Bremsstrahlung X-ray Emission Research Articles

Stability of electrons and X-rays generated in a pyroelectric accelerator

Conventional X-ray sources are far too bulky and require a high-power DC voltage. The pyroelectric X-ray generator technology has enabled us to develop portable, low-power X-ray sources for use in materials analysis, imaging, and other applications. Changing the temperature of single crystal lithium tantalate (LiTaO3) at moderate vacuum conditions gives an attractive possibility to generate and accelerate electron up to 100 keV. The electrons are ejected either from the crystal or from the target (depending on polarity). The electrons then generate X-rays via bremsstrahlung and characteristic X-ray emission processes. The aim of this experimental investigation is to explore the interesting feature of the pyroelectric accelerator that generates a monoenergetic electron flux with a stable value of peak energy for a long time. Here we present studies of features of electron flux in pyroelectric accelerator depending on the pressure of residual gas and the distance between the crystal and the target-collimator. We examine the correlation between monoenergetic electron production and avalanche discharge. We also studied outgassing from some accelerator components. The pyroelectric X-ray generator technology is currently being developed is a reliable, compact, stable, and reproducible X-ray source with controllable parameters, which does not require a high-power DC voltage or the use of hazardous (radioactive) materials.

Free-electron crystals for enhanced X-ray radiation

Bremsstrahlung—the spontaneous emission of broadband radiation from free electrons that are deflected by atomic nuclei—contributes to the majority of X-rays emitted from X-ray tubes and used in applications ranging from medical imaging to semiconductor chip inspection. Here, we show that the bremsstrahlung intensity can be enhanced significantly—by more than three orders of magnitude—through shaping the electron wavefunction to periodically overlap with atoms in crystalline materials. Furthermore, we show how to shape the bremsstrahlung X-ray emission pattern into arbitrary angular emission profiles for purposes such as unidirectionality and multi-directionality. Importantly, we find that these enhancements and shaped emission profiles cannot be attributed solely to the spatial overlap between the electron probability distribution and the atomic centers, as predicted by the paraxial and non-recoil theory for free electron light emission. Our work highlights an unprecedented regime of free electron light emission where electron waveshaping provides multi-dimensional control over practical radiation processes like bremsstrahlung. Our results pave the way towards greater versatility in table-top X-ray sources and improved fundamental understanding of quantum electron-light interactions.

Observing galaxy clusters and the cosmic web through the Sunyaev Zel’dovich effect with MISTRAL

Galaxy clusters and surrounding medium, can be studied using X-ray bremsstrahlung emission and Sunyaev Zel’dovich (SZ) effect. Both astrophysical probes, sample the same environment with different parameters dependance. The SZ effect is relatively more sensitive in low density environments and thus is useful to study the filamentary structures of the cosmic web. In addition, observations of the matter distribution require high angular resolution in order to be able to map the matter distribution within and around galaxy clusters. MISTRAL is a camera working at 90GHz which, once coupled to the Sardinia Radio Telescope (SRT), can reach 12″ angular resolution over 4′ field of view (f.o.v.). The forecasted sensitivity drives to a Noise Equivalent Flux Density of ≃ 10–15 mJy √s and the mapping speed is MS = 380′2 mJy−2 h−1. MISTRAL was recently installed at the focus of the SRT and soon will take its first photons.

The Efficiency of Electron Acceleration during the Impulsive Phase of a Solar Flare

Solar flares are known to be prolific electron accelerators, yet identifying the mechanism(s) for such efficient electron acceleration in solar flare (and similar astrophysical settings) presents a major challenge. This is due in part to a lack of observational constraints related to conditions in the primary acceleration region itself. Accelerated electrons with energies above ∼20 keV are revealed by hard X-ray (HXR) bremsstrahlung emission, while accelerated electrons with even higher energies manifest themselves through radio gyrosynchrotron emission. Here, we show, for a well-observed flare on 2017 September 10, that a combination of RHESSI HXR and and the Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA) EUV observations provides a robust estimate of the fraction of the ambient electron population that is accelerated at a given time, with an upper limit of ≲10−2 on the number density of nonthermal (≥20 keV) electrons, expressed as a fraction of the number density of ambient protons in the same volume. This upper limit is about 2 orders of magnitude lower than previously inferred from microwave observations of the same event. Our results strongly indicate that the fraction of accelerated electrons in the coronal region at any given time is relatively small but also that the overall duration of the HXR emission requires a steady resupply of electrons to the acceleration site. Simultaneous measurements of the instantaneous accelerated electron number density and the associated specific electron acceleration rate provide key constraints for a quantitative study of the mechanisms leading to electron acceleration in magnetic reconnection events.

Upgrades to the gamma ray imager on DIII-D enabling access to high flux hard x-ray measurements during the runaway electron plateau phase (invited).

The Gamma Ray Imager (GRI) is a pinhole camera providing 2D imaging of MeV hard x-ray (HXR) bremsstrahlung emission from runaway electrons (REs) over the poloidal cross section of the DIII-D tokamak. We report a series of upgrades to the GRI expanding the access to RE scenarios from the diagnosis of a trace amount of REs to high flux HXR measurements during the RE plateau phase. We present the implementation of novel gamma ray detectors based on LYSO and YAP crystals coupled to multi-pixel photon counters, enabling a count rate in excess of 1MHz. Finally, we highlight new insights into the RE physics discovered during the current quench and RE plateau phase experiments as the result of these upgrades.

Optical to X-Ray Signatures of Dense Circumstellar Interaction in Core-collapse Supernovae

Abstract Progenitors of core-collapse supernovae (SNe) can shed significant mass to circumstellar material (CSM) in the months to years preceding core collapse. The ensuing SN explosion launches ejecta that may subsequently collide with this CSM, producing shocks that can power emissions across the electromagnetic spectrum. In this work we explore the thermal signatures of dense CSM interaction when the CSM density profile is truncated at some outer radius. CSM with optical depth >c/v (where v is the shock velocity) will produce primarily ∼blackbody optical/UV emission, whereas lower optical depth CSM will power bremsstrahlung X-ray emission. Focusing on the latter, we derive light curves and spectra of the resulting X-ray transients that include a detailed treatment of Comptonization. Due to strong photoelectric absorption, the X-ray light curve is dominated by the postinteraction phase that occurs after the shock reaches the CSM truncation radius. We treat this regime here for the first time. Using these results, we present the phase space of optical, UV, and X-ray transients as a function of CSM properties, and discuss detectability prospects. We find that ROSAT would not have been sensitive to CSM X-ray transients but that eROSITA is expected to detect many such events. Future wide-field UV missions such as the Ultraviolet Transient Astronomy Satellite will dramatically enhance sensitivity to large optical depth CSM configurations. Finally, we present a framework within which CSM properties may be directly inferred from observable features of X-ray transients. This can serve as an important tool for studying stellar mass loss using SN X-ray detections.

Three-dimensional simulations of X-ray cavities inflated by radio galaxies

ABSTRACT Vast cavities in the intergalactic medium are excavated by radio galaxies. The cavities appear as such in X-ray images because the external medium has been swept-up, leaving a hot but low-density bubble surrounding the radio lobes. We explore here the predicted thermal X-ray emission from a large set of high-resolution three-dimensional simulations of radio galaxies driven by supersonic jets. We assume adiabatic non-relativistic hydrodynamics with injected straight and precessing jets of supersonic gas emitted from nozzles. Images of X-ray bremsstrahlung emission tend to generate oval cavities in the soft keV bands and leading arcuate structures in hard X-rays. However, the cavity shape is sensitive to the jet-ambient density contrast, varying from concave-shaped at η = 0.1 to convex for η = 0.0001 where η is the jet/ambient density ratio. We find lateral ribs in the soft X-rays in certain cases and propose this as an explanation for those detected in the vicinity of Cygnus A. In bi-lobed or X-shaped sources and in curved or deflected jets, the strongest X-ray emission is not associated with the hotspot but with the relic lobe or deflection location. This is because the hot high-pressure and dense high-compression regions do not coincide. Directed towards the observer, the cavity becomes a deep round hole surrounded by circular ripples. With short radio-mode outbursts with a duty cycle of 10 per cent, the intracluster medium simmers with low Mach number shocks widely dissipating the jet energy in between active jet episodes.

Forward Modeling of Particle Acceleration and Transport in an Individual Solar Flare

Abstract The aim of this study is to generate maps of the hard X-ray emission produced by energetic electrons in a solar flare and compare them with observations. The ultimate goal is to test the viability of the combined MHD/test-particle approach for data-driven modeling of active events in the solar corona and their impact on the heliosphere. Based on an MHD model of X-class solar flare observed on 2017 September 8, we calculate trajectories of a large number of electrons and protons using the relativistic guiding-center approach. Using the obtained particle trajectories, we deduce the spatial and energy distributions of energetic electrons and protons, and calculate bremsstrahlung hard X-ray emission using the “thin-target” approximation. Our approach predicts some key characteristics of energetic particles in the considered flare, including the size and location of the acceleration region, energetic particle trajectories and energy spectra. Most importantly, the hard X-ray bremsstrahlung intensity maps predicted by the model are in good agreement with those observed by RHESSI. Furthermore, the locations of proton and electron precipitation appear to be close to the sources of helioseismic response detected in this flare. Therefore, the adopted approach can be used for observationally driven modeling of individual solar flares, including manifestations of energetic particles in the corona, as well as the inner heliosphere.

A model for the energy and angular distribution of x rays emitted from an x-ray tube. Part II. Validation of x-ray spectra from 20 to 300kV.

To present and validate a complete x-ray emission model (bremsstrahlung and characteristic x-ray emission) for the energy range 20-300kV. An analytical x-ray spectrum model that combines the bremsstrahlung emission model developed in Part I with a previously developed characteristic x-ray emission model is validated by comparison with Monte Carlo calculations, published measured spectra, and models developed by other authors. Furthermore, the assumptions and limitations of previous spectrum models are summarized, and their predictions are compared with results obtained by Monte Carlo simulations of x rays emitted from tungsten and molybdenum targets. The model is able to reproduce narrow-beam Monte Carlo calculations to within 0.5% in terms of the first and second aluminum half-value layer thickness (HVL). Compared with measured spectra, the difference in HVL is < 2% for typical diagnostic and therapeutic beam qualities available at primary standard laboratories. Compared with previous spectrum models, the present model performs especially well for low kilovoltage x-ray beams (below 50kV), and is reliable for a wider range of take-off angles, that is, the angle between the target surface and the direction of emission. The difference in model and Monte Carlo predictions of the energy-fluence weighted air kerma (i.e., the photon energy absorption in air) is < 0.5% using the present model, while previous spectrum models can differ by more than 10%. The x-ray emission model developed in this work has been validated against Monte Carlo calculations and measured results. The model provides an efficient alternative to comprehensive Monte Carlo simulations and is an improvement over previous models. The model can be used to predict both central- and off-axis spectra, as well as off-axis effects such as the (anode) heel effect.

The STIX Aspect System (SAS): The Optical Aspect System of the Spectrometer/Telescope for Imaging X-Rays (STIX) on Solar Orbiter

The Spectrometer/Telescope for Imaging X-rays (STIX) is a remote sensing instrument on Solar Orbiter that observes the hard X-ray bremsstrahlung emission of solar flares. This paper describes the STIX Aspect System (SAS), a subunit that measures the pointing of STIX relative to the Sun with a precision of pm 4'', which is required to accurately localize the reconstructed X-ray images on the Sun. The operating principle of the SAS is based on an optical lens that images the Sun onto a plate that is perforated by small apertures arranged in a cross-shaped configuration of four radial arms. The light passing through the apertures of each arm is detected by a photodiode. Variations of spacecraft pointing and of distance from the Sun cause the solar image to move over different apertures, leading to a modulation of the measured lightcurves. These signals are used by ground analysis to calculate the locations of the solar limb, and hence the pointing of the telescope.

Enhanced bremsstrahlung X-ray emission from Ag nanoparticles irradiated by ultrashort laser pulses

In this work, an Ag nanoparticle colloidal suspension flowing in the form of a thin jet (250 μm) is irradiated by 150 femtosecond, 800 nm laser pulses to form a plasma which emits bremsstrahlung X-rays of up to 100 keV energy. The flowing jet ensures long-term durability of the plasma source during continuous laser irradiation. The laser pulse is p-polarized and the angle of incidence is normal to the jet surface, to optimize resonance absorption of laser radiation by the plasma electron density gradient. A 30-fold enhancement is observed in the X-ray yield in the nanoparticle suspension, compared to the precursor salt solution. This is because of the local field enhancement (LFE) associated with the localized surface plasmon resonance (LSPR) in Ag nanoparticles. Multiphoton ionization will be greatly enhanced in the presence of LFE, resulting in the generation of a relatively larger number of free electrons, which become “hot” electrons of high kinetic energy by resonance absorption. Bremsstrahlung in the X-ray regime occurs due to the deceleration of these hot electrons. Under identical excitation conditions the corresponding X-ray enhancement measured in Au nanoparticles is relatively lower at 18-fold. This decrease is due to the higher ionization potential of Au (9.22 eV) as compared to Ag (7.58 eV). On the other hand, absorption spectra and SEM images measured after continuous irradiation reveal that Au nanoparticles are more photostable compared to Ag nanoparticles. These studies show that Ag nanoparticles are better suited for X-ray generation compared to Au nanoparticles under the experimental conditions employed. Applications include dynamics studies, microscopy, and lithography.

Bremsstrahlung emission profile from intense laser-solid interactions as a function of laser focal spot size

The bremsstrahlung x-ray emission profile from high intensity laser-solid interactions provides valuable insight to the internal fast electron transport. Using penumbral imaging, we characterise the spatial profile of this bremsstrahlung source as a function of laser intensity by incrementally increasing the laser focal spot size on target. The experimental data shows a dual-source structure; one from the central channel of electrons, the second a larger substrate source from the recirculating electron current. The results demonstrate than an order of magnitude improvement in the intensity contrast between the two x-ray sources is achieved with a large focal spot, indicating preferable conditions for applications in radiography. An analytical model is derived to describe the transport of suprathermal electron populations that contribute to substrate and central channel sources through a target. The model is in good agreement with the experimental results presented here and furthermore is applied to predict laser intensities for achieving optimum spatial contrast for a variety of target materials and thicknesses.

Secondary fluorescence effects in microbeam analysis and their impacts on geospeedometry and geothermometry

Characteristic and bremsstrahlung X-ray emission during electron-specimen interactions in electron microprobe (EPMA) and scanning electron microscope (SEM) instruments causes secondary fluorescence X-ray effects from adjacent (boundary) phases. This is well-known, yet the impact of such effects in microbeam analysis of natural mineral-hosted inclusions and adjacent to mineral-mineral and mineral-glass boundaries are frequently neglected, especially in geospeedometry and geothermometry applications. To demonstrate the important influence of the secondary fluorescence effect on the measured concentration of elements and its consequences for geochemical applications, we consider the effect of mineral-mineral and mineral-glass boundaries in microanalysis of Cr, Zr and Ti both experimentally, using electron probe measurements on cold-pressed material couples, and computationally, using the software suite “CalcZAF/Standard” and its Graphical User Interface (GUI) for the semi-analytical model FANAL (Llovet et al., 2012). We demonstrate, for example, that apparent Cr contents of the order of ~3000 to 5000 ppm in chromite-hosted glass inclusions at 6 μm from the inclusion boundary can be entirely due to secondary fluorescence in the Cr-rich host phase. Because the spatial gradient in secondary fluorescence-induced X-ray emission superficially resembles a diffusion profile, we emphasize the need to quantitatively correct for such effects in any geospeedometry application involving measurement of diffusion profiles adjacent to grain boundaries with large concentration contrasts. We also provide a scheme for estimating analytical errors related to the secondary fluorescence effect when applying geothermometers such as Ti-in-zircon, Ti-in-quartz (TitaniQ) and Zr-in-rutile. Temperature estimates based on trace Ti, Zr and Cr contents in minerals and glasses affected by secondary fluorescence in nearby phases (e.g., rutile, zircon and chromite) can be severely overestimated, in some cases by hundreds of degrees Celsius.

High-temperature differential emission measure and altitude variations in the temperature and density of solar flare coronal X-ray sources

The detailed knowledge of plasma heating and acceleration region properties presents a major observational challenge in solar flare physics. Using the Ramaty High Energy Solar Spectroscopic Imager (RHESSI), the high temperature differential emission measure, DEM(T), and the energy-dependent spatial structure of solar flare coronal sources are studied quantitatively. The altitude of the coronal X-ray source is observed to increase with energy by ~+0.2 arcsec/keV between 10 and 25 keV. Although an isothermal model can fit the thermal X-ray spectrum observed by RHESSI, such a model cannot account for the changes in altitude, and multi-thermal coronal sources are required where the temperature increases with altitude. For the first time, we show how RHESSI imaging information can be used to constrain the DEM(T) of a flaring plasma. We develop a thermal bremsstrahlung X-ray emission model with inhomogeneous temperature and density distributions to simultaneously reproduce: i) DEM(T), ii) altitude as a function of energy, and iii) vertical extent of the flaring coronal source versus energy. We find that the temperature-altitude gradient in the region is ~+0.08 keV/arcsec (~1.3 MK/Mm). Similar altitude-energy trends in other flares suggest that the majority of coronal X-ray sources are multi-thermal and have strong vertical temperature and density gradients with a broad DEM(T).

The Spectrometer/Telescope for Imaging X-rays on Solar Orbiter: Flight design, challenges and trade-offs

STIX is the X-ray spectral imaging instrument on-board the Solar Orbiter space mission of the European Space Agency, and together with nine other instruments will address questions of the interaction between the Sun and the heliosphere. STIX will study the properties of thermal and accelerated electrons near the Sun through their Bremsstrahlung X-ray emission, addressing in particular the emission from flaring regions on the Sun. The design phase of STIX has been concluded. This paper reports the final flight design of the instrument, focusing on design challenges that were faced recently and how they were addressed.

Quasi-periodic oscillations in accreting magnetic white dwarfs

Quasi-periodic oscillations (QPOs) are observed in the optical flux of some polars with typical periods of 1 to 3 s but none have been observed yet in X-rays where a significant part of the accreting energy is released. QPOs are expected and predicted from shock oscillations. Most of the polars have been observed by the XMM-Newton satellite. We made use of the homogeneous set of observations of the polars by XMM-Newton to search for the presence of QPOs in the (0.5–10 keV) energy range and to set significant upper limits for the brightest X-ray polars. We extracted high time-resolution X-ray light curves by taking advantage of the 0.07 s resolution of the EPIC-PN camera. Among the 65 polars observed with XMM-Newton from 1998 to 2012, a sample of 24 sources was selected on the basis of their counting rate in the PN instrument to secure significant limits. We searched for QPOs using Fast Fourier Transform (FFT) methods and defined limits of detection using statistical tools. Among the sample surveyed, none shows QPOs at a significant level. Upper limits to the fractional flux in QPOs range from 7% to 71%. These negative results are compared to the detailed theoretical predictions of numerical simulations based on a 2D hydrodynamical code presented in Paper II. Cooling instabilities in the accretion column are expected to produce shock quasi-oscillations with a maximum amplitude reaching ~40% in the bremsstrahlung (0.5–10 keV) X-ray emission and ~20% in the optical cyclotron emission. The absence of X-ray QPOs imposes an upper limit of ~(5–10) g cm-2 s-1 on the specific accretion rate but this condition is found inconsistent with the value required to account for the amplitudes and frequencies of the observed optical QPOs. This contradiction outlines probable shortcomings with the shock instability model.

Interaction of high intensity laser with non-uniform clusters and enhanced X-ray emission

Laser irradiated clusters with non-uniform density variation are shown to broaden surface plasmon resonance very significantly. As the clusters get heated and expand hydro-dynamically, the Bremsstrahlung X-ray emission yield passes through a maximum in time. The maximum yield decreases with increase in non-uniformity in the electron density inside the clusters. At higher laser intensity, the nonlinearity in laser cluster interaction may arise even prior to electron heating, via the relativistic mass variation and the nonlinear restoration force on electrons. For clusters with radius less than one tenth of the laser wavelength, the restoration force nonlinearity dominates.

Note: Effect of photodiode aluminum cathode frame on spectral sensitivity in the soft x-ray energy band.

Silicon photodiodes used for soft x-ray detection typically have a thin metal electrode partially covering the active area of the photodiode, which subtly alters the spectral sensitivity of the photodiode. As a specific example, AXUV4BST photodiodes from International Radiation Detectors have a 1.0 μm thick aluminum frame covering 19% of the active area of the photodiode, which attenuates the measured x-ray signal below ~6 keV. This effect has a small systematic impact on the electron temperature calculated from measurements of soft x-ray bremsstrahlung emission from a high-temperature plasma. Although the systematic error introduced by the aluminum frame is only a few percent in typical experimental conditions on the Madison Symmetric Torus, it may be more significant for other instruments that use similar detectors.

Diagnosing residual motion via the x-ray self emission from indirectly driven inertial confinement implosions.

In an indirectly driven implosion, non-radial translational motion of the compressed fusion capsule is a signature of residual kinetic energy not coupled into the compressional heating of the target. A reduction in compression reduces the peak pressure and nuclear performance of the implosion. Measuring and reducing the residual motion of the implosion is therefore necessary to improve performance and isolate other effects that degrade performance. Using the gated x-ray diagnostic, the x-ray Bremsstrahlung emission from the compressed capsule is spatially and temporally resolved at x-ray energies of >8.7 keV, allowing for measurements of the residual velocity. Here details of the x-ray velocity measurement and fitting routine will be discussed and measurements will be compared to the velocities inferred from the neutron time of flight detectors.

On the lack of X-ray bright Type IIP supernovae

Type IIP supernovae (SNe) are expected to arise from red supergiant stars (RSGs). These stars have observed mass-loss rates that span more than two orders of magnitude, from less than | $10^{-6}\,{\rm M}_{\odot }$ | yr−1 to almost | $10^{-4}\,{\rm M}_{\odot }$ | yr−1. Thermal bremsstrahlung X-ray emission from at least some IIPs should reflect the larger end of the high mass-loss rates. Strangely, no IIP SNe are seen where the X-ray luminosity is large enough to suggest mass-loss rates greater than about | $10^{-5}\,{\rm M}_{\odot }$ | yr−1. We investigate if this could be due to absorption of the X-ray emission. After carefully studying all the various aspects, we conclude that absorption would not be large enough to prevent us from having detected X-ray emission from high mass-loss rate IIPs. This leads us to the conclusion that there may be an upper limit of | ${\sim } 10^{-5}\,{\rm M}_{\odot }$ | yr−1 to the mass-loss rate of Type IIP progenitors, and therefore to the luminosity of RSGs that explode to form Type IIPs. This in turn suggests an upper limit of | ${\lesssim } 19\,{\rm M}_{\odot }$ | for the progenitor mass of a Type IIP SN. This limit is close to that obtained by direct detection of IIP progenitors, as well as that suggested by recent stellar evolution calculations. Although the statistics need to be improved, many current indicators support the notion that RSGs above | ${\sim } 19\,{\rm M}_{\odot }$ | do not explode to form Type IIP SNe.