Two Monte Carlo-based simulators for imaging-system modeling and projection simulation of flat-panel X-ray source

The advantages of a flat-panel X-ray source(FPXS)make it a promising candidate for imaging applications.Accurate imaging-system modeling and projection simulation are critical for analyzing imaging performance and resolving overlapping projection issues in FPXS.The conventional analytical ray-tracing approach is limited by the number of patterns and is not applicable to FPXS-projection calculations.However,the computation time of Monte Carlo(MC)simulation is independent of the size of the patterned arrays in FPXS.This study proposes two high-efficiency MC projection simulators for FPXS:a graphics processing unit(GPU)-based phase-space sampling MC(gPSMC)simulator and GPU-based fluence sampling MC(gFSMC)simulator.The two simulators comprise three components:imaging-system modeling,photon initialization,and physical-interaction simulations in the phantom.Imaging-system modeling was performed by modeling the FPXS,imaging geometry,and detector.The gPSMC simulator samples the initial photons from the phase space,whereas the gFSMC simulator performs photon initialization from the calculated energy spectrum and fluence map.The entire process of photon interaction with the geometry and arrival at the detector was simulated in parallel using multiple GPU kernels,and projections based on the two simulators were calculated.The accuracies of the two simulators were evaluated by comparing them with the conventional analytical ray-tracing approach and acquired projections,and the efficiencies were evaluated by comparing the computation time.The results of simulated and realistic experiments illustrate the accuracy and efficiency of the proposed gPSMC and gFSMC simulators in the projection calculation of various phantoms.

Global hybrid simulations of soft X-ray emissions in the Earth’s magnetosheath

Earth’s magnetopause is a thin boundary separating the shocked solar wind plasma from the magnetospheric plasmas,and it is also the boundary of the solar wind energy transport to the magnetosphere.Soft X-ray imaging allows investigation of the large-scale magnetopause by providing a two-dimensional(2-D)global view from a satellite.By performing 3-D global hybrid-particle-in-cell(hybrid-PIC)simulations,we obtain soft X-ray images of Earth’s magnetopause under different solar wind conditions,such as different plasma densities and directions of the southward interplanetary magnetic field.In all cases,magnetic reconnection occurs at low latitude magnetopause.The soft X-ray images observed by a hypothetical satellite are shown,with all of the following identified:the boundary of the magnetopause,the cusps,and the magnetosheath.Local X-ray emissivity in the magnetosheath is characterized by large amplitude fluctuations(up to 160%);however,the maximum line-of-sight-integrated X-ray intensity matches the tangent directions of the magnetopause well,indicating that these fluctuations have limited impact on identifying the magnetopause boundary in the X-ray images.Moreover,the magnetopause boundary can be identified using multiple viewing geometries.We also find that solar wind conditions have little effect on the magnetopause identification.The Solar wind Magnetosphere Ionosphere Link Explorer(SMILE)mission will provide X-ray images of the magnetopause for the first time,and our global hybrid-PIC simulation results can help better understand the 2-D X-ray images of the magnetopause from a 3-D perspective,with particle kinetic effects considered.

Hybrid-Vlasov simulation of soft X-ray emissions at the Earth’s dayside magnetospheric boundaries

Solar wind charge exchange produces emissions in the soft X-ray energy range which can enable the study of near-Earth space regions such as the magnetopause,the magnetosheath and the polar cusps by remote sensing techniques.The Solar wind Magnetosphere Ionosphere Link Explorer(SMILE)and Lunar Environment heliospheric X-ray Imager(LEXI)missions aim to obtain soft Xray images of near-Earth space thanks to their Soft X-ray Imager(SXI)instruments.While earlier modeling works have already simulated soft X-ray images as might be obtained by SMILE SXI during its mission,the numerical models used so far are all based on the magnetohydrodynamics description of the space plasma.To investigate the possible signatures of ion-kinetic-scale processes in soft Xray images,we use for the first time a global hybrid-Vlasov simulation of the geospace from the Vlasiator model.The simulation is driven by fast and tenuous solar wind conditions and purely southward interplanetary magnetic field.We first produce global X-ray images of the dayside near-Earth space by placing a virtual imaging satellite at two different locations,providing meridional and equatorial views.We then analyze regional features present in the images and show that they correspond to signatures in soft X-ray emissions of mirrormode wave structures in the magnetosheath and flux transfer events(FTEs)at the magnetopause.Our results suggest that,although the time scales associated with the motion of those transient phenomena will likely be significantly smaller than the integration time of the SMILE and LEXI imagers,mirror-mode structures and FTEs can cumulatively produce detectable signatures in the soft X-ray images.For instance,a local increase by 30%in the proton density at the dayside magnetopause resulting from the transit of multiple FTEs leads to a 12%enhancement in the line-of-sight-and time-integrated soft X-ray emissivity originating from this region.Likewise,a proton density increase by 14%in the magnetosheath associated with mirror-mode structures can result in an enhancement in the soft X-ray signal by 4%.These are likely conservative estimates,given that the solar wind conditions used in the Vlasiator run can be expected to generate weaker soft X-ray emissions than the more common denser solar wind.These results will contribute to the preparatory work for the SMILE and LEXI missions by providing the community with quantitative estimates of the effects of small-scale,transient phenomena occurring on the dayside.

Simultaneous fluorescence and Compton scattering computed tomography based on linear polarization X-ray

Purpose To propose a method for simultaneous fluorescence and Compton scattering computed tomography by using linearly polarized X-rays.Methods Monte Carlo simulations were adopted to demonstrate the feasibility of the proposed method.In the simulations,the phantom is a polytetrafluoroethylene cylinder inside which are cylindrical columns containing aluminum,water,and gold(Au)-loaded water solutions with Au concentrations ranging between 0.5 and 4.0 wt%,and a parallel-hole collimator imaging geometry was adopted.The light source was modeled based on a Thomson scattering X-ray source.The phantom images for both imaging modalities were reconstructed using a maximumlikelihood expectation maximization algorithm.Results Both the X-ray fluorescence computed tomography(XFCT)and Compton scattering computed tomography(CSCT)images of the phantom were accurately reconstructed.A similar attenuation contrast problem for the different cylindrical columns in the phantom can be resolved in the XFCT and CSCT images.The interplay between XFCT and CSCT was analyzed,and the contrast-to-noise ratio(CNR)of the reconstruction was improved by correcting for the mutual influence between the two imaging modalities.Compared with K-edge subtraction imaging,XFCT exhibits a CNR advantage for the phantom.Conclusion Simultaneous XFCT and CSCT can be realized by using linearly polarized X-rays.The synergy between the two imaging modalities would have an important application in cancer radiation therapy.

NOVEL CELESTIAL NAVIGATION FOR SATELLITE USING X-RAY PULSARS

The X-ray pulsar-based navigation is a novel technology for the satellite autonomous navigation. The position and the velocity of the satellite are deterimined by using the pulse phases detected at the satellite and predicted by the pulse timing models. With the detected pulse phase, the satellite position with respect to the Earth center can be calculated along the line-of-sight to the pulsar. Using three pulsars, the satellite position in the in- ertial frame can be resolved. The extended Kalman filter （EKF） algorithm is designed to incorporate the range measurements with the satellite dynamics. Simulation verification shows that the proposed algorithm can accu- rately determine the satellite orbit, with the position error less than 100 m. Furthermore, the factors influencing the navigation performance are also discussed.

Simulations and software development for the Hard X-ray Imager onboard ASO-S

China’s first solar mission,the Advanced Space-based Solar Observatory(ASO-S),is now changing from Phase B to Phase C.Its main scientific objectives are summarized as’1M2B’,namely magnetic field and two types of bursts(solar flares and coronal mass ejections).Among the three scientific payloads,Hard X-ray Imager(HXI)observes images and spectra of X-ray bursts in solar flares.In this paper,we briefly report on the progresses made by the HXI science team(data and software team)during the design phase(till May 2019).These include simulations of HXI imaging,optimization of HXI grids,development of imaging algorithms,estimation of orbital background,as well as in-orbit calibration plan.These efforts provided guidance for the engineering,improved HXI’s imaging capability and reduced the cost of the instrument.

Numerical simulation and method study of X-ray litho-density logging

To effectively replace the isotope radiation source in litho-density logging,this study presents a method for measuring the formation density and photoelectric absorption index(Pe)using a switchable X-ray tube.First,the gamma-ray litho-density logging(GLD)method for measuring formation density and Pe using chemical sources is introduced.Then,a benchmark verification based on the X-ray litho-density logging tool prototype and data published by Simon(In:Paper presented at the SPWLA 59th annual logging symposium,London,UK,2018)was carried out using Monte Carlo numerical simulations.Second,the impacts of the photoelectric effect and detector statistical error on the GLD method were analyzed.Finally,based on a theoretical analysis,the formation density and Pe measurement algorithm(double energy window(DEW)method)was improved,which was found to be suitable for X-ray litho-density logging.Moreover,the results obtained using this algorithm were compared with those obtained using the GLD method.The results indicate that owing to the impact of photoelectric effect and detector statistical error on the density energy window,the accuracy of formation density and Pe measurement using the GLD method is relatively low,with the uncertainty in formation density and Pe measurement reaching 2.620±0.047 g/cm3 and 4.090±0.580 b/e,respectively.In comparison,the DEW method can improve the accuracy of density and Pe measurement to 0.006 g/cm3 and 0.065 b/e,respectively,as the photoelectric effect in the density window is corrected using the counts in the lithology window of the energy spectrum.This study aims to provide a new theoretical foundation for processing X-ray litho-density logs in the future.

2D hydrodynamic simulation of a line-focused plasma in Ni-like Ag x-ray laser research

In most collisional schemes of x-ray laser （XRL） experiments, a bow-like intensity distribution of XRL is often observed, and it is generally ascribed to the two-dimensional hydrodynamic behaviour of expanding plasma. In order to better understand its essence in physics, a newly developed two-dimensional non-equilibrium radiation hydrodynamic code XRL2D is used to simulate a quasi-steady state Ni-like Ag XRL experiment on ShenGuang-Ⅱfacility. The simulation results show that the bow-like distribution of Ni-like ions caused by over-ionization in the central area of plasma is responsible for the bow-like shape of the XRL intensity distribution observed.

Three-dimensional spatial structure of the macro-pores and flow simulation in anthracite coal based on X-ray μ-CT scanning data

The three-dimensional(3 D) structures of pores directly affect the CH4 flow.Therefore,it is very important to analyze the3 D spatial structure of pores and to simulate the CH4 flow with the connected pores as the carrier.The result shows that the equivalent radius of pores and throats are 1-16 μm and 1.03-8.9 μm,respectively,and the throat length is 3.28-231.25 μm.The coordination number of pores concentrates around three,and the intersection point between the connectivity function and the X-axis is 3-4 μm,which indicate the macro-pores have good connectivity.During the single-channel flow,the pressure decreases along the direction of CH4 flow,and the flow velocity of CH4 decreases from the pore center to the wall.Under the dual-channel and the multi-channel flows,the pressure also decreases along the CH4 flow direction,while the velocity increases.The mean flow pressure gradually decreases with the increase of the distance from the inlet slice.The change of mean flow pressure is relatively stable in the direction horizontal to the bedding plane,while it is relatively large in the direction perpendicular to the bedding plane.The mean flow velocity in the direction horizontal to the bedding plane(Y-axis) is the largest,followed by that in the direction horizontal to the bedding plane(X-axis),and the mean flow velocity in the direction perpendicular to the bedding plane is the smallest.

Effects of dynamo magnetic fields on observational properties of accreting millisecond X-ray pulsars

In this paper,we have investigated accreting millisecond X-ray pulsars,which are rapidly rotating neutron stars in low-mass X-ray binaries.These systems exhibit coherent X-ray pulsations that arise when the accretion flow is magnetically channeled to the stellar surface.Here,we have developed the fundamental equations for an accretion disk around accreting millisecond X-ray pulsars in the presence of a dynamo generated magnetic field in the inner part of the disk.We have also formulated the numerical method for the structure equations in the inner region of the disk and the highest accretion rate is enough to form the inner region of the disk,which is overpowered by radiation pressure and electron scattering.Finally,we have examined our results with the effects of dynamo magnetic fields on accreting millisecond X-ray pulsars.

Denoising of X-ray pulsar observed profile using biorthogonal lifting wavelet transform

In X-ray pulsar-based navigation, strong X-ray background noise leads to a low signal-to-noise ratio（SNR） of the observed profile, which consequently makes it very difficult to obtain an accurate pulse phase that directly determines the navigation precision. This signifies the necessity of denoising of the observed profile. Considering that the ultimate goal of denoising is to enhance the pulse phase estimation, a profile denoising algorithm is proposed by fusing the biorthogonal lifting wavelet transform of the linear phase characteristic with the thresholding technique. The statistical properties of X-ray background noise after epoch folding are studied. Then a wavelet-scale dependent threshold is introduced to overcome correlations between wavelet coefficients. Moreover, a modified hyperbola shrinking function is presented to remove the impulsive oscillations of the observed profile. The results of numerical simulations and real data experiments indicate that the proposed method can effectively improve SNR of the observed profile and pulse phase estimation accuracy, especially in short observation durations. And it also outperforms the Donoho thresholding strategy normally used in combination with the orthogonal discrete wavelet transform.

Simulation Study of Quantitative X-Ray Fluorescence Analysis of Ore Slurry Using Partial Least-Squares Regression

X-ray fluorescence （XRF） in combination with partial least-squares （PLS） regression was employed to analyze the ore slurry grade. Using the Monte Carlo simulation code PENELOPE, X-ray fluorescence spectra of ore samples were obtained. Good accuracy was achieved when this method was used to analyze elements with concentrations of several percent or above. It was demonstrated that the more the number of X-ray fluorescence spectra used to calibrate, the better the obtained accuracy. In this method detector resolution was found to have little or no effect on the results of quantitative analysis. The effect of the concentration of water was investigated as well, and it was found to have little influence on the results.

GEANT4 simulation study of over-response phenomenon of fiber x-ray sensor

The purpose of this article is to explore the cause of the over-response phenomenon of fiber x-ray sensor.The sensor is based on a length of PMMA fiber,whose end is filled with the scintillation material Gd_(2)O_(2)S:Tb.The Monte Carlo simulation software GEANT4 uses the phase space file provided by the International Atomic Energy Agency(IAEA),by irradiating the fiber x-ray sensor in the water phantom,counting the fluorescence signal of the optical fiber x-ray sensor after propagation through the fiber.In addition,the number of Cerenkov photons propagating through the fiber is also counted.Comparing this article with previous research,we believe that one of the reasons for the over-response of the fiber x-ray sensor is the non-linear response of the deposition energy of the scintillator to the fluorescence.By establishing a region of interest and counting the x-rays in this region,the simulation results show that the counted number of x-rays that may affect the fiber x-ray sensor is the biggest in the area of interest at a water depth of 5 cm.This result is close to the maximum dose point of the experimental and simulated percentage depth dose(PDD) curve of fiber x-ray sensor.Therefore,the second reason of the over-response phenomenon is believed to be fact that the inorganic materials such as Gd_(2)O_(2)S:Tb have larger effective atomic numbers,so the fiber x-ray sensors will cause more collisions with x-ray in a low energy region of 0.1 MeV-1.5 MeV.

Suzaku observation of Be / X-ray binary pulsar EXO 2030+375

We study the timing and spectral properties of Be/X-ray binary pulsar EXO 2030＋375 using a Suzaku observation taken on 2012 May 23, during a less intense Type I outburst. Pulsations were clearly detected in the X-ray light curves at a barycentric period of 41.2852 s, which suggest that the pulsar is spinning-up. The pulse profiles were found to be peculiar, e.g. unlike those obtained from the earlier Suzaku observation acquired on 2007 May 14. A single-peaked narrow profile at soft X-rays （0.5-10 keV range） changed to a double-peaked broad profile in the 12-55 keV energy range and again reverted back to a smooth single-peaked profile at hard X-rays （55-70 keV range）. The 1.0-100.0 keV broadband spectrum of the pulsar was found to be well described by three continuum models described as （i） a partial covering high energy cut-off power-law model, （ii） a partially absorbed power-law with high- energy exponential rolloff and （iii） a partial covering Negative and Positive power law with EXponential （NPEX） continuum model. Unlike the earlier Suzaku observation during which several low energy emission lines were detected, a weak and narrow Iron Ka emission line at 6.4 keV was only present in the pulsar spectrum during the 2012 May outburst. Non-detection of any absorption like feature in the 1-100 keV energy range supports the claim of the absence of the cyclotron resonance scattering feature in EXO 2030＋375 from the earlier Suzaku observation. Pulse-phase resolved spectroscopy revealed the presence of additional dense matter causing the absence of a second peak from the soft X-ray pulse profiles. The details of the results are described in the paper.

Modification of gravitational redshift of x-ray burst produced by pulsar surface magnetoplasma

In this paper, the propagation of x-ray bursts in the magnetoplasma of pulsar magnetosphere is discussed. The electromagnetic interaction between x-ray bursts and magnetoplasma is described as some geometry. The electromagnetic effects of surface superstrong magnetic field and dynamic effects of outflowing magnetoplasma of pulsars are treated as an optical metric. The Gordon metric is introduced to represent the gravitational metric and optical metric. So the propagation of x-ray bursts in magnetoplasma of pulsars can be described as x-ray bursts transmitting in an effective space characterized by Gordon metric. The modification of gravitational redshift, attributed to the flowing magnetoplasma of pulsars, is obtained and it is shown that the modification is of redshift and can reach the same magnitude as the gravitational redshift for ordinary pulsars.

Detectability of rotation-powered pulsars in future hard X-ray surveys

Recent INTEGRAL/IBIS hard X-ray surveys have detected about 10 young pulsars. We show hard X-ray properties of these 10 young pulsars, which have a luminosity of 10^33 - 10^37 erg s^-1 and a photon index of 1.6-2.1 in the energy range of 20-100 keV. The correlation between X-ray luminosity and spin-down power of Lx ∝ Lsd^1.31 suggests that the hard X-ray emission in rotation-powered pulsars is dominated by the pulsar wind nebula （PWN） component. Assuming spectral properties are similar in 20-100keV and 2-10 keV for both the pulsar and PWN components, the hard X-ray luminosity and flux of 39 known young X-ray pulsars and 8 millisecond pulsars are obtained, and a correlation of Lx ∝ Lsd^1.5 is derived. About 20 known young X-ray pulsars and 1 millisecond pulsars could be detected with future INTEGRAL and HXMT surveys. We also carry out Monte Carlo simulations of hard X-ray pulsars in the Galaxy and the Gould Belt, assuming values for the pulsar birth rate, initial position, proper motion velocity, period, and magnetic field distribution and evolution based on observational statistics and the Lx - Lsd reltions：Lx ∝Lsd^1.31 and Lx∝Lsd^1.5 More than 40 young pulsars （mostly in the Galactic plane） could be detected after ten years of INTEGRAL surveys and the launch of HXMT. So, the young pulsars would be a significant part of the hard X-ray source population in the sky, and will contribute to unidentified hard X-ray sources in present and future hard X-ray surveys by INTEGRAL and HXMT.

Simulation of the orbit and spin period evolution of the double pulsars PSR J0737-3039 from their birth to coalescence induced by gravitational wave radiation

The complete orbital and spin period evolutions of the double neutron star(NS)system PSR J0737-3039 are simulated from birth to coalescence,which include the two observed radio pulsars classified as primary NS PSR J0737-3039 A and companion NS PSR J0737-3039 B.By employing the characteristic age of PSR J0737-3039 B to constrain the true age of the double pulsar system,the initial orbital period and initial binary separation are obtained as 2.89 h and 1.44 x 106 km,respectively,and the coalescence age or the lifetime from the birth to merger of PSR J0737-3039 is obtained to be 1.38×10^(8)yr.At the last minute of coalescence,corresponding to the gravitational wave frequency changing from 20 Hz to1180 Hz,we present the binary separation of PSR J0737-3039 to be from 442 km to 30 km,while the spin periods of PSR J0737-3039 A and PSR J0737-3039 B are 27.10 ms and 4.63 s,respectively.From the standard radio pulsar emission model,before the system merged,the primary NS could still be observed by a radio telescope,but the companion NS had crossed the death line in the pulsar magnetic-field versus period(B-P)diagram at which point it is usually considered to cease life as a pulsar.This is the first time that the whole life evolutionary simulation of the orbit and spin periods for a double NS system is presented,which provides useful information for observing a primary NS at the coalescence stage.

Spectral Properties of Anomalous X-ray Pulsars

We examine the spectra of the persistent emission from anomalous X-ray pulsars (AXPs) and their variation with the spin-down rate . Based on an accretion-powered model, the influences of both the magnetic field and the mass accretion rate on the spectral properties of AXPs are addressed. We then investigate the relation between the spectral property of AXPs and mass accretion rate . The result shows that there exists a linear correlation between the photon index and the mass accretion rate: the spectral hardness increases with increasing . A possible emission mechanism for the explanation of the spectral properties of AXPs is also discussed.

Computer Simulation and X-ray Diffraction of Nanocrystals

X-ray diffraction of structure in nanocrystalline alpha -Fe and Cu was studied by atomistic simulation. Atomic position equilibrium was reached by using molecular dynamics method to simulate nanocrystalline structure with Finnis potentials to model interatomic interactions. It was found that the boundary component exhibits short-range order, and the distortion in crystalline component increases with the decrease of grain size.

Simulation of X-ray Diffraction Line Broadening Caused by Stress Gradients and Determination of Stress Distribution by Fourier Analysis

Different physical, mechanical and chemical processes, such as： ion implantation, oxidation, nitridation and others create on the surface of materials residual stress state, characterized by high level and strong gradient. X-ray diffraction method widely used for stress measurements has some difficulties in interpretation of experimental data, when the depth of X-ray penetration is compared with thickness of surface layer where inhomogeneous stress distribution is localized. Early it has been shown by authors that diffraction line broadening occurs when analyzed surface is characterized by strong gradient. The interest to study the diffraction line broadening is connected to the possibility of obtaining information about parameters of surface stress distribution. In the present paper the convolution and deconvolution concepts of Fourier analysis were applied to study X ray diffraction line broadening caused by surface stress gradients. Developed methodology allows determining of stress distribution in superficial layers of materials.