Unit Solid Angle Research Articles

Formulation of sound propagation and scattering in forests as a radiative transfer theory

Several approaches have been used in the literature to characterize sound scattering in forests. In this presentation, an approach based on the radiative transfer equation (RTE) is considered. Although the RTE has been used for other types of scattering problems, it is new to forest acoustics. The RTE is an integro-differential equation for the specific intensity, which is the average power flux density within a unit solid angle in a certain direction of propagation. It describes the processes of sound scattering and absorption by discrete scatterers such as tree trunks, branches, and leaves. RTE is applied to sound propagation in a four-layer forest model (ground, trunks, canopy, and open air). The trunk layer is modelled with vertical cylinders, while the canopy layer is modeled by spheres. The scattering amplitudes and total scattering cross sections of these scatterers are identified. At the interface of the trunk layer and ground, the specific intensity is specularly reflected. Although the RTE is ba...

Quantitative characterization of ZnS:Mn embedded polyurethane optical emission in three mechanoluminescent regimes

ZnS:Mn is one of the brightest mechanoluminescent (ML) materials known, which makes it a prime candidate for use in Structural Health Monitoring (SHM) applications. For this work crystalline ZnS:Mn powder was embedded in a clear polyurethane (PU) matrix. The total emitted energy per unit solid angle per unit area, as well as the average radiance values of different samples are measured with a calibrated camera to determine the concentration and thickness of ZnS:Mn/PU for optimal emission. A trend is observed where emission from tensile loading appears stronger than that from compressive loading, and emission from fracture is stronger than that from plastic loading which itself is stronger than that from elastic loading. Also, the ML emissions by elastic and plastic loadings tend to increase with material thickness and ZnS:Mn concentration. Both tensile- and bending-induced fracture modes were tested. While for tensile fracture the emission is approximately independent of thickness and concentration, the emission accompanying an abrupt bending fracture increases with the ZnS:Mn concentration. This work presents a first quantitative absolute measurement of ML emission of ZnS:Mn, while also introducing the use of PU as potential host material, that can benefit many SHM and other applications.

The Application of the Genetic Algorithm-Back Propagation Neural Network Algorithm in the High-Energy Physics

Multiparticle production mechanism is one of the most phenomena that the high-energy physics concerns. In this work, the evolutionary genetic algorithm (GA) is used to optimize the parameters of the back-propagation neural networks (BPNN). The hybrid evolutionary-neuro model (GA-BPNN) was trained to simulate the rapidity distribution 1/N(dN/dY) of positive and negative pions p-Au, p-Ag and p-Xe for p-Ar, p-Xe interactions at lab momentum Plab =100 GeV/c. Also, for total charged, positive and negative pions for interactions at Plab = 200 GeV/c. Finally, total charged particles for p- Pb collision at center-of-mass energy sqrt(s) = 5.02 TeV are simulated. An efficient ANN network with different connection parameters (weights and biases) have been designed by the GA to calculate and predict the rapidity distribution as a function of the lab momentum Plab, mass number (A) and the number of particles per unit solid angle (Y). Our simulated results have been compared with the experimental data and the matching has been clearly found. It is indicated that the developed GA-BPNN model for rapidity distribution was more successful.

Modeling and Simulation for High Energy Sub-Nuclear Interactions Using Evolutionary Computation Technique

High energy sub-nuclear interactions are a good tool to dive deeply in the core of the particles to recognize their structures and the forces governed. The current article focuses on using one of the evolutionary computation techniques, the so-called genetic programming (GP), to model the hadron nucleus (h-A) interactions through discovering functions. In this article, GP is used to simulate the rapidity distribution of total charged, positive and negative pions for p--Ar and p--Xe interactions at 200 GeV/c and charged particles for p-pb collision at 5.02 TeV. We have done so many runs to select the best runs of the GP program and finally obtained the rapidity distribution as a function of the lab momentum , mass number (A) and the number of particles per unit solid angle (Y). In all cases studied, we compared our seven discovered functions produced by GP technique with the corresponding experimental data and the excellent matching was so clear.

Structure of gamma-ray burst jets: intrinsic versus apparent properties

With this paper we introduce the concept of apparent structure of a gamma-ray burst (GRB) jet, as opposed to its intrinsic structure. The latter is customarily defined specifying the functions ϵ(θ) (the energy emitted per jet unit solid angle) and Γ(θ) (the Lorentz factor of the emitting material); the apparent structure is instead defined by us as the isotropic equivalent energy Eiso(θv) as a function of the viewing angle θv. We show how to predict the apparent structure of a jet given its intrinsic structure. We find that a Gaussian intrinsic structure yields a power-law apparent structure: this opens a new viewpoint on the Gaussian (which can be understood as a proxy for a realistic narrow, well-collimated jet structure) as a possible candidate for a quasi-universal GRB jet structure. We show that such a model (a) is consistent with recent constraints on the observed luminosity function of GRBs; (b) implies fewer orphan afterglows with respect to the standard uniform model; (c) can break out the progenitor star (in the collapsar scenario) without wasting an unreasonable amount of energy; (d) is compatible with the explanation of the Amati correlation as a viewing angle effect; (e) can be very standard in energy content, and still yield a very wide range of observed isotropic equivalent energies.

A simulation study on the quantitative assessment of tissue microstructure with photoacoustics.

A detailed derivation of a quantity, defined as the acoustic power per unit solid angle far from the illuminated volume divided by the intensity of the incident light beam and termed as differential photoacoustic (PA) cross section, is presented. The expression for the differential PA cross section per unit absorbing volume retains two terms, namely, the coherent and the incoherent parts. The second part based on a correlation model can be employed to analyze the PA signal power spectrum for tissue characterization. The performances of the fluid sphere, Gaussian, and exponential correlation models in assessing the mean size and the variance in the optical absorption coefficients of absorbers were investigated by performing in silico experiments. It was possible to evaluate diameters of solid spherical absorbers with radii ≥ 20 μm with an accuracy of 10% for an analysis bandwidth of 5 to 50 MHz using the first two correlation models. The accuracy of estimation was about 22% for fluid spheres mimicking erythrocytes for the third correlation model for an analysis bandwidth of 5 to 100 MHz. The extracted values of average variance in the optical absorption coefficients demonstrated good correlation with the nominal values. This study suggests that the method presented here may be developed as a potential tissue characterization tool.

Luminosity function and jet structure of Gamma-Ray Burst

The structure of Gamma Ray Burst (GRB) jets impacts on their prompt and afterglow emission properties. The jet of GRBs could be uniform, with constant energy per unit solid angle within the jet aperture, or it could instead be structured, namely with energy and velocity that depend on the angular distance from the axis of the jet. We try to get some insight about the still unknown structure of GRBs by studying their luminosity function. We show that low (1e46-1e48 erg/s) and high (i.e. with L > 1e50 erg/s) luminosity GRBs can be described by a unique luminosity function, which is also consistent with current lower limits in the intermediate luminosity range (1e48-1e50} erg/s). We derive analytical expressions for the luminosity function of GRBs in uniform and structured jet models and compare them with the data. Uniform jets can reproduce the entire luminosity function with reasonable values of the free parameters. A structured jet can also fit adequately the current data, provided that the energy within the jet is relatively strongly structured, i.e. E propto theta^{-k} with k > 4. The classical E propto theta^{-2} structured jet model is excluded by the current data.

Instantaneous power radiated from magnetic dipole moments

We compute the power radiated per unit solid angle of a moving magnetic dipole moment, and its instantaneous radiated power, both non-relativistically and relativistically. This is then applied to various interesting situations: solar neutrons, electron synchrotrons and cosmological Dirac neutrinos. Concerning the latter, we show that hypothesized early-universe Big Bang conditions allow for neutrino radiation cooling and provide an energy loss-mechanism for subsequent neutrino condensation.

StarDriver: A Flexible Laser Driver for Inertial Confinement Fusion and High Energy Density Physics

We propose a novel method to minimize laser–plasma instabilities and improve laser–plasma coupling by the use of multi-beam laser architecture with a large system frequency bandwidth and many beamlets per unit solid angle. The StarDriver™, laser driver is constructed from 104 to 105 individual lasers, each delivering nominally 100 J in pulses of ~3–30 ns at a nominal wavelength of ~355 nm with better than 3–5 diffraction-limited performance. The beamlets are individually relatively narrowband to facilitate maximum laser performance, but the ensemble of beamlets span a wide frequency range. Currently available laser media enable Δω/ω ~ 2 % at 355 nm with the possibility of system bandwidths approaching 10 % in the future. The many beamlets of StarDriver™ provide optimal asymptotic smoothing for hydrodynamic instabilities (0–1 %), innovative focusing strategies including zooming, and the large bandwidth enables extremely rapid hydrodynamic smoothing times ~30 fs. The distribution of frequencies among the beamlets allows flexibility for fine control of the seeding of the Rayleigh–Taylor instability. The ultra-broad bandwidth combined with the large total k-spectrum of the laser drive in the plasma corona may enable complete suppression of the most problematic laser–plasma instabilities such as stimulated Brillouin backscatter, stimulated Raman scatter, cross-beam energy transfer, and the two plasmon decay instability. StarDriver™ offers potentially superior flexibility in laser drivers for inertial confinement fusion, enabling almost arbitrary sequencing of wavelength, polarization, focus, and fine control of the spatio-temporal properties of the drive in the corona. The highly modular strategy of StarDriver™ should enable an attractive development pathway as well as maximizing overall system efficiency.

A discrepancy observed in the dipole anisotropy in the radio sky at 1.4 GHz and that in the CMBR – A threat to the cosmological principle?

According to the cosmological principle, the sky brightness at any frequency should appear uniform in all directions to an observer considered to be fixed in the co-moving coordinate system of the expanding universe. However a peculiar motion of the observer introduces a dipole anisotropy in the observed sky brightness, which should be independent of the observing frequency. We have examined the angular distribution in the radio-sky brightness, i.e., an integrated emission from discrete sources per unit solid angle, from the NVSS sample containing 1.8 million discrete radio sources at 1.4 GHz. Our results give a dipole anisotropy which is in the same direction as that of the CMBR from the COBE or WMAP, but the magnitude we find is about 4 times larger at a statistically significant (about 3σ) level. A genuine difference between the two dipoles cannot arise from the observer's motion alone, and it would imply intrinsically anisotropic universe, with anisotropy changing with the epoch. This would violate the cosmological principle where isotropy of the universe is assumed for all epochs, and on which the whole modern cosmology is based upon.

UTILITY OF GALAXY CATALOGS FOR FOLLOWING UP GRAVITATIONAL WAVES FROM BINARY NEUTRON STAR MERGERS WITH WIDE-FIELD TELESCOPES

The first detections of gravitational waves from binary neutron star mergers with advanced LIGO and Virgo observatories are anticipated in the next five years. These detections could pave the way for multi-messenger gravitational-wave (GW) and electromagnetic (EM) astronomy if GW triggers are successfully followed up with targeted EM observations. However, GW sky localization is relatively poor, with expected localization areas of ~10--100 square degrees; this presents a challenge for following up GW signals from compact binary mergers. Prioritizing wide-field follow-ups based on the probability of successful imaging is important when it is impossible to tile the entire gravitational-wave localization region. Galaxy catalogs were effective at narrowing down regions of the sky to search in initial attempts at joint GW/EM observations. The next generation of GW detectors will have a ten-fold increase in range thereby increasing the expected number of galaxies per unit solid angle a thousand-fold. As an additional complication, catalogs may be highly incomplete. We show how to quantify the advantages of using galaxy catalogs as a function of only two parameters: the three-dimensional volume within the field of view of a telescope after accounting for the GW distance measurement uncertainty, and the fraction of the GW sky localization uncertainty region that can be followed up. We find that the use of galaxy catalogs can improve success rates by ~10% to a factor of 4 relative to follow-up strategies that do not utilize such catalogs for the scenarios we considered. We determine that catalogs with a 75% completeness perform comparably to complete catalogs in most cases, while 33%-complete catalogs can lead to lower follow-up success rates than complete catalogs for small fields of view, though still providing an advantage over strategies that do not use a catalog at all.

Anisotropy of energy losses in high-current Z-pinches produced by the implosion of cylindrical tungsten wire arrays

Results are presented from measurements of the anisotropy of energy losses in high-current Z-pinches produced by the implosion of wire arrays at the ANGARA-5-1 facility at load currents of up to 4MA. The energy losses were measured in the radial direction and along the pinch axis from the anode side. The main diagnostics were time-integrated thermocouple calorimeters, nanosecond X-ray diodes (XRDs) with different filters, and a foil radiation calorimeter with a time resolution of 2 μs. The azimuthal anisotropy of energy losses was measured for different wire array configurations and different shapes of the high-voltage electrode. The presence of strong initial azimuthal inhomogeneity of the wire mass distribution (sectioned arrays), as well as the use of conical electrodes instead of plane ones, does not increase the azimuthal inhomogeneity of the total energy losses. For cylindrical wire arrays, energy losses in the radial direction are compared with those along the pinch axis. According to XRD and calorimetric measurements, the radiation yield per unit solid angle along the pinch axis is two to three times lower than that in the radial direction. In the axial direction, the energy flux density of the expanding plasma is two to three times lower than the radiation intensity. The measured radiation yield across the pinch is 2.5–5 kJ/sr, while that along the pinch axis is 1–2 kJ/sr. The results obtained by means of XRDs agree to within measurement errors with those obtained using the radiation calorimeter. It is found that the energy per unit solid angle carried by the expanding plasma in the radial direction does not exceed 10% of the soft X-ray yield. Analysis of the structure of time-integrated pinhole images and signals from the radial and axial XRDs shows that radiation emitted in the radial direction from the hot central region of the pinch is partially screened by the less dense surrounding plasma halo, whereas radiation emitted in the axial direction is a superposition of the emissions from the hot dense central region of the pinch and the colder less dense peripheral plasma.

A “BOOSTED FIREBALL” MODEL FOR STRUCTURED RELATIVISTIC JETS

We present a model for relativistic jets which generates a particular angular distribution of Lorentz factor and energy per solid angle. We consider a fireball with specific internal energy E/M launched with bulk Lorentz factor \gamma_B. This "boosted fireball" model is motivated by the phenomenology of collapsar jets, but is applicable to a wide variety of relativistic flows. In its center-of-momentum frame the fireball expands isotropically, converting its internal energy into radially expanding flow with asymptotic Lorentz factor \eta_0 ~ E/M. In the lab frame the flow is beamed, expanding with Lorentz factor \Gamma = 2 \eta_0 \gamma_B in the direction of its initial bulk motion and with characteristic opening angle \theta_0 ~ 1/\gamma_B. The flow is jet-like with \Gamma \theta_0 ~ 2 \eta_0 such that jets with \Gamma > 1/\theta_0 are naturally produced. The choice \eta_0 ~ \gamma_B ~ 10 yields a jet with \Gamma ~ 200 on-axis and angular structure characterized by opening angle \theta_0 ~ 0.1 of relevance for cosmological GRBs, while \gamma_B >~ 1 may be relevant for low-luminosity GRBs. The model produces a family of outflows, of relevance for different relativistic phenomena with structures completely determined by \eta_0 and \gamma_B. We calculate the energy per unit solid angle for the model and use it to compute light curves for comparison with the widely used top-hat model. The jet break in the boosted fireball light curve is greatly subdued when compared to the top-hat model because the edge of the jet is smoother than for a top-hat. This may explain missing jet breaks in afterglow light curves.

Compensating for Target Motion Between Seed Pulse and Phase Conjugate Beam Arrival Times at the Target

In nonmechanical beam steering with phase conjugate (PC) mirrors, the scattered and reflected beams from a foam target are utilized. A scattering and reflection model has been made that coincides with the experimental results with an error between ±15%. In the case where four probe beams irradiated a foam target, the three-dimensional intensity distribution was simulated. The peak-to-minimum variation of the reflected plus the scattered energy per unit solid angle was [approximately]20% of the average energy.The PC beam direction needs to compensate for target motion (v [approximately] 300 m/s) to accurately irradiate an injected target with laser beams in the method of beam steering with PC mirrors. The compensation with four-wave mixing utilized as a PC mirror is shown. A PC beam can be adjusted by properly setting the angle between two pump beams.

Inhibition of the dynamical Casimir effect with Robin boundary conditions

We consider a real massless scalar field in $3+1$ dimensions satisfying a Robin boundary condition at a nonrelativistic moving mirror. Considering vacuum as the initial field state, we compute explicitly the number of particles created per unit frequency and per unit solid angle, exhibiting in this way the angular dependence of the spectral distribution. The well-known cases of Dirichlet and Neumann boundary conditions may be reobtained as particular cases from our results. We show that the particle creation rate can be considerably reduced (with respect to the Dirichlet and Neumann cases) for particular values of the Robin parameter. Our results extend, for $3+1$ dimensions, previous results found in the literature for $1+1$ dimensions. Further, we also show that this inhibition of the dynamical Casimir effect occurs for different angles of particle emission.

Ultrafast AMO physics at the LCLS x-ray FEL

The Linac Coherent Light Source at the SLAC National Accelerator Laboratory, Stanford University, began operation in 2009 as the world's first hard x-ray free electron laser. Early experiments have concentrated on atomic physics, and have demonstrated several key features of the ultrafast high field x-ray-atom interaction. This paper reviews some of these early results. 1. INTRODUCTION: WORLD'S BRIGHTEST ULTRAFAST X-RAY SOURCE, LCLS The new class of so-called fourth generation synchrotron light sources are a revolutionary departure from storage ring-based electron synchrotrons. They are true x-ray lasers, which use relativistic electrons as a gain medium. The Linac Coherent Light Source (LCLS) at the SLAC National Accelerator Laboratory in Menlo Park California is the world's first hard x-ray free electron laser (XFEL) (1). It produces coherent x-ray fields that exceed the brightness of any previous source (measured in energy per unit time per unit solid angle per unit frequency interval) by a factor of one billion or more. The increase of peak brightness is due to a combination of increased pulses energy and decreased pulse duration. These novel lasers allow us to observe the fastest timescales in atoms and molecules, which are due to the interactions among electrons. The timescales involved can be characterized by electronic energy separations, which can be many electron volts. Any ultrafast laser source is limited to pulse durations no shorter than a single cycle of the electromagnetic field. For the conventional Ti : Sapphire laser ( = 800nm), this is about 2fs. But for x-rays, the pulse duration can be much shorter. Simulations show that the LCLS is capable of sub-femtosecond pulses, which could capture electronic coherences in molecules. X-rays can reveal multielectron processes inside atoms and molecules. Furthermore, these sources have not yet reached their intrinsic limits in pulse duration, where sub-femtosecond pulses are possible (2). Accelerator and x-ray laser scientists and engineers at SLAC have also begun to explore techniques that will one day provide routine operation with pulses whose time-bandwidth product is near the Fourier transform limit (3). Such control over x-ray fields could be exploited just as in the field of quantum optics with visible and ultraviolet laser pulses. This short review will highlight some of the first results, and the opportunities for further work in strong-field x-ray physics of atoms and molecules. The work described here was all done at the LCLS x-ray free electron laser by the Stanford PULSE Institute AMO group in collaboration with other AMO research groups around the world (4-9). Some of the experiments described form the thesis research of two Stanford students, Mike Glownia and James Cryan (10, 11).

Pure cyclotron spectra of V405 Aur

In this study we investigate the pure cyclotron spectra of an Intermediate Polar (IP), V405 Aur, which has the highest magnetic field strength in its class. Recent studies have shown that cyclotron harmonics are seen in the spectra of V405 Aur. We assume that cyclotron harmonics are produced by electrons having a streaming bi-Maxwellian velocity profile found in the lower portions of the accretion column connecting to the magnetic poles of the white dwarf in V405 Aur system. Then we applied the Green function for a dispersive medium such as the one found in V405 Aur. Further we assume that the cyclotron radiation is produced at Ordinary and Extraordinary wave modes. We find the general solution of the Green function for the Extraordinary wave mode and a dispersive medium and we derived the pure cyclotron spectra from the Green function. This function enabled us to calculate energy radiated per unit wavelength per unit solid angle as a function of wavelength which revealed the sixth, seventh and the eighth harmonics which correspond to the observed values. Our model produces the observed cyclotron spectrum of V405 Aur.

LARGE PECULIAR MOTION OF THE SOLAR SYSTEM FROM THE DIPOLE ANISOTROPY IN SKY BRIGHTNESS DUE TO DISTANT RADIO SOURCES

According to the cosmological principle, the Universe should appear isotropic, without any preferred directions, to an observer whom we may consider to be fixed in the co-moving co-ordinate system of the expanding Universe. Such an observer is stationary with respect to the average distribution of the matter in the Universe and the sky brightness at any frequency should appear uniform in all directions to such an observer. However a peculiar motion of such an observer, due to a combined effect of Doppler boosting and aberration, will introduce a dipole anisotropy in the observed sky brightness; in reverse an observed dipole anisotropy in the sky brightness could be used to infer the peculiar velocity of the observer with respect to the average Universe. We determine the peculiar velocity of the solar system relative to the frame of distant radio sources, by studying the anisotropy in the sky brightness from discrete radio sources, i.e., an integrated emission from discrete sources per unit solid angle. Our results give a direction of the velocity vector in agreement with the Cosmic Microwave Background Radiation (CMBR) value, but the magnitude ($\sim 1600\pm 400$ km/s) is $\sim 4$ times the CMBR value ($369\pm 1$ km/s) at a statistically significant ($\sim 3\sigma$) level. A genuine difference between the two dipoles would imply anisotropic Universe, with the anisotropy changing with the epoch. This would violate the cosmological principle where the isotropy of the Universe is assumed for all epochs, and on which the whole modern cosmology is based upon.

The Absolute Calibration of a Small-Angle Scattering Instrument with a Laboratory X-ray Source

Absolute calibration of small-angle scattering data (in units of differential cross-section per unit sample volume per unit solid angle) is necessary for the determination of molecular weights, the number density of particles, the scattering-length densities of phases in multiphased systems, volume fraction, the specific surface area of the scatterers and to restrict the parameters of a given model to the set which reproduces the observed intensity. It is also a useful means for the detection of artefacts in SAS experiments. Absolute intensities from the same sample also allows intercalibration among different instruments. This work details the absolute calibration procedure of a small-angle X-ray scattering instrument, the Rigaku S-Max3000. Absolute calibration is achieved by using two standards: homogeneous and stable glassy carbon and water. The scattering intensity of glassy carbon is calibrated by comparison with the absolute-calibration measurements taken on the USAXS instrument located at the 32ID beamline of the Advanced Photon Source in Argonne National Laboratory. This instrument has primary calibration capability. Scattering from water is angle-independent and only depends on the physical property of isothermal compressibility. Absolute calibrations using two standards were compared. The agreement of scale factors obtained using two standards suggests that precalibrated glassy carbon can serve as a convenient standard for all types of materials under study.

Poynting–Stokes tensor and radiative transfer in discrete random media: the microphysical paradigm

This paper solves the long-standing problem of establishing the fundamental physical link between the radiative transfer theory and macroscopic electromagnetics in the case of elastic scattering by a sparse discrete random medium. The radiative transfer equation (RTE) is derived directly from the macroscopic Maxwell equations by computing theoretically the appropriately defined so-called Poynting-Stokes tensor carrying information on both the direction, magnitude, and polarization characteristics of local electromagnetic energy flow. Our derivation from first principles shows that to compute the local Poynting vector averaged over a sufficiently long period of time, one can solve the RTE for the direction-dependent specific intensity column vector and then integrate the direction-weighted specific intensity over all directions. Furthermore, we demonstrate that the specific intensity (or specific intensity column vector) can be measured with a well-collimated radiometer (photopolarimeter), which provides the ultimate physical justification for the use of such instruments in radiation-budget and particle-characterization applications. However, the specific intensity cannot be interpreted in phenomenological terms as signifying the amount of electromagnetic energy transported in a given direction per unit area normal to this direction per unit time per unit solid angle. Also, in the case of a densely packed scattering medium the relation of the measurement with a well-collimated radiometer to the time-averaged local Poynting vector remains uncertain, and the theoretical modeling of this measurement is likely to require a much more complicated approach than solving an RTE.