Isotropic Source Research Articles

Calculation of dose point kernel values for monoenergetic electrons and beta emitting radionuclides: Intercomparison of Monte Carlo codes

Targeted radionuclide therapy (TRT) and beta-emitting seeds brachytherapy (BSBT) exploit the characteristics of energy deposited by beta-emitting radionuclides. Monte Carlo (MC) modelling of electron transport is crucial for calculations of absorbed dose for TRT and BSBT. However, computer codes capable of providing consistent results are still limited. Since experimental validations show several difficulties, the estimation of electron dose point kernel (DPK) is often used to verify the accuracy of different MC codes. In this work, we compared DPK calculations for various point, isotropic and monoenergetic electron sources and several beta-emitting radioisotopes using the codes MCNP, EGSnrc, PENELOPE and TOPAS with different simulation options. The simulations were performed using latest versions of EGSnrc and Penelope, TOPAS version 3.3.1 and MCNP version 6.1 Monte Carlo codes. In our simulations, the geometrical model consists of a point electron source placed at the center of a water sphere emitting isotropically. The water sphere was divided into 28 shells and the energy deposition was scored within these shells. The radius of the outermost shell was 1.2R0, where R0 is the continuous slowing down approximation (CSDA) range. Five monoenergetic beta sources with energies of 0.05, 0.1, 0.5, 1 and 3 MeV were studied. Six beta-emitting radionuclides were also simulated: Lu-177, Sm-153, Ho-166, Sr-89, I-131 and Y-90. Monoenergetic electron simulations showed large deviations among the codes, larger than 13% depending on the electron energy and the distance from the source. In the cases where beta spectra of radionuclides were simulated, all MC codes showed differences from EGSnrc (used as reference value - RV) less than 3% within rE90 range (radius of the sphere in which 90% of the energy of the spectrum electrons would be deposited). TOPAS showed results comparable to EGSnrc and PENELOPE. DPK values for 0.1 MeV monoenergetic electrons, calculated using MCNP6, led to differences higher than ±5% from RV despite our attempts to tune electron transport algorithms and physics parameters.

Effect of scattering anisotropy on the properties of photon density waves.

The frequency characteristics of spherical photon density waves excited in media with different degrees of scattering anisotropy are studied. Statistical modeling of the frequency and phase responses of the spatial irradiance of the light field emitted by a point-sized isotropic source were performed employing the Monte Carlo technique. The scattering anisotropy of the medium was determined by the Henyey-Greenstein phase function with different values of the mean scattering cosine. It is shown that the scattering anisotropy factor determines the frequency range, in which the effect of the photon path length distribution on the magnitude of the photon density wave dispersion is maximal. In media with quasi-isotropic scattering, dispersion effects are manifested at lower frequencies as compared to those for anisotropic media. The simulation results are compared with the analytical solution for the asymptotic regime of the light field in an isotropically scattering medium.

Ultrasound modulated bioluminescence tomography with a single optical measurement

Ultrasound modulated bioluminescence tomography is an imaging method which can be formulated as a hybrid inverse source problem. In the regime where light propagation is modeled by a radiative transfer equation, previous approaches to this problem require large numbers of optical measurements (Bal et al 2016 SIAM J. Math. Anal. 48 1332). Here we propose an alternative solution for this inverse problem which requires only a single optical measurement in order to reconstruct the isotropic source. Specifically, we derive two inversion formulae based on Neumann series and Fredholm theory respectively, and prove their convergence under sufficient conditions. The resulting numerical algorithms are implemented and experimented to reconstruct both continuous and discontinuous sources in the presence of noise.

Monte Carlo assessment of low energy electron range in liquid water and dosimetry effects

The effects of low energy electrons in biological tissues have proved to lead to severe damages at the cellular and sub-cellular level. It is due to an increase in the relative biological effectiveness (RBE) of these electrons with a decrease in their penetration range. That is, lower the range higher will be its RBE.Therefore, accurate determination of low energy electron range becomes a key issue for radiation dosimetry. This work reports on in-water electron tracks evaluated at low kinetic energy (1–50 keV) using isotropic mono-energetic point source approach suitably implemented by different general-purpose Monte Carlo codes. For this aim, simulations were performed using PENELOPE, EGSnrc, MCNP6, FLUKA and Geant4-DNA Monte Carlo codes to obtain the particle range, R,R90,R50. Finally, evaluation of dose point kernel (DPK), as used for internal dosimetry, was carried out as an application example. Scaled dose point kernels (sDPK) were estimated for a range of mono-energetic low energy electron sources. The non-negligible differences among the calculated sDPK using different codes were obtained for energy electrons up to 5 keV. It was also observed that differences of in-water range for low-energy electrons, due to the different general-purpose Monte Carlo codes, affected the DPKs used for dosimetry by convolution approach. Finally, the 3D dosimetry was found to be almost not affected at macroscopic clinical scale, whereas non-negligible differences appeared at the microscopic level. Hence, a thorough validation of the used sDPKs have to be performed before they could be used in applications to derive any conclusions.

Effect of Random 3D Correlated Velocity Perturbations on Numerical Modeling of Ground Motion from the Source Physics Experiment

ABSTRACTExplosions are traditionally discriminated from earthquakes, using the relative amplitude of compressional and shear waves at regional and teleseismic distances known as the P/S discriminant. Pyle and Walter (2019) showed this technique to be less robust at shorter distances, in detecting small-magnitude earthquakes and low-yield explosions. The disparity is largely due to ground motion from small, shallow sources being significantly impacted by near-surface structural complexities. To understand the implications of wave propagation effects in generation of shear motion and P/S ratio during underground chemical explosions, we performed simulations of the Source Physics Experiment (SPE) chemical explosions using 1D and 3D velocity models of the Yucca Flat basin. All simulations used isotropic point sources in the frequency range 0–5 Hz. We isolate the effect of large-scale geological structure and small-scale variability at shallow depth (<5 km), using a regional 3D geologic framework model (GFM) and the GFM-R model derived from the GFM, by adding correlated stochastic velocity perturbations. A parametric study of effects of small-scale velocity variations on wave propagation, computed using a reference 1D velocity model with stochastic perturbations, shows that the correlation length and depth of stochastic perturbations significantly impact wave scattering, near-surface wave conversions, and shear-wave generation. Comparisons of recorded and simulated waveforms for the SPE-5 explosion, using 3D velocity models, demonstrate that the shallow structure of the Yucca Flat basin contributes to generation of observed shear motion. The inclusion of 3D wave scattering, simulated by small-scale velocity perturbations in the 3D model, improves the fit between the simulated and recorded waveforms. In addition, a relatively low intrinsic attenuation, combined with small-scale velocity variations in our models, can confirm the observed wave trapping and its effect on duration of coda waves and the spatial variation of P/S ratio at basin sites.

Cascaded neural network based small array synthesis with robustness to noise

A cascaded neural network approach has been presented in this paper to estimate the excitation for the desired field distribution using a radial basis function neural network (RBFNN). The article has employed an electromagnetic design example consisting of 5 × 5 and 6 × 6 planar antenna array of isotropic sources with inter element-distance of 0.5λ to show the adaptation of the neural network model in estimating the desired output. A neural network is trained using a dataset of suitable excitation voltages and its corresponding radiation patterns, which proves to be efficient in predicting the excitation voltages required to generate the desired pattern. A set of techniques based on a cascaded neural network is adopted for pattern synthesis using magnitude and phase, magnitude only, and template-based input data. The robustness of the method has also been tested by considering noise with different SNR levels. The results found in each case have a close fit with the desired pattern.

A Monte Carlo study on the gamma-ray buildup factors for the linear sources embedded in a cylindrical shield

While the studies on the photon buildup factors for point gamma sources have been widely made, studies for linear gamma sources, for which the spent nuclear fuel rods and/or brachytherapy seeds could be a relatively good approximation, have become extremely scarce. Buildup factors for linear gamma sources are computed here by means of the MCNPX Monte Carlo code. The exposure buildup factors were calculated for isotropic linear gamma sources with different lengths (1, 10, and 100 cm) and also for different energies (0.1, 0.5, 1, 5, and 10 MeV) in a cylindrical concrete shield up to depths of 10 mean free paths. A number of 45 simulations were done to reach the appropriate results for the linear gamma sources and 5 more simulations were performed for point gamma sources to be compared with the linear sources of photons at the same energies. For validation, we calculated the total mass attenuation coefficients, mass energy absorption coefficients, and uncollided photon flux for some photon energy in concrete and we showed that the simulation results have a good agreement with NIST-XCOM database and theory. Finally, comparisons are shown for the exposure buildup factors of the linear source in terms of the gamma-rays energy, of the length of the linear source and of the cylindrical shield, which can be useful in the design of a suitable shield.

Design of metacontinua in the aeroacoustic spacetime

The effect of background flows on the response of acoustic metamaterials is a key aspect that prevented the full disclosure of their potential in those applications where an aerodynamic velocity field strongly influences the propagation of acoustic disturbances. Indeed, the classic approaches for metamaterial design do not consider the aeroacoustic interaction, and the resulting metamaterials cannot preserve their response when operating in flows. So far, only few authors have addressed the problem, mostly focusing on understanding the phenomenon or identifying corrective techniques with limited usability in practical applications. The present study proposes a general method for the modification of the mechanical properties of acoustic metacontinua to preserve their response in presence of a background flow. The method is based on the application of spacetime coordinate transformations exploiting the spacetime formal invariance of the generalised d’Alembertian. This methodology applies to the equation governing the propagation of acoustic disturbances in a metamaterial having arbitrary constitutive equations independently on the method used for its original design. The approach is validated through numerical simulations, using as a benchmark the problem of the acoustic cloaking of a cylinder impinged by a perturbation generated by an isotropic point source within a flowing medium. Numerical results are obtained for an asymptotic Mach number M_infty le 0.35.

Anisotropic deformations in a class of projectively-invariant metric-affine theories of gravity

Among the general class of metric-affine theories of gravity, there is a special class conformed by those endowed with a projective symmetry. Perhaps the simplest manner to realise this symmetry is by constructing the action in terms of the symmetric part of the Ricci tensor. In these theories, the connection can be solved algebraically in terms of a metric that relates to the spacetime metric by means of the so-called deformation matrix that is given in terms of the matter fields. In most phenomenological applications, this deformation matrix is assumed to inherit the symmetries of the matter sector so that in the presence of an isotropic energy–momentum tensor, it respects isotropy. In this work we discuss this condition and, in particular, we show how the deformation matrix can be anisotropic even in the presence of isotropic sources due to the non-linear nature of the equations. Remarkably, we find that Eddington-inspired-Born–Infeld (EiBI) theories do not admit anisotropic deformations, but more general theories do. However, we find that the anisotropic branches of solutions are generally prone to a pathological physical behaviour.

The Effects of Assumed Source Depth and Shear-Wave Velocity on Moment Tensors Estimated for Small, Contained Chemical Explosions in Granite

ABSTRACTThe Source Phenomenology Experiment (SPE-Arizona) included of a series of chemical explosions detonated within a copper mine in Arizona. This study focuses on ground motions from detonations in the copper mine, which are analyzed to assess the uniqueness of the resulting source representation when the source region propagation characteristics have a range of possible models. P-wave velocities are well constrained by refraction data with less constraint of the S-wave velocities. The effects of explosion source depth and VS are assessed with Green’s functions for a range of models in which VP is held constant. Propagation models with a Poisson’s value of 0.25 and a source depth 30–60 m most accurately replicate the data. The explosion was detonated at a centroid depth of 30 m, so trade-offs in depth are demonstrated. The compensated linear vector dipole and explosion components of the Green’s functions convolved with a Mueller–Murphy source function are compared. Both produce significant energy in the 2–12 Hz band, due to surface-wave contributions with no clear depth dependencies above 20 Hz. The range of propagation models is used with the observational data to invert for the frequency-domain moment tensor. Fits to the data from these inversions have cross-correlation values of 0.64, demonstrating effectiveness in replicating the observations with the assumed propagation path effects and resulting source function. Inversions produce horizontal dipoles (Mxx and Myy), roughly half the maximum amplitude of Mzz, consistent with a compensated linear vector dipole source, which is frequency dependent. Denny and Johnson, Mueller–Murphy, Walter and Ford, and the revised Mueller–Murphy source models, parameterized for granite, are compared to the moment tensors. Despite a nonisotropic moment tensor source, the revised Mueller–Murphy isotropic source model best replicates the long-period moments, overshoot, and corner frequency.

Buildup with Bremsstrahlung in the Martian Atmosphere

Photon buildup is a function of energy, medium, and geometry and therefore must be specifically calculated for the case of interest. The Martian atmosphere, mostly comprising carbon dioxide, is becoming more relevant to radiation researchers and therefore warrants the study of this gas mixture’s buildup properties for ionizing photon flux resulting from the secondary effects of galactic cosmic rays and solar flares. Specifically, this work uses the MCNP6 code to develop energy absorption buildup factors in finite slab models for energies ranging from 40 keV to 15 MeV with Martian regolith as the backscattering medium. The Martian carbon dioxide cycle is accounted for by determining maximum and minimum mean densities as a function of orbital position. An isotropic point source model for the atmosphere is also developed using the geometric progression fitting function. Buildup is bounded to a factor of approximately 23 at 100 keV for normally incident photons at the top of the atmosphere. For conservatism, the design problem neglects coherent scattering but assumes bremsstrahlung effects and uses Klein-Nishina free-electron cross sections for Compton scattering.

Extreme intra-hour variability of the radio source J1402+5347 discovered with Apertif

The propagation of radio waves from distant compact radio sources through turbulent interstellar plasma in our Galaxy causes these sources to twinkle, a phenomenon called interstellar scintillation. Such scintillations are a unique probe of the micro-arcsecond structure of radio sources as well as of the sub-AU-scale structure of the Galactic interstellar medium. Weak scintillations (i.e. an intensity modulation of a few percent) on timescales of a few days or longer are commonly seen at centimetre wavelengths and are thought to result from the line-of-sight integrated turbulence in the interstellar plasma of the Milky Way. So far, only three sources were known that show more extreme variations, with modulations at the level of some dozen percent on timescales shorter than an hour. This requires propagation through nearby (d ≲ 10 pc) anomalously dense (ne ∼ 102 cm−3) plasma clouds. Here we report the discovery with Apertif of a source (J1402+5347) showing extreme (∼50%) and rapid variations on a timescale of just 6.5 min in the decimetre band (1.4 GHz). The spatial scintillation pattern is highly anisotropic, with a semi-minor axis of about 20 000 km. The canonical theory of refractive scintillation constrains the scattering plasma to be within the Oort cloud. The sightline to J1402+5347, however, passes unusually close to the B3 star Alkaid (η UMa) at a distance of 32 pc. If the scintillations are associated with Alkaid, then the angular size of J1402+5347 along the minor axis of the scintels must be smaller than ≈10 μas, yielding an apparent brightness temperature for an isotropic source of ≳1014 K.

Wormholes in 4D Einstein\u2013Gauss\u2013Bonnet gravity

Recent times witnessed a significant interest in regularizing, a D rightarrow 4 limit, of EGB gravity initiated by Glavan and Lin [Phys. Rev. Lett. 124, 081301 (2020)] by re-scaling GB coupling constant as alpha /(D-4) and taking limit D rightarrow 4, and in turn these regularized 4D gravities have nontrivial gravitational dynamics. Interestingly, the maximally or spherically symmetric solution to all the regularized gravities coincides in the 4D case. In view of this, we obtain an exact spherically symmetric wormhole solution in the 4D EGB gravity for an isotropic and anisotropic matter sources. In this regard, we consider also a wormhole with a specific radial-dependent shape function, a power-law density profile as well as by imposing a particular equation of state. To this end, we analyze the flare-out conditions, embedding diagrams, energy conditions and the volume integral quantifier. In particular our −ve branch results, in the limit alpha rightarrow 0, reduced exactly to vis-grave{a}-vis 4D Morris-Thorne of GR.

Focal construct geometry for high-intensity x-ray diffraction from laser-shocked polycrystalline.

An increasing number of dynamic experiments, especially those involving laser drive, are employing in situ x-ray diffraction as a probe to interrogate structure evolution between states of matter under extreme pressure and temperature. We present an alternative configuration, focal construct geometry, for in situ x-ray diffraction to measure the structure and evolution of dynamically compressed polycrystalline materials on a laser platform. This configuration makes full use of the isotropically emitted He-α x rays by employing an annular (or semi-annular) collimator rather than a regular pinhole collimator and thus increases the flux of incident x rays reaching the sample as well as the intensity of the diffracted x rays, enabling the detection of a diffraction pattern with less laser energy. Its effectiveness and applicability are validated against the conventional Debye-Scherrer geometry through direct molecular dynamics simulations and x-ray diffraction simulations for two representative shock-induced phase transition events, solid-solid and solid-liquid (or melting). This configuration reproduces all the Debye-Scherrer diffraction profiles in good accuracy and demonstrates superior efficiency in utilizing the isotropic x-ray source and harvesting diffracted x rays while preserving the angular resolution.

Nonstationary angular distribution of optical field radiance from an isotropic source in sea water

The spatial-angular and temporal characteristics of the radiance of the light field emitted by a nonstationary point isotropic source in sea water are studied. Using the Monte Carlo method, we calculated the pulse transfer functions and frequency responses of the angular radiance distributions at various distances from the source. Particular integral characteristics of the angular radiance distributions are estimated. It is shown that with an increase in the delay time, measured from the time of arrival of ballistic photons, the angular radiance distribution asymptotically tends to be isotropic. The frequency and phase responses of the alternating radiance component from a source modulated by power at a high frequency are studied. It is shown that with an increase in the modulation frequency, the angular distribution of the alternating radiance component is concentrated close to the direction to the source.

Modeling of a Spherical Tokamak as an Extended Neutron Source Using ASTRA and MCNP

Fast neutrons can perform two functions that are critical to the nuclear power industry: breed fissile material from fertile material and burn minor actinides in spent fuel. The fusion reaction in a fusion reactor between deuterium and tritium nuclei yields an alpha particle and a neutron with 14.5-MeV energy. These neutrons can easily convert fertile material (such as 238U and 232Th) into fissile material (such as 239Pu and 233U) and destroy minor actinides (Pu, Np, Am, and Cm) present in the spent nuclear fuel and significantly reduce its radiotoxicity. However, each neutron generated implies one tritium consumed and tritium is generated by nuclear reactions between a neutron and lithium nuclei. Therefore, neutron economy becomes a crucial issue and an adequate neutronic modeling of the neutron source becomes critical. In this article, the Automatic System for TRansport Analysis (ASTRA) and Monte Carlo N-Particle (MCNP) codes are used in conjunction to characterize the neutron flux coming out of a low-aspect-ratio tokamak machine. ASTRA is first used to determine temperature and density profiles within the plasma volume and MCNP is used to evaluate the neutron flux. Since the simulation of an extended neutron source with complex geometry such as the confined plasma inside the tokamak is difficult, steps are taken to simplify the geometry by defining two simple sources, a cylindrical wall and a disk, which should have similar neutron emission distributions to the volumetric source. This reduction is done using a preprocessing Monte Carlo algorithm to establish probability density functions of neutron emission for the cylindrical wall as a function of the height and for the disk as a function of the radius, based on the emission characteristics of the extended source. The results that compare the neutron flux leaving a volume for four different configurations of neutron sources (isotropic source at the geometrical origin, ring source at the plasma geometrical center, cylindrical wall source, and distributed multipoint source) are presented. It is found that the processing of the two simple sources has a higher computational cost in terms of both processing time and random number consumption.

Reconstruction of the Lambert Curve in a Scattering Medium by Using Pulsed Sounding

Under study is some mathematical model of the radiation transfer process in a scattering medium initiated by a pulsed point isotropic source. We inspect the inverse problem of finding a diffusely reflecting curve by using the two integral overdetermination conditions on the solution of the radiative transfer equation. Some nonlinear differential equation is obtained for the function that describes the shape of the desired curve in the case of the single scattering approximation. Numerical analysis of the inverse problem solution is carried out to check the stability under perturbations of the initial data .

The Impact of Tissue Type and Density on Dose Point Kernels for Patient-Specific Voxel-Wise Dosimetry: A Monte Carlo Investigation.

We report the generation of dose point kernels for clinically-relevant radionuclide beta decays and monoenergetic electrons in various tissues to understand the impact of tissue type on dose point kernels. Currently available voxel-wise dosimetry approaches using dose point kernels ignore tissue composition and density heterogeneities. Therefore, the study on the impact of tissue type on dose point kernels is warranted. Simulations were performed using the GATE Monte Carlo toolkit, which encapsulates GEANT4 libraries. Dose point kernels were simulated in phantoms of water, compact bone, lung, adipose tissue, blood and red marrow for radionuclides 90Y, 188Re, 32P, 89Sr, 186Re, 153Sm and 177Lu and monoenergetic electrons (0.015-10 MeV). All simulations were performed by assuming an isotropic point source of electrons at the center of a homogeneous spherical phantom. Tissue-specific differences between kernels were investigated by normalizing kernels for effective pathlength. Transport of 20 million particles was found to provide sufficient statistical precision in all simulated kernels. The simulated dose point kernels demonstrate excellent agreement with other Monte Carlo packages. Deviation from kernels reported in the literature did not exceed a 10% global difference, which is consistent with the variability among published results. There are no significant differences between the dose point kernel in water and kernels in other tissues that have been scaled to account for density; however, tissue density predictably demonstrated itself to be a significant variable in dose point kernel distribution.

On the mass of bootstrapped Newtonian sources

We show that the bootstrapped Newtonian potential generated by a uniform and isotropic source does not depend on the one-loop correction for the matter coupling to gravity. The latter, however, affects the relation between the proper mass and the Arnowitt–Deser–Misner (ADM) mass and, consequently, the pressure needed to keep the configuration stable.

Source of Unusual Monochromatic Wave Packets Recorded Globally in the Seismograms of 11 November 2018

On 11 November 2018, all global seismic stations recorded unusual long-period wave packets without any discernible body waves. Our analyses show that the wave packets are monochromatic with period 15.6 sec and are fundamental-mode non-dispersive Rayleigh waves. Using the arrival times of wave packets, the source is approximately located at the northeastern edge of the recent earthquake swarm zone, off the east coast of Mayotte, a volcanic island between North Madagascar and East Africa. Synthetic seismograms for an isotropic source at 15 km depth with a transient oscillation of the same periodas that observed in the wave packets, are consistent with the recorded seismograms. We estimate an equivalent magnitude (M w ) of ~4.8 for this source and suggest that a volcanic fluid sphere of radius ~5 km might have generated such radial oscillations. Interestingly, similar monochromatic waves with lesser amplitude were also recorded on many occasions from the same source during 21 June 2018 to 10 September 2019. The period of waves increased from 15.2 sec in June to 15.6 sec in October– November 2018 and then decreased to 15.0 sec in September 2019, which implies an increase and then decrease in radius of the fluid sphere at the source.