A waveguide high-pass filter system for measuring the spectrum of pulsed terahertz sources

We propose a system for measuring spectra of terahertz (THz) pulses, including single pulses, which is based on high-pass filters (HPFs). The system consists of channels for measuring amplitudes of pulses (initial pulses and those transmitted via HPFs with different cutoff frequencies) and an algorithm for processing of the obtained data. The pulse spectrum is restored by using the iteration method or the amplitude–frequency method. The iteration method of spectrum restoration is applicable in the range of THz pulse durations from 10⁻⁹ s to 10⁻⁷ s. The amplitude–frequency method is applicable to THz pulses with durations exceeding 10⁻⁸ s. The system for measuring of THz pulse spectra was simulated by using the characteristics of specially developed waveguide HPFs. The relative simulation error of determining the central frequency by the amplitude–frequency method is equal to 2 · 10⁻⁶ for THz pulse durations of 10⁻⁵ s and longer.     

Evaluation of new large area PMT with high quantum efficiency

The neutrino detector of the Jiangmen Underground Neutrino Observatory(JUNO) is designed to use20 kilotons of liquid scintillator and approximately 16000 20 inch photomultipliers(PMTs).One of the options is to use the 20 inch R12860 PMT with high quantum efficiency which has recently been developed by Hamamatsu Photonics.The performance of the newly developed PMT preproduction samples is evaluated.The results show that its quantum efficiency is 30%at 400 nm.Its Peak/Valley(P/V) ratio for the single photoelectron is 4.75 and the dark count rate is 27 kHz at the threshold of 3 mV while the gain is at 1 × 10~7.The transit time spread of a single photoelectron is 2.86 ns.Generally the performances of this new 20 inch PMT are improved over the old one of R3600.     

Untersuchung der Elektronenkomponente von Einzellawinen mit dem Photomultiplier

With a photomultiplier single electron avalanches of 10³ carriers can be observed in nitrogen. 2.5% of all photons (in the range 3000≦λ≦7000 Å) of an avalanche produce a photoelectron at the cathode of the photomultiplier. The influence of the variable solid angle on the photomultiplier pulse is given. Trouble by noise is calculated and it is shown that the information depends on the number of photoelectrons only. The electron components of avalanches in methane, nitrogen, and mixtures have an exponential rise and the measured rise time constant τ_beob is in accordance with the theoretical value 1/αυ_. For carrier numbers >10⁶ space charge influence is observed, as given by theory. The quantum efficiencyQ per ionizing impact is found to beQ～10^?3 in vapours andQ～1 in gases. Values of the electron drift velocity in mixtures of N₂ and CH₄ are given.     

A method to obtain pulse contrast on a single shot

A novel method for obtaining a single shot multi-point high dynamic range pulse contrast measurement is presented. We use Dammann gratings to generate multiple beamlets by division of amplitude on ultrashort laser pulses. The analysis results show that this method can achieve high dynamic range in pulse contrast measurement on a single shot by using photomultiplier tube （PMT） detectors and the long working distances to minimize cross talk between channels. Some distortion of pulse shape is also analyzed detailedly with the Dammann grating and its compensation grating, which may degrade the pulse contrast measurement in some degree by pulse stretching and spectrum clipping.     

PMT waveform modeling at the Daya Bay experiment

Detailed measurements of Hamamatsu R5912 photomultiplier signals are presented, including the single photoelectron charge response, waveform shape, nonlinearity, saturation, overshoot, oscillation, prepulsing, and afterpulsing. The results were used to build a detailed model of the PMT signal characteristics over a wide range of light intensities. Including the PMT model in simulated Daya Bay particle interactions shows no significant systematic effects that are detrimental to the experimental sensitivity.     

Multistop analyzer for time-of-flight measurements at a resolution of ±5 ns

A multichannel multistop time analyzer that provides the measurement of the time intervals between start and stop pulses and the accumulation of their time distribution in the memory of its unit is described. The time of arrival of a stop signal is recorded at a resolution of ±5 ns and a dead time of 30 ns. The number of stops per start can achieve 6000. The number of channels is 131072 (2¹⁷). The time analyzer is based on a CAMAC unit and can be used to design measurement-information systems for solving problems in neutron spectrometry and mass spectrometry by time-of-flight methods and for studying the time characteristics of luminescence radiation.     

高能电子与超强激光束作用产生的阿秒脉冲列

利用非线性汤姆孙散射的理论，从理论和数值模拟上研究了单电子在横向穿越高斯激光束束 腰时所辐射的x射线阿秒脉冲列的性质. 主要分析了电子以初始能量γ₀=1M eV—100M eV横向穿越激光振幅参数为a₀=1—10的高斯光束束腰获得的阿秒辐射脉冲的 时间 和空间性质. 计算表明，辐射呈现脉冲列的形式. 脉冲列的包络宽度取决于激光强度、束腰 的宽度以及入射电子能量. 电子的初始能量比激光强度对电子辐射脉冲的影响更大. 辐射脉 宽、脉冲间隔和脉冲包络宽度都正比于1/γ²₀，辐射功率正比于 γ⁶₀，辐射能 量正比于γ⁴₀. 当改变激光振幅a₀时，辐射功率正比 于a²₀、辐射包络中单 个脉冲脉宽正比于1/a₀、脉冲之间的间隔正比于a₀. 当保持激光强 度不变，而改变光束 束腰半径w₀时，辐射的脉冲数量、包络和辐射能量正比于w₀. 当 激光功率保 持不变时而改变激光强度和束腰半径时，脉冲包络宽度和最大辐射能量都基本不变. 当激光 振幅参数a₀=1，电子初始能量为10MeV时，激光束腰为两个激光波长时，电子 辐 射脉冲包络宽度只有14×10^-3τ₀（τ₀为入 射激光周期），达到几个阿秒的量级. 关键词： 阿秒脉冲 非线性汤姆孙散射 高斯激光光束     

Dynamical theory for photon and photoelectric pulse distributions in single molecule fluorescence

A single two-level molecule driven by CW-laser field and a photomultiplier tube (PMT) are considered as two parts of the united dynamical system connected with each other by photons of molecular fluorescence. Each PMT is characterized by a rate α of photo-effect and by a rate β of PMT recovery. A theory for the photon distribution function w_N(t) and for the photoelectric pulse distribution function f_n(t) for such a system is built up. If times 1/ α and 1/ β characterizing PMT are much shorter as compared to the average time interval 1/ k between two successively emitted photons of fluorescence, the photon and the photoelectron distribution functions coincide with each other, i.e. f_n(t) ≅ w_N(t). A relation between w_N(t) and f_n(t) is studied in detail for the case in which PMT works slower as compared to the rate k of photon emission, i.e. at 1/ α, 1/ β ≥ 1/ k.     

光纤模式混合器在差分吸收光谱系统中的应用研究

差分吸收光谱技术(DOAS)已经发展为监测对流层痕量气体的重要技术，一般采用光电倍增管(PMT)作为探测器.由于光电二极管阵列(PDA)在多道探测及像元灵敏性方面的优势，采用PDA代替扫描探测装置将能改善系统剩余噪声，提高系统性能.但实际应用中由于使用收发一体Cassegrain望远镜造成灯谱与大气谱结构不统一，在消除PDA像元间差异的过程中给系统带来误差，从而导致剩余噪声的标准偏差达到1.4×10^-3.根据光纤扰模原理在系统中加入自制光纤模式混合器很好的解决了上述难题，在实际应用中起到良好的作用，系统剩余噪声的标准偏差为3.4×10^-4. 关键词： 差分吸收光谱 扰模 模式混合器 剩余噪声     

Development of human hand phantom containing radiophotoluminescence material

A radiophotoluminescence (RPL) material with high radiation sensitivity was made of polyurethane resin, silver-activated metaphosphate glass particles, and hollow glass microspheres. The density was adjusted to be 1.1 g/cm³ by controlling the amount of hollow glass microspheres. The response to high-energy photons over 100 keV was similar to that of the tissue-equivalent material (polymethylmethacrylate) because the two electron densities were similar. The RPL response had satisfactory linearity in the dose range from 10 to 6 × 10⁴ mGy.An RPL scanner for three-dimensional (3-D) dose measurement was composed of an XYZθ motorized stage, a UV pulse laser, a gated photomultiplier tube (PMT), a red-laser displacement sensor, and an integrating ammeter. The surface profile was measured by the red-laser-displacement sensor. The UV laser was used as an excitation source, and the RPL responses were effectively detected with the gated PMT.An RPL material hand phantom was fabricated to understand the extremity dosimetry of a radiation worker's hand. The hand phantom was exposed to X-rays, and its surface dose profile was obtained by the RPL scanner. Subsequently, the hand phantom was sliced into dozens of square plates using a diamond wire saw. Each inner dose profile was obtained with the RPL scanner. The inner dose profiles were roughly consistent with the computational simulation results. These results indicated that RPL imaging of the hand phantom was useful to understand extremity dosimetry.     

An analogue front-end ASIC prototype designed for PMT signal readout

The Water Cherenkov Detector Array(WCDA) is one of the core detectors in the Large High Altitude Air Shower Observatory(LHAASO),and it consists of 3600 photomultiplier tubes(PMTs).Both high resolution time and charge measurement are required over a large dynamic range from 1 photoelectron(P.E.) to 4000 P.E.The prototype of an analogue front-end Application Specific Integrated Circuit(ASIC) fabricated using Global Foundry 0.35 μm CMOS technology is designed to read out the PMT signal in the WCDA.This ASIC employs leading edge discrimination and an(RC)~4 shaping structure.Combined with the following Time-to-Digital Converter(TDC) and Analog-to-Digital Converter(ADC),both the arrival time and charge of the PMT signal can be measured.Initial test results indicate that time resolution is better than 350 ps and charge resolution is better than 10% at 1 P.E.and better than 1% with large input signals(300 P.E.to 4000 P.E.).Besides,this ASIC has a good channel-to-channel isolation of more than 84 dB and the temperature dependency of charge measurement is less than 5% in the range 0-50°C.     

The halogenation of polycrystalline zinc studied by He(I)/(II) photoelectron spectroscopy

He(Iα)/(IIα) photoelectron spectra have been recorded for the surface reaction between clean polycrystalline zinc and molecular chlorine, bromine and iodine. For saturation exposures > 10⁻³ Pa a continuous layer of the ZnX₂ halide is formed. For low exposures < 2 × 10⁻⁴ Pa there is evidence for a precursor species which is assumed to be chemisorbed atomic halogen atoms formed by dissociative adsorption.     

Prototype of readout electronics for the LHAASO KM2A electromagnetic particle detectors

The KM2A(one kilometer square extensive air shower array) is the largest detector array in the LHAASO(Large High Altitude Air Shower Observatory) project. The KM2 A consists of 5242 EDs(Electromagnetic particle Detectors) and 1221 MDs(Muon Detectors). The EDs are distributed and exposed in the wild. Two channels, anode and dynode, are employed for the PMT(photomultiplier tube) signal readout. The readout electronics designed in this paper aims at accurate charge and arrival time measurement of the PMT signals, which cover a large amplitude range from 20 P.E.(photoelectrons) to 2×10~5 P.E. By using a "trigger-less" architecture, we digitize signals close to the PMTs. All digitized data is transmitted to DAQ(Data Acquisition) via a simplified White Rabbit protocol.Compared with traditional high energy experiments, high precision of time measurement over such a large area and suppression of temperature effects in the wild become the key techniques. Experiments show that the design has fulfilled the requirements in this project.     

Pseudo-Lock-in Light Detection Method for a Sinusoidally-Gain-Modulated Photomultiplier Tube

In order to detect a weak signal varied in an intensive background light, we propose a novel pseudo-lock-in light detection scheme used for a photomultiplier tube (PMT) whose gain is modulated sinusoidally. Let the signal light incident on the PMT be modulated sinusoidally at a frequency f. In this situation, one of the dynode voltages applied for the PMT is deeply biased and a sinusoidal reference signal with a frequency f is superimposed on the bias voltage. Then, we detect a frequency component of 2f at the output of the PMT selectively. To pick up only the 2f component, we connect a digital oscilloscope directly to the output of the PMT. Thereafter the digitized waveform stored on a temporal memory of the digital oscilloscope is fast-Fourier-transformed (FFT). Two numerical values obtained from the resultant amplitude and the phase spectrum at the frequency 2f, respectively, correspond to the intensity and the phase of the modulated signal light incident on the PMT. A fundamental performance test of the proposed scheme was carried out.     

Visualization of the spatio-temporal structure of a pulsed photoelectron beam formed by femtosecond laser radiation

A method based on an original electron microscope created for investigating photoelectron beams is presented. It ensures a nanometer spatial resolution and picosecond time resolution. Electrons appearing when a metal needle is irradiated by femtosecond laser pulses are transmitted through a dielectric microcapillary and are subjected to a ponderomotive potential created by femtosecond laser radiation focused near the capillary tip. The position-sensitive detection scheme allows for the detection of the spatial profile of a photo-electron beam with a magnification of K ≅ 4 × 10⁴. The time structure of the photoelectron beam is visualized by scanning the delay time between laser pulses irradiating the needle and a laser pulse focused near the capillary tip.     

Optical fiber-based gamma-ray spectroscopy with cerium-doped lutetium yttrium orthosilicate crystal

A fiber-optic radiation sensor (FORS), which is composed of an inorganic scintillator, a plastic optical fiber (POF), a photomultiplier tube (PMT), and a multichannel analyzer (MCA), was developed for gamma-ray energy spectroscopy. In this study, we selected a cerium-doped lutetium yttrium orthosilicate (LYSO:Ce) as the sensitive element of the FORS and found that LYSO:Ce with dimensions of 3 × 3 × 15mm³ gives the best performance in obtaining a gammaray energy spectrum. The FORS allows us to measure the energy spectra of Co-60, Na-22, and Cs-137.     

Probe Measurements of Parameters of Streamers of Nanosecond Frequency Crown Discharge

Investigations of the parameters of single streamers of nanosecond frequency corona discharge, creating a voluminous low-temperature plasma in extended coaxial electrode systems, are performed. Measurements of the parameters of streamers were made by an isolated probe situated on the outer grounded electrode. Streamers were generated under the action of voltage pulses with a front of 50–300 ns, duration of 100–600 ns, and amplitude up to 100 kV at the frequency of 50–1000 Hz. The pulse voltage, the total current of the corona, current per probe, and glow in the discharge gap were recorded in the experiments. It was established that, at these parameters of pulse voltage, streamers propagate at an average strength of the electric field of 4–10 kV/cm. Increasing the pulse amplitude leads to an increase in the number of streamers hitting the probe, an increase in the average charge of the head of a streamer, and, as a consequence, an increase in the total streamer current and the energy introduced into the gas. In the intervals up to 3 cm, streamer breakdown at an average field strength of 5–10 kV/cm is possible. In longer intervals, during the buildup of voltage after generation of the main pulse, RF breakdown is observed at Е_av ≈ 4 kV/cm.     

Cosmological effects of superconducting strings

Superconducting cosmic strings are a plausible consequence of symmetry breaking in grand unified gauge theories. The luminosity in electromagnetic radiation of an oscillating current-carrying loop may substantially exceed the luminosity in gravitational radiation. In the typical case considered, the energy released electromagnetically is 10⁴⁹ erg s⁻¹, or 10⁶⁶ erg in toto. Several consequences follow from this, the most interesting of which is the possibility that such loops may heat their surroundings, generating large, dense spherical shells of gas. Galaxies forming on these gravitationally unstable shells at moderate redshift will be seen at the present epoch to lie on bubbles having radii in the range 10–20h⁻¹ Mpc if the initial ratio of luminosity in electromagnetic waves to that in gravitational wavess is > 10⁻³ for mass/length 10²² g cm⁻¹. The required primordial energy density in magnetic fields is > 3 × 10⁻⁹ of the radiation energy density, if the charge carriers are bosons or superheavy fermions. Since these shells fill up space, the galaxies will have a distribution similar to that found in a recent survey of the northern sky. When the current saturates, a loop will emit particles copiously, and may be seen as an X-ray source at z ∼ 10–50. Such loops may also contribute significantly to the hard X-ray and γ-ray backgrounds and to 10²⁰ eV cosmic rays.     

Picosecond X-ray plasma radiation measurements

Direct picosecond measurements of X-ray laser plasma radiation were performed with a high-speed X-ray image-converter camera (ICC). This camera operates in the single-frame mode with an exposure time ranging from 5 ns to 0.5 μs and in the streak mode with 5 × 10⁹ to 5 × 10⁷ cm/s streak velocities. Its temporal resolution in the streak mode was calculated to be about 7 ps. A plasma was created by focussing 10 ps 1 to 2 joule laser pulses onto a titanium target placed in a vacuum chamber. The halfwidth of the recorded X-ray pulses varied from 30 to 60 ps.     

On the nature of emissions of polymethyl methacrylate excited by an electron beam of subnanosecond or nanosecond duration

The results of studies of the physical nature of emissions produced in polymethyl methacrylate excited by electron beams of a subnanosecond or a nanosecond duration are presented. The spatial, amplitude, and spectral-kinetic properties of emissions have been examined under an electron beam energy density varying from 10^–4 to 4 × 10^–1 J/cm². It has been found that cathodoluminescence is the primary type of emission under low energy densities of the electron beam. When the energy density of a nanosecond electron beam and/or the number of pulses of excitation by a subnanosecond electron beam were increased, an electrical breakdown of polymethyl methacrylate occurred in the irradiated region. This process was accompanied by a burst of emission of dense, low-temperature plasma.