arXiv subjects

Koichi Ueda

Publications and source records attributed to Koichi Ueda.

4 recordsLinked to original sources

arXiv2026

Studies on photon-feedback and LaB$_6$ photocathode for the GasPM development

We present new developments, based on beam tests and cosmic rays, on the gaseous photomultiplier (GasPM). The GasPM detects photons by combining a photocathode with a resistive-plate-chamber avalanche. It achieves $\mathcal{O}$(10) ps time resolution with affordable scalability. The GasPM provides precise and efficient Cherenkov-based charged-particle identification too when combined with a radiator. Our target application in a future Belle II upgrade aims at suppressing beam-induced background photons, which are typically detected off-collision time, that spoil the electromagnetic calorimeter performance. We reached 25 ps single-photon time-resolution at 3.3x10$^6$ gain in 2022, using a picosecond-pulse laser and a LaB$_6$ photocathode. However, electrons entering through a MgF$_2$ window upstream of a CsI photocathode showed a worsening to 70 ps in a 2023 test. Here we aim at addressing the chief causes of the observed degradation. We focus on ultraviolet-photon emission from the de-excitation of the gas molecules, which generates a secondary "photon-feedback" signal overlapping the primary one, and degrading time resolution. We conceive and operate an improved beam test that, along with multiple device-configuration changes, employes a new 10 GSPS frequency digitizer to separate the photon-feedback signal from the genuine signal. We also use cosmic-rays on a LaB$_6$ photocathode, which has higher than CsI's resistance to air and to ions drifting backwards onto the photocathode, to explore its quantum efficiency.

physics.ins-det↗

arXiv2026

Recent GasPM advances: photon-feedback mitigation and LaB$_{6}$ photocathode studies

We report recent developments and tests with beams and cosmic rays of the gaseous photomultiplier (GasPM). The GasPM is a photosensor that combines a photocathode with the avalanche-multiplication mechanism of a resistive-plate chamber, offering excellent time resolution and cost-effective scalability. In addition, the GasPM provides precise and efficient Cherenkov-based charged-particle identification if combined with a radiator. Our primary use case aims at an upgrade of the Belle II detector to suppress beam-induced background photons, preferably detected off-collision time, that degrade the performance of the electromagnetic calorimeter. In 2022 we achieved a promising single-photon time-resolution of 25 ps at 3.3 x 10$^6$ gain, using a picosecond-pulse laser and a LaB$_6$ photocathode. However, a 2023 beam test with electrons impinging on a MgF$_2$ window attached to a CsI photocathode showed a worsening to 70 ps. This work aims at addressing the principal causes of the time-resolution degradation. We primarily target ultraviolet-photon emission during excitation and de-excitation of the gas molecules, which leads to a secondary signal that overlaps the primary signal, spoiling time resolution (photon feedback). We design and execute an improved beam test. Along with several GasPM configuration changes, we introduce a new 10 GSPS frequency digitizer to better discriminate primary from secondary signals thus enabling the study of photon feedback. We also conduct a cosmic-ray test using a LaB$_6$ photocathode, which is known to have higher than CsI's resistance to ions drifting backwards onto the photocathode and to air exposure, to probe quantum efficiency in view of an upcoming beam test.

physics.ins-det↗

arXiv2022

NMR evidence for energy gap opening in thiol-capped platinum nanoparticles

When the particle size of a metal is reduced, it is expected that an energy gap will open due to the quantum size effect. However, the energy gap in platinum (Pt) metal nanoparticles has not been observed directly by nuclear magnetic resonance (NMR). To investigate the particle size dependence of the electronic state of Pt nanoparticles, we performed 195Pt NMR experiments on thiol-capped Pt nanoparticles with three different average diameters of less than 3 nm. For the nanoparticles with a diameter of 2.8 nm, we observed usual metallic behavior with a smaller density of states than that of the bulk Pt. In contrast, the temperature dependence of 1/T1T in nanoparticles less than 2.5 nm in diameter is an activation-energy form above 150 K, which is semiconducting behavior with an energy gap of the order of 2000 K. The significant decrease in 1/T1T by more than two orders of magnitude in the smaller Pt nanoparticles compared to the bulk Pt is attributable to the disappearance of the density of states at the Fermi energy, which is consistent with the opening of an energy gap. These results indicate a metal-insulator transition below 2.5 nm in diameter is present in our thiol-capped Pt nanoparticle samples. The effect of the thiol capping on the electronic structure suggested by the experimental results is also discussed.

cond-mat.mes-hall↗

arXiv2014

Conformance Testing as Falsification for Cyber-Physical Systems

In Model-Based Design of Cyber-Physical Systems (CPS), it is often desirable to develop several models of varying fidelity. Models of different fidelity levels can enable mathematical analysis of the model, control synthesis, faster simulation etc. Furthermore, when (automatically or manually) transitioning from a model to its implementation on an actual computational platform, then again two different versions of the same system are being developed. In all previous cases, it is necessary to define a rigorous notion of conformance between different models and between models and their implementations. This paper argues that conformance should be a measure of distance between systems. Albeit a range of theoretical distance notions exists, a way to compute such distances for industrial size systems and models has not been proposed yet. This paper addresses exactly this problem. A universal notion of conformance as closeness between systems is rigorously defined, and evidence is presented that this implies a number of other application-dependent conformance notions. An algorithm for detecting that two systems are not conformant is then proposed, which uses existing proven tools. A method is also proposed to measure the degree of conformance between two systems. The results are demonstrated on a range of models.

eess.SY↗