arXiv subjects

K. Okai

Publications and source records attributed to K. Okai.

4 recordsLinked to original sources

arXiv2022

Lattice constants and magnetism of L10-ordered FePt under high pressure

We studied the relationship between the lattice constant and magnetism of L10-ordered FePt under high pressure by means of first-principles calculations and synchrotron x-ray measurements. Based on our calculations, we found that the c/a ratio shows an anomaly at ~ 20 GPa and that the Pt magnetic moment is sharply suppressed at ~ 60 GPa. As for the c/a, we experimentally verified the anomaly at ~ 20 GPa by powder x-ray diffraction. We also measured the x-ray magnetic circular dichroism at the Pt L edge up to ~ 20 GPa. Any significant change of the Pt magnetic moment was not observed, in agreement with the calculations. These results thus indicate the possibility that novel magnetic states can be created in L10-ordered FePt by lattice deformation under high pressure.

cond-mat.mtrl-sci↗

arXiv2018

Energy response of X-rays under high flux conditions using a thin APD for the energy range of 6-33 keV

This paper reports on the demonstration of a high-rate energy measurement technique using a thin depletion layer silicon avalanche photodiode (Si-APD). A dedicated amplitude-to-time converter is developed to realize simultaneous energy and timing measurement in a high rate condition. The energy response of the system is systematically studied by using monochromatic X-ray beam with an incident energy ranging from 6 to 33 keV. The obtained energy spectra contain clear peaks and tail distributions. The peak fraction monotonously decreases as the incident photon energy increases. This phenomenon can be explained by considering the distribution of the energy deposit in silicon, which is investigated by using a Monte Carlo simulation.

physics.ins-det↗

arXiv2017

Fast X-ray detector system with simultaneous measurement of timing and energy for a single photon

We developed a fast X-ray detector system for nuclear resonant scattering (NRS) experiments. Our system employs silicon avalanche photo-diode (Si-APD) as a fast X-ray sensor. The system is able to acquire both timing and energy of a single X-ray photon simultaneously in a high rate condition, $10^6$ counts per second for one Si-APD. The performance of the system was investigated in SPring-8, a synchrotron radiation facility in Japan. Good time resolution of \SI{120}{ps} (FWHM) was achieved with a slight tail distribution in the time spectrum by a level of $10^{-9}$ at 1 ns apart from the peak. Using this system, we successfully observed the NRS from the 26.27-keV level of mercury-201, which has a half-life of $630 \pm 50$ ps. We also demonstrated the reduction of background events caused by radioactive decays in a radioactive sample by discriminating photon energy.

physics.ins-det↗

arXiv2017

Nuclear resonant scattering experiment with fast time response: new scheme for observation of $^{229\rm m}$Th radiative decay

Nuclear resonant excitation of the 29.19-keV level in $^{229}$Th with high-brilliance synchrotron- radiation and detection of its decay signal, are proposed with the aim of populating the extremely low-energy isomeric state of $^{229}$Th.The proposed experiment, known as nuclear resonant scattering (NRS), has the merit of being free from uncertainties about the isomer level energy. However, it requires higher time resolution and shorter tail in the response function of the detector than that of conventional NRS experiments because of the short lifetime of the 29.19-keV state. We have fabricated an X-ray detector system which has a time resolution of 56 ps and a shorter tail function than the previously reported one. We have demonstrated an NRS experiment with the 26.27-keV nuclear level of $^{201}$Hg for feasibility assessment of the $^{229}$Th experiment. The NRS signal is clearly distinct from the prompt electronic scattering signal by the implemented detector system. The half-life of the 26.27-keV state of $^{201}$Hg is determined as 629 $\pm$ 18 ps which is better precision by a factor three than that reported to date.

nucl-ex↗