Energy-dependent Polarization Research Articles

Polarization of Compton-Scattered Prompt Gamma-Ray Technique for HEU Detection at 186 keV

Prompt gamma-ray polarization is a practical method for detecting highly enriched uranium (HEU) in concealed sources. It also provides information on their geometry, magnetic fields, and radiation mechanisms. However, prompt gamma-ray polarization measurements have rarely been applied in nuclear nonproliferation areas to detect HEU. In this study, the feasibility of detecting the characteristic energy peak of 186 keV, which is associated with the asymmetry of the activation mechanism and the detection of energy-dependent polarization from concealed HEU sources, was evaluated using the Compton scattering approach. A Monte Carlo N-particle transport code simulation was used to realize the activation mechanism of HEU via two 1.4-mm strips of converter material [i.e., cesium lead tribromide (CsPbBr3)], transported by secondary scattered gamma rays during the three-stage process of Compton scattering, polarization, and detection. This paper presents the mathematical model, the physics of Compton scattering, and the polarization mechanism for the detection technique. In this case, the physics is relevant to both processes in which the emitted secondary scattered gamma rays undergo initial orthogonal polarization. Specifically, to meet the objective of testing the technical protocol for the enhanced detection of energy peaks associated with HEU, particularly 186 keV, simulations were conducted to quantify the HEU volume and neutron source strength in the MCNP data card to perform error analysis. The detector system had the potential to acquire good resolved photopeak with a 4.5% relative error or less, with a 1 Ci source activity, and a peak-to-background ratio of 1.15. This resolution took 163 s for high-purity germanium detection, which is comparable to current methods used for material detection placed within 100 to 900 s to completion. The small error difference was due to the attributes of the phenomenal enhancement properties of cesium tribromide and polarimetry. The identified photo peaks included K-shell X-rays from 235U, 61 keV from fission, 511-keV annihilation, and the peak of interest at 186 keV. The result from spectral analyses showed clear signatures related to pure and adulterated HEU. HEU detection with the low neutron yield and the easiness of shielding the yield of the HEU sample showed that the HEU characterization was feasible when shielded, with the highest success rate under both enhancement approaches. The optimization and scale-up of this technique are expected to enhance its applications in a large-scale HEU detection design.

Impact of the Accretion Disk Thickness on the Polarization of the Thermal Emission from Stellar Mass Black Holes

The Imaging X-ray Polarimetry Explorer, launched in 2021 December, enables precision measurements of the energy-dependent polarization of the X-ray emission from stellar mass and supermassive black holes. In this paper, we study the impact of the accretion disk geometry on the polarization of the thermal emission of mass-accreting stellar-mass black holes. We present a ray-tracing code that allows us to predict how the X-ray polarization energy spectra change as we dial up the thickness of the accretion disk from a geometrically thin accretion disk to a torus-shaped geometrically thick accretion disk. The results show that thicker disks can produce higher polarization degrees as the thick disk geometries lead to a larger fraction of X-rays reflecting off portions of the disk. We study the observational degeneracies between the disk shape on the one hand and the black hole spin and disk inclination on the other hand. We conclude with a discussion of the implications of our studies for black hole spin measurements.

Detection of significant X-ray polarization from transient NS-LMXB XTE J1701−462 with IXPE and its implication on the coronal geometry

ABSTRACT In this paper, we performed a spectro-polarimetric analysis of the transient NS-LMXB XTE J1701−462 using IXPE, NICER, and NuSTAR data during its 2022 outburst. We report significant detection of energy-dependent polarization in the X-ray signal from the source on 2022 September 29 in the 2–4, 4–8, and 2–8 keV energy bands with a polarization degree of 3.9 ± 0.3, 5.5 ± 0.6, and 4.5 ± 0.4 per cent, respectively. The polarization angle in the overall 2–8 keV band was ∼143° ± 2°. The spectra were modelled using a combination of thermal emission from an accretion disc, Comptonized emission from a hot electron plasma (or corona) and a Gaussian line. From spectro-polarimetric analysis, the polarization degree due to the disc emission had an upper limit of ∼11.5 per cent, and that of the Comptonized emission was constrained at 7.7 ± 2.5 per cent (at the 3σ level). The results suggest that the Comptonized component probably originates from a spreading-layer/boundary-layer above the neutron star surface. IXPE observation of the source on 2022 October 8 does not show significant polarization which can be attributed to a weakening of the coronal Comptonized emission. The implication of these results are discussed.

Relativistic distorted wave approach to electron impact excitation of argon gas using a complex potential

In this study, we apply relativistic effects to excitation of the lowest lying resonance states of argon gas with inclusion of absorption, polarization and exchange effects to the electrostatic distortion potential to form an overall complex distortion potential. The atomic wave functions are constructed in the multi-configuration Dirac Fork approach by modifying the general-purpose relativistic atomic structure code GRASP for numerical procedures, while differential cross sections (DCS) and integral cross sections (ICS) have been obtained using our new code RDWBA1. Our results are compared with available experimental and theoretical results in literature. Present results from this study predict that use of a complex distortion potential in the relativistic approach to excitation of argon generally lowers integral and differential cross sections as impact energies of the incident electron increases, specifically beyond 50 eV, and that the energy dependent polarization potential adopted in this work plays a major role in improving shapes of cross-sections at near threshold impact energies up to around 30 eV, where available distorted-wave methods fail to give satisfactory results when compared to experiments.

Polarization Predictions in the GRB Prompt Phase with the Internal Shock Model

Abstract As the standard gamma-ray burst (GRB) prompt-emission model, the internal shock (IS) model can reproduce the fast-rise and slow-decay features of the pulses in the GRB light curve. The time- and energy-dependent polarization can deliver important physical information on the emission region and can be used to test models. Polarization predictions for the GRB prompt phase with the magnetized IS model should be investigated carefully. The magnetic field of the magnetized IS model is very likely to be mixed and decays with radius. The synchrotron emission in the presence of such a decaying magnetic field can recover the Band-like spectrum of the GRB prompt phase. We investigate the dependence of the polarization of GRB prompt emission on both time and energy in the framework of the magnetized IS model. Due to the large range of parameters, it is hard to distinguish the magnetized IS model and the magnetic-reconnection model through polarization degree (PD) curves. The energy-dependent PD could increase toward the high-energy band for the magnetized IS model, while it decreases to zero above the megaelectronvolt band for the dissipative photosphere model. Therefore, we conclude that the energy dependence of PD can be used to distinguish these two models for the GRB prompt emission. Finally, we find that, independent of the observational energy band, the profiles of the ξ B − PD curve for the time-integrated and time-resolved PDs are very similar, where ξ B is the magnetic field strength ratio of the ordered component to the random component.

An unfolding method for X-ray spectro-polarimetry

X-ray polarimetry has great scientific potential and new experiments, such as X-Calibur, PoGOLite, XIPE, and GEMS, will not only be orders of magnitude more sensitive than previous missions, but also provide the capability to measure polarization over a wide energy range. However, the measured spectra depend on the collection area, detector responses, and, in case of balloon-borne experiments, the absorption of X-rays in the atmosphere, all of which are energy dependent. Combined with the typically steep source spectra, this leads to significant biases that need to be taken into account to correctly reconstruct energy-resolved polarization properties. In this paper, we present a method based on an iterative unfolding algorithm that makes it possible to simultaneously reconstruct the energy spectrum and the polarization properties as a function of true photon energy. We apply the method to a simulated X-Calibur data set and show that it is able to recover both the energy spectrum and the energy-dependent polarization fraction.

A mobile UHV-system designed for femtosecond time, energy and spin resolved photoelectron spectroscopy

We report on the design and tests of a completely mobile photoemission end station for femtosecond time, energy and spin resolved magnetodynamics experiments. The setup is designed for the use at free-electron laser (FEL) x-ray beamlines as well as at synchrotron facilities. In situ sample preparation in an adjacent UHV chamber is performed. The assembly can be altered to obtain energy resolved measurements with the help of a hemispherical electron energy analyzer or to obtain energy integrated measurements by inserting an electron transfer lens system. The magnetic information is obtained by spin analysis with a 50 kV Mott detector. The setup is tested in house with an electron gun, at the Swiss Light Source (SLS) and has been used at the FEL at FLASH in Hamburg. Results show the practicability of the system and exemplarily the kinetic energy dependent polarization of the low energy secondary electrons emitted from a 15 ML iron film on W(110) is presented.

Interaction spatial barrier distributions for the proton-halo system 8B+58Ni

Radial barrier distributions for the proton-halo system 8B+58Ni are obtained by the use of Woods-Saxon polarization potentials calculated from a fit to elastic scattering angular distribution data. The strength, diffuseness and reduced radii of the polarization potentials, besides fitting the data, are such that reproduce the barrier height and position as extracted from a Wong's fit to the universal behavior of reduced interaction cross sections for halo systems. It is found that, for the proton-halo system 8B+58Ni, the interaction barrier height VI and the interaction barrier radius RI result correspondingly lower and larger than the Coulomb nominal barrier height VB and barrier position RB as expected from Barrier Penetration Models and double folding density dependent interactions such as the São Paulo Potential. It is also shown that, the determined energy dependent polarization potentials, besides satisfying the dispersion relation, present a behavior consistent with the Breakup Threshold Anomaly. Keywords. Nuclear Reactions, weakly bound nuclei, threshold anomaly.

TESTS OF GENERAL RELATIVITY IN THE STRONG-GRAVITY REGIME BASED ON X-RAY SPECTROPOLARIMETRIC OBSERVATIONS OF BLACK HOLES IN X-RAY BINARIES

Although General Relativity (GR) has been tested extensively in the weak gravity regime, similar tests in the strong gravity regime are still missing. In this paper we explore the possibility to use X-ray spectropolarimetric observations of black holes in X-ray binaries to distinguish between the Kerr metric and the phenomenological metrics introduced by Johannsen and Psaltis (2011) (which are not vacuum solutions of Einstein's equation) and thus to test the no-hair theorem of GR. To this end, we have developed a numerical code that calculates the radial brightness profiles of accretion disks and parallel transports the wave vector and polarization vector of photons through the Kerr and non-GR spacetimes. We used the code to predict the observational appearance of GR and non-GR accreting black hole systems. We find that the predicted energy spectra and energy dependent polarization degree and polarization direction do depend strongly on the underlying spacetime. However, for large regions of the parameter space, the GR and non-GR metrics lead to very similar observational signatures, making it difficult to observationally distinguish between the two types of models.

Dispersion-equation approach to obtaining complex optical potentials for electron scattering

The absorption part of a local optical potential for electron-He scattering is obtained from an energy-dependent polarization potential by numerically solving the dispersion relation relating the real and imaginary parts of the optical potential. The resulting optical model, with no empirical parameters, yields reasonably accurate total scattering cross sections for elastic plus inelastic scattering at impact energies in the 30–100 eV range.

Local effective polarization and absorption potentials from cross-sections for e–atom scattering

A possibility of obtaining local, effective, energy-dependent polarization and absorption potentials from experimental cross-sections for electron scattering is investigated. Potentials have been fitted with the help of the genetic algorithm on large sets of experimental data for e–He, e–Ne and e–Ar elastic scattering at impact energies 20–3000 eV. The obtained potentials reproduce the cross-sections within experimental errors.

Effective non-empirical absorption potentials based on quasifree-scattering model

Several recently proposed modifications and improvements of the quasifree-scattering model for absorption potentials have been tested and compared. The possibility of obtaining energy-dependent polarization potentials from these absorption potentials by the dispersion relation is investigated. Numerical calculations for the elastic scattering of electrons by neon atoms at impact energies 20–3000 eV are performed and the results are compared with experimental data.

Measuring energy dependent polarization in soft γ-rays using Compton scattering in PoGOLite

Linear polarization in X- and γ-rays is an important diagnostic of many astrophysical sources, foremost giving information about their geometry, magnetic fields, and radiation mechanisms. However, very few X-ray polarization measurements have been made, and then only mono-energetic detections, whilst several objects are assumed to have energy dependent polarization signatures. In this paper, we investigate whether detection of energy dependent polarization from cosmic sources is possible using the Compton technique, in particular with the proposed PoGOLite balloon-experiment, in the 25–100 keV range. We use Geant4 simulations of a PoGOLite model and input photon spectra based on Cygnus X-1 and accreting magnetic pulsars (100 mCrab). Effective observing times of 6 and 35 h were simulated, corresponding to a standard and a long duration flight, respectively. Both smooth and sharp energy variations of the polarization are investigated and compared to constant polarization signals using chi-square statistics. We can reject constant polarization, with energy, for the Cygnus X-1 spectrum (in the hard state), if the reflected component is assumed to be completely polarized, whereas the distinction cannot be made for weaker polarization. For the accreting pulsar, constant polarization can be rejected in the case of polarization in a narrow energy band with at least 50% polarization, and similarly for a negative step distribution from 30% to 0% polarization.

Theory of Magnetic Circular Dichroism of the CeL2,3X-ray Absorption Spectra in Mixed Valence Vompound CeFe2

Magnetic circular dichroism (MCD) of x-ray absorption spectroscopy (XAS) at Ce L 2,3 edges in the typical mixed valence compound CeFe 2 is theoretically studied by using the impurity Anderson model combined with a cluster approach. It is found that the energy-dependent polarization of Ce 5 d states due to mixing with spin-polarized Fe 3 d states is indispensable for explaining the spectra. Our calculations reproduce well the two-peak structure observed in XAS and MCD experiments, especially their energy position, spectral width and energy separation of the two peaks. For a detailed analysis of these, the Coulomb interactions - U d c (between a 5 d electron and a core hole) and U f d (between a 4 f electron and a 5 d electron) play a special role.

Polarization properties of nonpolar GaN films and (In,Ga)N/GaN multiple quantum wells

We have grown GaN films and (In,Ga)N/GaN multiple quantum wells (MQWs) by plasma-assisted molecular-beam epitaxy on γ-LiAlO2(100) substrates. Due to the crystal symmetry of γ-LiAlO2(100), GaN films and (In,Ga)N/GaN MQWs can be realized in a nonpolar (M-plane) configuration, i.e., the c axis of the wurtzite unit cell lies in the growth plane. For compressively, anisotropically strained M-plane GaN films, the band structure of the valence band changes in such a way that the optical transmittance becomes 100% linearly polarized for two orthogonal in-plane directions, where one of these directions is parallel to the c axis of the GaN film. The photoluminescence properties of M-plane In0.1Ga0.9N/GaN MQWs exhibit a strong in-plane optical anisotropy for linear polarization with an energy-dependent polarization degree of up to 96%, which is presumably also due to a large valence-band splitting induced by the large compressive strain in the QWs.

Polarized x-ray emission from magnetized neutron stars: signature of strong-field vacuum polarization.

In the atmospheric plasma of a strongly magnetized neutron star, vacuum polarization can induce a Mikheyev-Smirnov-Wolfenstein type resonance across which an x-ray photon may (depending on its energy) convert from one mode into the other, with significant changes in opacities and polarizations. We show that this vacuum resonance effect gives rise to a unique energy-dependent polarization signature in the surface emission from neutron stars. The detection of polarized x rays from neutron stars can provide a direct probe of strong-field quantum electrodynamics and constrain the neutron star magnetic field and geometry.

Polarization anisotropy of the photoluminescence of M-plane (In,Ga)N/GaN multiple quantum wells

We investigate the polarization anisotropy of the photoluminescence of an M-plane (11̄00) In0.1Ga0.9N/GaN multiple quantum well grown on γ-LiAlO2 (100) by molecular-beam epitaxy. In contrast to C-plane (0001) structures, a strong in-plane optical anisotropy with an energy-dependent polarization degree of up to 96% is observed for this M-plane sample. An apparent spectral shift of the emission with polarization angle is attributed to the impact of exciton localization on the polarization degree. The presence of localized states manifests itself further in the anomalous temperature dependence of the photoluminescence linewidth.

Polarization potential for e-atom scattering by DCS minimization at intermediate energies

The differential scattering cross section (DCS) for electrons scattered elastically by neon, argon, and krypton atoms is studied using a model potential. In the present study the long-range polarization potential is represented by an energy-dependent function in the form: Vp(E,r)=αd/[r+ξ(a,b,E)]4, and the short-range part is constructed from the nonrelativistic Hartree–Fock wave function of the target atom in the form \documentclass{article}\pagestyle{empty}\begin{document}$V_{s}=(2Z/r)\sum_{k}\sum_{n}A_{n}R_{n}(r)\exp(-B_{k}r)$\end{document}. Accurate phase shifts have been computed for angular momentum in the range 0<l<13 from the Schrödinger equation and the Born approximation for higher values of l<1000. The computed differential cross section obtained using the approximate effective interaction potential for electrons scattered by the target atoms in their ground state is compared with available published results. In the present study the parameters contained in the energy-dependent polarization potential are determined by the minimization of the DCS with respect to angle θ and the incident energy of E. The critical values of angle and energy (θc, Ec) are found to be (98.1°, 66.5 eV) for neon, (117.3°, 120.5 eV) for argon, and (80.5°, 188.0 eV) for krypton. The resulting DCS in the angular range 2°<θ<178° is found to be an improvement over earlier results and in better agreement with the available experimental data in the intermediate energy range. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 80: 989–998, 2000

Polarization potential for e‐atom scattering by DCS minimization at intermediate energies

The differential scattering cross section (DCS) for electrons scattered elastically by neon, argon, and krypton atoms is studied using a model potential. In the present study the long-range polarization potential is represented by an energy-dependent function in the form: Vp(E,r)=αd/[r+ξ(a,b,E)]4, and the short-range part is constructed from the nonrelativistic Hartree–Fock wave function of the target atom in the form \documentclass{article}\pagestyle{empty}\begin{document}$V_{s}=(2Z/r)\sum_{k}\sum_{n}A_{n}R_{n}(r)\exp(-B_{k}r)$\end{document}. Accurate phase shifts have been computed for angular momentum in the range 0<l<13 from the Schrödinger equation and the Born approximation for higher values of l<1000. The computed differential cross section obtained using the approximate effective interaction potential for electrons scattered by the target atoms in their ground state is compared with available published results. In the present study the parameters contained in the energy-dependent polarization potential are determined by the minimization of the DCS with respect to angle θ and the incident energy of E. The critical values of angle and energy (θc, Ec) are found to be (98.1°, 66.5 eV) for neon, (117.3°, 120.5 eV) for argon, and (80.5°, 188.0 eV) for krypton. The resulting DCS in the angular range 2°<θ<178° is found to be an improvement over earlier results and in better agreement with the available experimental data in the intermediate energy range. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 80: 989–998, 2000

Polarized cathodoluminescence study of selectively grown self-assembled InAs/GaAs quantum dots

We have examined the optical properties of self-assembled InAs quantum dots (QDs) grown on prepatterned GaAs(001) substrates with polarization sensitive and time-resolved cathodoluminescence (CL) imaging and spectroscopy techniques. The InAs QDs were formed using a novel application in self-assembled molecular beam epitaxial growth, which entailed the growth of InAs on preformed [11̄0]-oriented stripe mesas. Interfacet In adatom migration occurred along the stripe side-walls during growth, enabling the selective formation of linear arrays of InAs QDs on the stripe mesas. The total InAs deposition needed to induce the two-dimensional to three-dimensional morphology change on the stripes is less than that required to initiate QD formation on the unpatterned substrates. The QDs formed on the mesa top were found with a luminescence distribution redshifted relative to QDs in the valley region, indicating that QDs with a larger average size were formed on the mesa top. The lower density of QDs in the valley region led to a weaker emission and sharper δ-like transitions at lower beam currents, relative to emission from QDs on the mesa. CL imaging was employed to study the spatial distribution of luminescence and identified the presence of relatively small QDs situated near the edges of the valley region along the lower surface of the stripe edges. An excitation- and energy-dependent polarization anisotropy relative to the 〈110〉 directions was observed in CL emission. A polarization anisotropy reversal was found between CL from QDs on the mesa and in the valley regions, revealing the importance of the stress anisotropy in both the formation of QDs and their subsequent optical properties. The CL was examined as a function of temperature to evaluate the thermal re-emission of carriers and the associated activation energies. The carrier relaxation kinetics were studied with time-resolved CL to measure differences in the CL onset and decay rates for QDs in the valley and mesa regions.