Analysis of Electromagnetic Calorimeters and Application to the Deeply Virtual Compton Scattering Experiment at Hall C of Jefferson Lab.
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Hamdi, Wassim [Monastir University]↗
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Boltzmann equation for the electron gas of a nondegenerate plasma
The collision terms in the Boltzmann equation associated with various processes are derived. For processes having a Fokker-Planck (F-P) limit, the associated F-P operator is derived by means of physical arguments to determine the form of the operator; its multiplying constant is fixed by calculating the total energy exchange rate and comparing with the rate computed by other means. In this manner, the F-P operator is derived for electron-ion scattering, electron-electron scattering in the high-velocity limit, electron-atom elastic scattering, Compton scattering, and the high-velocity limit for inelastic scattering. Other processes considered are bremsstrahlung, radiative-recombination, photoionization, collisional ionization of atoms, and suprathermal-particle ionization of atoms.
Gould, R. J.↗
Multiple resonant scattering in the Compton upscatter model of gamma-ray bursts
Resonant Compton scattering, an increasingly popular mechanism for suppressing X-rays and producing gamma rays, must be treated as a multiple-scattering process for conditions thought characteristic of gamma-ray bursts. Photons that multiply scatter with a beamed power-law electron distribution in a uniform magnetic field produce a flat spectrum between the cyclotron frequency and an optical-depth-dependent critical energy; this critical energy ranges between several hundred keV and several MeV. Above this critical energy, the gamma-ray spectrum has a shape determined by the electron distribution and described by a single-scattering model. Only electron distributions that are nearly proportional to the electron momentum are able to simultaneously suppress X-rays and produce a single-scattering spectrum. As the Thomson optical depth approaches unity, photons that experience multiple scatterings often spawn additional photons at a rate that makes the model unphysical.
Brainerd, J. J.↗
Soft-photon emission effects and radiative corrections for electromagnetic processes at very high energies
Higher-order electromagnetic processes involving particles at ultrahigh energies are discussed, with particular attention given to Compton scattering with the emission of an additional photon (double Compton scattering). Double Compton scattering may have significance in the interaction of a high-energy electron with the cosmic blackbody photon gas. At high energies the cross section for double Compton scattering is large, though this effect is largely canceled by the effects of radiative corrections to ordinary Compton scattering. A similar cancellation takes place for radiative pair production and the associated radiative corrections to the radiationless process. This cancellation is related to the well-known cancellation of the infrared divergence in electrodynamics.
Gould, R. J.↗
Gamma rays of 1-30 MeV from the Galactic Center Region
Preliminary results are reported for gamma rays from the Galactic Center Region at 1-30 MeV made with a balloon-borne double Compton scatter gamma ray telescope. The Compton scatter angle, energy, and arrival time of the incident gamma ray and direction of the Compton scattered secondary for downward-moving gamma rays were measured. The handling of the data is described, and the results of Monte Carlo simulation angular resolutions and absolute efficiencies for different zenith angles of incident gamma rays are given. A contour sky map of the weighted fluxes for a southern sky region is shown whose primary feature is a significant enhancement in the Galactic Center Region. It is concluded that several discrete sources contribute to broaden the maximum in the horizontal or RA direction. Less broadening is indicated along the galactic plane.
Oneill, T.↗
Synchrotron and inverse Compton emission from expanding sources in jets - Application to SS 433
The formalism for an expanding source is extended to include continuous injection of electrons and the production of X-rays by inverse Compton scattering of external photons and synchrotron-self-Compton scattering. The model is applied to the expanding nonthermal sources entrained in the jets of SS 433. These sources are found capable of producing both the flaring radio component and the X-ray fluxes only if the relativistic electrons and the magnetic field are far out of equipartition. If the expanding source in the jets is in equipartition, the X-rays may originate from a stationary source within the binary system, which is self-absorbed at radio frequencies and produces X-rays by inverse Compton scattering of optical photons from the binary.
Band, David L.↗
Density scaling approximation for Monte-Carlo simulations of radioactive plumes
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. Here, 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 135 Xe 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 135 Xe. 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.
46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
X-Ray Spectra from General Relativistic Radiation Magnetohydrodynamic Simulations of Thin Disks
We compare X-ray emission from several general relativistic, multifrequency, radiation magnetohydrodynamic simulations of thin black hole accretion disks with different accretion rates and spins. The simulations were performed using the M1 closure scheme, resolved with 12 frequency (energy) bins logarithmically spaced from 5 × 10 −3 to 5 × 10 3 keV. We apply a general relativistic Monte Carlo transport code to postprocess the simulation data with greater fidelity in frequency resolution and Compton scattering treatment. Despite the relatively few energy bins and Kompaneets approximation to Compton scattering utilized in the M1 method, we find generally good agreement between the methods. Both produce prominent thermal profiles with peaks around 2–2.5 keV, where agreement is particularly strong and representative of the soft state. Both also find weaker (lower-luminosity) thermally sourced emission extending out to 100 keV due to the hotter innermost regions of the disks. Inverse Compton scattering becomes increasingly effective at hardening spectral outputs with increasing black hole spin, and becomes the dominant mechanism for photons that escape with energies between 10 to several hundred keV. At very high rates of spin, the radiation flux in this upscattered component becomes comparable to the thermal flux, a phenomenon typically associated with intermediate states. Beyond 10 4 keV, we observe faint, free–free emission from hot, optically thin coronal regions developing near the horizon, common to both spinning and nonspinning black holes.
79 ASTRONOMY AND ASTROPHYSICS↗
The Transition Density Formalism in the First Compton Computation on $^4$He
The method and results of the first theory description of 4He Compton scattering at nuclear energies is presented, with a focus on figures. It uses the same Compton kernels familiar from proton, deuteron and 3He Compton scattering in Chiral Effective Field Theory with explicit Delta degrees of freedom, applicable between about 50 and 130MeV. The result compares well to data from HIγS, MAXlab and Illinois. The sensitivity of the cross section on the (static) scalar-isoscalar polarisabilities of the nucleon is explored. The project is part of the synergetic international effort of experimentalists and theorists in Compton scattering on one- and few-nucleon systems.
Griesshammer, Harald [The George Washington Univer↗
The energy loss of relativistic electrons in energetic photon gases and its relation to the equilibrium spectrum of electrons
The energy loss of relativistic electrons is considered for astrophysical objects which contain energetic photon gases in the XUV-range. Such objects have been suggested as possible candidates for gamma-ray point sources through the mechanism of inverse Compton scattering. In certain cases where inverse Compton scattering is the dominant energy loss mechanism, an equilibrium spectrum results which can deviate from a strict power law. Such an effect can have consequences on the observed radiation spectrum from such objects.
Prince, T.↗
Development of a Silicon Tracker for the All-Sky Medium Energy Gamma-ray Observatory Prototype
The gamma-ray sky from several hundred keV to a hundred MeV has remained largely unexplored due to the challenging nature of detecting gamma rays in this regime. At lower energies, Compton scattering is the dominant interaction process whereas at higher energies pair production dominates, with a crossover at a few MeV. Thus, an instrument designed to work in this energy range must be optimized for both Compton and pair-production events. AMEGO, the All-sky Medium Energy Gamma-ray Observatory, a Probe-class mission in consideration for the 2020 decadal survey, is designed to operate at energies from 200 keV to > 10 GeV with over an order of magnitude increase in sensitivity and with superior angular and energy resolution compared to previous instruments. AMEGO comprises four major subsystems: a plastic anticoincidence detector for rejecting cosmic-ray events, a silicon tracker for tracking pair-production products and tracking and measuring the energies of Compton-scattered electrons, a cadmium-zinc-telluride (CZT) calorimeter for measuring the energy and location of Compton scattered photons, and a CsI calorimeter for measuring the energy of the pair-production products at high energies. A prototype instrument, known as ComPair, is under development at NASA’s Goddard Space Flight Center and the US Naval Research Laboratory. In this contribution, we provide details on the development of the silicon tracker subsystem.
Griffin, Sean↗
Neutron-induced 2.2 MeV background in gamma ray telescopes
Neutron-induced gamma ray production is an important source of background in Compton scatter gamma ray telescopes where organic scintillator material is used. Most important is deuteron formation when atmospheric albedo and locally produced neutrons are thermalized and subsequently absorbed in the hydrogenous material. The resulting 2.2 MeV gamma ray line radiation essentially represents a continuous isotropic source within the scintillator itself. Interestingly, using a scintillator material with a high hydrogen-to-carbon ratio to minimize the scintillator material with a high hydrogen-to-carbon ratio to minimize the neutron-induced 4.4 MeV carbon line favors the np reaction. The full problem of neutron-induced background in Compton scatter telescopes has been previously discussed. Results are presented of observations with the University of California balloon-borne Compton scatter telescope where the 2.2 MeV induced line emission is prominently seen.
Zanrosso, E. M.↗
Physical processes in the strong magnetic fields of accreting neutron stars
Analytical formulae are fitted to observational data on physical processes occurring in strong magnetic fields surrounding accreting neutron stars. The propagation of normal modes in the presence of a quantizing magnetic field is discussed in terms of a wave equation in Fourier space, quantum electrodynamic effects, polarization and mode ellipticity. The results are applied to calculating the Thomson scattering, bremsstrahlung and Compton scattering cross-sections, which are a function of the frequency, angle and polarization of the magnetic field. Numerical procedures are explored for solving the radiative transfer equations. When applied to modeling X ray pulsars, a problem arises in the necessity to couple the magnetic angle and frequency dependence of the cross-sections with the hydrodynamic equations. The use of time-dependent averaging and approximation techniques is indicated.
Meszaros, P.↗
Global effects of thermal conduction on two-phase media
The evolution of two-phase systems of astrophysical gases which change mass between the phases is studied to see whether a steady state is ever reached. The criterion for thermal instability in a cloudy medium is derived. The evolution of the pressure and density of the intercloud medium under the combined effects of heating and radiative cooling of the intercloud gas on the one hand and evaporation and condensation of the clouds on the other is determined. The equilibrium density to which the system evolves is determined for the case when the pressure is fixed. The theory is illustrated by the case in which the intercloud gas is heated by Compton scattering in a hard radiation field and cooled by bremsstrahlung and inverse Compton scattering.
Begelman, Mitchell C.↗
Coronally Fluoresced Stellar Photospheric X-Ray Spectra
Monte Carlo computations of photospheric fluorescent x-ray spectra are described. X-rays emitted from a hot corona incident on an underlying photosphere can undergo either scattering events by Compton scattering or photoabsorption events through the ionization of atoms or weakly ionized species. Through scattering events, photons can be reflected back in a direction towards the stellar surface where they have a finite chance of escape. Photoionization events involving x-ray photons directed toward the photosphere are predominantly inner shell interactions, Observable fluorescent lines can arise as a result of the finite escape probabilities of photons emitted in outward directions by hole transitions in these atoms. For a given stellar composition, fluorescent line strengths are dependent on the height of the fluorescing source above the photosphere and on the angle between the source and the observer. Although the only fluorescent lines likely to be detected in stellar coronae are Fe Ka and perhaps O Ka, these lines could provide powerful and direct spectroscopic diagnostics of coronal geometry.
Drake, Jeremy J.↗
High-Energy 3D Calorimeter for Use in Gamma-Ray Astronomy Based on Position-Sensitive Virtual Frisch-Grid CdZnTe Detectors
We will present a concept for a calorimeter based on a novel approach of 3D position-sensitive virtual Frisch-grid CdZnTe (hereafter CZT) detectors. This calorimeter aims to measure photons with energies from approximately 100 keV to 20 - 50 MeV . The expected energy resolution at 662 keV is better than 1% FWHM, and the photon interaction position-measurement accuracy is better than 1 mm in all 3 dimensions. Each CZT bar is a rectangular prism with typical cross-section from 5 x 5 to 7 x 7 mm2 and length of 2 - 4 cm. The bars are arranged in modules of 4 x 4 bars, and the modules themselves can be assembled into a larger array. The 3D virtual voxel approach solves a long-standing problem with CZT detectors associated with material imperfections that limit the performance and usefulness of relatively thick detectors (i.e., greater than 1 cm). Also, it allows us to use the standard (unselected) grade crystals, while achieving the energy resolution of the premium detectors and thus substantially reducing the cost of the instrument. Such a calorimeter can be successfully used in space telescopes that use Compton scattering of gamma rays, such as AMEGO, serving as part of its calorimeter and providing the position and energy measurement for Compton-scattered photons (like a focal plane detector in a Compton camera). Also, it could provide suitable energy resolution to allow for spectroscopic measurements of gamma ray lines from nuclear decays.
Moiseev, A.↗
Faraday rotation in the M87 radio/X-ray halo
Comparison of polarization maps at various wavelengths demonstrates the existence of a large Faraday rotation uniform over the radio core of M87. Much of this rotation must be external to the core, lest it appear completely depolarized when the rotation is about 90 degrees. The Faraday rotation is shown to occur primarily in the surrounding radio/X-ray halo. Using the electron density inferred from X-ray observations, the magnetic field in the halo is found to be 2.5 microgauss. The deduced magnetic field strength permits an evaluation of the importance of Compton scattering of 3 K background photons by relativistic electrons in the radio halo. The emergent Compton-scattered spectrum is calculated, and its contribution to the observed X-ray flux is small, probably about a percent or so, while the rest is due to thermal bremsstrahlung.
Dennison, B.↗
Nonlinear X-ray Optics
Novel X-ray free-electron lasers (XFELs) are capable of producing radiation with unprecedented properties. Specifically, their ultrashort coherent X-ray pulses can generate extreme peak intensities where ordinary rules of light-matter interaction may no longer apply, and nonlinear processes start to become important. XFELs have made it possible to observe some of the most important coherent non-sequential nonlinearities, including X-ray-optical sum frequency generation (SFG), X-ray second harmonic generation (XSHG), two-photon absorption (TPA) and nonlinear two-photon Compton scattering (2PCS). In particular, the 2PCS experiment has led to unexpected results, namely the observation of a substantial anomalous red shift in the energy of photons generated by nonlinear two-photon X-ray Compton scattering in beryllium. The energy shift is in addition to the predicted nonlinear Compton shift and was neither expected from extrapolations from linear X-ray interactions nor from nonlinear effects at optical wavelengths. Our results suggest a novel nonlinear scattering mechanism that can only be observed at high fields at X-ray wavelengths, where the photon energy is a significant fraction of the electron rest mass (and thus a significant momentum transfer to the electron occurs during the scattering) and where the interaction is not dominated by the quiver motion of the electron in the field (ponderomotive potential) as is usually the case at longer (optical) wavelengths. The goal of this research project was the investigation of fundamental nonlinear X-ray-matter interactions. As these processes are typically weak, part of this research is the exploration of more efficient nonlinear X-ray processes by studying novel mechanisms including solid-state effects, the assistance of optical radiation and novel phase-matching techniques. These experiments require extremely large X-ray field strengths, such as produced by XFELs. Nonlinear X-ray effects can lead to important applications in many research fields. For example, it could lead to instantaneous plasma diagnostics for materials in extreme conditions, as a method for combining atomic-scale structural sensitivity with chemical specificity, or as a probe of electron dynamics in solids on the attosecond (1 as = 10 -18 s) time- and Ångstrom (10 -10 m) length scales. In addition, a detailed understanding of nonlinear X-ray matter interactions is crucial for understanding and extending the limits of structural determination at high intensity such as in serial femto-crystallography. The results could have a profound impact on advanced XFEL light sources such as the LCLS-II.
47 OTHER INSTRUMENTATION↗