大口径チェレンコフ望遠鏡に搭載する高速カメラの開発

宇宙空間には様々なエネルギーを持つ粒子が無数に存在している。その中でも数十keV(keV:103eV)を越えるエネルギーを持つ光子はガンマ線と呼ばれ、宇宙空間に存在する様々な天体現象から放出されている。ガンマ線は中性粒子であるため、荷電粒子である宇宙線とは違い宇宙空間での磁場との相互作用によって曲げられない。そのため宇宙の奥深くまで見渡す事のできるプローブとして注目されている。そこで長らく地上からのガンマ線観測を行ってきた各国のグループが手を組み、世界で唯一の次世代ガンマ線天文台を建設するCherenkov Telescope Array(CTA)計画が立ち上がった。この計画は、従来から10倍近い感度向上を達成し、20GeVから100TeV以上にわたる極めて広大なエネルギー領域でのガンマ線観測を目指す国際共同実験である。CTA計画では観測するエネルギー領域にあわせて3種類の大気チェレンコフ望遠鏡が建設される予定である。一番大きな大口径望遠鏡(Large-sized Telescope, LST)は直径23mの反射鏡を持ち、焦点面検出器（カメラ）には1855本の光電子増倍管(Photo Multiplier Tube, PMT)が搭載される。LSTの初号機はスペイン領カナリア諸島ラ・パルマ島に2018年に完成しており、現在本稼働に向けた最終調整と、これに続くLST2-4号機の開発準備が行われている。私はこのLSTに搭載されるカメラの開発や性能評価に携わってきた。カメラに搭載するPMTの性能評価を高精度かつ迅速に行うために、カメラ内に搭載するモジュール単位で使用できる暗箱や発光時間幅の短いパルス光源、そのパルス光量を幅広いダイナミックレンジで制御できる光源ボックス等を開発しており、これらを統合して光検出器の時間応答性や出力波形を詳細に評価できる測定セットアップを完成させた。本論文では、LSTに設定された要求値を満たすカメラ装置を開発するために私が行ってきたハードウェアや性能評価セットアップの設計・開発、またこれらを利用して得られた評価結果について述べる。またカメラの光検出器となるPMTの性能評価用光源として開発した発光時間幅がピコ秒オーダー(ピコ秒:10-12秒)の極めて高速なレーザーパルス光源装置については、その動作原理や詳細な時間応答特性についても述べるとともに、PMT以外に速度応答性の高い半導体光検出器や光電管も併用して光源自体の性能評価を行い、光検出器の評価用光源として高い性能を発揮できる結果が得られているのでこれも併せて述べる。甲南大学令和4年度(2022年度

Development of the photomultiplier tube readout system for the first Large-Sized Telescope of the Cherenkov Telescope Array

The Cherenkov Telescope Array (CTA) is the next generation ground-based very high energy gamma-ray observatory. The Large-Sized Telescope (LST) of CTA targets 20 GeV -- 1 TeV gamma rays and has 1855 photomultiplier tubes (PMTs) installed in the focal plane camera. With the 23 m mirror dish, the night sky background (NSB) rate amounts to several hundreds MHz per pixel. In order to record clean images of gamma-ray showers with minimal NSB contamination, a fast sampling of the signal waveform is required so that the signal integration time can be as short as the Cherenkov light flash duration (a few ns). We have developed a readout board which samples waveforms of seven PMTs per board at a GHz rate. Since a GHz FADC has a high power consumption, leading to large heat dissipation, we adopted the analog memory ASIC "DRS4". The sampler has 1024 capacitors per channel and can sample the waveform at a GHz rate. Four channels of a chip are cascaded to obtain deeper sampling depth with 4096 capacitors. After a trigger is generated in a mezzanine on the board, the waveform stored in the capacitor array is subsequently digitized with a low speed (33 MHz) ADC and transferred via the FPGA-based Gigabit Ethernet to a data acquisition system. Both a low power consumption (2.64 W per channel) and high speed sampling with a bandwidth of $>$300 MHz have been achieved. In addition, in order to increase the dynamic range of the readout we adopted a two gain system achieving from 0.2 up to 2000 photoelectrons in total. We finalized the board design for the first LST and proceeded to mass production. Performance of produced boards are being checked with a series of quality control (QC) tests. We report the readout board specifications and QC results.Comment: In Proceedings of the 34th International Cosmic Ray Conference (ICRC2015), The Hague, The Netherlands. All CTA contributions at arXiv:1508.0589

Quality Control of High-Speed Photon Detectors

High-speed-photon detectors are some of the most important tools for observations of high energy cosmic rays. As technologies of photon detectors and their read-out electronics improved rapidly, the time resolution of some cosmic ray detectors became better than one nanosecond. To utilize such devices effectively, calibrations using a short-pulse light source are necessary. We have developed a pulsed laser of 80 picosecond width and adjustable peak intensity up to 100 mW. This pulsed laser is composed of a simple electric circuit and a laser diode. Details of this pulsed laser and its application for quality controls of photon detectors are reported in this contribution

Quality Control of High-Speed Photon Detectors

High-speed-photon detectors are some of the most important tools for observations of high energy cosmic rays. As technologies of photon detectors and their read-out electronics improved rapidly, the time resolution of some cosmic ray detectors became better than one nanosecond. To utilize such devices effectively, calibrations using a short-pulse light source are necessary. We have developed a pulsed laser of 80 picosecond width and adjustable peak intensity up to 100 mW. This pulsed laser is composed of a simple electric circuit and a laser diode. Details of this pulsed laser and its application for quality controls of photon detectors are reported in this contribution

Coherent radio emission from the electron beam sudden appearance

International audienceWe report on radio frequency measurements of the electron beam sudden appearance signal from the Telescope Array Electron Light Source (TA-ELS). The TA-ELS is constructed to calibrate the Telescope Array fluorescence telescope, and as such it can be used to mimic a cosmic-ray or neutrino induced particle cascade. This makes the TA-ELS the perfect facility to study new detection techniques such as the radio detection method. We report on the data obtained by four independent radio detection set-ups. Originally searching for either the direct Askaryan radio emission, or a radar echo from the induced plasma, all these experiments measured a very strong transient signal when the beam exits the accelerator. Due to the different scope of the individual experiments, we have detected the beam sudden appearance signal at different frequencies, ranging between 50 MHz and 12.5 GHz. The direct application in nature for this signal is found in cosmic-ray or neutrino induced particle cascades traversing through different media, such as air, ice, and rock. These measurements are compared to the theoretical predictions for this signal, where it follows that theory and experiment match very well over the full spectrum

Calibration and performance of the readout system based on switched capacitor arrays for the Large-Sized Telescope of the Cherenkov Telescope Array

International audienceThe Cherenkov Telescope Array1 (CTA) is the next-generation ground-based observatory for very-high-energy gamma rays. The CTA consists of three types of telescopes with different mirror areas to cover a wide energy range (20 GeV–300 TeV) with an order of magnitude higher sensitivity than the predecessors. Among those telescopes, the Large-Sized Telescope (LST) is designed to detect low-energy gamma rays between 20 GeV and a few TeV with a 23 m diameter mirror. To make the most of such a large light collection area (about 400 m2), the focal plane camera must detect as much reflected Cherenkov light as possible. We have developed each camera component to meet the CTA performance requirements for more than ten years and performed quality-control tests before installing the camera to the telescope.2, 3 The first LST (LST-1) was inaugurated in October 2018 in La Palma, Spain (Figure 1).4 After the inauguration, various calibration tests were performed to adjust hardware parameters and verify the camera performance. In parallel, we have been developing the analysis software to extract physical parameters from low-level data, taking into account some intrinsic characteristics of the switched capacitor arrays, Domino Ring Sampler version 4 (DRS4), used for sampling the waveform of a Cherenkov signal. In this contribution, we describe the hard- ware design of the LST camera in Section 2, a procedure for low-level calibration in Section 3, and the readout e of the LST camera after the hardware calibration with a dedicated analysis chain in Section 4

Chasing Gravitational Waves with the Chereknov Telescope Array

Presented at the 38th International Cosmic Ray Conference (ICRC 2023), 2023 (arXiv:2309.08219)2310.07413International audienceThe detection of gravitational waves from a binary neutron star merger by Advanced LIGO and Advanced Virgo (GW170817), along with the discovery of the electromagnetic counterparts of this gravitational wave event, ushered in a new era of multimessenger astronomy, providing the first direct evidence that BNS mergers are progenitors of short gamma-ray bursts (GRBs). Such events may also produce very-high-energy (VHE, > 100GeV) photons which have yet to be detected in coincidence with a gravitational wave signal. The Cherenkov Telescope Array (CTA) is a next-generation VHE observatory which aims to be indispensable in this search, with an unparalleled sensitivity and ability to slew anywhere on the sky within a few tens of seconds. New observing modes and follow-up strategies are being developed for CTA to rapidly cover localization areas of gravitational wave events that are typically larger than the CTA field of view. This work will evaluate and provide estimations on the expected number of of gravitational wave events that will be observable with CTA, considering both on- and off-axis emission. In addition, we will present and discuss the prospects of potential follow-up strategies with CTA