Fine Time Structure Research Articles

Performance of LGAD strip detectors for particle counting of therapeutic proton beams

Objective. The performance of silicon detectors with moderate internal gain, named low-gain avalanche diodes (LGADs), was studied to investigate their capability to discriminate and count single beam particles at high fluxes, in view of future applications for beam characterization and on-line beam monitoring in proton therapy. Approach. Dedicated LGAD detectors with an active thickness of 55 μm and segmented in 2 mm2 strips were characterized at two Italian proton-therapy facilities, CNAO in Pavia and the Proton Therapy Center of Trento, with proton beams provided by a synchrotron and a cyclotron, respectively. Signals from single beam particles were discriminated against a threshold and counted. The number of proton pulses for fixed energies and different particle fluxes was compared with the charge collected by a compact ionization chamber, to infer the input particle rates. Main results. The counting inefficiency due to the overlap of nearby signals was less than 1% up to particle rates in one strip of 1 MHz, corresponding to a mean fluence rate on the strip of about 5 × 107 p/(cm2·s). Count-loss correction algorithms based on the logic combination of signals from two neighboring strips allow to extend the maximum counting rate by one order of magnitude. The same algorithms give additional information on the fine time structure of the beam. Significance. The direct counting of the number of beam protons with segmented silicon detectors allows to overcome some limitations of gas detectors typically employed for beam characterization and beam monitoring in particle therapy, providing faster response times, higher sensitivity, and independence of the counts from the particle energy.

Technical note: Measurement of the bunch structure of a clinical proton beam using a SiPM coupled to a plastic scintillator with an optical fiber.

Recent proposals of high dose rate plans in protontherapy as well as very short proton bunches may pose problems to current beam monitor systems. There is an increasing demand for real-time proton beam monitoring with high temporal resolution, extended dynamic range and radiation hardness. Plastic scintillators coupled to optical fiber sensors have great potential in this context to become a practical solution towards clinical implementation. In this work, we evaluate the capabilities of a very compact fast plastic scintillator with an optical fiber readout by a SiPM and electronics sensor which has been used to provide information on the time structure at the nanosecond level of a clinical proton beam. A 3×3×3mm3 plastic scintillator (EJ-232Q Eljen Technology) coupled to a 3×3mm2 SiPM (MicroFJ-SMA-30035, Onsemi) has been characterized with a 70 MeV clinical proton beam accelerated in a Proteus One synchrocyclotron. The signal was read out by a high sampling rate oscilloscope (5 GS/s). By exposing the sensor directly to the proton beam, the time beam profile of individual spots was recorded. Measurements of detector signal have been obtained with a time sampling period of 0.8 ns. Proton bunch period (16 ns), spot (10 μs) and interspot (1 ms) time structures could be observed in the time profile of the detector signal amplitude. From this, the RF frequency of the accelerator has been extracted, which is found to be 64 MHz. The proposed system was able to measure the fine time structure of a clinical proton accelerator online and with ns time resolution.

Subsecond X-ray Spikes in Solar Flares: Kinematics of Acceleration and Transport of Accelerated Electrons

Hard X-rays time structure of solar flares can be caused by the timing of electrons acceleration, the kinematics of their propagation in the flaring plasma, and possible changes in plasma parameters and the loop magnetic field. In this paper the subsecond hard X-ray emission features were studied in a model of collapsing magnetic trap. The injection of 50-millisecond pulses of accelerated electrons in looptop with specified distributions of plasma density and magnetic field structure was considered. The magnetic field fluctuations and ion-acoustic turbulence influence on the fine time structure of hard X-ray emission was shown. The simulation results that it is possible to observe a super-thin time structure in the hard X-ray emission from accelerated electrons only if: ion-acoustic turbulence develops in magnetic loops and/or the pitch-angular distribution of electrons is sharply anisotropic. Moreover, the amplitude of individual spike is greater for harder electron spectra.

Nonlinear cochlear mechanics without direct vibration-amplification feedback.

Recent in vivo recordings from the mammalian cochlea indicate that although the motion of the basilar membrane appears actively amplified and nonlinear only at frequencies relatively close to the peak of the response, the internal motions of the organ of Corti display these same features over a much wider range of frequencies. These experimental findings are not easily explained by the textbook view of cochlear mechanics, in which cochlear amplification is controlled by the motion of the basilar membrane (BM) in a tight, closed-loop feedback configuration. This study shows that a simple phenomenological model of the cochlea inspired by the work of Zweig [J. Acoust. Soc. Am. 138, 1102 (2015)] can account for recent data in mouse and gerbil. In this model, the active forces are regulated indirectly, through the effect of BM motion on the pressure field across the cochlear partition, rather than via direct coupling between active-force generation and BM vibration. The absence of strong vibration-amplification feedback in the cochlea also provides a compelling explanation for the observed intensity invariance of fine time structure in the BM response to acoustic clicks.

Simulation of Accelerated Electron Beam Transport with a Fine Time Structure in Flare Loop Plasma

This work discusses the results of the analysis of the pulsed time structure of X-ray radiation for the SOL1996-07-09T09:09:01, GOES X2.6-class flare recorded by CGRO/BATSE and the numerical simulation of the propagation of accelerated electrons in the plasma of a magnetic loop with a time structure at injection that reflects the structure of hard X-ray radiation. Initially, preliminary processing of the data series was carried out, and the spectral methods of Wavelet and Fourier transforms of X-ray series were then used in energy ranges: 22–56 keV and 56–105 keV. In the time series (22–56 keV and 56–105 keV), the time structures with a duration of ~0.1 to ~1 s are selected; their spectral power exceeds the red-noise level with a probability of 99%. The simulation of the thin time structure of the X-ray radiation of the accelerated electron beam was carried out on the basis of the solution of the kinetic equation for the given plasma parameter distributions and the magnetic field along the flare loop. It is shown that for plasma density values of less than ~1011 cm–3 in the loop top, the structure of hard X-ray radiation in the top does not reflect the millisecond structure of accelerated electrons. A fine structure of hard X-rays can occur in footpoints with a high plasma density (n0 > 1011 cm–3), and more likely with a fairly soft electron spectrum (δ ~ 7) and/or with increasing angular anisotropy in the distribution of accelerated electrons at the time of their injection into the top of the magnetic loop.

On the High‐Energy Spectral Component and Fine Time Structure of Terrestrial Gamma Ray Flashes

AbstractTerrestrial gamma ray flashes (TGFs) are very short bursts of gamma radiation associated to thunderstorm activity and are the manifestation of the highest‐energy natural particle acceleration phenomena occurring on Earth. Photon energies up to several tens of megaelectronvolts are expected, but the actual upper limit and high‐energy spectral shape are still open questions. Results published in 2011 by the AGILE team proposed a high‐energy component in TGF spectra extended up to ≈100 MeV, which is difficult to reconcile with the predictions from the Relativistic Runaway Electron Avalanche (RREA) mechanism at the basis of many TGF production models. Here we present a new set of TGFs detected by the AGILE satellite and associated to lightning measurements capable to solve this controversy. Detailed end‐to‐end Monte Carlo simulations and an improved understanding of the instrument performance under high‐flux conditions show that it is possible to explain the observed high‐energy counts by a standard RREA spectrum at the source, provided that the TGF is sufficiently bright and short. We investigate the possibility that single high‐energy counts may be the signature of a fine‐pulsed time structure of TGFs on time scales ≈4 μs, but we find no clear evidence for this. The presented data set and modeling results allow also for explaining the observed TGF distribution in the (Fluence × duration) parameter space and suggest that the AGILE TGF detection rate can almost be doubled.

Harmonic mode-locking and sub-round-trip time nonlinear dynamics of electro-optically controlled solid state laser

Harmonic mode-locking in a solid state laser due to optoelectronic control is studied numerically on the basis of two methods. The first one is detailed numeric simulation taking into account laser radiation fine time structure. It is shown that optimally chosen feedback delay leads to self-started mode-locking with generation of desired number of pulses in the laser cavity. The second method is based on discrete maps for short laser pulse energy. Both methods show that the application of combination of positive and negative feedback loops allows to reduce the period of regular nonlinear dynamics down to a fraction of a laser cavity round trip time.

Multichannel Signal Processing With Deep Neural Networks for Automatic Speech Recognition

Multichannel automatic speech recognition (ASR) systems commonly separate speech enhancement, including localization, beamforming, and postfiltering, from acoustic modeling. In this paper, we perform multichannel enhancement jointly with acoustic modeling in a deep neural network framework. Inspired by beamforming, which leverages differences in the fine time structure of the signal at different microphones to filter energy arriving from different directions, we explore modeling the raw time-domain waveform directly. We introduce a neural network architecture, which performs multichannel filtering in the first layer of the network, and show that this network learns to be robust to varying target speaker direction of arrival, performing as well as a model that is given oracle knowledge of the true target speaker direction. Next, we show how performance can be improved by factoring the first layer to separate the multichannel spatial filtering operation from a single channel filterbank which computes a frequency decomposition. We also introduce an adaptive variant, which updates the spatial filter coefficients at each time frame based on the previous inputs. Finally, we demonstrate that these approaches can be implemented more efficiently in the frequency domain. Overall, we find that such multichannel neural networks give a relative word error rate improvement of more than 5% compared to a traditional beamforming-based multichannel ASR system and more than 10% compared to a single channel waveform model.

RELEC mission: Relativistic electron precipitation and TLE study on-board small spacecraft

The main goal of the Vernov mission is the study of magnetospheric relativistic electron precipitation and its possible influence on the upper atmosphere as well as the observation of Transient Luminous Events (TLE) and Terrestrial Gamma Flashes (TGF) across a broad range of the electromagnetic spectrum.The RELEC (Relativistic Electrons) instrument complex onboard the Vernov spacecraft includes two identical X- and gamma-ray detectors of high temporal resolution and sensitivity (DRGE-1 and DRGE-2), three axis position detectors for high-energy electrons and protons (DRGE-3), a UV TLE imager (MTEL), a UV detector (DUV), a low frequency analyser (LFA), a radio frequency analyser (RFA), and AN electronics module responsible for control and data collection (BE).The RELEC mission conducts the following experiments:–simultaneous observations of high-energy electron and proton fluxes (within the energy range of ∼0.1–10.0MeV) and low-frequency (∼0.1–10kHz) electromagnetic wave field intensity variations with high temporal resolution (∼1ms);–fine time structure (∼1μs) measurements of transient atmospheric events in UV, X- and gamma rays with an optical imaging capability with a resolution of ∼1km in wide field of view (FOV);–measurements of electron flux pitch-angle distributions in dynamical ranges from ∼0.1 up to 105 part/cm2/s;–monitoring of charged and neutral background particles in different areas of near-Earth space.

Influence of longitudinal mode beating on laser-induced damage in fused silica

In our study, the laser-induced damage densities on a fused silica surface produced by multiple longitudinal mode (MLM) pulses are found to be higher than those produced by single longitudinal mode pulses at 1064 nm. This behavior is explained by the enhancement of the three-photon absorption due to the intensity spikes related to longitudinal mode beating. At 355 nm, the absorption is linear and an opposite behavior occurs. It can be explained with the help of a process involving thermomechanics coupled with the fine time structure of MLM pulses, leading to the possible annealing of part of the absorbent defects.

Oscillating Magnetic Trap and Non-Thermal Emission from Solar Flares

Abstract A comparative analysis of the fine time structure of microwave and hard X-ray emissions from the flare event on 1992 November 5 is given. Based on wavelet analysis, quasi-periodic pulsations with a fundamental period of 6 s in both wave bands have been revealed. The anticorrelation of time profiles of microwave and hard X-ray emissions as well as time delays between the emission peaks in different wave bands are explained by the excitation of sausage oscillations of a flare loop modelling by a magnetic trap. It has been shown that the intermediate pitch angle diffusion regime of trapped electrons into the loss-cone should be realized in flare loops. The relation between time delays of emission peaks at different wave bands and the pulse duration has been investigated.

Time delay in pulsations of the nonthermal radiation of solar flares

The oscillations with a period of about 6 and 12 s in the nonthermal radiation of a solar flare occurred on November 5, 1992, are identified. The time-translated profiles of hard X-ray and microwave radiation flux are characterized by an anticorrelation. The specific features of the radiation fine time structure are interpreted using the model of the coronal magnetic mirror where fast magnetoacoustic modes are excited.

CARRIER AND ENVELOPE TRIGGERED COCHLEAR STIMULATION

Generation of electrode stimulation signals for an implanted electrode array is described. An acoustic audio signal is processed to generate band pass signals which include a fine structure carrier signal and a modulator envelope signal. For each band pass signal, fine time structure information is extracted from the carrier signal to determine a sequence of stimulation event signals. For one or more low frequency band pass signals, the modulator envelope signal is sampled synchronously with the carrier signal to create envelope weighted stimulation event signals. For one or more higher frequency band pass signals, if and only if the modulator envelope signal exceeds a sampling threshold value, then the modulator envelope signal is sampled synchronously with the carrier signal to create envelope weighted stimulation event signals. The envelope weighted stimulation event signals are then processed to produce electrode stimulation signals for the implanted electrode array.

Statistical distribution of rainy events characteristics and instantaneous hyetographs generation (Merguellil watershed in central Tunisia)

In semi-arid regions, variability of rainfall inputs is high at different time and space scales. Thus, intense and highly variable hydro-meteorological events, which generate both potential resource and hazards, are of major interest. The Merguellil watershed is characterized by a large majority of rainfall in the form of storms, particularly violent in spring and autumn. Storms of summer are short and violent and can be the origin of important floods. Due to the strong rainfall variability, precipitation and hydrological regime are very irregular. This is what led us to think of a detailed study of rainy events. It was founded that two statistical distributions of parameters characterizing events are dominated (log normal, exponential).Thus, our objective consists in studying, in its globality, the collected pluviographic data in order to make a synthesis and analysis which should make it possible the generation of fictitious events to better control the rainfall risk and to study the response of the watershed faces to several types of rainfall. As result, we obtained hyetographs of the height studied rain gauges. These hyetographs show several types of events that are well in conformity with the observed reality. This generation is an advantageous tool in terms of both its robustness and its performance. Its relevance is based on the analysis of the fine time structure of recorded rainfall events. It can be made over several years, and would permit us to avoid being constrained by sample size. Generating rainfall events provides extensive time-related rainfall data that are consistent at all time steps, and can be seen as solution, even partially, of data lack in hydrological study.

Effects of Reverberation on the Directional Sensitivity of Auditory Neurons across the Tonotopic Axis: Influences of Interaural Time and Level Differences

In reverberant environments, acoustic reflections interfere with the direct sound arriving at a listener's ears, distorting the binaural cues for sound localization. We investigated the effects of reverberation on the directional sensitivity of single neurons in the inferior colliculus (IC) of unanesthetized rabbits. We find that reverberation degrades the directional sensitivity of single neurons, although the amount of degradation depends on the characteristic frequency (CF) and the type of binaural cues available. When interaural time differences (ITDs) are the only available directional cue, low-CF cells sensitive to ITDs in the waveform fine time structure maintain better directional sensitivity in reverberation than high-CF cells sensitive to ITDs in the envelope induced by cochlear filtering. Conversely, when both ITD and interaural level difference (ILD) cues are available, directional sensitivity in reverberation is comparable throughout the tonotopic axis of the IC. This result suggests that, at high frequencies, ILDs provide better directional information than envelope ITDs, emphasizing the importance of the ILD-processing pathway for sound localization in reverberation.

Femtosecond pulse splitting effect in the linear transfer regime

The parametric boundaries and physical mechanism of the origination of the femtosecond laser pulse splitting effect in a strongly scattering medium are ascertained on the basis of a Monte Carlo numerical solution of a nonstationary transfer equation. It is shown that the splitting effect resulting in a bimodal configuration of a pulse envelope is pronounced in a limited range of values of the scattering coefficient of the disperse medium and the anisotropy factor of the phase scattering function. The effect has been registered at pulses of less than 800 fs in length; the geometrical conditions of the signal recording are of essential importance. Under the optimal choice of parameters, the time configuration of the computed signals is in good qualitative agreement with the well-known experimental data. The accounting for the fine time structure of a transmitted signal and the multiparameter dependence of the expected effect require a certain modification of the local statistical modeling algorithms given in the Appendix.

From TORTORA to MegaTORTORA—Results and Prospects of Search for Fast Optical Transients

To study short stochastic optical flares of different objects (GRBs, SNs, etc.) of unknown localizations as well as NEOs it is necessary to monitor large regions of sky with high‐time resolution. We developed a system consisting of widefield camera with field of view of 400–600 sq.deg. which uses TV‐CCD with 0.13 s temporal resolution to record and classify optical transients, and a fast robotic telescope aimed to perform their spectroscopic and photometric investigation just after detection. Such two‐telescope complex, combining wide‐field camera TORTORA and robotic telescope REM, operated from May 2006 at La Silla ESO observatory. Some results of its operation, including first high time resolution study of optical transient accompanying GRB and discovery of its fine time structure, are presented. Also, prospects for improving the efficiency of such observations are given, and a project of a next generation wide field monitoring system, the MegaTORTORA, is described.

Auditory-motivated techniques for detection and classification of passive sonar signals

Hydrophone arrays used to make passive sonar recordings are becoming increasingly complex, thus placing great demands on sonar operators who are required to identify transient and tonal events. Machine hearing systems can mitigate this problem, by performing initial classification and detection of passive sonar signals based on perceptually relevant principles. We describe two such systems. In the first system, transient sounds are characterised using measures of timbre and acoustic pattern. Acoustic features relating to timbre were identified by a multidimensional scaling study, in which human listeners judged the similarity of transient sonar sounds. The perceptual features give improved classification performance compared to commonly-used statistical measures derived from the power spectrum. The second system is an auditory-motivated approach to narrowband detection, which is based on information in zero-crossing intervals rather than signal power. This is motivated by the observation that the fine time structure of an acoustic stimulus is preserved by the phase locking of auditory nerve fibres. We show that a power detector outperforms the interval detector when the signal is centred on the analysis filter. However, when the signal frequency deviates from the centre of the analysis band, interval-based detection improves and power-based detection worsens.

Measurement of the time structure of a coherent synchrotron radiation burst in NewSUBARU

The time structure of a burst of coherent synchrotron radiation (CSR) emitted from a high-peak-current beam in the electron storage ring NewSUBARU has been investigated. The source of this CSR burst is fine time structure in a bunch produced by longitudinal beam instability. This burst is unstable but very easy to obtain, so it can be used for some experimental applications with appropriate averaging of data. With an averaging period of 10ms, the fluctuation of the integrated power was about 10%.

Source Structures of the 1998 November 28 X-Class Flare and Electron Acceleration

We studied the 1998 November 28 X-class flare, which showed long-duration, two-ribbon $\mathrm{H}\alpha$ emission. (1) A soft X-ray loop system developed along the major magnetic neutral line in the impulsive phase. Hard X-ray and microwave emissions due to nonthermal electrons were located in some limited regions. While the compact hard X-ray loop was associated with weak, diffuse soft X-ray emission, two large microwave loops did not have soft X-ray counterparts. (2) Time profiles of two large microwave loops were similar to that of hard X-ray total emission, which emanated mostly from the compact source, even in the fine time structure in the rising phase. (3) A super-hot thermal hard X-ray source appeared around the impulsive peak and was located along the major magnetic neutral line in the declining phase, coinciding with the bright soft X-ray emission. The thermal component can be explained within the standard reconnection model to the extent that the super-hot thermal plasma was heated in the large soft X-ray loop. However, the nonthermal electrons were accelerated in a localized region where three loops interacted with each other. The inferred configuration may be a more complicated form of the double-loop interaction model proposed by Hanaoka (1996) and Nishio et al. (1997).