Intense Pulsed Irradiation Research Articles

Unsupervised deep learning method for single image super-resolution of the thick pinhole imaging system using deep image prior

Thick pinhole imaging system is widely used for diagnosing intense pulsed radiation sources. However, owing to the trade-off among spatial resolution, field of view (FOV) and signal-to-noise ratio (SNR), the imaging system normally falls short in achieving high-precision spatial diagnosis. In this paper, we propose an unsupervised deep learning method for single image super-resolution (SISR) of the thick pinhole imaging system. The point spread function (PSF) of the imaging system is obtained by analytical calculation and Monte Carlo simulation methods, and the mathematical model of the imaging system is established using a linear equation. To solve the ill-posed inverse problem, we adopt randomly initialized deep convolutional neural networks (DCNNs) as an image prior without pre-training, which is named deep image prior (DIP). The results demonstrate that, by utilizing the SISR technique to increase the number of pixels in reconstructed images, the proposed DIP algorithm can mitigate the spatial resolution degradation caused by an insufficient spatial sampling frequency of the camera. Compared with various classical algorithms, the proposed DIP algorithm exhibits superior capabilities in recovering high-frequency signals and suppressing ringing artifacts. Furthermore, the convergence and robustness of the proposed DIP algorithm under different random seeds and SNR conditions are also verified.

Super-resolution image reconstruction of a neutron thick pinhole imaging system using image denoiser prior based on half quadratic splitting method

Thick pinhole imaging system is a powerful tool to diagnose intense pulsed radiation sources. However, high spatial resolution of a thick pinhole imaging system cannot be achieved due to the limitation of field of view (FOV) and detection efficiency. In this paper, we propose a novel super-resolution image reconstruction algorithm to improve the spatial resolution of a practical thick pinhole imaging system designed for 14.1 MeV neutrons with 100 mm FOV. Point spread function (PSF) of the imaging system is obtained through complete simulation methods, indicating that the spatial resolution of the imaging system is about 500 μ m. A 4 × super-resolution image reconstruction algorithm using image denoiser as implicit image prior, named image denoiser prior, based on half quadratic splitting (HQS) method is firstly applied to restore more details of neutron sources. The results show that the proposed algorithm performs better than the Richardson-Lucy (RL) algorithm with fewer artifacts and sharper boundaries in reconstructed images. Notably, at a low noise level, the proposed algorithm can surpass the Nyquist sampling limit of 200 μ m, which is determined by the equivalent camera pixel size, to achieve a spatial resolution of 150 μ m.

Solving the spectral gap problem with a four-mirror bow-tie cavity in waveguide free-electron laser oscillators

The generation of intense coherent radiation pulses in the far-infrared and terahertz regimes is of considerable interest to the free-electron laser (FEL) radiation user community. At long wavelengths, the diffraction effect can be quite severe, therefore, an optical waveguide is required to confine the radiation field. However, it will also bring about some new phenomena, and the most noteworthy one is the spectral gap phenomenon: at some particular wavelengths, regardless of electron beam adjustments, the coupling efficiency and output power of waveguide FEL oscillators drop significantly. Such spectral gap has an adverse effect on experimental results since numerous experiments require continuous spectral scanning. In this paper, we propose to utilize a bow-tie cavity instead of conventional cavities to the waveguide FEL to solve the spectral gap problem. The simulation was carried out based on the parameters of FELiChEM, a newly built user facility in China. Numerical simulation code OPC combining with modified GENESIS is used to enable the modelling, for the first time, of a bow-tie cavity based FEL in the far-infrared wavelength regime. The simulation results indicate that this novel structure can effectively eliminate the spectral gaps and substantially enhance long-wavelength laser performance.

Measurements in pulsed neutron fields.

Dosimetry in pulsed and mixed radiation fields represents an important challenge in radiation measurements, because in several accelerator technologies, the acceleration occurs in bunches of particles with short time duration, producing intense radiation pulses spaced by a relatively long time of beam off. The stray mixed radiation field generated around these installations maintains the same time structure as the primary beam and causes a problem in workplace fields' monitoring. Active neutron detectors normally used in steady neutron fields, specifically REM-counters and Bonner sphere spectrometers, can suffer pulsed fields because of the high dead time losses during the bursts and are often inadequate for pulsed field monitoring. This work reviews the efforts of European Radiation Dosimetry Group (EURADOS), and in particular of the Working Group 11 'High Energy Radiation Fields', to define the problem, characterize instrumentation and to propose solutions to the issue of pulsed and mixed radiation fields. Despite the EURADOS initiative, several open issues still remain. A metrological traceability for pulsed neutron fields is missing. There is still room for the optimization and better characterization of available instruments. Initiatives of education and training are required.

Laser Intensity Effect on Polyyne Synthesis in Liquid Hydrocarbons

Laser synthesis of polyyne molecules C2nH2 (n > 2) in liquid hydrocarbons is a complex process in which intense pulsed radiation decomposes the initial carbon-containing substance (the hydrocarbon solvent itself or the solid carbon particles in a suspension). Notwithstanding the fact that the mechanism of pulsed laser ablation in liquids (PLAL) is widely accepted, the effect of the laser parameters on laser-driven polyyne formation is still not understood in detail. Here, we report a study of the polyyne yield as a function of the laser field intensity and exposure dose. Several carbon-containing liquids, including pure n-hexane, pure ethanol, and graphite powder suspended in ethanol, were treated with tightly focused picosecond IR radiation (wavelength of 1064 nm, pulse duration of 10 ps). The synthesis rate was characterized by UV-vis optical absorption spectroscopy. The yields of the polyynes were found to vary in exact accordance with the value of the absorbed laser energy, following specific nonlinear or linear laws. The influence of the laser intensity on the partial concentration of polyynes in the solution was analyzed.

Initial characterization of a dynamic hohlraum radiation source tailored for high-temperature opacity measurements at an 8-MA facility

This paper reports some important properties of a dynamic hohlraum radiation source intended to study the high-temperature opacity of medium-Z atoms. The time-resolved axial radiation power in two x-ray diodes gives the time-evolution of an equivalent black-body temperature that peaks at ∼260 eV at stagnation. Time-gated framing pinhole images show that the source comprises an intense high-temperature core that lasts for ∼2 ns preceded by a 10-ns-long lower-temperature implosion phase that emits mostly softer x rays. Combining pinhole images with soft x-ray power gives a time-resolved brightness radiation temperature that reaches 130 eV. Thus, the lower-temperature source could ionize an opacity sample, then the intense high-temperature radiation pulse could measure its opacity. Likewise, the time-integrated spectrum measured with a spherically bent crystal spectrometer is compatible with multiple blackbodies with different temperatures, from 176 to 185 eV. These characterizations suggest that this dynamic hohlraum can be used for high-temperature opacity measurements.

Validation of ablation model for polyethylene using pulsed x-ray and proton exposures

The surface erosion of polyethylene is interrogated using pulsed x rays at the Z Machine (Sandia National Laboratories) and with proton beams at the Gamble II generator (Naval Research Laboratory) to validate a coupled model for volumetric thermal ablation, photoionization, finite-rate decomposition, and molecular recombination of radicals. The intense radiation pulses (up to ∼1014W/m2 over tens of nanoseconds) are used to generate one-dimensional vapor flows with low ionization fractions and a simplified geometry compared to typical laser ablation, allowing for evaluation of the model under local thermal equilibrium conditions. Areal momentum carried by the ensuing uniaxial hydrodynamic shock is used to indicate the extent of ablation. The threshold fluence for ablation is found to be in close correspondence with the bulk melt transition, and reasonable agreement with the model is obtained for peak temperatures in polyethylene up to 5500 K and heating rates up to 1011K/s where thermal decomposition reactions are also active.

Experimental evaluation of accuracy in determining the direction to a pulsed source of gamma-radiation by a spherical absorber with CdTe detectors in a system of nuclear situation monitoring

The necessity and possibility of creating a dosimetric channel for determining the direction to the source of penetrating gamma-radiation in the nuclear situation monitoring system have been substantiated. The experimental assessment of a reduced error of the device for determining the direction to pulsed sources of radiation using a spherical absorber and CdTe detectors was carried out. These detectors can be used effectively only if there is appropriate electronic equipment, constructed in accordance with their characteristics and spectral-temporal characteristics of highly intense pulsed radiation. The direction to high-intensity pulsed radiation of linear accelerator Varian Clinac 600C (USA) by telluride-cadmium detectors was determined in the pulsed mode. This allowed conducting an experiment to determine the dependence of the coefficient of proportionality of recording pulses from each detector on the angle of direction to the source of pulsed radiation of the linear accelerator. A reduced error was assessed by comparing the received experimental data with a theoretical dependence based on a physical and mathematical model. It was recommended to divide the entire range of angles from 0° to 360° into five sub-ranges, in which three or two proportionality coefficients are responsible for determining the angle in space on gamma sources. The maximum reduced error does not exceed 10 % and the maximum angle error is not more than 8.4°. The most accurate determining of the angle to a radiation source can be carried out in the presence of a theoretically calculated and experimental database of proportionality coefficients for all angles in space and energy of radiation sources

Flash light synthesis of noble metal nanoparticles for electrochemical applications: silver, gold, and their alloys

The synthesis of noble metal nanoparticles (i.e., silver and gold) for electrochemical applications has been widely studied using mainly wet synthesis chemical methods or electrochemical deposition methods. In this work, we propose a single-step flash photochemical method for synthesizing noble metal nanoparticles (i.e., silver, gold, and Au:Ag) in solution for electrochemical applications. The method is based on the intense pulsed radiation emitted from a xenon flashlamp, which induce the reduction of metal cations in solution to form metallic nanoparticles. The present method is based on the selective light absorption of the metal precursor salts combined with the decomposition of polyvinylpyrrolidone added to the solution. This approach can be termed as flash light synthesis.

Attosecond pulse generation from $H_2^ + $H2+ ions using a multicolor beam superposition method.

The behavior of the high-order harmonics and output attosecond pulses from hydrogen molecule ions with various internuclear distances that are exposed to high intensity incoming pulses are investigated. The incoming pulses that are spectrally wide yield from a superposition of monochromatic beams with a constant frequency distance. Our simulations show that the most intense and shortest attosecond pulses can result from hydrogen molecular ions with large internuclear distances which are exposed to irradiation of intense pulses with a frequency width greater than 0.03 a.u.

Noninvasive bunch length measurements exploiting Cherenkov diffraction radiation

We present the observation and the detailed investigation of coherent Cherenkov diffraction radiation (CChDR) in terms of spectral-angular characteristics. Electromagnetic simulations have been performed to optimize the design of a prismatic dielectric radiator and the performance of a detection system with the aim of providing longitudinal beam diagnostics. Successful experimental validations have been organized on the CLEAR and the CLARA facilities based at CERN and Daresbury laboratory respectively. With ps to sub-ps long electron bunches, the emitted radiation spectra extend up to the THz frequency range. Bunch length measurements based on CChDR have been compared to longitudinal bunch profiles obtained using a radio frequency deflecting cavity or coherent transition radiation (CTR). The retrieval of the temporal profile of both Gaussian and non-Gaussian bunches has also been demonstrated. The proposed detection scheme paves the way to a new kind of beam instrumentation, simple and compact for monitoring short bunches of charged particles, particularly well-adapted to novel accelerator technologies, such as dielectric and plasma accelerators. Finally, CChDR could be used for generating intense THz radiation pulses at the MW level in existing radiation facilities, providing broader opportunities for the user community.

A guide to sample delivery systems for serial crystallography.

Crystallography has made a notable contribution to our knowledge of structural biology. For traditional crystallography experiments, the growth of crystals with large size and high quality is crucial, and it remains one of the bottlenecks. In recent years, the successful application of serial femtosecond crystallography (SFX) provides a new choice when only numerous microcrystals can be obtained. The intense pulsed radiation of X-ray free-electron lasers (XFELs) enables the data collection of small-sized crystals, making the size of crystals no longer a limiting factor. The ultrafast pulses of XFELs can achieve 'diffraction before destruction', which effectively avoids radiation damage and realizes diffraction near physiological temperatures. More recently, the SFX has been expanded to serial crystallography (SX) that can additionally employ synchrotron radiation as the light source. In addition to the traditional ones, these techniques provide complementary opportunities for structural determination. The development of SX experiments strongly relies on the advancement of hardware including the sample delivery system, the X-ray source, and the X-ray detector. Here, in this review, we categorize the existing sample delivery systems, summarize their progress, and propose their future prospectives.

Interactions of Intense Extremely Short Pulses with Quantum Objects

A simple description of the action of an extremely intense and short radiation pulse on a quantum object is proposed. The general approximate solution of the transient Schrodinger equation is concretized for the case of electric-dipole transitions in a hydrogen atom.

Apparatus complex based on liquid xenon detector for gamma spectrometry in the intervals between pulses of intense radiation

To investigate the effects of intense radiation on the operation of the liquid xenon spectrometer we have created apparatus complex on the basis of the liquid xenon detector. The experimental setup consists of a multifunctional chamber, gas system, cooling system, temperature control system, X-ray generator, a special preamp, passive protection, scintillation monitor of the accelerator beam, thermoluminescent dosimeters, copper monitor bremsstrahlung, Ge(Li) detector. Multifunctional chamber includes a detecting unit (flat or cylindrical ionization chamber), the cleaning unit of the xenon, control unit of the purity of liquid xenon. The liquid xenon detector was irradiated by bremsstrahlung pulses of the microtron. The frequency of irradiation pulses was 400 Hz. The absorbed dose was varied from 10-7 to 0.1 Gy per pulse. The electronic and ionic processes in liquid xenon at different radiation doses were investigated. The recovery time of the spectrometric mode of operation of the liquid xenon detector after intense pulse irradiation has been studied. Stable operation of the liquid xenon spectrometer in the intervals between the pulses of the accelerator shown for a long time.

Liquid-xenon detector under the intensive pulse irradiation conditions

The effect of intense pulsed irradiation on the operation of the liquid xenon spectrometer was studied. The ionization chamber filled with liquid xenon was irradiated by bremsstrahlung pulses of the microtron. The pulse repetition rate was 400 Hz. The absorbed dose ranged from 10-7 to 0.1 Gy per pulse. Stable operation of the liquid xenon spectrometer in the intervals between the pulses of the accelerator was shown for a long time.

Photoionized argon plasmas induced with intense soft x-ray and extreme ultraviolet pulses

In this work, photoionized plasmas were created by irradiation of gaseous argon with soft x-ray (SXR) and extreme ultraviolet (EUV) intense radiation pulses. Two different laser-produced plasma sources, employing a low energy Nd:YAG laser and a high energy iodine laser system (PALS), were used for creation of photoionized plasmas. In both cases the EUV or SXR beam irradiated the Ar stream, injected into a vacuum chamber synchronously with the radiation pulse. Emission spectra, measured for the Ar photoionized plasmas indicated strong differences in ionization degree for plasmas produced using low and high energy systems. In case of the the EUV driving pulses, emission lines corresponding to neutral atoms and singly charged ions were observed. In case of the SXR pulses utilized for the photoionized plasma creation, only Ar V–VIII emission lines were recorded. Additionally, electron density measurements were performed by laser interferometry employing a femtosecond laser system synchronized with the irradiating system. Maximum electron density for the Ar photoionized plasma, induced using the high energy system, reached 1.9 · 1018 cm−3. Interferometric measurements performed for the moment of maximum intensity of the main laser pulse (t = 0) revealed no fringe shift. Detection limit for the interferometric measurements was estimated. It allowed to estimate the upper limit for electron density at t = 0 as 5 · 1016 cm−3.

Dichroism in the photoionisation of atoms at XUV free-electron lasers

Two-color photoionization of atomic He has been investigated by angle-integrated and angle-resolved electron spectroscopy. The combined action of intense radiation pulses from the XUV free-electron laser (FEL), FERMI or FLASH, and a synchronized optical laser on the target atom gives rise to a rich sideband structure in the photoemission spectrum. Measurements of the angular distribution parameters and the determination of the circular and linear dichroism for the two-color photoionization enable a detailed analysis of the symmetry of the outgoing electron waves and of the dynamics underlying the multi-photon processes. The experimental results are in excellent agreement with theoretical results obtained using perturbation theory (low intensity regime) and the strong field approximation. For the particular case of two-photon ionization the measurements represent an ideal tool for characterizing certain FEL parameters, here for example the degree and the sign of circular polarization. Finally, new features of the dichroism are theoretically predicted originating from the non-dipole contribution into the photoionization amplitudes.

Preface for Special Issue on Biology with X-ray Lasers 2.

In 1970, Breedlove and Tramel and later Solem and Baldwin (1982) suggested that the problem of radiation damage in structural biology might be solved by destroying the sample with a short pulse of intense radiation, because a few photons should be scattered before the onset of radiation damage. Following pioneering work by many, this outlandish idea is now a reality (see, e.g., Neutze et al., 2000; Gaffney and Chapman, 2007; Spence, 2008; Chapman et al., 2011; and Boutet et al., 2012)). This Special Issue, the proceedings of a meeting organized by the NSF Science & Technology Center for Biology with X-ray Lasers, constitutes a timely report on the latest developments, and a tantalizing view of what is yet to come. There has been progress on all fronts: instrumental, technical, and scientific. These developments have transformed heroic “campaigns” to focused experiments with acceptable requirements in beamtime and materials. Structure determinations, which in 2011 took 18 months from experiment to submitted paper, can now be completed in a few weeks, with a factor of 10 less beamtime, a factor of 4 less time for data analysis, and a factor of 100 less protein. Critical steps, such as de novo structure determination, have been demonstrated (Barends et al., 2014), and a wide variety of alternative approaches are under development (see this issue). The sheer range of systems shown to be amenable to Serial Femtosecond Crystallography has left little doubt about the validity of this approach, and its ability to deliver hitherto inaccessible insights into difficult systems and processes. The experimental and algorithmic methods developed for X-ray laser applications are increasingly migrating to synchrotrons. Recent experiments reported here also demonstrate the power of X-ray laser methods for time-resolved work, where the use of protein nanocrystals smaller than the optical absorption length of the pump light has produced the largest differences in a density map ever seen (Tenboer et al., 2014). Indeed, the “diffract-before-destroy” approach is now poised to extend serial femtosecond methods to non-cyclic processes, including enzymology, and sub-picosecond timescales inaccessible at synchrotrons. For longer times, mixing jets using nanocrystals smaller than diffusion lengths promise to allow diffusive mixing for the first time. More generally, the elucidation of femtosecond processes linking chemistry to biology is an enticing prospect under active investigation. The promise of determining structure without crystals has proved difficult to realize, but initial results are now emerging (Ekeberg et al., 2015), with more anticipated from an international collaboration involving many leading groups (Aquila et al., 2015). Recently, electron-based single-particle methods have begun to produce conformational movies and energy landscapes (Dashti et al., 2014) but remain constrained by the number of available snapshots. The X-ray laser has the capability to produce extremely large datasets under physiological conditions. As reported in this issue, it is imperative to develop means for compiling large datasets of useful snapshots obtained under tightly controlled and reproducible imaging conditions, and to correct any remaining imaging artifacts algorithmically. In a short time, the application of X-ray lasers to biology has moved from a debate on validity to a rapidly growing demonstration of unprecedented access to a treasure trove of new information. The reports in this Special Issue make it clear that the best is yet to come.

IL LASER: FULCRO DI GRANDI INFRASTRUTTURE DI RICERCA

An important characteristic of the laser sources consists in the ability to generate short and intense radiation pulses. This feature has stimulated the development of large-scale laser infrastructures mainly dedicated to nuclear fusion. This paper presents the realization of a new research infrastructure “Extreme Light Infrastructure” (ELI) which has conceptually different purposes compared to the others. The main feature consists essentially in the production of an extremely high peak power by using pulses of the duration of a few femtoseconds and of high energy. Focusing these pulses over an area of the size of the laser wavelength one can achieve extremely high intensities, which will enable new research in particle physics, gravitational physics, nonlinear field theory, physics of high pressures, astrophysics and cosmology.

Dynamic shear and tensile strength of iron: Continual and atomistic simulation

In this paper, continual and atomistic simulations are used to investigate the shear and spall strength of iron under high-rate strain conditions. The continual simulation is based on the use of models of dislocation plasticity and fracture due to formation and growth of microcracks and cavities; the molecular-dynamic simulation is based on the use of the LAMMPS software. The obtained results are analyzed in the light of experimental data for the high-speed impact and irradiation of iron films by ultrashort pulses of intense laser radiation.