arXiv subjects

M. Kimura

Publications and source records attributed to M. Kimura.

At least 19 recordsLinked to original sources

arXiv2026

Toward High-Resolution Detection of Target Fragmentation: TEA-Sensitized NIT for Proton Therapy Applications

Nano Imaging Trackers (NIT) are fine-grained nuclear emulsions capable of tracking charged particles with sub-micrometric spatial resolution. The DAMON (Direct Measurement of Target Fragmentation) experiment recently employed NIT to detect target-fragmentation events relevant to proton therapy in direct kinematics. These measurements showed that the small crystal size, while providing high spatial resolution, limits the reconstruction efficiency for primary proton tracks and high-energy secondary protons. This work reports an enhancement of NIT sensitivity achieved through alternative chemical sensitization methods, namely gold-plus-sulfur (Au-S) and triethanolamine (TEA), together with the use of the GR-1 developer. The detector response was evaluated with gamma-ray, carbon-ion and proton exposures, including clinically relevant proton energies.

physics.ins-det↗

arXiv2026

A diagnostic system of 5.7 keV muon beam for muon accelerator

Realization of a low-emittance muon beam through the acceleration of keV-scale muons requires the injection of a suitably matched beam into an accelerator, since beam mismatch can lead to emittance growth and reduced acceleration efficiency. In one such scheme, muons are first thermalized to room temperature and then injected into a linear accelerator. Non-destructive diagnostics are challenging because of the low energy and low intensity. We developed a compact low-energy muon diagnostic system compatible with the accelerator under construction at J-PARC. The system is designed to evaluate beam conditions required for precise tuning prior to acceleration. Commissioning with low-energy muon sources shows the system's capability to identify low-energy muon signals and measure beam profiles.

physics.acc-ph↗

arXiv2026

Deformation, halo, and bubble structure: A paradigm shift of exotic phenomena in light to medium mass nuclei

The emergence of exotic nuclear structures, such as deformation, one- and two-neutron halos, and bubble configurations, marks a paradigm shift in our understanding of light- to medium-mass nuclei far from stability, particularly near and within the island of inversion extending across $N=20-28$. In this review, we integrate microscopic structure calculations using the antisymmetrized molecular dynamics method with reaction theories such as the Glauber model for high-energy collisions, and highlight the use of the fully quantum mechanical finite-range distorted wave Born approximation for calculating both inclusive and exclusive Coulomb breakup observables for these medium mass systems. These theoretical frameworks enable precise probing of nuclear density profiles through observables such as total reaction cross sections, neutron removal cross sections, relative energy spectra, parallel momentum distributions, and angular distributions. Applications to several nuclei in the island of inversion reveal enhanced halo extensions, neutron-neutron correlations in Borromean nuclei, and central density depletions in bubbles, challenging traditional shell-model paradigms. Furthermore, the sensitivity of astrophysical reaction rates to these exotic inputs is explored, demonstrating their role in the refinement of r-process nucleosynthesis models and elemental abundance predictions. This unified approach not only bridges nuclear structure and reactions, but also highlights the driplines as frontiers for unraveling nuclear matter under extreme conditions, with implications for rare-isotope beam experiments and beyond.

nucl-th↗

arXiv2026

Investigating nuclear density profiles to reveal particle-hole configurations in the island of inversion

Background: In the mass regions with an abnormal shell structure, the so-called ``island of inversion," the spin-parity of odd-mass nuclei provides quantitative insights into the shell evolution. However, the experimental determination of the spin-parity is often challenging, leaving it undetermined in many nuclei. Purpose: We discuss how the shell structure affects the density profiles of nuclei in the island of inversion and investigate whether these can be probed from the total reaction and elastic scattering cross sections. Method: The antisymmetrized molecular dynamics (AMD) is employed to generate various particle-hole configurations and predict the energy levels of these nuclei. The obtained density distributions are used as inputs to the Glauber model, which is employed to calculate the total reaction and elastic scattering cross sections for revealing their relationship to the particle-hole configurations. Results: In addition to the well-known correlation between nuclear deformation and radius, we show the correlations between the particle-hole configurations and both central density and diffuseness. We show that different particle-hole configurations are well reflected in the total reaction and elastic scattering cross sections. Conclusion: The total reaction and elastic scattering cross sections are useful probes to identify the spin-parity of nuclei when different particle-hole configurations coexist.

nucl-th↗

arXiv2026

First Nuclear Ultra-Heavy Dark Matter Search in Argon Time Projection Chambers with the DarkSide-50 Experiment

We report the first search for nuclear ultra-heavy dark matter (UHDM) in a dual-phase liquid argon time projection chamber using the DarkSide-50 experiment. Unlike conventional weakly interacting massive particles (WIMPs), nuclear UHDM candidates may be composed of many dark nucleons and scatter numerous times while passing through the detector. Accounting for energy loss through the Earth's overburden, we apply selection criteria optimized for multi-scatter event topologies using the 532-day low-radiation campaign of the DarkSide-50 detector. Excluded limits on the UHDM-nucleon scattering cross section for dark nucleon masses of $m_χ= 10, 50, 100, 500 \, \mathrm{GeV/c^2}$ are presented.

hep-ex↗

arXiv2026

Effect of neutron-proton asymmetry on the $^3$H clustering in Boron isotopes

To investigate the influence of neutron-proton asymmetry on the formation of asymmetric clusters, we perform a systematic comparative study of $^{3}$H and $α$ cluster preformation in the Boron isotopic chain ($^{11-14}$B). Within the framework of Antisymmetrized Molecular Dynamics (AMD), we compute the nuclear wave functions and subsequently extract the reduced width amplitudes (RWA) and spectroscopic factors (SF). The results show that the $α$ cluster SF exhibits a monotonic decrease with increasing neutron number, consistent with the established suppression effect of the neutron skin. In contrast, the $^{3}$H cluster SF displays a non-monotonic behavior, peaking at $^{12}$B. This distinct trend indicates that the formation of the asymmetric $^{3}$H cluster is subject to a competition between suppression from the neutron skin and an enhancement driven by the neutron-proton asymmetry of the parent nucleus. We successfully isolate this enhancement effect by analyzing the ratio of the SFs, SF($^{3}$H)/SF($α$). This approach not only quantifies the enhancement but also proposes the SF ratio as a robust experimental observable for probing insights into asymmetric clustering phenomena.

nucl-th↗

arXiv2026

Characterization of argon recoils at the keV scale with ReD and ReD+

The ReD experiment measured the ionization yield Qy of argon for nuclear recoils in the 2-10 keV range using a dual-phase Time Projection Chamber irradiated with neutrons from a Cf-252 fission source. The measurement extends coverage below 7 keV, confirms consistency with previous data above 7 keV, and indicates a higher Qy at lower energies. These results are relevant for argon-based experiments searching for dark matter in the form of low-mass Weakly Interacting Massive Particles, which are very sensitive to the modeling of the detector response in this energy range.

hep-ex↗

arXiv2026

Fermionic sub-GeV Dark Matter from evaporating Primordial Black Holes at DarkSide-50

We present a search for boosted dark matter from Primordial Black Holes (PBH) evaporation using the DarkSide-50 ionization-signal-only dataset corresponding to the experiment's ($12202\pm180$) ${\rm kg\: d}$ exposure. We focus on evaporation of PBHs with masses in the range [$10^{14},\,10^{16}$] g producing Dirac fermionic dark matter particles with sub-GeV kinetic energy. These relativistic particles, with energies up to hundreds of MeV, can generate detectable signals for masses below $\mathcal{O}(100)$ MeV. The absence of a signal enables setting complementary limits to those derived from cosmological observations and direct detection searches for cosmic ray-boosted dark matter.

hep-ph↗

arXiv2025

Characterization of the ionization response of argon to nuclear recoils at the keV scale with the ReD experiment

In the recent years, argon-based experiments looking for Dark Matter in the Universe have explored the non-standard scenario in which Dark Matter is made by low-mass Weakly Interacting Massive Particles, of mass in the range of 1-10 GeV instead of the canonical hundreds of GeV. Detecting such particles is challenging, as their expected signatures are nuclear recoils with energies below 10 keV, observable solely via ionization. This necessitates a precise understanding of the detector response in this energy regime, which remains incomplete for argon. To address this, the ReD experiment was developed within the framework of the DarkSide-20k Collaboration to produce and characterize few-keV nuclear recoils. A compact dual-phase argon Time Projection Chamber (TPC) was irradiated with neutrons from a Cf252 source, to produce Ar recoils in the energy range of interest via (n,n') elastic scattering. A downstream spectrometer composed of 18 plastic scintillators detected the neutrons scattered off Ar nuclei, enabling recoil energy reconstruction via two-body kinematics. The ionization yield Qy of argon, defined as the number of electrons produced per unit energy deposit, was measured in a model-independent way between 2 and 10 keV. These measurements extend direct experimental coverage well below the previous limit of approximately 7 keV. The results are consistent with existing data above 7 keV, while they indicate a higher Qy at lower energies.

nucl-ex↗

arXiv2025

Microscopic study of the asymptotic behavior of the reduced width amplitude in $^7$Li and $^7$Be

We investigate the effects of different basis model spaces on the calculation of reduced width amplitude (RWA) and asymptotic normalization coefficient (ANC) for the $^{7}$Li and $^{7}$Be nuclei. The two-cluster model ($α+t/^3$He) and three-cluster model ($α+d+n/p$) with the generator coordinates method (GCM) are applied to calculate the wave function of $^7$Li and $^7$Be. Specifically, the model space for the three-cluster model is constructed upon a sufficiently broad space of two-cluster configuration bases by further including three-cluster configuration bases. We compare the impact on the results from two basis sets for these added three-cluster bases: one with a compact and one with a broad spatial distribution. The final results reveal that the two-cluster model cannot accurately reproduce the binding energies of $^7$Li and $^7$Be, and tends to overestimate their ANCs. Regarding the calculations with the three-cluster model, while the two basis sets do not give significant differences in energy or energy spectrum, the basis set with a compact model space fails to describe the asymptotic behavior of the RWA adequately. This introduces excessive uncertainty into the ANC calculation. In the end, we conclude that when calculating ANC via a microscopic framework, particular attention must be paid to ensuring sufficient model space, especially for the components describing the breakup channels. This approach provides ANC values for $^7$Li and $^7$Be that agree well with experimental results.

nucl-th↗

arXiv2025

Characterization of spurious-electron signals in the double-phase argon TPC of the DarkSide-50 experiment

Spurious-electron signals in dual-phase noble-liquid time projection chambers have been observed in both xenon and argon Time Projection Chambers (TPCs). This paper presents the first comprehensive study of spurious electrons in argon, using data collected by the DarkSide-50 experiment at the INFN Laboratori Nazionali del Gran Sasso (LNGS). Understanding these events is a key factor in improving the sensitivity of low-mass dark matter searches exploiting ionization signals in dual-phase noble liquid TPCs. We find that a significant fraction of spurious-electron events, ranging from 30 to 70% across the experiment's lifetime, are caused by electrons captured from impurities and later released with delays of order 5-50 ms. The rate of spurious-electron events is found to correlate with the operational condition of the purification system and the total event rate in the detector. Finally, we present evidence that multi-electron spurious electron events may originate from photo-ionization of the steel grid used to define the electric fields. These observations indicate the possibility of reduction of the background in future experiments and hint at possible spurious electron production mechanisms.

hep-ex↗

arXiv2025

Acceleration of positive muons by a radio-frequency cavity

Acceleration of positive muons from thermal energy to $100~$keV has been demonstrated. Thermal muons were generated by resonant multi-photon ionization of muonium atoms emitted from a sheet of laser-ablated aerogel. The thermal muons were first electrostatically accelerated to $5.7~$keV, followed by further acceleration to 100 keV using a radio-frequency quadrupole. The transverse normalized emittance of the accelerated muons in the horizontal and vertical planes were $0.85 \pm 0.25 ~\rm{(stat.)}~^{+0.22}_{-0.13} ~\rm{(syst.)}~π~$mm$\cdot$mrad and $0.32\pm 0.03~\rm{(stat.)} ^{+0.05}_{-0.02} ~\rm{(syst.)}~π~$mm$\cdot$mrad, respectively. The measured emittance values demonstrated phase space reduction by a factor of $2.0\times 10^2$ (horizontal) and $4.1\times 10^2$ (vertical) allowing good acceleration efficiency. These results pave the way to realize the first-ever muon accelerator for a variety of applications in particle physics, material science, and other fields.

physics.acc-ph↗

arXiv2025

Bridging the Gap: OPTICAM Reveals the Hidden Spin of the WZ Sge Star GOTO 065054.49+593624.51

WZ Sge stars are highly evolved accreting white dwarf systems (AWDs) exhibiting remarkably large amplitude outbursts (a.k.a. super-outbursts), typically followed by short rebrightenings/echo outbursts. These systems have some of the lowest mass transfer rates among AWDs, making even low magnetic fields dynamically important. Such magnetic fields are often invoked to explain the phenomenology observed in these systems, such as their X-ray luminosity and long periods of quiescence (30+ years). However, the detection of these is very elusive given the quenching of the accretion columns during outburst and the low luminosity of these systems during quiescence. Here we present high-cadence multi-band observations with {\it OPTICAM} of the recent outburst of the recently discovered WZ Sge star GOTO065054.49+593624.51, during the end of the main outburst and the dip in-between rebrightenings, covering 2 orders of magnitude in brightness. Our observations reveal the presence of a statistically significant signal with $P_ω\simeq148$ seconds in the bluer ($g$) band which is detected only during the dip between the main outburst and the rebrigthenings. We interpret this signal as the spin period of the AWD. If confirmed, GOTO 0650 would bridge the gap between intermediate- and fast-rotating intermediate polars (IPs) below the period gap.

astro-ph.HE↗

arXiv2025

Emulating multi-channel scattering based on the eigenvector continuation in the discrete basis formalism

We construct an emulator for a multi-channel scattering problem based on the eigenvector continuation. To this end, we employ the Kohn variational principle formulated in the discrete basis formalism. We apply this to one-dimensional scattering problems with a Gaussian barrier. Both for a single-channel and two-channel problems, we demonstrate that the penetration probability as well as the wave functions are well reproduced by the emulator. In particular, the energy dependence of the penetrability in a wider range of energy from well below the barrier to well above the barrier is successfully reproduced.

nucl-th↗

arXiv2025

The candidates of 2$α$ condensate around the 16O nucleus studied by the real-time evolution method

Background: Searching for alpha condensation around a core nucleus, a new class of nuclear clustering, is an interesting topic. Previous theoretical studies predicted 16O + 2$α$ condensed states. However, in those studies, the strong mixing with non-resonant states made the identification of true resonant states non-trivial. Purpose: To address this issue, we aim to provide a more robust theoretical verification of the 16O + 2$α$ condensation. Our goal is to clearly identify the resonant states and predict their properties, such as decay widths. Method: We employ the real-time evolution method (REM), which generates physically important basis states using the equation of motion, minimizing contamination from the continuum. The analytical continuation in the coupling constant (ACCC) was used to estimate the $α$-decay widths. Results: The present calculations show much better convergence of eigenstates, and the $0^+_3$ and $0^+_4$ states showed remarkable isoscalar monopole transition strengths, which were in good agreement with the predictions of Ichikawa et al. The small alpha-decay widths for these states suggest that experimental observation appears feasible. Conclusion: The present results show the reasonable agreement with a previous work, firmly establishing the $0^+_3$ and $0^+_4$ states approximately 5 and 1 MeV below the 16O + 2$α$ threshold as candidates for the 2$α$ condensation around the core nucleus 16O. The REM proved effective in identifying these states.

nucl-th↗

arXiv2024

Search for dark matter annual modulation with DarkSide-50

Dark matter induced event rate in an Earth-based detector is predicted to show an annual modulation as a result of the Earth's orbital motion around the Sun. We searched for this modulation signature using the ionization signal of the DarkSide-50 liquid argon time projection chamber. No significant signature compatible with dark matter is observed in the electron recoil equivalent energy range above $40~{\rm eV_{ee}}$, the lowest threshold ever achieved in such a search.

hep-ex↗

arXiv2024

Long-term temporal stability of the DarkSide-50 dark matter detector

The stability of a dark matter detector on the timescale of a few years is a key requirement due to the large exposure needed to achieve a competitive sensitivity. It is especially crucial to enable the detector to potentially detect any annual event rate modulation, an expected dark matter signature. In this work, we present the performance history of the DarkSide-50 dual-phase argon time projection chamber over its almost three-year low-radioactivity argon run. In particular, we focus on the electroluminescence signal that enables sensitivity to sub-keV energy depositions. The stability of the electroluminescence yield is found to be better than 0.5%. Finally, we show the temporal evolution of the observed event rate around the sub-keV region being consistent to the background prediction.

hep-ex↗

arXiv2024

Isospin-forbidden electric dipole transition of the 9.64 MeV state of $^{12}$C

The electric dipole transition of the 3$^-$ state at 9.64 MeV of $^{12}$C to the 2$^+$ state at 4.44 MeV is speculated to play a key role in the triple-$α$ reaction at high temperatures. A theoretical prediction of its transition width is a challenge to nuclear theory because it belongs to a class of isospin-forbidden transitions. We extend a microscopic 3$α$ cluster-model to include isospin 1 impurity components, and take into account both isovector and isoscalar electirc dipole operators. Several sets of $2^+$ and $3^-$ wave functions are generated by solving a radius-constrained equation of motion with the stochastic variational method, resulting in reproducing very well the electric quadrupole and octupole transition probabilities to the ground state. The electric dipole transition width is found to be 7--31 meV, 16 meV on the average, and more than half of the width is contributed by the isospin mixing of $α$ particles.

nucl-th↗