arXiv subjects

A. Kimura

Publications and source records attributed to A. Kimura.

At least 19 recordsLinked to original sources

arXiv2026

Improved $^{94}$Mo neutron resonance parameters from neutron capture and transmission measurements at n_TOF and GELINA

We report high-resolution measurements of the $^{94}\mathrm{Mo}(\mathrm{n},γ)^{95}\mathrm{Mo}$ cross section in the neutron energy range from a few eV up to about 250 keV, performed at the n_TOF facility (CERN), and of the $^{94}\mathrm{Mo}(\mathrm{n},\mathrm{tot})$ cross section up to 32 keV, measured at GELINA (JRC Geel). A combined R-matrix analysis of capture and transmission data yields significantly improved neutron-resonance parameters for $^{94}$Mo. A total of 186 resonances were observed in this analysis of which 127 reported here for the first time. The resulting Maxwellian-averaged cross section at stellar temperatures relevant to the slow neutron-capture process (s-process) is approximately 25% lower than previous evaluations and literature values. To assess the astrophysical impact of the revised cross section, we performed s-process nucleosynthesis calculations for low-mass asymptotic giant branch stars. Despite the substantial reduction in the $^{94}\mathrm{Mo}(\mathrm{n},γ)^{95}\mathrm{Mo}$ rate, the final yields vary by only 3-4%. This demonstrates that the isotopic budget of $^{94}$Mo in AGB stars is primarily governed by the branching flow at $^{94}\mathrm{Nb}$ rather than by the neutron-capture destruction on $^{94}$Mo itself. The new cross section thus helps disentangle nuclear cross-section uncertainties from branching-flow effects in the s-process production of $^{94}$Mo.

nucl-ex↗

arXiv2026

Realization of a parity-violating antiferromagnetic state in LaMnSi

Spontaneous symmetry breaking underlies functional electronic phenomena in quantum materials. Breaking space-inversion ($\mathcal{P}$) or time-reversal ($\mathcal{T}$) symmetry can generate spin-split electronic bands central to modern spintronics. By contrast, parity-violating antiferromagnetic (AFM) order breaks both $\mathcal{P}$ and $\mathcal{T}$ while preserving the combined $\mathcal{PT}$ symmetry, enabling spin-degenerate yet momentum-asymmetric electronic bands. This momentum asymmetry has been proposed as a microscopic origin of unconventional nonreciprocal and nonlinear responses but its experimental verification has remained challenging because it requires establishing both the symmetry-breaking magnetic order and the associated electronic structure. Here we combine soft x-ray angle-resolved photoemission spectroscopy (ARPES) and polarization-resolved optical second-harmonic generation (SHG) microscopy to study LaMnSi, a candidate parity-violating AFM metal. Soft x-ray ARPES resolves the three-dimensional bulk band structures in agreement with density functional theory calculations for the AFM phase, whereas SHG microscopy detects sign-reversing nonlinear optical responses from opposite AFM domains that carry $\mathcal{T}$-odd parity-violating order. Together, these results provide direct evidence for parity-violating AFM state in LaMnSi, establish LaMnSi as a parity-violating AFM metal, and identify this class of AFMs as a promising platform for symmetry-controlled nonreciprocal and nonlinear electronic responses.

cond-mat.mtrl-sci↗

arXiv2026

Energy-differential measurement of the $^{\mathrm{nat}}$C(n,p) and $^{\mathrm{nat}}$C(n,d) reactions at the n_TOF facility at CERN

Energy-differential cross section of the $^{\mathrm{nat}}$C(n,p) and $^{\mathrm{nat}}$C(n,d) reactions was measured at the neutron time of flight facility n_TOF at CERN. The measurement was performed in the first experimental area (EAR1; flight path of 182.5 m). Two position-sensitive $ΔE$-$E$ silicon telescopes were used. Two naturally occurring carbon isotopes, $^{12}$C and $^{13}$C, contribute to the reactions on natural carbon, with the (n,p) reaction threshold at 13.7 MeV and the (n,d) threshold at 14.9 MeV (determined by the $^{12}$C isotope for both reactions). This paper provides the details of the analysis leading to the final results published previously as a Letter. The cross section results are reported up to 25 MeV. During the data analysis the population of the excited states in the daughter nuclei $^{11}$B, $^{12}$B, $^{13}$B had to be considered, requiring the adoption of the branching ratios and angular distributions of the emitted particles from an external source of information. TALYS-2.0 calculations were used as the main source and an in-depth analysis of the model-related uncertainties was performed. The n_TOF results are largely inconsistent with the major evaluation libraries. On the other hand, an unexpected agreement is found with TALYS-2.0 calculations. Specifically, the obtained cross section for the (n,p) reaction is significantly higher than in the available evaluations, fully supporting the earlier finding from an integral measurement at n_TOF.

nucl-ex↗

arXiv2026

First $^{94}$Nb($n,γ$) Measurement: Constraining the Nucleosynthetic Origin of $^{94}$Mo in Presolar Grains

Isotopic measurements of presolar silicon carbide grains from dying stars have revealed a puzzling overabundance of $^{94}$Mo that stellar nucleosynthesis models have failed to reproduce for two decades. This discrepancy challenged our understanding of the slow neutron-capture process ($s$-process) that forges approximately half of the elements heavier than iron. The key uncertainty lies at $^{94}$Nb, a radiactive branching point where competition between neutron capture and beta decay governs the $^{94}$Mo production, yet the neutron-capture cross section had never been measured. Here we report the first experimental determination of the $^{94}$Nb(n,$γ$)$^{95}$Nb cross section important for Mo isotopic abundances. The measurement was enabled by a coordinated effort involving high-purity target preparation at Institute of Solid State and Materials Research (IFW) Dresden, radioactive sample production at the Institut Laue-Langevin (ILL) Grenoble, radiochemical characterization at Paul Scherrer Institute (PSI) Villigen, and the Time-of-Flight CERN n$\_$TOF facility using for the first time segmented total-energy detectors. Incorporation of the resulting Maxwellian-averaged cross section into fully coupled nucleosynthesis models of low-mass asymptotic giant branch (AGB) stars brings them into agreement with the presolar grain data. These results remove a major nuclear-physics input uncertainty at the $^{94}$Nb branching point and provide a firmer foundation for understanding the origin of $^{94}$Mo in the solar system.

nucl-ex↗

arXiv2026

Radiative neutron capture cross section of $^{242}$Pu measured at n_TOF-EAR1 in the unresolved resonance region up to 600 keV

Accurate neutron capture cross sections are essential for the design and operation of fast reactors using MOX fuels. For $^{242}$Pu, the Nuclear Energy Agency (NEA) recommends 8--12% accuracy in the fast energy region (2--500 keV), compared to the current uncertainty of 35%. Moreover, integral experiments and previous measurements suggest the evaluated $^{242}$Pu(n,$γ$) cross section is overestimated, particularly in the JEFF-3.3 library, which shows a 14% overestimation between 1 keV and 1 MeV. Recent measurements from LANSCE reported a 20--30% reduction in the 1--40 keV range relative to evaluations. To solve these discrepancies, the $^{242}$Pu(n,$γ$) cross section was measured from 1 to 600 keV at CERN n_TOF-EAR1 facility using a 95(4) mg $^{242}$Pu target, enriched to 99.959%. Gamma rays from neutron capture were detected with an array of C$_6$D$_6$ scintillators and a novel application of the Pulse Height Weighting Technique was employed. The resulting cross section presents a systematic uncertainty between 8 and 12%, reducing the current uncertainties of 35\% and achieving the accuracy requested by the NEA. Analysis using FITACS produced average resonance parameters, consistent with the analysis of the resolved resonance region. Our data align well with Wisshak and Kaeppeler, and are 10--14\% lower than JEFF-3.3 in the 1--250 keV range, helping to achieve consistency with integral benchmarks. At higher energies, our results are in reasonable agreement with ENDF/B-VIII.1 and JEFF-3.3. In contrast, DANCE results appear to underestimate the cross section by a factor of 2--3 above a few keV.

nucl-ex↗

arXiv2025

Measurement of the $^\text{nat}$C(n,p) and $^\text{nat}$C(n,d) reactions from n_TOF at CERN

The energy dependence of the cross section of the (n,p) and (n,d) reactions on $^\text{nat}$C has been studied for the first time at the n_TOF facility at CERN, from the particle detection threshold up to 25 MeV. The measurement was performed with two telescopes made of position-sensitive silicon $ΔE$-$E$ detectors, covering the angular range from 20° to 140°. A detector efficiency has been determined by means of Monte Carlo simulations of the experimental setup. Various assumptions on the angular distributions and branching ratios of the excited levels of the residual $^{11}$B, $^{12}$B, $^{13}$B nuclei were considered. In particular, theoretical calculations based on the TALYS-2.0 code were used and the systematic uncertainties in the analysis results were determined from the variations in these distributions. The n_TOF data on the (n,p) and (n,d) reaction on carbon are characterized by a higher accuracy and wider energy range than currently available in literature. A comparison with current evaluations from different libraries reveals a rather significant disagreement with the n_TOF results, while a remarkable agreement is observed with the prediction of TALYS-2.0 for this light element.

nucl-ex↗

arXiv2024

Towards a new generation of solid total-energy detectors for neutron-capture time-of-flight experiments with intense neutron beams

Challenging neutron-capture cross-section measurements of small cross sections and samples with a very limited number of atoms require high-flux time-of-flight facilities. In turn, such facilities need innovative detection setups that are fast, have low sensitivity to neutrons, can quickly recover from the so-called $γ$-flash, and offer the highest possible detection sensitivity. In this paper, we present several steps toward such advanced systems. Specifically, we describe the performance of a high-sensitivity experimental setup at CERN n\_TOF EAR2. It consists of nine sTED detector modules in a compact cylindrical configuration, two conventional used large-volume C$_{6}$D$_{6}$ detectors, and one LaCl$_{3}$(Ce) detector. The performance of these detection systems is compared using $^{93}$Nb($n$,$γ$) data. We also developed a detailed \textsc{Geant4} Monte Carlo model of the experimental EAR2 setup, which allows for a better understanding of the detector features, including their efficiency determination. This Monte Carlo model has been used for further optimization, thus leading to a new conceptual design of a $γ$ detector array, STAR, based on a deuterated-stilbene crystal array. Finally, the suitability of deuterated-stilbene crystals for the future STAR array is investigaged experimentally utilizing a small stilbene-d12 prototype. The results suggest a similar or superior performance of STAR with respect to other setups based on liquid-scintillators, and allow for additional features such as neutron-gamma discrimination and a higher level of customization capability.

physics.ins-det↗

arXiv2024

Measurement and analysis of the $^{246}$Cm and $^{248}$Cm neutron capture cross-sections at the EAR2 of the n TOF facility

The $^{246}$Cm(n,$γ$) and $^{248}$Cm(n,$γ$) cross-sections have been measured at the Experimental Area 2 (EAR2) of the n_TOF facility at CERN with three C$_6$D$_6$ detectors. This measurement is part of a collective effort to improve the capture cross-section data for Minor Actinides (MAs), which are required to estimate the production and transmutation rates of these isotopes in light water reactors and innovative reactor systems. In particular, the neutron capture in $^{246}$Cm and $^{248}$Cm open the path for the formation of other Cm isotopes and heavier elements such as Bk and Cf and the knowledge of (n,$γ$) cross-sections of these Cm isotopes plays an important role in the transport, transmutation and storage of the spent nuclear fuel. The reactions $^{246}$Cm(n,$γ$) and $^{248}$Cm(n,$γ$) have been the two first capture measurements analyzed at n_TOF EAR2. Until this experiment and two recent measurements performed at J-PARC, there was only one set of data of the capture cross-sections of $^{246}$Cm and $^{248}$Cm, that was obtained in 1969 in an underground nuclear explosion experiment. In the measurement at n_TOF a total of 13 resonances of $^{246}$Cm between 4 and 400 eV and 5 of $^{248}$Cm between 7 and 100 eV have been identified and fitted. The radiative kernels obtained for $^{246}$Cm are compatible with JENDL-5, but some of them are not with JENDL-4, which has been adopted by JEFF-3.3 and ENDF/B-VIII.0. The radiative kernels obtained for the first three $^{248}$Cm resonances are compatible with JENDL-5, however, the other two are not compatible with any other evaluation and are 20% and 60% larger than JENDL-5.

nucl-ex↗

arXiv2024

Circular polarization measurement for individual gamma rays in capture reactions with intense pulsed neutrons

Measurements of circular polarization of $γ$-ray emitted from neutron capture reactions provide valuable information for nuclear physics studies. The spin and parity of excited states can be determined by measuring the circular polarization from polarized neutron capture reactions. Furthermore, the $γ$-ray circular polarization in a neutron capture resonance is crucial for studying the enhancement effect of parity nonconservation in compound nuclei. The $γ$-ray circular polarization can be measured using a polarimeter based on magnetic Compton scattering. A polarimeter was constructed, and its performance indicators were evaluated using a circularly polarized $γ$-ray beam. Furthermore, as a demonstration, the $γ$-ray circular polarization was measured in $^{32}$S($\vec{\textrm{n}}$,$γ$)$^{33}$S reactions with polarized neutrons.

physics.ins-det↗

arXiv2024

Zone-selection effect of photoelectron intensity distributions in a nonsymmorphic system RAlSi (R : Ce and Nd)

We investigate the electronic structures of noncentrosymmetric Weyl semimetals RAlSi (R: Ce and Nd) using soft x-ray angle-resolved photoemission spectroscopy. We find that the photoelectron intensity distribution observed in the momentum-resolved electronic bands is highly sensitive to the covered Brillouin zone (BZ) due to the zone-selection effect arising from the nonsymmorphic crystal structure of RAlSi. Our data reconstruct the photoelectron distributions varied according to the zone-selection effect, and reveal comprehensive information about the electronic band structures reproduced by band calculations. This detailed information enables us to experimentally trace the Weyl-cone dispersion throughout three-dimensional momentum space, providing valuable insights into the unique properties of RAlSi.

cond-mat.str-el↗

arXiv2024

Transverse asymmetry of individual $γ$-rays in the $^{139}$La($\vec{n}$, $γ$)$^{140}$La reaction

The enhancement of the parity-violating asymmetry in the vicinity of $p$-wave compound nuclear resonances was observed for a variety of medium-heavy nuclei. The enhanced parity-violating asymmetry can be understood using the $s$-$p$ mixing model. The $s$-$p$ mixing model predicts several neutron energy-dependent angular correlations between the neutron momentum $\vec k_n$, neutron spin $\vecσ_n$, $γ$-ray momentum $\vec k_γ$, and $γ$-ray polarization $\vecσ_γ$ in the $(n,γ)$ reaction. In this paper, the improved value of the transverse asymmetry of $γ$-ray emissions, corresponding to a correlation term $\vecσ_n\cdot(\vec k_n\times\vec k_γ)$ in the $^{139}\mathrm{La}(\vec n,γ)^{140}\mathrm{La}$ reaction, and the transverse asymmetries in the transitions to several low excited states of $^{140}\mathrm{La}$ are reported.

nucl-ex↗

arXiv2023

High sensitivity of a future search for P-odd/T-odd interactions on the 0.75 eV $p$-wave resonance in $\vec{n}+^{139}\vec{\rm La}$ forward transmission determined using pulsed neutron beam

Neutron transmission experiments can offer a new type of highly sensitive search for time-reversal invariance violating (TRIV) effects in nucleon-nucleon interactions via the same enhancement mechanism observed for large parity violating (PV) effects in neutron-induced compound nuclear processes. In these compound processes, the TRIV cross-section is given as the product of the PV cross-section, a spin-factor $κ$, and a ratio of TRIV and PV matrix elements. We determined $κ$ to be $0.59\pm0.05$ for $^{139}$La+$n$ using both $(n, γ)$ spectroscopy and ($\vec{n}+^{139}\vec{\rm La}$) transmission. This result quantifies for the first time the high sensitivity of the $^{139}$La 0.75~eV $p$-wave resonance in a future search for P-odd/T-odd interactions in ($\vec{n}+^{139}\vec{\rm La}$) forward transmission.

nucl-ex↗

arXiv2023

Angular Distribution of $γ$-rays from Neutron-Induced Compound States of $^{140}$La

Angular distribution of individual $γ$-rays, emitted from a neutron-induced compound nuclear state via radiative capture reaction of ${}^{139}$La(n,$γ$) has been studied as a function of incident neutron energy in the epithermal region by using germanium detectors. An asymmetry $A_{\mathrm{LH}}$ was defined as $(N_{\mathrm L}-N_{\mathrm H})/(N_{\mathrm L}+N_{\mathrm H})$, where $N_{\mathrm L}$ and $N_{\mathrm H}$ are integrals of low and high energy region of a neutron resonance respectively, and we found that $A_{\mathrm{LH}}$ has the angular dependence of $(A\cosθ_γ+B)$, where $θ_γ$ is emitted angle of $γ$-rays, with $A= -0.3881\pm0.0236$ and $B=-0.0747\pm0.0105$ in 0.74 eV p-wave resonance. This angular distribution was analyzed within the framework of interference between s- and p-wave amplitudes in the entrance channel to the compound nuclear state, and it is interpreted as the value of the partial p-wave neutron width corresponding to the total angular momentum of the incident neutron combined with the weak matrix element, in the context of the mechanism of enhanced parity-violating effects. Additionally we used the result to quantify the possible enhancement of the breaking of the time-reversal invariance in the vicinity of the p-wave resonance.

nucl-ex↗

arXiv2023

Pushing the high count rate limits of scintillation detectors for challenging neutron-capture experiments

One of the critical aspects for the accurate determination of neutron capture cross sections when combining time-of-flight and total energy detector techniques is the characterization and control of systematic uncertainties associated to the measuring devices. In this work we explore the most conspicuous effects associated to harsh count rate conditions: dead-time and pile-up effects. Both effects, when not properly treated, can lead to large systematic uncertainties and bias in the determination of neutron cross sections. In the majority of neutron capture measurements carried out at the CERN n\_TOF facility, the detectors of choice are the C$_{6}$D$_{6}$ liquid-based either in form of large-volume cells or recently commissioned sTED detector array, consisting of much smaller-volume modules. To account for the aforementioned effects, we introduce a Monte Carlo model for these detectors mimicking harsh count rate conditions similar to those happening at the CERN n\_TOF 20~m fligth path vertical measuring station. The model parameters are extracted by comparison with the experimental data taken at the same facility during 2022 experimental campaign. We propose a novel methodology to consider both, dead-time and pile-up effects simultaneously for these fast detectors and check the applicability to experimental data from $^{197}$Au($n$,$γ$), including the saturated 4.9~eV resonance which is an important component of normalization for neutron cross section measurements.

physics.ins-det↗

arXiv2023

Switching on surface conduction in a topological insulator

The protected surface conduction of topological insulators is in high demand for the next generation of electronic devices. What is needed to move forward are robust settings where topological surface currents can be controlled by simple means, ideally by the application of external stimuli. Surprisingly, this direction is only little explored. In this work we demonstrate that we can boost the surface conduction of a topological insulator by both light and electric field. This happens in a fully controlled way, and the additional Dirac carriers exhibit ultra-long lifetimes. We provide a comprehensive understanding, namely that carriers are injected from the bulk to the surface states across an intrinsic Schottky barrier. We expect this mechanism to be at play in a broad range of materials and experimental settings.

cond-mat.mtrl-sci↗

arXiv2023

Laser-based angle-resolved photoemission spectroscopy with micrometer spatial resolution and detection of three-dimensional spin vector

We have developed a state-of-the-art apparatus for laser-based spin- and angle-resolved photoemission spectroscopy with micrometer spatial resolution (micro-SARPES). This equipment is achieved through the combination of a high-resolution photoelectron spectrometer, a 6-eV laser with high photon flux that is focused down to a few micrometers, a high-precision sample stage control system, and a double very-low-energy-electron-diffraction spin detector. The setup achieves an energy resolution of 1.5 (5.5) meV without (with) the spin detection mode, compatible with a spatial resolution better than 10 micrometers. This enables us to probe both spatially-resolved electronic structures and vector information of spin polarization in three dimensions. The performance of micro-SARPES apparatus is demonstrated by presenting ARPES and SARPES results from topological insulators and Au photolithography patterns on a Si (001) substrate.

cond-mat.mtrl-sci↗

arXiv2023

First measurement of the $^{94}$Nb($n$,$γ$) cross section at the CERN n\_TOF facility

One of the crucial ingredients for the improvement of stellar models is the accurate knowledge of neutron capture cross-sections for the different isotopes involved in the $s$-,$r$- and $i$- processes. These measurements can shed light on existing discrepancies between observed and predicted isotopic abundances and help to constrain the physical conditions where these reactions take place along different stages of stellar evolution.In the particular case of the radioactive $^{94}$Nb, the $^{94}$Nb($n$,$γ$) cross-section could play a role in the determination of the $s$-process production of $^{94}$Mo in AGB stars, which presently cannot be reproduced by state-of-the-art stellar models. There are no previous $^{94}$Nb($n$,$γ$) experimental data for the resolved and unresolved resonance regions mainly due to the difficulties in producing high-quality samples and also due to limitations in conventional detection systems commonly used in time-of-flight experiments.Motivated by this situation, a first measurement of the $^{94}$Nb($n$,$γ$) reaction was carried out at CERN n\_TOF, thereby exploiting the high luminosity of the EAR2 area in combination with a new detection system of small-volume C6D6-detectors and a high quality $^{94}$Nb-sample. The latter was based on hyper-pure $^{93}$Nb material activated at the high-flux reactor of ILL-Grenoble. An innovative ring-configuration detection system in close geometry around the capture sample allowed us to significantly enhance the signal-to-background ratio. This set-up was supplemented with two conventional C$_{6}$D$_{6}$ detectors and a high-resolution LaCl$_{3}$(Ce)-detector, which will be employed for addressing reliably systematic effects and uncertainties.At the current status of the data analysis, 18 resonance in $^{94}$Nb+$n$ have been observed for the first time in the neutron energy range from thermal up to 10 keV.

nucl-ex↗

arXiv2023

Evidence of 3D Dirac conical bands in TlBiSSe by optical and magneto-optical spectroscopy

TlBiSSe is a rare realization of a 3D semimetal with a conically dispersing band that has an optical response which is well isolated from other contributions in a broad range of photon eneries. We report optical and magneto-optical spectroscopy on this material. When the compound is chemically tuned into a state of the lowest carrier concentration, we find a nearly linear frequency dependence of the optical conductivity below 0.5~eV. Landau level spectroscopy allows us to describe the system with a massive Dirac model, giving a gap $2Δ=32$~meV and an in-plane velocity parameter $v= 4.0\times 10^5$~m/s. % Finally, we provide a theoretical recipe to extract all parameters of the anisotropic Dirac band, including the Fermi energy and band degeneracy.

cond-mat.mtrl-sci↗