Experimental Attenuation Data Research Articles

Characterization of selected additive manufacturing materials for synchrotron monochromatic imaging and broad-beam radiotherapy at the Australian synchrotron-imaging and medical beamline

Objective. This study aims to characterize radiological properties of selected additive manufacturing (AM) materials utilizing both material extrusion and vat photopolymerization technologies. Monochromatic synchrotron x-ray images and synchrotron treatment beam dosimetry were acquired at the hutch 3B and 2B of the Australian Synchrotron-Imaging and Medical Beamline. Approach. Eight energies from 30 keV up to 65 keV were used to acquire the attenuation coefficients of the AM materials. Comparison of theoretical, and experimental attenuation data of AM materials and standard solid water for MV linac was performed. Broad-beam dosimetry experiment through attenuated dose measurement and a Geant4 Monte Carlo simulation were done for the studied materials to investigate its attenuation properties specific for a 4 tesla wiggler field with varying synchrotron radiation beam qualities. Main results. Polylactic acid (PLA) plus matches attenuation coefficients of both soft tissue and brain tissue, while acrylonitrile butadiene styrene, Acrylonitrile styrene acrylate, and Draft resin have close equivalence to adipose tissue. Lastly, PLA, co-polyester plus, thermoplastic polyurethane, and White resins are promising substitute materials for breast tissue. For broad-beam experiment and simulation, many of the studied materials were able to simulate RMI457 Solid Water and bolus within ±10% for the three synchrotron beam qualities. These results are useful in fabricating phantoms for synchrotron and other related medical radiation applications such as orthovoltage treatments. Significance and conclusion. These 3D printing materials were studied as potential substitutes for selected tissues such as breast tissue, adipose tissue, soft-tissue, and brain tissue useful in fabricating 3D printed phantoms for synchrotron imaging, therapy, and orthovoltage applications. Fabricating customizable heterogeneous anthropomorphic phantoms (e.g. breast, head, thorax) and pre-clinical animal phantoms (e.g. rodents, canine) for synchrotron imaging and radiotherapy using AM can be done based on the results of this study.

Novel techniques for in situ estimation of shear-wave velocity and damping ratio through MASW testing part II: a Monte Carlo algorithm for the joint inversion of phase velocity and phase attenuation

SUMMARY This paper deals with in situ characterization of the small-strain shear-wave velocity VS and damping ratio DS from an advanced interpretation of Multi-channel Analysis of Surface Waves (MASW) surveys. A new approach based on extracting Rayleigh wave data using the CFDBFa method has been discussed in the companion paper. This paper focuses on mapping the experimental Rayleigh wave phase velocity and attenuation into profiles of VS and DS versus depth, which is achieved through a joint inversion procedure. The joint inversion of phase velocity and attenuation data utilizes a newly developed Monte Carlo global search algorithm, which implements a smart sampling procedure. This scheme exploits the scaling properties of the solution of the Rayleigh eigenvalue problem to modify the trial earth models and improve the matching with the experimental data. Thus, a reliable result can be achieved with a limited number of trial ground models. The proposed algorithm is applied to the inversion of synthetic data and of experimental data collected at the Garner Valley Downhole Array site, as described in the companion paper. In general, inverted soil models exhibit well-defined VS profiles, whereas DS profiles are affected by larger uncertainties. Greater uncertainty in the inverted DS profiles is a direct result of higher variability in the experimental attenuation data, the limited wavelength range at which reliable values of attenuation parameters can be retrieved, and the sensitivity of attenuation data to both DS and VS. Nonetheless, the resulting inverted earth models agree well with alternative in situ estimates and geological data. The results stress the feasibility of retrieving both stiffness and attenuation parameters from active-source MASW testing and the effectiveness of extracting in situ damping ratio estimates from surface wave data.

A study of the diffusive properties of a modified compressible Navier-Stokes model

The aim of this study is to provide further validation for the weakly well-posed modified compressible Navier-Stokes system proposed in Svärd (Phys A 506:350–375, 2018) when applied to ideal gases. We do so by considering sound attenuation, both theoretically and numerically for argon and oxygen, and make comparisons with experimental values in the literature. Furthermore, we compute shock profiles for argon and nitrogen, and compare them with experiments in the literature. Our numerical simulations have revealed problems when using experimental attenuation data, as presented in the available literature, for validation and determination of diffusion coefficients. However, comparisons with the shock data, suggest that the modified system may benefit from an additional heat diffusive term. In view of these and previously published validation tests, the model proposed in Svärd (Phys A 506:350–375, 2018) is equally accurate as the standard compressible system. However, with more complete experimental information for the attenuation case at hand, it might be possible to further improve the accuracy by more precise determination of the diffusion coefficients. We propose a tentative adjustment of the model that may be tested/validated, if more detailed experimental information becomes available.

Using ultrasonic attenuation in cortical bone to infer distributions on pore size

In this work we infer the underlying distribution on pore radius in human cortical bone samples using ultrasonic attenuation data. We first discuss how to formulate polydisperse attenuation models using a probabilistic approach and the Waterman Truell model for scattering attenuation. We then compare the Independent Scattering Approximation and the higher-order Waterman Truell models’ forward predictions for total attenuation in polydisperse samples. Following this, we formulate an inverse problem under the Prohorov Metric Framework coupled with variational regularization to stabilize this inverse problem. We then use experimental attenuation data taken from human cadaver samples and solve inverse problems resulting in nonparametric estimates of the probability density function on pore radius. We compare these estimates to the “true” microstructure of the bone samples determined via microCT imaging. We find that our methodology allows us to reliably estimate the underlying microstructure of the bone from attenuation data.

Diffusion of gravity waves by random space inhomogeneities in pancake-ice fields. Theory and validation with wave buoys and synthetic aperture radar

We study the diffusion of ocean waves by ice bodies much smaller than a wavelength, such as pancakes and small ice floes. We argue that inhomogeneities in the ice cover at scales comparable to that of the wavelength significantly increase diffusion, producing a contribution to wave attenuation comparable to what is observed in the field and usually explained by viscous effects. The resulting attenuation spectrum is characterized by a peak at the scale of the inhomogeneities in the ice cover, which could explain the rollover of the attenuation profile at small wavelengths observed in field experiments. The proposed attenuation mechanism leads to the same behaviors that would be produced by a viscous wave model with effective viscosity linearly dependent on the ice thickness. This may explain recent findings that viscous wave models require a thickness-dependent viscosity to fit experimental attenuation data. Experimental validation is carried out using wave buoy attenuation data and synthetic aperture radar image analysis.

Ultrasonic propagation in concentrated colloidal dispersions: improvements in a hydrodynamic model

Recent theoretical and experimental findings demonstrate that modeling ultrasonic attenuation of a concentrated colloidal suspension requires inclusion of shear-induced contributions, an element being unaccounted for by most scattering models. Herein, we extend a hydrodynamic model from low to high particle volume fraction by effectively relating the single particle dynamic drag to particle concentration to account for hydrodynamic interparticle interactions. We calculate an expression for the complex-valued effective dynamic mass density at high concentrations, which is then combined with a viscosity-corrected effective bulk modulus to estimate ultrasonic velocity and attenuation for a monodisperse suspension of solid spherical particles in a viscous liquid. The effective velocity and attenuation are functions of particle volume fraction, frequency, and physical properties of particles and liquid. We compare our results with those from two recently developed scattering models: a multi-modal multiple scattering model and a core-shell effective medium model, each taking into account the viscosity of the host fluid through shear wave influences. Finally, we find that our extended model predicts experimental attenuation data the best for a silica in water suspension compared to the results of other models.

Feasibility study of using a compact deuterium-deuterium (D-D) neutron generator for energy-selective transmission tomography

This article presents a feasibility study of using a compact deuterium-deuterium (D-D) fast neutron generator for element-sensitive fast neutron tomography. The energy range of such neutron generators is quasi-monoenergetic from about 2.2–2.8 MeV, depending on the angle of emission. Unlike X-rays and gamma photons, fast neutron cross-sections have unique energy sensitivity in this range depending on the nuclide, making differentiation of materials in principle possible by performing tomographic attenuation measurements at multiple energies. In this context, detailed neutron spectrum models were created and attenuation versus emission angle data were simulated for several uniform test samples. An automated mechanical apparatus and plastic scintillator fast neutron detectors were used to interrogate these samples over the full range of emission angles around a custom neutron generator located at the Paul Scherrer Institute. The experimental attenuation data were compared with simulations. Simulations and experiments were found to be generally in good agreement, demonstrating the fundamental feasibility of the approach.

Study on physico-mechanical and gamma-ray shielding characteristics of new ternary nanocomposites

In this study, a series of “Epoxy-Clay-PbO” nanocomposites under the name of ECPNCs were prepared by the molding method, and their physico-mechanical properties were investigated by different techniques. Focus of the work, was on the shielding ability of the ECPNCs for the gamma rays, emitted from Ir-192, Cs-137 and Co-60 with a wide range of energy. Scanning electron microscopy, and X-ray diffractometry demonstrated that clay platelets were fully exfoliated, and the PbO particles were homogenously distributed in the polymeric matrix. Thermo-gravimetric analysis and standard tensile tests revealed that PbO content has an “increasing/decreasing” effect on “thermal stability/mechanical strength” of the nanocomposites. Gamma shielding experiments showed that efficacy of ECPNCs containing 30 wt% PbO was 47% better than that of concrete. Experimental attenuation data were confirmed by theoretical calculations, so that the maximum difference between them was 14.1%. Furthermore, a correlation was developed between PbO content of the ECPNCs and their mass attenuation coefficient for all gamma sources.

Sensitivity of seismic measurements to frequency‐dependent attenuation and upper mantle structure: An initial approach

AbstractThis study addresses the sensitivity of seismic attenuation measurements to dissipative mechanisms and structure in the Earth's upper mantle. The Andrade anelastic model fits experimental attenuation data with a mild power law frequency dependence and can be scaled from laboratory to Earth conditions. We incorporate this anelastic model into 400km 1‐D thermal profiles of the upper mantle. These continuous‐spectrum models are approximated by multiple relaxation mechanisms that are implemented within a finite‐difference scheme to perform wave propagation simulations in 1‐D domains. In two sets of numerical experiments, we evaluate the measurable signature of the intrinsic attenuation structure. The two sets are defined by thermal profiles with added step functions of temperature, varying in (i) amplitude and depth or (ii) amplitude and sharpness. The corresponding synthetic data are processed using both the conventional t* approach, i.e., a linear regression of the displacement frequency spectrum, and an alternative nonlinear fit to identify the integrated value of attenuation and its frequency dependence. The measured sensitivity patterns are analyzed to assess the effects of the anelastic model and its spatial distribution on seismic data (in the absence of scattering effects). We have two straightforward results: (1) the frequency dependence power law is recoverable from the measurements; (2) t* is sensitive to both the depth and the amplitude of the step, and it is insensitive to the sharpness of the step, in the 0.25 to 2 Hz band. There is much potential for gaining information about the upper mantle thermodynamic state from careful interpretation of attenuation.

Measurement of Broadband Temperature-Dependent Ultrasonic Attenuation and Dispersion Using Photoacoustics

The broadband ultrasonic characterization of biological fluids and tissues is important for the continued development and application of high-resolution ultrasound imaging modalities. Here, a photoacoustic technique for the transmission measurement of temperature-dependent ultrasonic attenuation and dispersion is described. The system uses a photoacoustic plane wave source constructed from a polymethylmethacrylate substrate with a thin optically absorbent layer. Broadband ultrasonic waves are generated by illuminating the absorbent layer with nanosecond pulses of laser light. The transmitted ultrasound waves are detected by a planar 7-microm high-finesse Fabry-Perot interferometer. Temperature-induced thickness changes in the Fabry-Perot interferometer are tracked to monitor the sample temperature and maintain the sensor sensitivity. The measured -6 dB bandwidth for the combined source and sensor is 1 to 35 MHz, with an attenuation corrected signal level at 100 MHz of -10 dB. The system is demonstrated through temperature-dependent ultrasound measurements in castor oil and olive oil. Power law attenuation parameters are extracted by fitting the experimental attenuation data to a frequency power law while simultaneously fitting the dispersion data to the corresponding Kramers-Krönig relation. The extracted parameters are compared with other calibration measurements previously reported in the literature.

Improving the accuracy of pulsed field gradient NMR diffusion experiments: Correction for gradient non-uniformity

Pulsed field gradient NMR is a well-established technique for the determination of self-diffusion coefficients. However, a significant source of systematic error exists in the spatial variation of the applied pulsed field gradient. Non-uniform pulsed field gradients cause the decay of peak amplitudes to deviate from the expected exponential dependence on gradient squared. This has two undesirable effects: the apparent diffusion coefficient will deviate from the true value to an extent determined by the choice of experimental parameters, and the error estimated by the nonlinear least squares fitting will contain a significant systematic contribution. In particular, the apparent diffusion coefficient determined by exponential fitting of the diffusional attenuation of NMR signals will depend both on the exact pulse widths used and on the range of gradient amplitudes chosen. These problems can be partially compensated for if experimental attenuation data are fitted to a function corrected for the measured spatial dependence of the gradient and signal strength. This study describes a general alternative to existing methods for the calibration of NMR diffusion measurements. The dominant longitudinal variation of the pulsed field gradient amplitude and the signal strength are mapped by measuring pulsed field gradient echoes in the presence of a weak read gradient. These data are then used to construct a predicted signal decay function for the whole sample, which is parameterised as the exponential of a power series. Results are presented which compare diffusion coefficients obtained using the new calibration method with previous literature values.

A new fractal modification of ultrasonic attenuation model for measuring particle size in mineral slurries

Weight percentages of different size incremental particles in a mineral slurry is integrated into the ultrasonic attenuation model to derive a relation between ultrasonic attenuation and particle sizes. However, in the inertial and scattering regimes of frequency–radius space, irregularity and aggregation of particles can results in values of ultrasonic attenuation that are significantly different from those predicted by the derived model, which is based on the assumption of separated smooth spherical particles in mineral slurries. Experimental attenuation data obtained previously from mineral slurries of iron ores particles are compared with predicted values. It is shown that there is a negligible difference between predictions and experimental data. A new modification of theoretical model for ultrasonic attenuation is derived. The theory uses ωτ v or κR as a fractal scale. It requires an empirical determination of the difference between the fractal dimension of the measured mineral slurry and that of a hypothetical slurry of spheres with the same particle size distribution. The new fractal modification of ultrasonic attenuation model is found to enable better agreement with measured attenuation as a function concentration for irregular particles than the theoretical model. Moreover, the fractal approach is found to discriminate between the effects of particle irregularity and aggregation.

Physical mechanisms implied in ultrasound propagation through trabecular bones

Measurements of quantitative ultrasound parameters represent an established mean of skeletal status assessment in osteoporosis. However, to date, the exact physical mechanisms underlying ultrasound attenuation in cancellous bone have not been clearly documented. This study reviews the different mechanisms underlying propagation of ultrasound in trabecular bone, based on results obtained by our group and others. Bone is a porous, anisotropic, and scattering medium. Anisotropy of phase velocity has been explained with a multilayer model. Several attempts have been made to model the propagation with the Biot’s theory for poroelastic media. Encouraging results have been obtained for velocity. However, the viscous absorption mechanism implied in this theory disagrees with the experimental attenuation data and with the observed influence of fluid viscosity. Negative dispersion has been recorded, in agreement with some multiple scattering ideas. Several models of scattering (analytical and numerical) have given good predictions of backscattering coefficient. Despite the good results observed with these models, none of these could explain the experimental data, and especially the attenuation data. In conclusion, a more complex model will be discussed, to describe both absorption and scattering effects, as in geophysics for rock studies.

Reconstruction of megavoltage photon spectra from electronic portalimager derived transmission measurements

Three variants of the Schiff equation are investigated to model the spectraproduced by megavoltage linear accelerators. These models are testedagainst well-validated Monte Carlo (MC) generated spectra on the centralaxis of large-area fields, and show excellent agreement. Numericalreconstructions of 6 and 10 MV spectra using the same models are thenpresented, using experimental attenuation data derived from an electronicportal imager. The process of deriving spectra from experimentalattenuation data is shown to be inherently badly constrainedmathematically, with the derived spectrum being highly sensitive to noisein the source data, and non-unique. By placing a priori constraintson the Schiff model from both physical knowledge of the construction of theaccelerator and MC data, physically useful results are gained and presentedfor both the energy dependence and off-axis behaviour of photon spectra.

Comparison between the ultrasonic and the low frequency Bordoni peaks in copper

A comparison between properties of the Bordoni peaks at low and high frequency is realised from experimental data of attenuation and velocity obtained at Hz and MHz frequencies. The measurements were obtained in crystalline samples, all of them cut from the same crystal lot. Significant differences on secondary properties are obtained for the low and the high frequency peaks. The high frequency relaxation is interpreted as produced by a single relaxation process due to kink pair formation with kink diffusion in dislocation lines, but the low frequency relaxation must be considered as a more complicated process.

Contact law effects on wave propagation in particulate materials using distinct element modeling

The microstructural wave propagation behavior of a granular medium is modeled using the distinct element method. This technique simulates the discrete behavior of the medium by assuming that the motion of each particle may be modeled using Newtonian rigid-body mechanics with particular force-deformation and force-deformation rate contact laws. The present work provides a comparison of the effects of various contact laws on the wave propagational behaviors including wave attenuation and dispersion characteristics. Specific cases which have been studied include linear, non-linear and non-linear hysteretic force-deformation contact laws along with velocity proportional damping. Numerical results are compared with experimental data from dynamic photoelastic and strain gage experiments. Since velocity-dependent contact damping is not a reasonable model for dry cohesionless granular media, it was desired to determine if a non-linear hysteretic contact law could be used to replace the velocity damping. Results indicate that such a non-linear law does provide a damping mechanism which can predict experimental attenuation data, and that the dispersion characteristics are modeled more accurately with this hysteretic model.

Laplace reconstruction of experimental diagnostic x-ray spectra.

This paper displays the results of a blind study used to determine the capability of a Laplace transform pair model to accurately reconstruct diagnostic x-ray spectra from experimental attenuation data. Spectra reconstructed from attenuation measurements are compared to experimental spectra obtained on the same unit using an intrinsic germanium spectrometer system. The results show that when pure attenuation materials are used, good agreement is obtained between the experimental and computed spectra. If an alloy attenuator like 1100 aluminum is used, the proportion of contaminants must be included in the Laplace formulation for accurate reconstruction.

The determination of X-ray spectra from attenuation data: Part I: The potentials of various methods

The value of the analysis of so-called ideal attenuation curves for obtaining X-ray spectral information is investigated. Various methods are critically examined. Limitations with respect to their accuracy, the maximum number of energy intervals and the useful energy range are presented for two absorber materials. The influence of various types of errors and uncertainties, which are associated with experimental attenuation data and its analysis, is quantitatively examined. This allows requirements as to the accuracy of the input data to be derived.

A different … view … of attenuation … Part III, Conclusion

“A Different Point of View on the Role of Attenuation as a Component of Total Propagation Loss in Underwater Acoustics,” a paper given at the 96th Meeting of the Acoustical Society, and its sequel, Part II, presented at the 97th Meeting, presented a mathematical model for propagation on the axis of a sound channel which proposed simple random‐phase addition of multipath signals as the mechanism determining spreading loss, rather than the usually accepted cylindrical spreading. Interpretation of experimental attenuation data using the new expression results in lower coefficient values that those of Thorp, Browning et al., based on the same data. Earlier studies involved eye‐fitting to published plots, and least‐squares fits to calculated propagation data produced by Weinberg's generic model. The present paper compares “old and new” attenuation model analyses of actual raw data, as planned for Part II, but made possible only in July 1979, with obtaining of the original records. [Work supported by the Naval Underwater Systems Center.]

Hypersonic attenuation at low temperatures in Al2O3

Abstract A numerical calculation is undertaken to determine the anharmonic attenuation of both transverse modes and the longitudinal acoustic mode propagating along the a axis in Al2O3. This calculation takes full account of crystal anisotropy and the anharmonic coupling term and, in addition, finite thermal phonon lifetimes are used throughout. It is shown that the attenuation of the longitudinal mode by interactions with longitudinally polarized thermal phonons (Fitzgerald and Silverman 1967) dominates by a large factor the attenuation due to interactions with transversely polarized thermal phonons (Kalejs, Maris and Truell 1966). Good agreement with experimental attenuation data is found for the slow transverse and longitudinal modes, but considerable discrepancies are found to exist for the fast transverse mode. The effect of the wave intensity on the energy-conserving Landau and Rumer (1937) type of process is investigated and found to be of significance for acoustic intensities ∼10−3 watts cm−2 at t...