Hard X-ray Region Research Articles

High Harmonic Generation Seeding Echo-Enabled Harmonic Generation toward a Storage Ring-Based Tender and Hard X-ray-Free Electron Laser

To align with the global trend of integrating synchrotron light source (SLS) and free electron laser (FEL) facilities on one site, in line with examples such as SPring-8 and SACLA in Japan and ELETTRA and FERMI in Italy, we actively explore FEL options leveraging the ultralow-emittance electron beam of the NSLS-II upgrade. These options show promising potential for synergy with storage ring (SR) operations, thereby significantly enhancing our facility’s capabilities. Echo-enabled harmonic generation (EEHG) is well-suited to SR-based FELs, and has already been demonstrated with the capability of generating extremely narrow bandwidth as well as high brightness, realized using diffraction-limited short pulses in transverse planes and Fourier transform-limited bandwidth in the soft X-ray spectrum. However, regarding a conventional EEHG scheme, the combination of the shortest seed laser wavelength (256 nm) and highest harmonic (200) sets the short wavelength limit to λ = 1.28 nm. To further extend the short wavelength limit down to the tender and hard X-ray region, a vital option is to shorten the seed laser wavelength. Thanks to recent advances in high harmonic generation (HHG), packing 109 photons at one harmonic within a few-femtosecond pulse could turn such a novel HHG source into an ideal seeding for EEHG. Thus, compared to the cascaded EEHG, the HHG seeding option could not only lower the cost, but also free the SR space for accommodating more user beamlines. Moreover, to mitigate the SASE background noise on the sample and detector, we combine the HHG seeding EEHG with the crab cavity short pulse scheme for maximum benefit.

Bifocal photon sieve imaging in the hard x-ray region.

Hard x-rays are widely used for plasma diagnosis, nondestructive inspection, and high-resolution x-ray imaging. A typical x-ray source is a tabletop micro-focus x-ray source. Here, a bifocal photon sieve (PS) with the smallest diameter of 59.6 nm was designed and fabricated by electron-beam lithography to focus hard x-rays on variable-resolution array images. An imaging experiment at 8.39 keV demonstrates that the designed and fabricated PS has two different focal lengths. The numerous pinholes that can be optimized provide richer degrees of freedom to realize considerably more functionalities. A multi-focal PS provides the possibility of splitting x-rays and further extends interferometry from visible light to hard x-rays.

Application of transition-edge sensors for micro-X-ray fluorescence measurements and micro-X-ray absorption near edge structure spectroscopy: a case study of uranium speciation in biotite obtained from a uranium mine.

In this study, we successfully applied a transition-edge sensor (TES) spectrometer as a detector for microbeam X-ray measurements from a synchrotron X-ray light source in the hard X-ray region to determine uranium (U) distribution at the micro-scale and its chemical species in biotite obtained from a U mine. It is difficult to separate the fluorescent X-ray of the U Lα1 line at 13.615 keV from that of the Rb Kα line at 13.395 keV in the X-ray fluorescence spectrum with an energy resolution of approximately 220 eV using a conventional silicon drift detector (SDD). Meanwhile, the fluorescent X-rays of U Lα1 and Rb Kα were fully separated by a TES with 50 eV energy resolution at an energy of around 13 keV. The successful peak separation by the TES led to an accurate mapping analysis of trace U in micro-X-ray fluorescence measurements and a decrease in the signal-to-background ratio in micro-X-ray absorption near edge structure spectroscopy. Thus, it could be a powerful tool for studying the U distribution and speciation in various environmental samples.

Three-pole wigglers at NSLS-II

The National Synchrotron Light Source-II (NSLS-II) at Brookhaven National Lab (BNL) is a third-generation medium-energy storage ring with 3 GeV and sub-nm-rad horizontal emittance, equipped with a top-off capability of 500 mA. It is designed to deliver an extremely intense photon beam across a wide spectral range, from the far-infrared to the very hard X-ray region. This outstanding performance is achieved through a combination of bending magnets (BM), three-pole wigglers (3 PW), and advanced insertion device (ID) sources. Six three-pole wigglers have been constructed at the NSLS-II IDs Lab. These devices are used to produce broadband radiation with lower angular power density and to monitor the electron beam emittance and energy spread. This paper describes the R&D activities focused on developing the required magnetic and mechanical designs, along with magnetic field optimization and the final magnetic measurement results. It also covers the spectral brightness, flux, power density, spatial and angular output properties of the 3 PWs, including their commissioning results and their effects on the performance of the NSLS-II storage ring.

A linear gradient multilayer Laue lens

The tilted multilayer Laue lens (MLL) is not capable of effective focusing hard X-rays to below 10 nm. The wedged MLL has the theoretical capability to focus hard X-rays to below 1 nm with a high level of efficiency, however, it is more challenging to fabricate compared with the tilted MLL. In view of this, a novel linear gradient multilayer Laue lens in the hard X-ray region is proposed in this paper. Simulation at the energy of 12 keV reveals that the focusing performance of such MLL is better than that of the tilted MLL and close to that of the wedged MLL, it can achieve both high efficiency and small focus, meanwhile such new structure is easier to fabricate than wedged MLL.

Novel figuring method for a multilayer Laue lens.

A new, to the best of our knowledge, figuring method for a multilayer Laue lens (MLL) in the hard X-ray region is proposed in this paper. Theoretical simulation at 20 keV shows that the figuring method can compensate for the structure error. The phase errors of the first-order diffracted wave decrease from 0.85π to 0.26π after figuring. The spatial resolution changes from 45 nm to 26 nm after figuring, which is almost the same as that of the ideal MLL with a spatial resolution of 24 nm. The figured MLL can achieve 36% of the ideal MLL's first-order diffraction efficiency. Such method may reduce the requirements for the fabrication of the MLLs, and may make it possible to manufacture the larger numerical aperture MLL with the longer working distance in the future.

Four-type phase-contrast X-ray imaging at SAGA Light Source

Phase-contrast X-ray imaging (Phase imaging), which uses X-ray phase-shift caused by passing through a sample, is a powerful tool for non-destructive three-dimensional observation. Since the cross-section of the phase-shift for light elements in the hard X-ray region is more than 1,000 times larger than that of the absorption, detailed observation can be performed even for biological soft tissues and organic materials, mainly composed of light elements. Phase imaging for a large field of view can be classified into four kinds. The sensitivity and dynamic range of phase imaging are related to a trade-off: each method’s properties differ significantly. Therefore, an optimized phase imaging method needs to be selected for each sample’s density distribution. We have been developing all types of phase imaging for fine observations of various samples using the optimal phase imaging method at the SAGA Light Source. We report the details of each method and instrumentation and give example observations.

Single-order focus multilayer Laue lens.

A novel sinusoidal multilayer Laue lens (MLL) in the hard X-ray region is proposed, to the best of our knowledge. The theoretical design shows that the structure function of the MLL is a sine function of the radius such as that of a sinusoidal transmission zone plate. A numerical simulation at the energy of 12 and 24 keV reveals that the MLL can suppress higher-order diffractions effectively, the characteristic of single-order diffraction with spatial resolution is the same as that of the corresponding classical MLL, and the MLL can achieve the first-order diffraction efficiency of 6.8% at 12 and 8.7% at 24 keV. The sinusoidal MLL can also work for single-order focusing at other energies.

High power density soft x-ray GaAs photodiodes with tailored spectral response

GaAs photodiode arrays have been designed for non-destructive monitoring of x-ray beam position in soft coherent beamline front ends in synchrotron light sources. A shallow p-on-n junction was employed to reduce the device photocurrent density to optimize the operation with beam power densities up to 20 W mm−2, mainly coming from hard x-rays. With this approach, the photocurrent is primarily defined by the excess carriers generated by low-energy x-ray photons absorbed near the detector surface. The p–n junction structures were grown by molecular beam epitaxy and processed into 64-element linear photodiode arrays. The devices were characterized first in the visible range with a high-power Ar-ion laser and then tested in the soft and hard x-ray regions up to 10 keV at two beamlines of the National Synchrotron Light Source II. The responsivity was measured to be 0.16 A W−1 at 0.7 keV and 0.05 A W−1 at 6 keV in agreement with modeling. At higher x-ray energies the measured responsivity was lower than predicted in the framework of the carrier diffusion model; a possible explanation is discussed.

Effect of soft and hard x-rays on shock propagation, preheating, and ablation characteristics in pure and doped Be ablators

Detailed investigations are carried out on shock, preheat, and ablation characteristics in x-ray driven beryllium based targets, a candidate ablator material for many inertial confinement fusion studies due to its high mass ablation rate. The study involves extensive radiation hydrodynamic simulations performed on pure and 1% copper doped beryllium foils irradiated by a temperature drive source consisting of both Planckian and Gaussian distributions with peaks lying in soft and hard x-ray regions, respectively. The results of steady state x-ray driven ablation and radiant heat exchange in a sub-critical shock are extended to a non-Planckian source. Based on that, new scaling relations are proposed for shock velocity, shock breakout temperature, maximum preheat temperature, and mass ablation rate with the temperature (120−200 eV) and the fraction of total energy density due to Gaussian distribution (0−0.25) of the incident drive. All parameters increase with drive temperature strength, but the presence of hard x rays does not affect them uniformly. Among all, preheat and shock breakout temperature exhibit a strong dependence on fraction of hard x rays present in the drive spectrum. The effect of doping translates into a pronounced decrease in preheat and shock breakout temperature, while mass ablation rate reduces marginally. The resulting variations in different parameters are explained on the basis of distribution of total extinction coefficient over the spectral form of an incident drive source.

Comparison of photon beam characteristics of Korea-4GSR with PLS-II

The Korea fourth-generation storage ring (Korea-4GSR), with 4-GeV beam energy and 58-pm-rad emittance has been designed by the Pohang Accelerator Laboratory. The evaluated brilliance and coherence are approximately 100 times higher in the hard X-ray region in Korea-4GSR than in the third-generation storage ring at PLS-II. The coherence properties and preservation of coherence through beamline optical elements were studied using SPECTRA code and hybrid method. Result of this paper show essential information for planning experiments and designing beamlines using ultimate low emittance storage ring.

Shortening X-Ray Pulse Duration via Saturable Absorption.

To shorten the duration of x-ray pulses, we present a nonlinear optical technique using atoms with core-hole vacancies (core-hole atoms) generated by inner-shell photoionization. The weak Coulomb screening in the core-hole atoms results in decreased absorption at photon energies immediately above the absorption edge. By employing this phenomenon, referred to as saturable absorption, we successfully reduce the duration of x-ray free-electron laser pulses (photon energy: 9.000keV, duration: 6-7fs, fluence: 2.0-3.5×10^{5} J/cm^{2}) by ∼35%. This finding that core-hole atoms are applicable to nonlinear x-ray optics is an essential stepping stone for extending nonlinear technologies commonplace at optical wavelengths to the hard x-ray region.

Bulk-sensitive magnetic microscope utilizing x-ray magnetic circularly polarized emission

We report a bulk-sensitive x-ray magnetic microscope that exploits a new magneto-optical effect in x-ray emission, referred to as x-ray magnetic circularly polarized emission (XMCPE). An advantage of XMCPE is a large magnetic dichroic effect for 3d transition-metal elements in the hard x-ray region, which enables the realization of a bulk-sensitive microscope suited to iron- and cobalt-rich ferromagnetic materials. We constructed a scanning microscope with 10 μm lateral resolution. A key element is a Montel-type collimating mirror that widely collects the divergent x rays emitted from a sample and converts them into a well-collimated x-ray beam, which is required for circular polarization analysis. Owing to this mirror, the obtained XMCPE spectra of metallic iron exhibited strong intensity and a large magnetic dichroic effect. The performance of the microscope is also demonstrated by the acquisition of magnetization images of an electrical steel sheet with an insulating coating.

Experimental demonstration of two-color x-ray free-electron-laser pulses via wakefield excitation

Several beam manipulation methods have been studied and experimentally demonstrated to generate x-ray multipulses in free-electron-laser (FEL) facilities, advancing the fields of material science, molecular physics, and photochemistry by enabling x-ray pump/x-ray probe experiments. We have demonstrated a novel method to generate hard x-ray multipulses using a hybrid compression scheme. Compared to other methods, this approach minimizes the time jitter among the FEL subpulses, produces large power FEL subpulses, is suitable for high repetition rate machines, since it does not imply the generation of losses, and it works at short and hard x-ray FEL wavelengths as well. All of these aspects make this approach attractive for the novel FEL facilities aiming to explore the hard x-ray region and to work at high repetition rate.

Development and application of a tender X-ray ptychographic coherent diffraction imaging system on BL27SU at SPring-8.

Ptychographic coherent diffraction imaging (CDI) allows the visualization of both the structure and chemical state of materials on the nanoscale, and has been developed for use in the soft and hard X-ray regions. In this study, a ptychographic CDI system with pinhole or Fresnel zone-plate optics for use in the tender X-ray region (2-5 keV) was developed on beamline BL27SU at SPring-8, in which high-precision pinholes optimized for the tender energy range were used to obtain diffraction intensity patterns with a low background, and a temperature stabilization system was developed to reduce the drift of the sample position. A ptychography measurement of a 200 nm thick tantalum test chart was performed at an incident X-ray energy of 2.500 keV, and the phase image of the test chart was successfully reconstructed with approximately 50 nm resolution. As an application to practical materials, a sulfur polymer material was measured in the range of 2.465 to 2.500 keV including the sulfur K absorption edge, and the phase and absorption images were successfully reconstructed and the nanoscale absorption/phase spectra were derived from images at multiple energies. In 3 GeV synchrotron radiation facilities with a low-emittance storage ring, the use of the present system will allow the visualization on the nanoscale of the chemical states of various light elements that play important roles in materials science, biology and environmental science.

Controlling core-hole lifetime through an x-ray planar cavity

It has long been believed that core-hole lifetime (CHL) of an atom is an intrinsic physical property, and controlling it is significant yet is very hard. Here, CHL of the 2p state of W atom is manipulated experimentally through adjusting the emission rate of a resonant fluorescence channel with the assistance of an x-ray thin-film planar cavity. The emission rate is accelerated by a factor linearly proportional to the cavity field amplitude, that can be directly controlled by choosing different cavity modes or changing the angle offset in experiment. This experimental observation is in good agreement with theoretical predictions. It is found that the manipulated resonant fluorescence channel even can dominate the CHL. The controllable CHL realized here will facilitate the nonlinear investigations and modern x-ray scattering techniques in hard x-ray region.

Coherent and incoherent imaging of biological specimens with electrons and X-rays

Since radiation damage is proportional to fluence, radiation damage limits the spatial resolution of biological structures determined by either X-ray or electron scattering. If only elastic scattering is used for structural information then electrons are superior as the ratio of elastic to inelastic scattering is higher than for X-rays. For soft X-rays in the water window below the O K edge photoabsorption contrast might be better than elastic scattering for distinguishing different biological materials. Phase contrast elastic scattering is most effective in the hard X-ray region up to about 10 keV. Radiation damage limits spatial resolution for most X-ray imaging to 10-20 nm. Local molar concentrations of Na,K and Ca ions can be determined at somewhat lower spatial resolutions using relevant absorption edges. At higher energies resolution res is only limited by the fluence available from the light source, since energy deposition is small.

Two components model for TeV $\gamma $-ray emission from extreme high-energy BL Lac objects

Extreme high-energy peaked BL Lac objects (EHBLs) are an emerging class of blazars with exceptional spectral properties, e.g., enhanced emission in the hard X-ray region and a hard spectrum at very high $\gamma $ -ray energies. Currently, no suitable model exists to explain the high-energy radiation mechanism of EHBLs. Here we apply a new model of the emission process to describe the Spectral Energy Distribution (SED) of EHBLs composed of two separate components. The internal component occurs in the jet, describes synchrotron radiation and inverse Compton (IC) scattering of ultra-relativistic electrons; the two processes produce two humps of SED. The external component occurs out of the jet and considers the interaction of the proton accelerated in the jet with the cosmic microwave background (CMB), which generates electron pairs and, in turn, a cascade spectrum due to the IC scattering of those pairs with soft photons. We reproduce the SED of eight EHBLs by finding suitable values of the parameters, meanwhile, compare the $\gamma $ -ray flux with the sensitivity of Large High Altitude Air Shower Observatory (LHAASO) and verify if these objects will be observable with LHAASO. Furthermore, we discuss the influence of intergalactic magnetic fields (IGMFs) of strength $5\times 10^{-18}\;\mathrm{{G}\leq B_{IG}\leq 5\times 10^{-14}\;}\mathrm{{G}}$ and the region of proton’s turbulence acceleration. Finally, we find that the model may be employed to explain the high-energy emission mechanism and predict that LHAASO may detect the high-energy data of 1ES 0347-121, 1ES 1218+304, 1ES 1959+650, 1ES 2344+514 and RGB J0710+591 in the near future.

Time-dependent soft and hard x-ray measurements using streak and x-ray diode array diagnostic systems.

High-temperature, high-density experiments require a simultaneous understanding of temporal and spectral regions. The spectral x-ray streak camera (SXSC) is a new high-temporal-resolution spectral x-ray diagnostic system that allows researchers to differentiate between soft and hard x-ray regions. The diagnostic offers three spectral channels with a wide spectral range, one direct channel that includes a filter and two indirect channels that include both mirrors and filters. The opto-mechanical design positions the filtered radiation at three different locations along the streak photo-cathode (PC) slit to provide time-dependent spectral channels with pico-second temporal resolution. A moderate spatial resolution (150-700 μm) is achieved using slits perpendicular to the PC slit, while the slit width is optimized according to the central channel wavelength (for each channel). The diagnostic system covers a spectral range of 30-500eV for the mirror channels and >1300eV for the direct channel. The temporal and spatial axes of the streak camera are calibrated with respect to a sequence of x-ray pulses. The SXSC diagnostic system is tested and analyzed using Marshak-wave emission from an SiO2 foam that was heated by a laser-beam irradiated halfraum. The SXSC results are compared to measurements from an x-ray diode array with similar spectral channels.

Microstructure evolution and hard x-ray reflectance of ultrathin Ru/C multilayer mirrors with different layer thicknesses

Nanoscale Ru/C multilayers are essential reflective optics in the hard x-ray region of 7–20 keV. To understand the layer growth behavior and develop ultrathin Ru/C multilayer mirrors with periods smaller than 3.0 nm, multilayers with different periods of 6.2–1.5 nm were fabricated and studied. It is found that the average interface width started to increase obviously when the period became smaller than 2.5 nm while the surface roughness of different multilayers remained almost the same. The intrinsic stress of the multilayer gradually decreased with decreasing period and reached a very low value of −82 MPa at d = 2.3 nm. High reflectance of 54% and 65% (at E = 8.04 keV) were demonstrated for the multilayers with periods of 2.5 nm and 3.0 nm, respectively, whereas that for 1.9 nm period was significantly lower. To further analyze the layer microstructure, x-ray diffraction and transmission electron microscopy were used. The polycrystallized structure of Ru remained similar for the multilayers with period less than 2.5 nm, while a non-continuous layer growth and severe intermixing between Ru and C were observed for the multilayer with period of 1.9 nm. The increased intermixing between Ru and C was found to be the main reason for the larger interface width and lower reflectance of the multilayers with period smaller than 2.5 nm. It also indicated that the layer thickness threshold for a Ru/C multilayer growing with good layer quality is 1.0–1.2 nm.