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K. Oikawa

Publications and source records attributed to K. Oikawa.

7 recordsLinked to original sources
arXiv2026

Development of a Modular Current-Mode NaI(Tl) Detector Array for Parity Odd (n,{\gamma}) Cross Section Measurements

The Neutron Optics Parity and Time-Reversal Violation Experiment (NOPTREX) Collaboration has developed a modular array of 24 NaI(Tl) detectors to measure parity and time-reversal symmetry violation in neutron-nucleus interactions. These detectors feature custom electronics that allow for operation in pulse or current mode. This paper describes the design, construction, characterization, and testing of the detectors in this array. We demonstrate the ability of the array to detect parity-odd asymmetries in neutron resonances by observing the known 0.7 eV parity-violating resonance in 139La in measurements at LANSCE.

physics.ins-det↗
arXiv2025

Revisiting the ARM cut in Compton gamma-ray imaging and its application to the INSPIRE detector

The Compton camera is a gamma-ray imaging device developed in the 1970s. In the 1990s, the COMPTEL detector onboard the CGRO was the first to utilize a Compton camera for MeV all-sky survey observations. Recently, various Compton cameras have been developed using scintillators, semiconductors, and gas detectors, some of which are intended for future small satellite missions as well as medical applications. However, the image obtained by a Compton camera has strong artifacts owing to the overlap of the Compton cones or the arcs, which degrade the resolution and sensitivity of the image. In this study, we revisit the adaptive ARM cut that significantly reduces artifacts when the direction of gamma ray emitting source is already known. This approach complements the statistically well-defined method based on the response function in the three-dimensional data space of scattering direction (\chi, \psi) and scattering angle \theta, but it is more direct, intuitive, and simplifies the extraction of spectra in astronomical observations of point-like sources. Using a Compton camera, INSPIRE, onboard the ultra-small satellite GRAPHIUM as an example, we numerically evaluated the extent of background reduction to estimate simulation-based sensitivity. The method was also applied to actual measurements using a quarter-scale prototype of INSPIRE to extract spectra from multiple sources within the same field of view.

astro-ph.IM↗
arXiv2012

Observation of lattice waves through observation of the photoluminescence Blinking in InGaN Quantum Well devices

The photoluminescence of III-V wide band-gap semiconductors as InGaN is characterized by local intensity fluctuations, known as 'blinking points', that despite decades of research are not yet completely understood. In this letter we report experimental data and a theoretical interpretation that suggests they are caused by the interference of thermal vibrations of the Quantum Well lattice. With far-field optical tests we could observe the lower frequency tail of these interference waves and study their dynamics as they propagate up to distances of several tens of microns.

physics.optics↗
arXiv2004

Influence of intermartensitic transitions on transport properties of Ni2.16Mn0.84Ga alloy

Magnetic, transport, and x-ray diffraction measurements of ferromagnetic shape memory alloy Ni$_{2.16}$Mn$_{0.84}$Ga revealed that this alloy undergoes an intermartensitic transition upon cooling, whereas no such a transition is observed upon subsequent heating. The difference in the modulation of the martensite forming upon cooling from the high-temperature austenitic state [5-layered (5M) martensite], and the martensite forming upon the intermartensitic transition [7-layered (7M) martensite] strongly affects the magnetic and transport properties of the alloy and results in a large thermal hysteresis of the resistivity $ρ$ and magnetization $M$. The intermartensitic transition has an especially marked influence on the transport properties, as is evident from a large difference in the resistivity of the 5M and 7M martensite, $(ρ_{\mathrm{5M}} - ρ_{\mathrm{7M}})/ρ_{\mathrm{5M}} \approx 15%$, which is larger than the jump of resistivity at the martensitic transition from the cubic austenitic phase to the monoclinic 5M martensitic phase. We assume that this significant difference in $ρ$ between the martensitic phases is accounted for by nesting features of the Fermi surface. It is also suggested that the nesting hypothesis can explain the uncommon behavior of the resistivity at the martensitic transition, observed in stoichiometric and near-stoichiometric Ni-Mn-Ga alloys.

cond-mat.mtrl-sci↗
arXiv2003

Entropy change at the martensitic transformation in ferromagnetic shape memory alloys Ni_{2+x}Mn_{1-x}Ga

The entropy change $ΔS$ between the high-temperature cubic phase and the low-temperature tetragonally-based martensitic phase of Ni$_{2+x}$Mn$_{1-x}$Ga ($x = 0 - 0.20$) alloys was studied. The experimental results obtained indicate that $ΔS$ in the Ni$_{2+x}$Mn$_{1-x}$Ga alloys increases with the Ni excess $x$. The increase of $ΔS$ is presumably accounted for by an increase of magnetic contribution to the entropy change. It is suggested that the change in modulation of the martensitic phase of Ni$_{2+x}$Mn$_{1-x}$Ga results in discontinuity of the composition dependence of $ΔS$.

cond-mat.mtrl-sci↗
arXiv2002

Fermi surface of the heavy fermion superconductor PrOs$_{4}$Sb$_{12}$

We have investigated the de Haas-van Alphen effect in the Pr-based heavy fermion superconductor PrOs$_4$Sb$_{12}$.The topology of Fermi surface is close to the reference compound LaOs$_4$Sb$_{12}$ and well explained by the band structure calculation based on the FLAPW-LDA+U method, where the 4{\it f} electrons are localized. We have confirmed a highly enhanced cyclotron effective mass 2.4$\sim7.6m_{\rm 0}$ which is apparently large compared to the usual Pr-based compounds.

cond-mat.str-el↗