Finite Source Size Research Articles

A correction algorithm of geometric unsharpness for neutron radiographs via adaptive fusion and total variation

The mobility of accelerator-based neutron sources has laid the foundation for the miniaturization of neutron radiography facilities in response to the increasing industrial requirements. Compared to traditional reactors and spallation neutron sources, miniaturized neutron radiography facilities based on accelerator neutron sources face challenges in achieving a sufficiently high collimation ratio. It leads to a pronounced level of geometric unsharpness on neutron radiographs, which arises from a finite source size and beam divergence. The presence of blurriness in radiographs significantly impairs the ability to accurately visualize the internal structures of the test objects, thus hindering observation. An algorithm combining adaptive fusion with total variation was proposed to correct the geometric unsharpness for neutron radiographs in this paper. In contrast to most traditional restoration algorithms that relied on regularization terms or filters, our proposed method offered both initial restoration and further restoration. The initial restoration procedure involved the utilization of the Richardson-Lucy algorithm coupled with the l0 smoothing method. This adaptive fusion technique was employed to enhance the sharpness of edges and denoise the neutron radiographs. Finally, the result of the adaptive fusion was embedded into a deblurring model based on total variation to further preserve the edges and restore more details during further restoration. In the experiments, it was demonstrated that our correction algorithm effectively preserved the prominent edges and the details in both the simulated and real neutron radiographs of thin and thick objects, exhibiting fewer ringing effects than traditional algorithms. It is worth mentioning that our work is primarily applicable and significant to neutron generators, but not for reactors or spallation neutron sources that can deliver much better images.

Under Einstein’s Microscope: Measuring Properties of Individual Rotating Massive Stars from Extragalactic Microcaustic Crossings

Highly magnified stars residing in caustic crossing lensed galaxies at z ≃ 0.7–1.5 in galaxy cluster lensing fields inevitably exhibit recurrent brightening events as they traverse a microcaustic network cast down by foreground intracluster stars. The detectable ones belong to nature’s most massive and luminous class of stars, with evolved blue supergiants being the brightest ones at optical wavelengths. Considering single stars in this work, we study to what extent intrinsic stellar parameters are measurable from multifilter light curves, which can be obtained with optical/near-IR space telescopes during one or multiple caustic crossing events. We adopt a realistic model for the axisymmetric surface brightness profiles of rotating O/B stars and develop a numerical lensing code that treats finite source size effects. With a single microcaustic crossing, the ratio of the surface rotation velocity to the breakup value is measurable to a precision of ∼0.1–0.2 for feasible observation parameters with current space telescopes, with all unknown intrinsic and extrinsic parameters marginalized over and without a degeneracy with inclination. Equatorial radius and bolometric luminosity can be measured to 1/3 and 2/3 of the fractional uncertainty in the microcaustic strength, for which the value is not known at each crossing but an informative prior can be obtained from theory. Parameter inference precision may be further improved if multiple caustic crossing events for the same lensed star are jointly analyzed. Our results imply new opportunities to survey individual massive stars in star formation sites at z ≃ 0.7–1.5 or beyond.

Magnified x ray ghost imaging with tapered polycapillary optics free of the penumbra effect.

X ray ghost imaging (XGI) offers both radiation dose-reduction potential and cost-effective benefits owing to the utilization of a single-pixel detector. Most XGI schemes with laboratory x ray sources require a mechanically moving mask for either structured illumination or structured detection. In either configuration, however, its resolution remains limited by the source size and the unit size of the mask. Upon propagation, the details of the object can actually be magnified by the divergence of x rays, but at the same time, the penumbra effect produced by the finite source size is dramatically intensified, which ultimately leads to a degradation of image quality in XGI. To address these limitations, this work proposes a magnified XGI scheme using structured detection equipped with tapered polycapillary optics, which can efficiently suppress the object's penumbra as well as resolve the magnified details of the object. In general, the resolution of this scheme is no longer affected by the source size but by the microcapillary size of polycapillary. Our work fundamentally achieves cancellation of penumbra effect-induced aberration, thus paving the way for high-resolution magnified XGI.

Development and evaluation of a standard for absorbed dose to water from alpha-particle-emitting radionuclides

Targeted radionuclide therapy and brachytherapy with alpha particles has gained significant clinical relevance recently. Absorbed dose traceability to a standard is currently lacking in the dosimetry chain. The short range of alpha particles in water of <100 μm complicates the absorbed dose measurements in the form of significant attenuation and perturbation effects. The aim of this work was to develop and validate a standard for absorbed dose to water from alpha-particle-emitting radionuclides. A dosimetric formalism to realize surface absorbed dose to water per unit activity using a windowless cylindrical parallel-plate ion chamber (IC) was introduced. IC-based and Monte Carlo (MC)-based correction factors were calculated for a planar circular 210Po alpha-particle emitter. The measured absorbed dose to air was compared to the MC-calculated absorbed dose. A parallel-plate IC with a nominal 4 mm collector diameter composed of polystyrene-equivalent material was utilized as a standard. MC simulations were performed using the TOPAS MC code and finite source size, backscatter, and emission angle divergence correction factors were calculated by modeling the IC and the source. Multiple measurement trials were performed to measure ionization current at air gaps in the 0.3 mm to 0.525 mm range. The proposed dosimetric formalism was employed to calculate the surface absorbed dose to water from a point-like 210Po source. The recombination and polarity correction factors were measured to be <0.50% when a 150 V mm−1 electric field strength was applied. The MC-calculated and measured absorbed dose to air agreed within 2.05%. The finite source size and backscatter corrections were <10% with the emission angle divergence correction being in the 93%–239% range. The surface absorbed dose to water was measured to be 2.8913 × 10−6 Gy s−1 Bq−1 with a combined uncertainty of 4.23% at k = 1. This work demonstrated the ability of a windowless parallel-plate IC as a standard for absorbed dose from alpha-particle-emitting radionuclides.

Microlensing constraints on axion stars including finite lens and source size effects

A fraction of light scalar dark matter, especially axions, may organize into Bose-Einstein condensates, gravitationally bound clumps, "boson stars", and be present in large number in galactic halos today. We compute the expected number of gravitational microlensing events of clumps composed of the ordinary QCD axion and axion-like-particles and derive microlensing constraints from the EROS-2 survey and the Subaru Hyper Suprime-Cam observation. We perform a detailed lensing calculation, including the finite lens and source size effects in our analysis. We constrain the axion mass in terms of the fraction of dark matter collapsed into clumps, the individual clump densities, and the axion self-coupling. We also consider and constrain clumps composed of a generic scalar dark matter candidate with repulsive self-interactions. Our analysis opens up a new window for the potential discovery of dark matter.

A public code for astrometric microlensing with contour integration

ABSTRACT We present the first public code for the calculation of the astrometric centroid shift occurring during microlensing events. The computation is based on the contour integration scheme and covers single and binary lensing of finite sources with arbitrary limb darkening profiles. This allows for general detailed investigations of the impact of finite source size in astrometric binary microlensing. The new code is embedded in version 3.0 of vbbinarylensing, which offers a powerful computational tool for extensive studies of microlensing data from current surveys and future space missions.

Non-radially pulsating stars as microlensing sources

ABSTRACT We study the microlensing of non-radially pulsating (NRP) stars. Pulsations are formulated for stellar radius and temperature using spherical harmonic functions with different values of l, m. The characteristics of the microlensing light curves from NRP stars are investigated in relation to different pulsation modes. For the microlensing of NRP stars, the light curve is not a simple multiplication of the magnification curve and the intrinsic luminosity curve of the source star, unless the effect of finite source size can be ignored. Three main conclusions can be drawn from the simulated light curves. First, for modes with m ≠ 0 and when the viewing inclination is more nearly pole-on, the stellar luminosity towards the observer changes little with pulsation phase. In this case, high-magnification microlensing events are chromatic and can reveal the variability of these source stars. Secondly, some combinations of pulsation modes produce nearly degenerate luminosity curves (e.g. (l, m) = (3, 0), (5, 0)). The resulting microlensing light curves are also degenerate, unless the lens crosses the projected source. Finally, for modes involving m = 1, the stellar brightness centre does not coincide with the coordinate centre, and the projected source brightness centre moves in the sky with pulsation phase. As a result of this time-dependent displacement in the brightness centroid, the time of the magnification peak coincides with the closest approach of the lens to the brightness centre as opposed to the source coordinate centre. Binary microlensing of NRP stars and in caustic-crossing features are chromatic.

Entrainment flow of a jet emerging into a half-space with the no-slip boundary condition

The current development of micro-scale technologies increases the interest in viscous flows with low and moderate Reynolds numbers. This work theoretically studies the entrainment flow of a viscous jet emerging from a plane wall into a half-space with the objective to understand the conditions where a similarity model can approximate a realistic flow. Two similarity models having analytic solutions are considered: the flow dominated by the momentum flux and the flow dominated by the mass flux. Algorithms are proposed to evaluate the parameters of the similarity models from the mass and momentum balances. Distributions of flow parameters and stresses on the wall are calculated for the similarity models. They are compared with the corresponding distributions obtained by computational fluid dynamics for a more realistic model with a finite size of the jet source and competitive influence of the mass and momentum fluxes. This comparison validates the mass-dominated similarity model at the jet Reynolds number Re ≤ 10 and the momentum-dominated similarity model at Re ≥ 30. The obtained results are applied to the problem of laser evaporation in selective laser melting. It is shown that the theoretically estimated flow velocity corresponds to the experimentally observed one. The theory explains the formation of the experimentally observed denuded zone and its widening by decreasing the ambient gas pressure.

On the wave optics effect on primordial black hole constraints from optical microlensing search

ABSTRACT Microlensing of stars, e.g. in the Galactic bulge and Andromeda galaxy (M31), is among the most robust, powerful method to constrain primordial black holes (PBHs) that are a viable candidate of dark matter. If PBHs are in the mass range $M_{\rm PBH} {\,\,\lesssim \,\,}10^{-10}\mathrm{ M}_\odot$, its Schwarzschild radius (rSch) becomes comparable with or shorter than optical wavelength (λ) used in a microlensing search, and in this regime the wave optics effect on microlensing needs to be taken into account. For a lensing PBH with mass satisfying rSch ∼ λ, it causes a characteristic oscillatory feature in the microlensing light curve, and it will give a smoking gun evidence of PBH if detected, because any astrophysical object cannot have such a tiny Schwarzschild radius. Even in a statistical study, e.g. constraining the abundance of PBHs from a systematic search of microlensing events for a sample of many source stars, the wave effect needs to be taken into account. We examine the impact of wave effect on the PBH constraints obtained from the r-band (6210 Å) monitoring observation of M31 stars in Niikura et al., and find that a finite source size effect is dominant over the wave effect for PBHs in the mass range MPBH ≃ [10−11, 10−10]M⊙. We also discuss that, if a denser cadence (10 s), g-band monitoring observation for a sample of white dwarfs over a year time-scale is available, it would allow one to explore the wave optics effect on microlensing light curve, if it occurs, or improve the PBH constraints in $M_{\rm PBH} {\,\,\lesssim \,\,}10^{-11}\mathrm{ M}_\odot$ even from a null detection.

Revisiting constraints on asteroid-mass primordial black holes as dark matter candidates

As the only dark matter candidate that does not invoke a new particle that survives to the present day, primordial black holes (PBHs) have drawn increasing attention recently. Up to now, various observations have strongly constrained most of the mass range for PBHs, leaving only small windows where PBHs could make up a substantial fraction of the dark matter. Here we revisit the PBH constraints for the asteroid-mass window, i.e., the mass range 3.5× 10−17M⊙ < mPBH < 4× 10−12M⊙. We revisit 3 categories of constraints. (1) For optical microlensing, we analyze the finite source size and diffractive effects and discuss the scaling relations between the event rate, mPBH and the event duration. We argue that it will be difficult to push the existing optical microlensing constraints to much lower mPBH. (2) For dynamical capture of PBHs in stars, we derive a general result on the capture rate based on phase space arguments. We argue that survival of stars does not constrain PBHs, but that disruption of stars by captured PBHs should occur and that the asteroid-mass PBH hypothesis could be constrained if we can work out the observational signature of this process. (3) For destruction of white dwarfs by PBHs that pass through the white dwarf without getting gravitationally captured, but which produce a shock that ignites carbon fusion, we perform a 1+1D hydrodynamic simulation to explore the post-shock temperature and relevant timescales, and again we find this constraint to be ineffective. In summary, we find that the asteroid-mass window, which was previously constrained due to femtolensing, WD survival, optical microlensing, and neutron star capture is no longer constrained. Hence, the asteroid-mass window remains open for PBHs to account for all the dark matter.

Microlensing of x-ray pulsars: A method to detect primordial black hole dark matter

Primordial black holes (PBHs) with a mass from $10^{-16}$ to $10^{-11}\,M_\odot$ may comprise 100% of dark matter. Due to a combination of wave and finite source size effects, the traditional microlensing of stars does not probe this mass range. In this paper, we point out that X-ray pulsars with higher photon energies and smaller source sizes are good candidate sources for microlensing for this mass window. Among the existing X-ray pulsars, the Small Magellanic Cloud (SMC) X-1 source is found to be the best candidate because of its apparent brightness and long distance from Earth. We have analyzed the existing observation data of SMC X-1 by the RXTE telescope (around 10 days) and found that PBH as 100% of dark matter is close to but not yet excluded. Future longer observation of this source by X-ray telescopes with larger effective areas such as AstroSat, Athena, Lynx, and eXTP can potentially close the last mass window where PBHs can make up all of dark matter.

Variable Circumstellar Extinction in UX Ori Stars of Late Spectral Types: V695 Per

Eclipses of young stars by dust fragments in protoplanetary disks are the main reason for stochastic variability in the luminosity of UX Ori stars. In analyses and modeling of these kinds of eclipses, the star is often treated as a point light source. This approach is fully justified in the case of hot stars in this family, since the sublimation radius for the dust in their circumstellar disk is much greater than the star’s radius. This approach may not apply to the case of cold stars, since the evaporation zone for the circumstellar dust is close to the star. Here it is shown that accounting for the finite size of the radiation source leads to a change in the shape of a color track on a “color-magnitude” diagram. Calculations for the star V695 Per simultaneously illustrate the variability of the brightness of UX Ori and AA Tau stars for the coldest currently known star of this type.

The dynamic spectral signatures from lunar occultation: A simulation study

Lunar occultation, which occurs when the Moon crosses sight-lines to distant sources, has been studied extensively through apparent intensity pattern resulting from Fresnel diffraction, and has been successfully used to measure angular sizes of extragalactic sources. However, such observations till date have been mainly over narrow bandwidth, or averaged over the observing band, and the associated intensity pattern in time has rarely been examined in detail as a function of frequency over a wide band. Here, we revisit the phenomenon of lunar occultation with a view to study the associated intensity pattern as a function of both time and frequency. Through analytical and simulation approach, we examine the variation of intensity across the dynamic spectra, and look for chromatic signatures which could appear as discrete dispersed signal tracks, when the diffraction pattern is adequately smoothed by a finite source size. We particularly explore circumstances in which such diffraction pattern might closely follow the interstellar dispersion law followed by pulsars and transients, such as the Fast Radio Bursts (FRBs), which remain a mystery even after a decade of their discovery. In this paper, we describe details of this investigation, relevant to radio frequencies at which FRBs have been detected, and discuss our findings, along with their implications. We also show how a band-averaged light curve suffers from temporal smearing, and consequent reduction in contrast of intensity variation, with increasing bandwidth. We suggest a way to recover the underlying diffraction signature, as well as the sensitivity improvement commensurate with usage of large bandwidths.

Improved phase-attenuation duality method with space-frequency joint domain iterative regularization.

A common problem of in-line phase contrast imaging systems based on laboratory source and detector is the negative effects of finite source size, limited spatial resolution, and system noise. These negative effects swamp the fine phase contrast fringes and impede the precise retrieval of phase maps. This study aims to develop a novel phase retrieval method to restore phase information that is lost due to an imperfect system. An improved phase-attenuation duality (PAD) method based on space-frequency joint domain iterative regularization (JDIR) is proposed to overcome the problems of the analytical PAD method and the spatial-domain iterative regularization (SDIR) based PAD method. These problems include noise robustness and optical transfer function compensation. The proposed method was compared with the two former PAD methods through computer simulations and experiments for validation. Results reveal that JDIR method outperforms the other two methods in terms of improving the visibility of structures in the retrieved phase maps. Among all the phase retrieval algorithms, the TV-norm-based JDIR method performed the best in considering the contrast and noise performance. This paper provides a new method to investigate quantitative phase-contrast imaging when considering the negative effects of an imperfect system.

Tomography with extended sources: Theory, error estimates, and a reconstruction algorithm

Recently, an approach to tomography with extended anisotropic radiation sources has been introduced, which helps to overcome the challenges resulting from the low brilliance typical for x-ray laboratory sources. The method is based on the three-dimensional Radon transform (3DRT) which uses planar integrals instead of line integrals. By extending the source spot in one direction, more photons can contribute to image formation while the impact on the resolution is minor with the 3DRT approach. In this work we present a more comprehensive description of the method, derive quantitative error estimates for the extraction of these planar integrals measured with a finite source size, and validate the 3DRT scheme by analytical theory. We also demonstrate a simple and efficient reconstruction algorithm for 3D Radon data. Finally, we further substantiate the method with experimental results obtained at a microfocus x-ray source with an extremely anisotropic source spot.

Suomi NPP VIIRS solar diffuser screen transmittance model and its applications.

The visible infrared imaging radiometer suite on the Suomi National Polar-orbiting Partnership satellite calibrates its reflective solar bands through observations of a sunlit solar diffuser (SD) panel. Sunlight passes through a perforated plate, referred to as the SD screen, before reaching the SD. It is critical to know whether the SD screen transmittance measured prelaunch is accurate. Several factors such as misalignments of the SD panel and the measurement apparatus could lead to errors in the measured transmittance and thus adversely impact on-orbit calibration quality through the SD. We develop a mathematical model to describe the transmittance as a function of the angles that incident light makes with the SD screen, and apply the model to fit the prelaunch measured transmittance. The results reveal that the model does not reproduce the measured transmittance unless the size of the apertures in the SD screen is quite different from the design value. We attribute the difference to the orientation alignment errors for the SD panel and the measurement apparatus. We model the alignment errors and apply our transmittance model to fit the prelaunch transmittance to retrieve the "true" transmittance. To use this model correctly, we also examine the finite source size effect on the transmittance. Furthermore, we compare the product of the retrieved "true" transmittance and the prelaunch SD bidirectional reflectance distribution function (BRDF) value to the value derived from on-orbit data to determine whether the prelaunch SD BRDF value is relatively accurate. The model is significant in that it can evaluate whether the SD screen transmittance measured prelaunch is accurate and help retrieve the true transmittance from the transmittance with measurement errors, consequently resulting in a more accurate sensor data product by the same amount.

Laser-driven shock waves studied by x-ray radiography.

Multimegabar laser-driven shock waves are unique tools for studying matter under extreme conditions. Accurate characterization of shocked matter is for instance necessary for measurements of equation of state data or opacities. This paper reports experiments performed at the LULI facility on the diagnosis of shock waves, using x-ray-absorption radiography. Radiographs are analyzed using standard Abel inversion. In addition, synthetic radiographs, which also take into account the finite size of the x-ray source, are generated using density maps produced by hydrodynamic simulations. Reported data refer to both plane cylindrical targets and hemispherical targets. Evolution and deformation of the shock front could be followed using hydrodynamic simulations.

Astrometric microlensing

In the next future, astrometric microlensing will offer a new channel for investigating the nature of both lenses and sources involved in a gravitational microlensing event. The effect, corresponding to the shift of the position of the multiple image centroid with respect to the source star location, is expected to occur on scales from micro-arcoseconds to milli-arcoseconds depending on the characteristics of the lens-source system. Here, we consider different classes of events (single/binary lens acting on a single/binary source) also accounting for additional effects including the finite source size, the blending and orbital motion. This is particularly important in the era of Gaia observations which are making possible astrometric measurements with unprecedented quality.

Feasibility Study for Improving the Image Characteristics in Digital Tomosynthesis (DTS) Using a Compressed-Sensing (Cs)-Based Pre-Deblurring Scheme

ABSTRACTDigital tomosynthesis (DTS) has been widely used in both industrial nondestructive testing and medical x-ray imaging as a popular multiplanar imaging modality. However, although it provides some of the tomographic benefits of computed tomography (CT) at reduced dose and imaging time, the image characteristics are relatively poor due to blur artifacts originated from incomplete data sampling for a limited angular range and also aspects inherent to imaging system, including finite focal spot size of the x-ray source, detector resolution, etc. In this work, in order to overcome these difficulties, we propose an intuitive method in which a compressed-sensing (CS)-based deblurring scheme is applied to the projection images before common DTS reconstruction. We implemented the proposed deblurring algorithm and performed a systematic experiment to demonstrate its viability for improving the image characteristics in DTS. According to our results, the proposed method appears to be effective for the blurring problems in DTS and seems to be promising to our ongoing application to x-ray nondestructive testing.

EXtended CASA Line Analysis Software Suite (XCLASS)

The eXtended CASA Line Analysis Software Suite (XCLASS) is a toolbox for the Common Astronomy Software Applications package (CASA) containing new functions for modeling interferometric and single dish data. Among the tools is the myXCLASS program which calculates synthetic spectra by solving the radiative transfer equation for an isothermal object in one dimension, whereas the finite source size and dust attenuation are considered as well. Molecular data required by the myXCLASS program are taken from an embedded SQLite3 database containing entries from the Cologne Database for Molecular Spectroscopy CDMS) and JPL using the Virtual Atomic and Molecular Data Center (VAMDC) portal. Additionally, the toolbox provides an interface for the model optimizer package Modeling and Analysis Generic Interface for eXternal numerical codes (MAGIX), which helps to find the best description of observational data using myXCLASS (or another external model program), i.e., finding the parameter set that most closely reproduces the data.