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Megumi Niikura

Publications and source records attributed to Megumi Niikura.

7 recordsLinked to original sources
arXiv2026

Energy spectra of light charged particles emitted following muon nuclear capture on $^\mathrm{nat}$Si

Background: Charged-particle emission following muon nuclear capture (muNC) probes the de-excitation dynamics of highly excited nuclei, particularly the interplay between preequilibrium and evaporation processes. While proton emission has been relatively well studied, data on composite charged particles remain limited, especially for low-energy alpha particles. Purpose: This work aims to measure energy spectra for individual charged-particle species following muNC on silicon and constrain theoretical descriptions of preequilibrium, evaporation, and composite-particle emission. Method: An experiment was performed at the RIKEN-RAL Muon Facility. Charged particles were identified using Delta E-E telescopes and digital pulse-shape analysis with nTD-Si detectors. Initial energy spectra were reconstructed by unfolding and compared with the microscopic and evaporation model (MEM) and PHITS calculations incorporating the surface coalescence model and meson-exchange-current extension. Results: Energy spectra of protons, deuterons, tritons, and alpha particles were extracted over a broad energy range, including the first measurement of the low-energy alpha-particle spectrum. MEM more closely reproduces the proton, deuteron, and triton spectral shapes and describes the low-energy alpha-particle spectrum well. PHITS reproduces the overall slope of the alpha-particle spectrum but exhibits particle-dependent discrepancies in absolute yields, including an overestimation of the evaporation component for all four species. Conclusion: The results demonstrate particle-species-dependent differences in charged-particle emission following muNC. These spectra constrain descriptions of preequilibrium and evaporation processes and highlight the need for improved modeling of composite-particle emission.

nucl-ex↗
arXiv2026

Comprehensive table of calculated Huff factors

We present a systematic calculation of the Huff factor for nuclei with atomic numbers ($ Z $) in the range of $ 6 \leq Z \leq 94 $. The Huff factor quantifies the increase in the partial lifetime of the decay-in-orbit (DIO) of the muonic atom and serves as an essential correction factor for extracting the nuclear muon capture rate from the measured lifetimes of the muonic atom. However, previous calculations typically provided only the atomic number dependence and neglected isotope dependence -- an assumption whose reliability had not been examined, despite its importance for a comprehensive understanding of the nuclear muon capture rate. In this work, we calculate the Huff factor using nuclear charge distributions obtained from a fully self-consistent microscopic nuclear structure model that incorporates pairing and deformation effects. The resultant Huff factors exhibit a monotonic decrease with increasing $ Z $, while the isotope dependence is found to be small. Our results also show good agreement with previous calculations, supporting the reliability of the present framework. The comprehensive set of Huff factors presented here constitutes the first unified values currently available and will serve as a basis for future evaluations of muon nuclear data.

nucl-th↗
arXiv2026

Muon Nuclear Data Development Project

Negative muon-induced nuclear reactions play a critical role in a wide range of scientific and technological applications; however, comprehensive nuclear data for these processes remain unavailable. To address this gap, we have launched the Muon Nuclear Data (muND) Development Project in Japan, aiming to construct a dedicated data library for muon capture reactions. The library consists of four sub-libraries: muonic X-ray energies and intensities (XR), lifetimes of muonic atoms and nuclear capture rates (LT), energy spectra of emitted particles (ES), and production branching ratios of residual nuclei (BR). This project integrates experimental measurements, theoretical modeling, and machine learning techniques to compile and evaluate the data. We report the current status and recent progress of each sub-library.

nucl-ex↗
arXiv2024

Muon Nuclear Data

We plan to develop a new nuclear database for muon-induced nuclear reactions (muon nuclear data). The database will consist of (1) energies and intensities of the muonic X rays, (2) lifetimes of the muonic atom, (3) production branching ratio of the residual nuclei by muon capture, (4) emission probabilities of the particles after muon capture, and (5) energy spectra of the emitted particles after muon capture. In this paper, we review the present status and current investigations for the muon nuclear data.

nucl-ex↗
arXiv2023

Chiral symmetry restoration at high matter density observed in pionic atoms

Modern theories of physics tell that the vacuum is not an empty space. Hidden in the vacuum is a structure of anti-quarks $\bar{q}$ and quarks $q$. The $\bar{q}$ and $q$ pair has the same quantum number as the vacuum and condensates in it since the strong interaction of the quantum chromodynamics (QCD) is too strong to leave it empty. The $\bar{q}q$ condensation breaks the chiral symmetry of the vacuum. The expectation value $<\bar{q}q>$ is an order parameter. For higher temperature or higher matter-density, $|<\bar{q}q>|$ decreases reflecting the restoration of the symmetry. In contrast to these clear-cut arguments, experimental evidence is so far limited. First of all, the $\bar{q}q$ is nothing but the vacuum itself. It is neither visible nor perceptible. In this article, we unravel this invisible existence by high precision measurement of pionic atoms, $π^-$-meson-nucleus bound systems. Using the $π^-$ as a probe, we demonstrate that $|<\bar{q}q>|$ is reduced in the nucleus at 58% of the normal nuclear density by a factor of 77 $\pm$ 2% compared with that in the vacuum. This reduction indicates that the chiral symmetry is partially restored due to the extremely high density of the nucleus. The present experimental result clearly exhibits the existence of the hidden structure, the chiral condensate, in the vacuum.

nucl-ex↗
arXiv2023

Effect of large-angle incidence on particle identification performance for light-charged ($Z \le 2$) particles by pulse shape analysis with a pad-type nTD silicon detector

In recent years, particle discrimination methods based on digital waveform analysis techniques for neutron-transmutation-doped silicon (nTD-Si) detectors have become widely used for the identification of low-energy charged particles. Although the particle discrimination capability of this method has been well demonstrated for small incident angles, the particle discrimination performance may be affected by changes in the detector response when the detector is moved closer to the charged particle source and the incident position distribution and incident angle distribution to the detector become wide. In this study, we performed a beam test for particle discrimination in light-charged ($Z \le 2$) particles using the digital waveform analysis method with a pad-type nTD-Si detector and investigated the dependence of the performance of the particle discrimination on the incident position and incident angle. As the incident angle increased, a decrease in the maximum current was observed, which was sufficient to affect the performance of the particle discrimination. This decrease can be expressed as a function of the penetration depth of the charged particles into the detector, which varies for each nuclide.

physics.ins-det↗
arXiv2023

Response of germanium detectors for high-energy $γ$-rays by $^{27}$Al(p, $γ$)$^{28}$Si at Ep=992 keV

The performance of germanium detectors for high-energy $γ$-rays was evaluated using a 992-keV resonance in the $^{27}$Al(p, $γ$)$^{27}$Si reaction. The measurement was conducted at the RIKEN tandem accelerator. The energy of the excited state from the resonance was evaluated as 12540.7(2) keV. Using newly evaluated excitation energy, an energy calibration function and the photo-peak efficiency of Ge detectors up to 10.8-MeV photon were deduced. The energy accuracy is achieved at 0.3 keV for the overall energy region. This reaction provides reliable energy and efficiency standards for high-energy $γ$ rays.

physics.ins-det↗