Isoscalar Giant Monopole Resonance and the first observation of overtones of the Isoscalar Giant Quadrupole Resonance in rare-earth Nd isotopes

The strength distributions of the Isoscalar Giant Monopole Resonance (ISGMR) and Isoscalar Giant Quadrupole Resonance (ISGQR) in 142,146-150Nd have been determined via inelastic alpha-particle scattering with the Grand Raiden (GR) Spectrometer at the Research Center for Nuclear Physics (RCNP), Japan. In the deformed nuclei 146-150Nd, the ISGMR strength distributions exhibit a splitting into two components, while the nearly spherical nucleus 142Nd displays a single peak in the ISGMR strength distribution. A noteworthy achievement in this study is the first-time detection of overtones in the Isoscalar Giant Quadrupole Resonance (ISGQR) strength distributions within Nd isotopes at an excitation energy around 25 MeV obtained through Multipole Decomposition Analysis (MDA).

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arXiv2023

Increasing the rate capability for the cryogenic stopping cell of the FRS Ion Catcher

At the FRS Ion Catcher (FRS-IC), projectile and fission fragments are produced at relativistic energies, separated in-flight, energy-bunched, slowed down, and thermalized in the ultra-pure helium gas-filled cryogenic stopping cell (CSC). Thermalized nuclei are extracted from the CSC using a combination of DC and RF electric fields and gas flow. This CSC also serves as the prototype CSC for the Super-FRS, where exotic nuclei will be produced at unprecedented rates making it possible to go towards the extremes of the nuclear chart. Therefore, it is essential to efficiently extract thermalized exotic nuclei from the CSC under high beam rate conditions, in order to use the rare exotic nuclei which come as cocktail beams. The extraction efficiency dependence on the intensity of the impinging beam into the CSC was studied with a primary beam of 238U and its fragments. Tests were done with two different versions of the DC electrode structure inside the cryogenic chamber, the standard 1 m long and a short 0.5 m long DC electrode. In contrast to the rate capability of 10^4 ions/s with the long DC electrode, results show no extraction efficiency loss up to the rate of 2x10^5 ions/s with the new short DC electrode. This order of magnitude increase of the rate capability paves the way for new experiments at the FRS-IC, including exotic nuclei studies with in-cell multi-nucleon transfer reactions. The results further validate the design concept of the CSC for the Super-FRS, which was developed to effectively manage beams of even higher intensities.

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arXiv2023

Recent Upgrades of the Gas Handling System for the Cryogenic Stopping Cell of the FRS Ion Catcher

In this paper, the major upgrades and technical improvements of the buffer gas handling system for the cryogenic stopping cell of the FRS Ion Catcher at GSI/FAIR (in Darmstadt, Germany) are described. The upgrades include implementation of new gas lines and gas purifiers to achieve a higher buffer gas cleanliness for a more efficient extraction of reactive ions as well as suppression of the molecular background ionized in the stopping cell. Furthermore, additional techniques have been implemented for improved monitoring and quantification of the purity of the helium buffer gas.

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arXiv2023

Intruder configurations in $^{29}$Ne at the transition into the island of inversion: Detailed structure study of $^{28}$Ne

Detailed $γ$-ray spectroscopy of the exotic neon isotope $^{28}$Ne has been performed for the first time using the one-neutron removal reaction from $^{29}$Ne 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 $^{28}$Ne 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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arXiv2022

Border of the Island of Inversion: Unbound states in $^{29}$Ne

The nucleus $^{29}$Ne 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 $^{29}$Ne firmly inside the island of inversion, has yet been observed. Here, the first investigation of unbound states of $^{29}$Ne is reported. The states were populated in $^{30}\mathrm{Ne}(p,pn)$ and $^{30}\mathrm{Na}(p,2p)$ 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 $\ell$-assignments for the observed states. The lowest-lying resonance at an excitation energy of 1.48(4) MeV shows clear signs of a significant $\ell$=3-component, giving first evidence for $f_{7/2}$ single particle strength in $^{29}$Ne. 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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arXiv2021

Mass measurements of As, Se and Br nuclei and their implication on the proton-neutron interaction strength towards the N=Z line

Mass measurements of the $^{69}$As, $^{70,71}$Se and $^{71}$Br isotopes, produced via fragmentation of a $^{124}$Xe primary beam at the FRS at GSI, have been performed with the multiple-reflection time-of-flight mass spectrometer (MR-TOF-MS) of the FRS Ion Catcher with an unprecedented mass resolving power of almost 1,000,000. For the $^{69}$As isotope, this is the first direct mass measurement. A mass uncertainty of 22 keV was achieved with only 10 events. For the $^{70}$Se isotope, a mass uncertainty of 2.6 keV was obtained, corresponding to a relative accuracy of $δ$m/m = 4.0$\times 10^{-8}$, with less than 500 events. The masses of the $^{71}$Se and $^{71}$Br isotopes were measured with an uncertainty of 23 and 16 keV, respectively. Our results for the $^{70,71}$Se and $^{71}$Br isotopes agree with the 2016 Atomic Mass Evaluation, and our result for the $^{69}$As isotope resolves the discrepancy between previous indirect measurements. We measured also the mass of $^{14}$N$^{15}$N$^{40}$Ar (A=69) with a relative accuracy of $δ$m/m = 1.7$\times 10^{-8}$, the highest yet achieved with a MR-TOF-MS. Our results show that the measured restrengthening of the proton-neutron interaction ($δ$V$_{pn}$) for odd-odd nuclei at the N=Z line above Z=29 (recently extended to Z=37) is hardly evident at N-Z=2, and not evident at N-Z=4. Nevertheless, detailed structure of $δ$V$_{pn}$ along the N-Z=2 and N-Z=4 lines, confirmed by our mass measurements, may provide a hint regarding the ongoing $\approx$500 keV discrepancy in the mass value of the $^{70}$Br isotope, which prevents including it in the world average of ${Ft}$-value for superallowed 0$^+\rightarrow$ 0$^+$ $β$ decays. The reported work sets the stage for mass measurements with the FRS Ion Catcher of nuclei at and beyond the N=Z line in the same region of the nuclear chart, including the $^{70}$Br isotope.

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arXiv2021

Precision measurements of differential cross sections and analyzing powers in elastic deuteron-deuteron scattering at 65 MeV/nucleon

We present measurements of differential cross sections and analyzing powers for the elastic 2H(~d; d)d scattering process. The data were obtained using a 130 MeV polarized deuteron beam. Cross sections and spin observables of the elastic scattering process were measured at the AGOR facility at KVI using two independent setups, namely BINA and BBS. The data harvest at setups are in excellent agreement with each other and allowed us to carry out a thorough systematic analysis to provide the most accurate data in elastic deuteron-deuteron scattering at intermediate energies. The results can be used to confront upcoming state-of-the-art calculations in the four-nucleon scattering domain, and will, thereby, provide further insights in the dynamics of three- and four-nucleon forces in few-nucleon systems.

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arXiv2020

Signature of a possible $α$-cluster state in $N=Z$ doubly-magic $^{56}$Ni

An inelastic $α$-scattering experiment on the unstable $N=Z$, doubly-magic $^{56}$Ni nucleus was performed in inverse kinematics at an incident energy of 50 A.MeV at GANIL. High multiplicity for $α$-particle emission was observed within the limited phase-space of the experimental setup. This observation cannot be explained by means of the statistical-decay model. The ideal classical gas model at $kT$ = 0.4 MeV reproduces fairly well the experimental momentum distribution and the observed multiplicity of $α$ particles corresponds to an excitation energy around 96 MeV. The method of distributed $mα$-decay ensembles is in agreement with the experimental results if we assume that the $α$-gas state in $^{56}$Ni exists at around $113^{+15}_{-17}$ MeV. These results suggest that there may exist an exotic state consisting of many $α$ particles at the excitation energy of $113^{+15}_{-17}$ MeV.

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arXiv2020

A comprehensive study of analyzing powers in the proton-deuteron break-up channel at 135 MeV

A measurement of the analyzing powers for the $^2$H$(\vec{p},pp)n$ break-up reaction was carried out at KVI exploiting a polarized-proton beam at an energy of 135 MeV. The scattering angles and energies of the final-state protons were measured using the Big Instrument for Nuclear-polarization Analysis (BINA) with a nearly $4π$ geometrical acceptance. In this work, we analyzed a large number of kinematical geometries including forward-forward configurations in which both the final-state particles scatter to small polar angles and backward-forward configurations in which one of the final-state particles scatters to large polar angles. The results are compared with Faddeev calculations based on modern nucleon-nucleon (NN) and three-nucleon (3N) potentials. Discrepancies between polarization data and theoretical predictions are observed for configurations corresponding to small relative azimuthal angles between the two final-state protons. These configurations show a large sensitivity to 3N force effects.

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