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Fully polarized Compton scattering from a beam of spin-polarized electrons is investigated in plane-wave backgrounds in a broad intensity region from the perturbative to the nonperturbative regimes. In the perturbative regime, polarized linear Compton scattering is considered for investigating polarization transfer from a single laser photon to a scattered photon, and in the high-intensity region, the polarized locally monochromatic approximation and locally constant field approximation are established and are employed to study polarization transfer from an incoming electron to a scattered photon. The numerical results suggest an appreciable improvement of about 10% in the scattering probability in the intermediate-intensity region if the electron's longitudinal spin is parallel to the laser rotation. The longitudinal spin of the incoming electron can be transferred to the scattered photon with an efficiency that increases with laser intensity and collisional energy. For collision between an optical laser with frequency ∼1 eV and a 10 GeV electron, this polarization transfer efficiency can increase from about 20% in the perturbative regime to about 50% in the nonperturbative regime for scattered photons with relatively high energy. [ABSTRACT FROM AUTHOR]

Gamma-ray bursts (GRBs) are the phenomena of rapid energy release of enormous power associated with the collapse or merging of stars. As a result of internal processes, populations of nonthermal accelerated particles radiating in a wide energy range are formed in them. A number of observations have shown that photons with energies up to tens of TeV are detected from some GRBs. However, due to the great energy losses of radiating particles, the explanation of this high-energy radiation in terms of standard radiation mechanisms runs into great difficulties. In this paper, based on the model of adiabatic expansion for the GRB afterglow phase, we investigate the influence of magnetic inhomogeneities on the spectra within the electron and proton synchrotron radiation mechanism by taking into account the Compton scattering of synchrotron photons. We show that the magnetic inhomogeneity effect can increase the maximum energies of the synchrotron radiation from electrons and protons several fold without affecting the maximum energies of the Compton photons being produced in the Klein–Nishina regime. [ABSTRACT FROM AUTHOR]

Recently, a γ γ collider based on the existing 17.5 GeV linac of the European XFEL has been proposed. High-energy photons will be generated by Compton scattering of laser photons with a wavelength of 0.5–1 μ m on electrons. Such a photon collider covers the range of invariant masses W γ γ < 12 Ge V / c 2 . The physics program includes spectroscopy of C -even resonances (c-, b-quarkonia, 4-quark states, glueballs) in various J P states. Variable circular and linear polarizations will help in determining the quantum numbers. In this paper, we present a summary of measured and predicted two-photon widths of various resonances in the mass region 3–12 Ge V / c 2 and investigate the experimental possibility of observing these heavy two-photon resonances under the conditions of a large multi-hadron background. Registration of all final particles is assumed. The minimum values of Γ γ γ (W) are obtained at which resonances can be detected at a 5 σ confidence level in 1 year of operation. [ABSTRACT FROM AUTHOR]

We study the Fourier time lags due to the Comptonization of disc-emitted photons in a spherical, uniform, and stationary X-ray corona, which located on the rotational axis of the black hole. We use monk , a general relativistic Monte Carlo radiative transfer code, to calculate Compton scattering of photons emitted by a thin disc with a Novikov–Thorne temperature profile. We find that the model time lags due to Comptonization remain constant up to a characteristic frequency and then rapidly decrease to zero at higher frequencies. We provide equations that can be used to determine the time lags and cross-spectra for a wide range of values for the corona radius, temperature, optical depth, height, and for various accretion rates and black hole masses. We also provide an equation for the X-ray luminosity of a single corona, as a function of its characteristics and location above the disc. Remarkably, the observed X-ray time lags of nearby, bright active galaxies can be successfully reproduced by inverse Comptonization process of multiple dynamic coronae. [ABSTRACT FROM AUTHOR]

The features of estimating the energies of the magnetic field and relativistic electrons of synchrotron radio sources, the X-ray emission of which is produced by the invers Compton scattering of synchrotron photons (the synchrotron self-Compton process), are considered. Estimates of these physical parameters depend on the angular dimensions of the radiation sources. The effect of angular dimensions on the estimates of physical parameters is illustrated by an estimate of the magnetic induction of the hot spot A of the radio galaxy Cygnus A. We showed why it is impossible to use apparent angular dimensions of non-uniform radio sources for estimating their physical parameters. To obtain correct estimates of the energies of the field and particles, we need to use the angular size from the spherical model of the source under study, the spectrum of which coincides with the radio spectrum of the object under study. Taking such an angular size of the source, we obtain the same estimates of the physical parameters of this object, both from radio and X-ray observations. [ABSTRACT FROM AUTHOR]

First, photoelectric effect and Compton scattering show photons as particles, and interference and diffraction of particles show particles as waves, therefore, duality exists probably only under interactic,ns. Its test is that duality not exists when there are not interactions. Next, some nonlinear quantum equations and wave equations have soliton solutions, which correspond to particles. This Is mathematical description of wave-pa1·ticle duality, and whose condition is also interactions and nonlinearity. Third, based on the soliton-chaos double solutions of nonlinear equations, we propose the field-particle duality, and field (wave)-quanturn-chaos ternary. Further, if we develop mathematical form, more multiplicity will be obtained. Finally, in the quantum entangled states, velocities of different phases obey possibly general Lorentz transformation (GLT). [ABSTRACT FROM AUTHOR]

The exclusive photoproduction off nucleon of a large invariant mass lepton pair in the backward region specified by the small Mandelstam variable - u is discussed in the framework of collinear QCD factorization. The amplitude is factorized in terms of photon-to-nucleon Transition Distribution Amplitudes (TDAs). These new non-perturbative objects, complementary to the "photon-inside-the-nucleon PDF", lead to a tomographic picture of the nucleon's photon content after Fourier transforming to the impact parameter space. This allows to address the question of where in the transverse plane the nucleon emits a photon. A simplistic model-estimate of photon-to-nucleon TDAs is used to estimate the magnitude of the corresponding cross sections. The background due to the electromagnetic Bethe–Heitler process is shown to be negligible in the kinematical regime of interest. [ABSTRACT FROM AUTHOR]

The release of radioactive gas into the atmosphere can diffuse into large volumes of air downwind from the point of release. The extent of radioactivity can cover thousands of cubic meters of air. For such large volumes, the weather models used to predict the down-wind distribution of the plume and the radiation transport models used to predict the radiation reaching ground-level from the plume can take tens of hours of computer time on multi-node institutional High-Performance Computing facilities. In this paper we focus on the radiation transport aspect of plume modeling. We describe a phenomenological method for approximating the amounts of radiation that reach ground level from large volumes of a static radioactive plume that can be calculated on a stand-alone personal computer in much shorter computation times than those usually needed for such large volume evaluations. We refer to this method as the Density Scaling Approximation (DSA). Its ability to approximate ground-level count rates of large plumes comes from using a small-plume volume with a scaled-up value of air density to simulate the same number of scatterings that occur during transport in larger plume volumes at normal air density. We demonstrate the DSA by using a 100 m-diameter air-filled hemispherical dome geometry with a uniform volumetric activity of 135Xe gas throughout the air-filled volume. The DSA for a larger dome diameter is obtained by evaluating the 100 m dome with an air density scaled up by the linear ratio of the larger diameter to the 100 m diameter. We find that this approximation works well for dome diameters up to 1200 m – the largest diameter studied and a size more than sufficient for accounting for all the radiation from 135Xe. Moreover, most of our DSA results can be calculated over 500 times faster than corresponding full-sized geometry with normal air density. To help evaluate the accuracy of the DSA and gain insight into how well it can reproduce different regions of the spectra, we use three, easily understood regions of interest to compare the DSA results to the full-sized geometry at normal air density results. These regions are the full-energy peak, the region of single-Compton scattering, and the region of multiple-Compton scattering. We show how the dominance of the Compton scattering mechanism determines this division and thus provides insight into how Compton scattering is manifested in spectra from photon scattering through air in general, and how well the DSA approximation works. [ABSTRACT FROM AUTHOR]

A polarimeter was constructed to measure the longitudinal polarization of a spin-polarized electron beam at 5 and 7 MeV. The polarimeter takes advantage of Compton scattering between circularly polarized bremsstrahlung photons produced by a longitudinally polarized electron beam striking a copper radiator and the spin-polarized electrons orbiting the iron atoms of an analyzing magnet. This so-called Compton transmission polarimeter is compact and relatively inexpensive compared to Mott-scattering polarimeters because no spin manipulator is required. This work presents the design of the radiator, analyzing magnet, photon detector assembly, and data acquisition system of the Compton transmission polarimeter as well as beam commissioning results performed at the Upgraded Injector Test Facility at Jefferson Lab. [ABSTRACT FROM AUTHOR]

Repeated Compton scattering of photons with thermal electrons is one of the fundamental processes at work in many astrophysical plasma. Solving the exact evolution equations is hard and one common simplification is based on Fokker–Planck (FP) approximations of the Compton collision term. Here we carry out a detailed numerical comparison of several FP approaches with the exact scattering kernel solution for a range of test problems assuming isotropic media and thermal electrons at various temperatures. The Kompaneets equation, being one of the most widely used FP approximations, fails to account for Klein–Nishina corrections and enhanced Doppler boosts and recoil at high energies. These can be accounted for with an alternative FP approach based on the exact first and second moments of the scattering kernel. As demonstrated here, the latter approach works very well in dilute media, but inherently fails to reproduce the correct equilibrium solution in the limit of many scattering. Conditions for the applicability of the FP approximations are clarified, overall showing that the Kompaneets equation provides the most robust approximation to the full problem, even if inaccurate in many cases. We close our numerical analysis by briefly illustrating the solutions for the spectral distortions of the cosmic microwave background (CMB) after photon injection at redshift z  ≲ 105, when double Compton and Bremsstrahlung emission can be omitted. We demonstrate that the exact treatment using the scattering kernel computed with cspack is often needed. This work should provide an important step towards accurate computations of the CMB spectral distortions from high-energy particle cascades. [ABSTRACT FROM AUTHOR]

A feasible implementation of a novel X‐ray detector for highly energetic X‐ray photons with a large solid angle coverage, optimal for the detection of Compton X‐ray scattered photons, is described. The device consists of a 20 cm‐thick sensitive volume filled with xenon at atmospheric pressure. When the Compton‐scattered photons interact with the xenon, the released photoelectrons create clouds of secondary ionization, which are imaged using the electroluminescence produced in a custom‐made multi‐hole acrylic structure. Photon‐by‐photon counting can be achieved by processing the resulting image, taken in a continuous readout mode. Based on Geant4 simulations, by considering a realistic detector design and response, it is shown that photon rates up to at least 1011 photons s−1 on‐sample (5 µm water‐equivalent cell) can be processed, limited by the spatial diffusion of the photoelectrons in the gas. Illustratively, if making use of the Rose criterion and assuming the dose partitioning theorem, it is shown how such a detector would allow obtaining 3D images of 5 µm‐size unstained cells in their native environment in about 24 h, with a resolution of 36 nm. [ABSTRACT FROM AUTHOR]

We consider the Compton scattering in the optically thick uniform spherical corona around a neutron star in an X-ray binary. In the scattering, the low energy seed photons (0.1 ∼ 2.5 keV) are scattered in low energy electrons (2.5 ∼ 10 keV) in the corona in two conditions, i.e. initial seed photons are scattered in a whole corona and scattered in every layer of the corona that are supposed to be divided into many layers. When the same number of input seed photons, the same corona parameters, and the same energy distribution of all photons in the two conditions are considered, the approximately same number of output photons can be obtained, which means that there is approximately a transform invariance of layering the Comptonized corona. Thus the scattering in the layers of a multilayered corona is approximately equal to the scattering in the whole corona by dividing the whole corona into several layers. It means that Compton scattering for the initial seed photons scattered in a whole optically thick spherical corona with uniformly distributed electrons also can be considered as that the multiple Compton scatterings take place in the layers of a multilayered corona in order approximately, which can be used to explore some physical process in one part of a corona. [ABSTRACT FROM AUTHOR]

We report here results of the analysis of correlated flux variations between the optical and GeV γ-ray bands in three bright BL Lac objects, namely AO 0235+164, OJ 287, and PKS 2155−304. This was based on the analysis of about 10 yr of data from the Fermi Gamma-ray Space Telescope covering the period between 2008 August 8 and 2018 August 8 along with optical data covering the same period. For all the sources, during the flares analysed in this work, the optical and γ-ray flux variations are found to be closely correlated. From broad-band spectral energy distribution modelling of different epochs in these sources using the one-zone leptonic emission model, we found that the optical–ultraviolet emission is dominated by synchrotron emission from the jet. The γ-ray emission in the low synchrotron peaked sources AO 0235+164 and OJ 287 is found to be well fitted with external Compton (EC) component, while the γ-ray emission in the high synchrotron peaked source PKS 2155−304 is well fitted with synchrotron self-Compton component. Further, we note that the γ-ray emission during the high-flux state of AO 0235+164 (epochs A and B) requires seed photons from both the dusty torus and broad-line region, while the γ-ray emission in OJ 287 and during epochs C and D of AO 0235+164 can be modelled by EC scattering of infrared photons from the torus. [ABSTRACT FROM AUTHOR]

3D Compton scattering imaging (CSI) is an upcoming concept exploiting the scattering of photons induced by the electronic structure of the object under study. The so-called Compton scattering rules the collision of particles with electrons and describes their energy loss after scattering. Although physically relevant, multiple-order scattering was so far not considered and therefore, only first-order scattering is generally assumed in the literature. The purpose of this work is to argument why and how a contour reconstruction of the electron density map from scattered measurement composed of first- and second-order scattering is possible (scattering of higher orders are here neglected). After the development of integral representations for the first- and second-order scattering, we approximate these models by Fourier integral operators (FIO) and study their smoothness properties. The second-order scattered radiation reveals itself to be structurally smoother than the radiation of first-order indicating that the contours of the electron density are essentially encoded within the first-order part. This opens the way to contour-based reconstruction techniques when considering multiple scattered data. Our main results, modeling and reconstruction scheme, are successfully implemented on synthetic and Monte-Carlo data. [ABSTRACT FROM AUTHOR]

Combining the nuclear resonance fluorescence (NRF) transmission method with computed tomography (CT) can be a novel method for imaging the isotope distributions, which is indispensable in nuclear engineering. We performed an experiment to reconstruct a three-dimensional NRF-CT image with isotope selectivity of enriched lead isotope rods (208Pb) together with a set of different rods, including another enriched isotope (206Pb), iron, and aluminum rods, inserted into a cylindrical aluminum holder. Using a laser Compton scattering (LCS) gamma ray beam with a 5.528 MeV maximum energy, 2 mm beam size, and 10 photon·s−1·eV−1 flux density, which is available at the BL1U beamline in the ultraviolet synchrotron orbital radiation-III (UVSOR-III) synchrotron radiation facility at the Institute of Molecular Science at the National Institutes of Natural Sciences in Japan, and we excited the Jπ = 1− NRF level at 5.512 MeV in 208Pb. An isotope-selective three-dimensional NRF-CT image of the 208Pb isotope distribution was experimentally obtained for the first time with a pixel resolution of 4 mm in the horizontal plane. [ABSTRACT FROM AUTHOR]

We study Compton scattering at helium atom exposed to an electromagnetic field with a central frequency of 80 a.u. ( ∼ 2.18 keV). We consider the situation where the incident light and scattered light are polarized along the same direction with small relative propagation angle β . The energy of the emitted electron ranges from 0.2 to 2.1 a.u. ( ∼ 5.44 –57.14 eV). The approach is based on previous works on stimulated Compton scattering (see Bachau et al. Phys Rev Lett 112:073001, 2014). We consider a field intensity of 3.51 × 10 16 W/cm 2 , where stimulated Compton scattering can be treated in perturbative regime. In lowest order perturbation theory, the process results from the contribution of A · P in second order and A 2 in first order (P is the electron momentum operator and A the vector potential of the field); both terms induce two-photon transitions. The Compton matrix element | M fg | 2 is extracted numerically resolving the time-dependent Schrödinger equation, and in perturbation theory, emphasis is put on the calculation of the second-order amplitude associated with A · P . We investigate the cases of relative propagation angle β = 0 and 10 degrees. The photoelectron energy distributions are dominated by the nondipole term A 2 ; they increase by orders of magnitude when β grows from 0 to 10 degrees. At β = 0 degree, both A · P and A 2 are at play and we show that the electrons are emitted in the (forward) direction of the momentum transfer Q and to a lesser extent in the backward direction. When β increases, the nondipole contribution A 2 tends to dominate and the forward/backward asymmetry vanishes. [ABSTRACT FROM AUTHOR]

We predict theoretically the leading role and angular anisotropy of bremsstrahlung in resonant Compton scattering of a hard X-ray photon by an atom. [ABSTRACT FROM AUTHOR]

The potential to perform attenuation and scatter compensation (ASC) in single-photon emission computed tomography (SPECT) without a separate transmission scan is highly significant. In this context, attenuation in SPECT is primarily due to Compton scattering, where the probability of Compton scatter is proportional to the attenuation coefficient of the tissue and the energy of the scattered photon and the scattering angle are related. Based on this premise, we investigated whether the SPECT emission data, including the scatter-window data, acquired in list-mode (LM) format and including the energy information, can be used to estimate the attenuation map. For this purpose, we propose a Fisher-information-based method that yields the Cramér–Rao bound (CRB) for the task of jointly estimating the activity and attenuation distribution using only the SPECT emission data. In the process, a path-based formalism to process the LM SPECT emission data, including the scattered-photon data, is proposed. The Fisher information method was implemented on NVIDIA graphics processing units (GPUs) for acceleration. The method was applied to quantify the information content of SPECT LM emission data, which contains up to first-order scattered events, in a simulated SPECT system with parameters modeling a clinical system using realistic computational studies with 2D digital synthetic and anthropomorphic phantoms. Experiments with anthropomorphic phantoms simulated myocardial perfusion and dopamine transporter (DaT)-Scan SPECT studies. The method was also applied to LM data containing up to second-order scatter for a synthetic phantom. The results show that the CRB obtained for the attenuation and activity coefficients was typically much lower than the true value of these coefficients. An increase in the number of detected photons yielded lower CRB for both the attenuation and activity coefficients. Further, we observed that systems with better energy resolution yielded a lower CRB for the attenuation coefficient. Overall, the results provide evidence that LM SPECT emission data, including the scatter-window data, contains information to jointly estimate the activity and attenuation coefficients. [ABSTRACT FROM AUTHOR]

Scattered radiation unavoidably generated in the patient will negatively impact both kilovoltage (KV) and megavoltage (MV) imaging applications. Recently, 'hybrid' methods (i.e. combining analytical and Monte Carlo (MC) techniques) are being investigated as a solution to accurately yet quickly calculate the scattered contribution for both KV and MV images. We have developed a customized MC simulation user code for investigating the individual components of patient-scattered photon fluence, which serves as a valuable tool in this area of research. The MC tool is based on the EGSnrc/DOSXYZnrc user code. The IAUSFL flag options associated with subroutine AUSGAB, combined with LATCH tracking, are used to classify the various interactions of particles with the media. Photons are grouped into six different categories: primary, 1st Compton scatter, 1st Rayleigh scatter, multiple scatter, bremsstrahlung, and positron annihilation. We take advantage of the geometric boundary check in DOSXYZnrc, to write exiting photon particle information to a phase-space file. The tool is validated using homogeneous and heterogeneous phantom configurations with monoenergetic and polyenergetic beams under parallel and divergent beam geometry, comparing MC-simulated exit primary fluence and singly-scattered fluence to corresponding analytical calculations. This MC tool has been validated to separately score the primary and scatter fluence components of the KV and MV imaging applications in the field of radiation therapy. The results are acceptable for the various configurations and beam energies tested here. Overall, the mean percentage differences are less than 0.2% and standard deviations less than 1.6%. This will be a critical test instrument for research in photon scatter applications and particularly for the development of hybrid methods, and is freely available from the authors for research purposes.5 [ABSTRACT FROM AUTHOR]

The scrutiny of the resonant laser-assisted bremsstrahlung (LAB) of ultrarelativistic electrons within the laser plasma ambience in the field of a nucleus is presented. The kinematics of the explored process can develop within two probable channels. Thus, for the first channel an electron radiates a spontaneous photon and subsequently scatters on a nucleus. In contrast, for the second channel an electron scatters on a nucleus and consequently a spontaneous photon emits. It is important to underline, that within the laser field plasma the ultrarelativistic electron in the intermediate state emerges to the mass shell and transforms into the real particle. Furthermore, the second order effect functionally transmits into two sequential first order procedures (implementing the laser-stimulated Compton process and the laser-assisted Mott scattering) with respect to the fine structure constant. We evaluated the fluctuation of the spontaneous photon resonant frequency for the various initial parameters criteria. As a result, in the investigation a specific diapason of the resonant frequency modification for the second reaction scheme was highlighted when the ultrarelativistic electron at first scatters on a nucleus and then radiates a spontaneous photon. The designated interval consists of three frequency levels in alternative to the first channel interaction pattern. Moreover, the study delineates the occurrence of the particles propagation within a narrow angle cone in the vector direction parallel to the initial electron momentum. To summarize, the article estimates that the magnitude of the differential cross-section in the resonant conditions substantially exceeds the cross-section of the process without the laser plasma ambience. In addition, pulsed laser radiation laboratories (SLAC, FAIR, XFEL, ELI, XCELS) maintain the ability to verify the project calculations. [ABSTRACT FROM AUTHOR]

The leading role of the bremsstrahlung at resonant Compton scattering of a hard X-ray photon by a multielectron atom has been theoretically predicted. This result can be important in the interpretation of Kα, β spectra of X-ray emission of stars, galaxies, and clusters of galaxies. [ABSTRACT FROM AUTHOR]

Many PET scanner detectors use LSO or LYSO as their scintillation crystals. Since natural lutetium contains 2.6% radioactive 176Lu, which decays β− followed by the emission of three gamma rays or internal conversion electrons, its intrinsic radioactivity needs to be considered. This work compares the energy spectrum obtained with Monte Carlo simulations of ionizing radiation transport using the 176Lu ion source defined in GATE v8.1, for different LYSO crystal sizes with experimental and analytical data. In addition, we performed a set of simulations of 88, 202 and 307 keV monoenergetic photons, the gamma rays of the isomeric transitions of the 176Hf excited states, to quantify the Compton scattered photons interacting in or escaping the crystal. Even though the optical transport effects were not simulated, the results show a very good agreement with analytical and experimental data and support the accuracy for modeling the intrinsic radioactivity of LSO/LYSO crystals with GATE. [ABSTRACT FROM AUTHOR]

A pronounced K α1,2 X-ray emission spectrum is theoretically predicted for the resonant inelastic scattering of two photons in the energy region near the 1s shell ionization threshold of a free multiply charged atomic ion. The observed double differential scattering cross section is estimated quantitatively. The results for the Fe16+ ion may be important in interpreting the X-ray emission spectra from galactic clusters, active galactic nuclei, and stars. [ABSTRACT FROM AUTHOR]

Almost 10 yr of γ-ray observations with the Fermi Large Area Telescope have revealed extreme γ-ray outbursts from flat spectrum radio quasars (FSRQs), temporarily making these objects the brightest γ-ray emitters in the sky. Yet, the location and mechanisms of the γ-ray emission remain elusive. We characterize long-term γ-ray variability and the brightest γ-ray flares of six FSRQs. Consecutively zooming in on the brightest flares, which we identify in an objective way through Bayesian blocks and a hill-climbing algorithm, we find variability on subhour timescales and as short as minutes for two sources in our sample (3C 279 and CTA 102) and weak evidence for variability at timescales less than the Fermi satellite's orbit of 95 minutes for PKS 1510–089 and 3C 454.3. This suggests extremely compact emission regions in the jet. We do not find any signs of γ-ray absorption in the broad-line region (BLR), which indicates that γ-rays are produced at distances greater than hundreds of gravitational radii from the central black hole. This is further supported by a cross-correlation analysis between γ-ray and radio/millimeter light curves, which is consistent with γ-ray production at the same location as the millimeter core for 3C 273, CTA 102, and 3C 454.3. The inferred locations of the γ-ray production zones are still consistent with the observed decay times of the brightest flares if the decay is caused by external Compton scattering with BLR photons. However, the minute-scale variability is challenging to explain in such scenarios. [ABSTRACT FROM AUTHOR]

Compton scatter photons are generally considered problematic in nuclear medicine imaging. Therefore, efforts are being made to minimize the involvement of these photons by employing some special strategies in daily practice. Basically, photons scattering at a small angle and traveling in the proper direction stand a chance of getting recorded and thereby participate in the image formation. These photons may create artifactual hot spots in the vicinity of a region with high concentration of radioactivity. The present study focuses on the negative impact of such photons during routine imaging in clinical setting, through an artifact detected in technetium-99m-IDA hepatobiliary scintigraphy, with the purpose of highlighting this issue to the nuclear medicine practitioners. [ABSTRACT FROM AUTHOR]

Background: Compton-scattered photons transfer incorrect spatial information. These photons are detected in used photo-peak energy window. In this study, three scatter correction procedures including dual-energy window (DEW), three energy window (TEW), and new approach were evaluated, and then the best procedure based on simulation and clinical conditions introduced. Materials and Methods: In this study, simulation projections and three-dimensional nonuniform rational B-spline-based Cardiac-Torso phantoms were produced by GEANT4 application for emission tomography simulation code. For clinical study, 2-day stress/rest myocardial perfusion imaging (MPI) protocol was performed with99mTc-sestamibi for 46 patients. Image quality parameters including contrast, signal-to-noise ratio (SNR), and relative noise of the background (RNB) were evaluated. Results: The simulation results showed that contrast values for DEW, TEW, and new approach were (0.45 ± 0.07, 0.5 ± 0.08, and 0.63 ± 0.09), SNR values (4.74 ± 0.94, 5.58 ± 1.08, and 6.56 ± 1.24), and RNB values (0.33 ± 0.06, 0.33 ± 0.07, and 0.33 ± 0.05), respectively. In clinical study, the contrast values for DEW, TEW, and new approach were 0.53 ± 0.03, 0.57 ± 0.07, and 0.62 ± 0.04 in rest MPI and were 0.52 ± 0.04, 0.57 ± 0.06, and 0.6 ± 0.05 in stress MPI, respectively. Moreover, for the rest images, the SNR values were 7.65 ± 1.9, 9.08 ± 2.2, and 10.2 ± 1.75 and for stress images were 7.76 ± 1.99, 9.12 ± 2.25, and 10.17 ± 2.04, respectively. Finally, RNB values for rest and stress images were 0.12 ± 0.03, 0.13 ± 0.03, and 0.13 ± 0.03, respectively. Conclusion: The simulation and the clinical studies showed that the new approach could be better performance than DEW, TEW methods, according to values of the contrast, and the SNR for scatter correction. [ABSTRACT FROM AUTHOR]

We present the design of a photon–photon collider based on conventional Compton gamma sources for the observation of secondary γ γ production. Two symmetric electron beams, generated by photocathodes and accelerated in linacs, produce two primary gamma rays through Compton back scattering with two high energy lasers. Tuning the system energy to the energy of the photon–photon cross section maximum, a flux of secondary gamma photons is generated. The process is analyzed by start-to-end simulations from the photocathodes to the propagation of the QED photons towards the detector. The new Monte Carlo code ‘Rate Of Scattering Events’ (ROSE) has been developed ad hoc for the counting of the QED events. Realistic numbers of the secondary gamma yield, referring to existing or approved set-ups and a discussion of the feasibility of the experiment are presented. [ABSTRACT FROM AUTHOR]

A theoretical and phenomenological consideration is given to higher order, strong field effects in electron/laser interactions. A consistent strong field theory is the Furry interaction picture of intense field quantum field theory. In this theory, fermions are embedded in the strong laser field and the Volkov wavefunction solutions that result, are exact with respect to the strong field. When these Volkov fermions interact with individual photons from other sources, the transition probability is enhanced in a series of resonances when the kinematics allow the virtual fermion to go on-shell. An experiment is proposed in which, for the first time, resonances could be used to generate high energy photons from relativistic electrons at rates orders of magnitude in excess of usual mechanisms. [ABSTRACT FROM AUTHOR]

We present a one-zone homogeneous self-consistent model for the multiwavelength emission features of PKS 1424+240 that allows the effects of automatic photon quenching to be studied. In the model, a steady γ-ray injection rate in the source is assumed and the non-linear time-dependent kinematic equations describing the evolution of both electrons and photons are then acquired; these self-consistently involve photon-photon interaction, synchrotron radiation of electron/positron pairs, inverse Compton scattering and synchrotron self-absorption. The model is applied to reproduce the multiwavelength spectrum of PKS 1424+240; our results indicate that the observed spectral energy distribution (SED) of PKS 1424+240 can be reproduced well in the case of both electron injection and photon injection. The injected electrons mainly influence the SED over radio, optical and X-ray bands and the SED in the high-energy γ-ray band is determined mainly by automatic photon quenching; in particular, the break form of the spectrum in the energy range ≥ a few GeV is caused by this. It should be noted that the observed SED from X-ray to γ-ray bands can be reproduced only through quenching, provided the observed data in the optical band are produced through some other mechanism, e.g. electron synchrotron radiation. [ABSTRACT FROM AUTHOR]

Intense, quasi-monochromatic, polarized γ -ray beams with high and tunable energy produced by Compton scattering of laser photons against relativistic electrons are used for fundamental studies and applications. Following a series of photoneutron cross section measurements in the Giant Dipole Resonance (GDR) energy region performed at the NewSUBARU synchrotron radiation facility, we have developed the eliLaBr Monte Carlo simulation code for characterization of the scattered γ -ray photon beams. The code is implemented using Geant4 and is available on the GitHub repository (https://github.com/dan-mihai-filipescu/eliLaBr). Here we report the validation of the eliLaBr code on NewSUBARU LCS γ -ray beam flux and spectral distribution data and two applications performed with it for asymmetric transverse emittance profiles electron beams, characteristic for synchrotrons. The first application is based on a systematic investigation of transverse collimator offsets relative to the laser and electron beam axis. We show that the maximum energy of the LCS γ -ray beam is altered by vertical collimator offsets, where the edge shifts towards lower energies with the increase in the offset. Secondly, using the eliLaBr code, we investigate the effect of the laser polarization plane orientation on the properties of the LCS γ -ray beams produced with asymmetric emittance electron beams. We show that: 1. The use of vertically polarized lasers contributes to the preservation of the LCS γ -ray beam maximum energy edge by increasing the precision in the vertical collimator alignment. 2. Under identical conditions for the electron and laser beams phase-space distributions, the energy spectrum of the scattered LCS γ -ray beam changes with the laser beam polarization plane orientation. More precisely, the use of vertically polarized laser beams slightly deteriorates the LCS γ -ray beam energy resolution. [ABSTRACT FROM AUTHOR]

We investigate the effects of Lorentzian astrophysical plasmas on resonant Compton scattering of photons by the helium ground and excited states. The bound-excited states energies in the plasma environments are obtained by using highly correlated exponential wave functions in the framework of Ritz variational method. The resonance Compton scattering cross sections in Lorentzian plasmas between the $$\hbox {1s}^{2}{\,}^{1}\hbox {S}$$ and 1s2p $$^{1}\hbox {P}$$ , 1s2s $$^{1}\hbox {S}$$ and 1s3p $$^{1}\hbox {P}$$ , 1s3s $$^{1}\hbox {S}$$ and 1s3d $$^{1}\hbox {D}$$ states are reported as a function of the spectral index and plasma parameter. The nonthermal character of the Lorentzian plasmas shows interesting features on the resonant Compton scattering cross sections. [ABSTRACT FROM AUTHOR]

Purpose: Dedicated cardiac SPECT scanners have improved performance over standard gamma cameras allowing reductions in acquisition times and/or injected activity. One approach to improving performance has been to use pinhole collimators, but this can cause position-dependent variations in attenuation, sensitivity, and spatial resolution. CT attenuation correction (AC) and an accurate system model can compensate for many of these effects; however, scatter correction (SC) remains an outstanding issue. In addition, in cameras using cadmium-zinc-telluride-based detectors, a large portion of unscattered photons is detected with reduced energy (low-energy tail). Consequently, application of energy-based SC approaches in these cameras leads to a higher increase in noise than with standard cameras due to the subtraction of true counts detected in the low-energy tail. Model-based approaches with parallel-hole collimator systems accurately calculate scatter based on the physics of photon interactions in the patient and camera and generate lower-noise estimates of scatter than energy-based SC. In this study, the accuracy of a model-based SC method was assessed using physical phantom studies on the GE-Discovery NM530c and its performance was compared to a dual energy window (DEW)-SC method. Methods: The analytical photon distribution (APD) method was used to calculate the distribution of probabilities that emitted photons will scatter in the surrounding scattering medium and be subsequently detected. APD scatter calculations for 99mTc-SPECT (140±14 keV) were validated with point-source measurements and 15 anthropomorphic cardiac-torso phantom experiments and varying levels of extra-cardiac activity causing scatter inside the heart. The activity inserted into the myocardial compartment of the phantom was first measured using a dose calibrator. CT images were acquired on an Infinia Hawkeye (GE Healthcare) SPECT/CT and coregistered with emission data for AC. For comparison, DEW scatter projections (120±6 keV) were also extracted from the acquired list-mode SPECT data. Either APD or DEW scatter projections were subtracted from corresponding 140 keV measured projections and then reconstructed with AC (APD-SC and DEW-SC). Quantitative accuracy of the activity measured in the heart for the APD-SC and DEW-SC images was assessed against dose calibrator measurements. The difference between modeled and acquired projections was measured as the root-mean-squared-error (RMSE). APD-modeled projections for a clinical cardiac study were also evaluated. Results: APD-modeled projections showed good agreement with SPECT measurements and had reduced noise compared to DEW scatter estimates. APD-SC reduced mean error in activity measurement compared to DEW-SC in images and the reduction was statistically significant where the scatter fraction (SF) was large (mean SF = 28.5%, T-test p = 0.007). APD-SC reduced measurement uncertainties as well; however, the difference was not found to be statistically significant (F-test p > 0.5). RMSE comparisons showed that elevated levels of scatter did not significantly contribute to a change in RMSE (p > 0.2). Conclusions: Model-based APD scatter estimation is feasible for dedicated cardiac SPECT scanners with pinhole collimators. APD-SC images performed better than DEW-SC images and improved the accuracy of activity measurement in high-scatter scenarios. [ABSTRACT FROM AUTHOR]

Purpose: In dual-isotope (Tc-99m/In-111) small-animal single-photon emission computed tomography (SPECT), quantitative accuracy of Tc-99m activity measurements is degraded due to the detection of Compton-scattered photons in the Tc-99m photopeak window, which originate from the In-111 emissions (cross talk) and from the Tc-99m emission (self-scatter). The standard triple-energy window (TEW) estimates the total scatter (self-scatter and cross talk) using one scatter window on either side of the Tc-99m photopeak window, but the estimate is biased due to the presence of unscattered photons in the scatter windows. The authors present a modified TEW method to correct for total scatter that compensates for this bias and evaluate the method in phantoms and in vivo. Methods: The number of unscattered Tc-99m and In-111 photons present in each scatter-window projection is estimated based on the number of photons detected in the photopeak of each isotope, using the isotope-dependent energy resolution of the detector. The camera-head-specific energy resolutions for the 140 keV Tc-99m and 171 keV In-111 emissions were determined experimentally by separately sampling the energy spectra of each isotope. Each sampled spectrum was fit with a Linear + Gaussian function. The fitted Gaussian functions were integrated across each energy window to determine the proportion of unscattered photons from each emission detected in the scatter windows. The method was first tested and compared to the standard TEW in phantoms containing Tc-99m:In-111 activity ratios between 0.15 and 6.90. True activities were determined using a dose calibrator, and SPECT activities were estimated from CT-attenuation-corrected images with and without scatter-correction. The method was then tested in vivo in six rats using In-111-liposome and Tc-99m-tetrofosmin to generate cross talk in the area of the myocardium. The myocardium was manually segmented using the SPECT and CT images, and partial-volume correction was performed using a template-based approach. The rat heart was counted in a well-counter to determine the true activity. Results: In the phantoms without correction for Compton-scatter, Tc-99m activity quantification errors as high as 85% were observed. The standard TEW method quantified Tc-99m activity with an average accuracy of -9.0%±0.7%, while the modified TEW was accurate within 5% of truth in phantoms with Tc-99m:In-111 activity ratios =0.52. Without scatter-correction, In-111 activity was quantified with an average accuracy of 4.1%, and there was no dependence of accuracy on the activity ratio. In rat myocardia, uncorrected images were overestimated by an average of 23%±5%, and the standard TEW had an accuracy of -13.8%±1.6%, while the modified TEW yielded an accuracy of -4.0%±1.6%. Conclusions: Cross talk and self-scatter were shown to produce quantification errors in phantoms as well as in vivo. The standard TEW provided inaccurate results due to the inclusion of unscattered photons in the scatter windows. The modified TEW improved the scatter estimate and reduced the quantification errors in phantoms and in vivo. [ABSTRACT FROM AUTHOR]

Image-reconstruction with Compton-scattered photons is a nonlinear problem, because it involves both the linear process of scattering and the exponential pre- and post-scatter attenuation. For thin and light objects and with high-energy photons, the exponential terms can be ignored, rendering the problem linear. We show that when attenuation is significant, linearity can also be accomplished by considering scattering as a scaling factor and with subsequent logarithmic transformation, the image-reconstruction problem is converted into a transmission-like problem along the travel lines of incident and scattered radiation. It is shown that this formulation produces images qualitatively reflecting object features in a manner similar to that of backprojection in conventional computed tomography. This is done without the need for calibration or scaling of measurements. The theoretical aspects of the introduced mathematical formulation are developed for idealized pencil-beams, adapted approximately to fan-beam configurations, and applied to imaging with laboratory-acquired measurements. The introduced transmission-like formulation and calibration-free aspects should promote wider use of scatter imaging. [ABSTRACT FROM AUTHOR]

Purpose: Compton cameras (CCs) use electronic collimation to reconstruct the images of activity distribution. Although this approach can greatly improve imaging efficiency, due to complex geometry of the CC principle, image reconstruction with the standard iterative algorithms, such as ordered subset expectation maximization (OSEM), can be very time-consuming, even more so if resolution recovery (RR) is implemented. We have previously shown that the origin ensemble (OE) algorithm can be used for the reconstruction of the CC data. Here we propose a method of extending our OE algorithm to include RR. Methods: To validate the proposed algorithm we used Monte Carlo simulations of a CC composed of multiple layers of pixelated CZT detectors and designed for imaging small animals. A series of CC acquisitions of small hot spheres and the Derenzo phantom placed in air were simulated. Images obtained from (a) the exact data, (b) blurred data but reconstructed without resolution recovery, and (c) blurred and reconstructed with resolution recovery were compared. Furthermore, the reconstructed contrast-to-background ratios were investigated using the phantom with nine spheres placed in a hot background. Results: Our simulations demonstrate that the proposed method allows for the recovery of the resolution loss that is due to imperfect accuracy of event detection. Additionally, tests of camera sensitivity corresponding to different detector configurations demonstrate that the proposed CC design has sensitivity comparable to PET. When the same number of events were considered, the computation time per iteration increased only by a factor of 2 when OE reconstruction with the resolution recovery correction was performed relative to the original OE algorithm. We estimate that the addition of resolution recovery to the OSEM would increase reconstruction times by 2-3 orders of magnitude per iteration. Conclusions: The results of our tests demonstrate the improvement of image resolution provided by the OE reconstructions with resolution recovery. The quality of images and their contrast are similar to those obtained from the OE reconstructions from scans simulated with perfect energy and spatial resolutions. [ABSTRACT FROM AUTHOR]

It has often been thought that the northern radio relic in the galaxy cluster CIZA J2242.8+5301 (the "Sausage" Cluster) is associated with cosmic ray (CR) electrons that are accelerated at a shock through the diffusive shock acceleration (DSA) mechanism. However, recent radio observations have shown that the radio spectrum is curved, which is inconsistent with the prediction of a simple DSA model. Moreover, the CR electron spectrum before being affected by radiative cooling seems to be too hard for DSA. In this study, we show that these facts are natural consequences if the electrons are reaccelerated in turbulence downstream of the shock. In this model, DSA is not the main mechanism for generating high-energy electrons. We find that the mean free path of the electrons should be much shorter than the Coulomb mean free path for efficient reacceleration. The scale of the turbulent eddies must be smaller than the width of the relic. We also predict hard X-ray spectra of inverse Compton scattering of photons. [ABSTRACT FROM AUTHOR]

X-ray scattering is typically used as a weak linear atomic-scale probe of matter. At high intensities, such as produced at free-electron lasers, nonlinearities can become important, and the probe may no longer be considered weak. Here we report the observation of one of the most fundamental nonlinear X-ray-matter interactions: the concerted nonlinear Compton scattering of two identical hard X-ray photons producing a single higher-energy photon. The X-ray intensity reached 4 × 1020 W cm−2, corresponding to an electric field well above the atomic unit of strength and within almost four orders of magnitude of the quantum-electrodynamic critical field. We measure a signal from solid beryllium that scales quadratically in intensity, consistent with simultaneous non-resonant two-photon scattering from nearly-free electrons. The high-energy photons show an anomalously large redshift that is incompatible with a free-electron approximation for the ground-state electron distribution, suggesting an enhanced nonlinearity for scattering at large momentum transfer. [ABSTRACT FROM AUTHOR]

Purpose: In digital breast tomosynthesis (DBT), scatter correction is highly desirable, as it improves image quality at low doses. Because the DBT detector panel is typically stationary during the source rotation, antiscatter grids are not generally compatible with DBT; thus, a software-based scatter correction is required. This work proposes a fully iterative scatter correction method that uses a novel fast Monte Carlo simulation (MCS) with a tissue-composition ratio estimation technique for DBT imaging. Methods: To apply MCS to scatter estimation, the material composition in each voxel should be known. To overcome the lack of prior accurate knowledge of tissue composition for DBT, a tissue-composition ratio is estimated based on the observation that the breast tissues are principally composed of adipose and glandular tissues. Using this approximation, the composition ratio can be estimated from the reconstructed attenuation coefficients, and the scatter distribution can then be estimated by MCS using the composition ratio. The scatter estimation and image reconstruction procedures can be performed iteratively until an acceptable accuracy is achieved. For practical use, (i) the authors have implemented a fast MCS using a graphics processing unit (GPU), (ii) the MCS is simplified to transport only x-rays in the energy range of 10-50 keV, modeling Rayleigh and Compton scattering and the photoelectric effect using the tissue-composition ratio of adipose and glandular tissues, and (iii) downsampling is used because the scatter distribution varies rather smoothly. Results: The authors have demonstrated that the proposed method can accurately estimate the scatter distribution, and that the contrast-to-noise ratio of the final reconstructed image is significantly improved. The authors validated the performance of the MCS by changing the tissue thickness, composition ratio, and x-ray energy. The authors confirmed that the tissue-composition ratio estimation was quite accurate under a variety of conditions. Our GPU-based fast MCS implementation took approximately 3 s to generate each angular projection for a 6 cm thick breast, which is believed to make this process acceptable for clinical applications. In addition, the clinical preferences of three radiologists were evaluated; the preference for the proposed method compared to the preference for the convolution-based method was statistically meaningful (p < 0.05, McNemar test). Conclusions: The proposed fully iterative scatter correction method and the GPU-based fast MCS using tissue-composition ratio estimation successfully improved the image quality within a reasonable computational time, which may potentially increase the clinical utility of DBT. [ABSTRACT FROM AUTHOR]

Purpose: Spectral information of the output of x-ray tubes in (dual source) computer tomography (CT) scanners can be used to improve the conversion of CT numbers to proton stopping power and can be used to advantage in CT scanner quality assurance. The purpose of this study is to design, validate, and apply a compact portable Compton spectrometer that was constructed to accurately measure x-ray spectra of CT scanners. Methods: In the design of the Compton spectrometer, the shielding materials were carefully chosen and positioned to reduce background by x-ray fluorescence from the materials used. The spectrum of Compton scattered x-rays alters from the original source spectrum due to various physical processes. Reconstruction of the original x-ray spectrum from the Compton scattered spectrum is based on Monte Carlo simulations of the processes involved. This reconstruction is validated by comparing directly and indirectly measured spectra of a mobile x-ray tube. The Compton spectrometer is assessed in a clinical setting by measuring x-ray spectra at various tube voltages of three different medical CT scanner x-ray tubes. Results: The directly and indirectly measured spectra are in good agreement (their ratio being 0.99) thereby validating the reconstruction method. The measured spectra of the medical CT scanners are consistent with theoretical spectra and spectra obtained from the x-ray tube manufacturer. Conclusions: A Compton spectrometer has been successfully designed, constructed, validated, and applied in the measurement of x-ray spectra of CT scanners. These measurements show that our compact Compton spectrometer can be rapidly set-up using the alignment lasers of the CT scanner, thereby enabling its use in commissioning, troubleshooting, and, e.g., annual performance check-ups of CT scanners. [ABSTRACT FROM AUTHOR]

The Rotating Scatter Mask (RSM) system is an inexpensive retrofit that provides imaging capabilities to scintillating detectors. Unlike traditional collimator systems that primarily absorb photons in order to form an image, this system primarily scatters the photons. Over a single rotation, there is a unique, smooth response curve for each defined source position. Testing was conducted using MCNPX simulations. Image reconstruction was performed using a chi-squared reconstruction technique. A simulated 100 uCi, Cs-137 source at 10 meters was detected after a single, 50-second rotation when a uniform terrestrial background was present. A Cs-137 extended source was also tested. The RSM field-of-view is 360 degrees azimuthally as well as 54 degrees above and 54 degrees below the horizontal plane. Since the RSM is built from polyethylene, the overall cost and weight of the system is low. The system was designed to search for lost or stolen radioactive material, also known as the orphan source problem. [ABSTRACT FROM PUBLISHER]

Purpose: In hybrid medical imaging devices combining positron emission tomography (PET) with magnetic resonance imaging, PET attenuation correction remains challenging. Known approaches to estimating attenuation (μ-)maps from PET emission data, especially maximum-likelihood reconstruction of activity and attenuation (MLAA), take into account true coincidences only and exhibit two kinds of ambiguities: First, the attenuation sinogram can only be determined up to a constant offset (sinogram ambiguity). Second, the attenuation sinogram is unknown outside of the support of the activity sinogram and does not completely define a μ-map (image-space ambiguity). In this work, the authors aim at using additional information from scattered coincidences to resolve these ambiguities-information that is unavailable using true coincidences. Methods: The authors propose a two-level scheme for combining measurements of true and scattered coincidences. On the top level, scatter-to-attenuation (S2A) reconstruction recovers the μ-map from a measurement of scattered coincidences and results available from trues-based algorithms. On the lower level, S2A reconstruction is implemented by iterative scatter simulation and a proposed (simplified) S2A back-projection. The S2A back-projection is based on determining possible scattering locations in image space from energy measurements (via Compton-scattering angles) and summing contributions from scattered coincidences in image space. S2A back-projection is validated with GATE simulations of activity-attenuation configurations with both sinogram and image-space ambiguities. The authors further evaluate the impact of asymmetric source and activity distributions, extended source distributions, and energy uncertainty to demonstrate the limitations of the simplified noniterative S2A back-projection approach. Feasibility of the iterative S2A reconstruction is evaluated in a low-resolution, analytical 2D problem. Results: S2A back-projection of scattered coincidences with scattered-photon energies in the range of 248-478 keV provides image-space information about the attenuation distribution, even in challenging cases of perfect spherical symmetry of attenuation and activity distributions as well as attenuation outside of the activity support. Realistic energy uncertainties (5% and 10% full width at half maximum at 511 keV) deteriorate spatial image resolution in the proposed noniterative method. The iterative S2A reconstruction is able to recover the full μ-map (errors less than 3.7×10-5/cm) as well as the unknown scaling factor (error smaller than 0.0005%) from scattered coincidences and an activity distribution with unknown scaling. Conclusions: Scattered coincidences provide information to complement existing PET attenuationcorrection approaches such as MLAA. The proposed scatter-to-attenuation back-projection and reconstruction may constitute a missing piece for resolving ambiguities in the simultaneous reconstruction of activity and attenuation, and improving the quality of μ-maps reconstructed from PET emission data. [ABSTRACT FROM AUTHOR]