arXiv subjects

S. Nozaki

Publications and source records attributed to S. Nozaki.

5 recordsLinked to original sources

arXiv2025

Development and Quality Control of PMT Modules for the Large-Sized Telescopes of the Cherenkov Telescope Array Observatory

The camera of the Large-Sized Telescopes (LSTs) of the Cherenkov Telescope Array Observatory (CTAO) consists of 1855 pixels that are grouped into 265 high-performance photomultiplier tube (PMT) modules. Each module comprises a seven-light-guide plate, seven PMT units, a slow control board, and a readout board with a trigger board. %In this paper we describe The requirements for the PMT modules include various aspects, such as photon detection efficiency, dynamic range, buffer depth, and test pulse functionality. We have developed a high-performance PMT module that fulfills all these requirements. Mass-production and quality control (QC) of modules for all four LSTs of the northern CTAO have been completed. Here we report on the technical details of each element of the module and its performance, together with the methods and results of QC measurements.

astro-ph.IM↗

arXiv2023

Extraordinary physical properties of superconducting YBa$_{1.4}$Sr$_{0.6}$Cu$_3$O$_6$Se$_{0.51}$ in a multiphase ceramic material

We report on a novel material obtained by modifying pristine YBCO superconductor in solid phase synthesis via simultaneous partial substitution of Ba by Sr and O by Se. Simultaneous application of EDX and EBSD confirmed that Se atoms indeed enter the crystalline lattice cell. The detailed XRD analysis further confirmed this conclusion and revealed that the obtained polycrystalline material contains 5 phases, with the major phase ($>$30\%) being a cuprate YBa$_{1.4}$Sr$_{0.6}$Cu$_{3}$O$_{6}$Se$% _{0.51}$. The obtained superconductor demonstrates unique properties, including i) two superconducting transitions with $T_{c1}\approx$ 35 K (granular surface phase) and $T_{c2}\approx$ 13 K (bulk granular phase) - this granular phase arrangement naturally yields the Wohlleben effect; ii) reentrant diamagnetism and resistive state; iii) strong paramagnetism with Curie-Weiss behavior (% $θ_{CW} \approx$ 4 K) and the ferromagnetic phase overruled by superconductivity; iv) Schottky anomaly visible in the heat capacity data and most likely delivered by small clusters of magnetic moments. Thorough analysis of the heat capacity data reveals a strong-coupling $d-$wave pairing in its bulk phase (with $2Δ/T_{c}\approx 5$), and, most importantly, a very unusual anomaly in this cuprate. There are reasons to associate this anomaly with the quantum criticality observed in traditional cuprate superconductors at much higher fields (achievable only in certain laboratories). In our case, the fields leading to quantum criticality are much weaker ($\sim $7-9 T) thus opening avenues for exploration of the interplay between superconductivity and pair density waves by the wider research community.

cond-mat.supr-con↗

arXiv2021

Physics Performance of the Large-Sized Telescope prototype of the Cherenkov Telescope Array

The Large-Sized Telescope (LST) prototype of the future Cherenkov Telescope Array (CTA) is located at the Northern site of CTA, on the Canary Island of La Palma. It is designed to provide optimal performance in the lowest part of the energy range covered by CTA, observing gamma rays down to energies of tens of GeV. The LST prototype started performing astronomical observations in November 2019 during the commissioning of the telescope and it has been taking data since then. In this contribution, we will present the tuning of the characteristics of the telescope in the Monte Carlo (MC) simulations to describe the data obtained, the estimation of its angular and energy resolution, and an evaluation of its sensitivity, both with simulations and with observations of the Crab Nebula.

astro-ph.HE↗

arXiv2021

Commissioning of the camera of the first Large Size Telescope of the Cherenkov Telescope Array

The first Large Size Telescope (LST-1) of the Cherenkov Telescope Array has been operational since October 2018 at La Palma, Spain. We report on the results obtained during the camera commissioning. The noise level of the readout is determined as a 0.2 p.e. level. The gain of PMTs are well equalized within 2\% variation, using the calibration flash system. The effect of the night sky background on the signal readout noise as well as the PMT gain estimation are also well evaluated. Trigger thresholds are optimized for the lowest possible gamma-ray energy threshold and the trigger distribution synchronization has been achieved within 1~ns precision. Automatic rate control realizes the stable observation with 1.5\% rate variation over 3 hours. The performance of the novel DAQ system demonstrates a less than 10\% dead time for 15 kHz trigger rate even with sophisticated online data correction.

astro-ph.IM↗

arXiv2018

A Technique for Estimating the Absolute Gain of a Photomultiplier Tube

Detection of low-intensity light relies on the conversion of photons to photoelectrons, which are then multiplied and detected as an electrical signal. To measure the actual intensity of the light, one must know the factor by which the photoelectrons have been multiplied. To obtain this amplification factor, we have developed a procedure for estimating precisely the signal caused by a single photoelectron. The method utilizes the fact that the photoelectrons conform to a Poisson distribution. The average signal produced by a single photoelectron can then be estimated from the number of noise events, without requiring analysis of the distribution of the signal produced by a single photoelectron. The signal produced by one or more photoelectrons can be estimated experimentally without any assumptions. This technique, and an example of the analysis of a signal from a photomultiplier tube, are described in this study.

astro-ph.IM↗