Variation Of Attenuation Coefficient Research Articles

Attenuation and dispersion phenomena of torsional waves in self-weighted, inhomogeneous, pre-stressed poro-elastic and poro-viscoelastic stratified structure

ABSTRACT This paper studies torsional wave propagation in a poro-viscoelastic medium of finite thickness. The considered medium is clamped between an inhomogeneous poro-elastic medium of finite thickness and a poro-elastic half-space. Both finite layers are initially stressed and the half-space is self-weighted. Classical dynamical coupled theories have been used to explore the problem. Interior deformations are calculated analytically. A complex frequency equation has been obtained with the aid of suitable boundary conditions. The decomposition of the complex frequency equation into real and imaginary parts provides dispersion and damping equations. These frequency equations exhibited their reliance on the variation of attenuation coefficient, dissipation factor, inhomogeneity, initial stresses, Biot's gravity, porosity and thickness ratio parameters. The impacts of the aforesaid parameters have been extensively studied and demonstrated graphically.

Variations of the Frequency-Dependent Attenuation Coefficient with the Bone Mineral Density and the Architectural Parameters in Bovine Trabecular Bone

The present study aims to investigate the variations of the frequency-dependent attenuation coefficient with the bone mineral density (BMD) and two architectural parameters, the bone volume fraction (BV/TV) and the trabecular thickness (Tb.Th), in bovine trabecular bone in vitro. The attenuation coefficient was measured over a frequency range from 0.2 to 0.6 MHz in 22 bovine femoral trabecular bone samples. The frequency variation of the attenuation coefficient was characterized by using a nonlinear power law, α(f) = α0 + α1fn, at frequencies between 0.2 and 0.6 MHz. The parameters α (the attenuation coefficient at 0.4 MHz) and α1 exhibited high correlations with the BMD and the BV/TV and relatively low correlations with the Tb.Th. The parameter α1 was found to have weak correlations with the BMD and the Tb.Th and a very weak correlation with the BV/TV. In contrast, the exponent n showed no correlations with the BMD and the architectural parameters. These results suggest that measurements of the parameters α and α1 in trabecular bone may be useful for the assessment of bone status and osteoporosis. A scattering model based on velocity fluctuations in a binary mixture (cortical matrix and marrow fat) was successfully applied to predict the variations of the frequency-dependent attenuation coefficient with the architectural parameters observed in the 22 bone samples.

Three-dimensional linearized stability analysis of Burnett equations for a monatomic gas

Burnett equations were originally derived in 1935 by Burnett by employing the Chapman-Enskog expansion to Classical Boltzmann equation to second order in Knudsen number Kn. Since then several variants of these equations have been proposed in the literature; these variants have differing physical and numerical properties. In this papers, we considered four such variants which are known in the literature as ‘the Original Burnett (OB) equations, the Conventional Burnett (CB)'equations, ‘the Augmented Burnett (AB)’ equations and the recently formulated by the authors ‘the Simplified Conventional (SCB) equations.’ One of the most important issues in obtaining numerical solutions of the Burnett equations is their stability under small perturbations. In this paper, we perform the linearized stability (known as the Bobylev Stability) analysis of three-dimensional Burnett equations for all the four variants (OB, CB, AB, and SCB) as far as the authors are aware for the first time in the literature on this subject. By introducing small perturbations in the steady state flow field, the trajectory curve and the variation in attenuation coefficient with wave frequency of the characteristic equation are obtained for all four variants of Burnett equations to determine their stability. The results show that the 3-D Augmented Burnett (AB) equations and the Simplified Conventional Burnett (SCB) equations are unconditionally stable under small wavelength perturbations. However, the Original Burnett (OB) and the Conventional Burnett (CB) equations are unstable when the Knudsen number becomes greater than a critical value and the stability condition worsens in 3-D when compared to the stability condition for 1-D and 2-D equations. The critical Knudsen number for 3-D OB and CB equations is 0.061 and 0.287 respectively. It should be noted that although both AB and SCB equations are unconditionally stable, SCB equations are much simpler to use numerically compared to AB equations without compromising accuracy.

Analysis of serial and parallel cascaded microring resonators: An FDTD approach

In this paper, we describe coupled microring resonators in serial and parallel fashion based on the mathematical analysis in terms of electric field equations at different ports of a microring resonator. The mathematical equations are supported by simulation results of the modelling of coupled resonators using finite difference time domain method. The variation of coupling coefficients between the ring and the bus waveguide is studied. Variation of attenuation coefficients with transmission is also analyzed. These integrated coupled ring resonators can be useful for routing applications as the route of the electric field propagation can be varied to different ports at different wavelengths. Also, the values of Q factor and on – off ratio achieved signify better performance of the modelled devices.

Monte Carlo calculation of shielding properties of newly developed heavy concretes for megavoltage photon beam spectra used in radiation therapy

Introduction: Globally, the need for radiotherapy as a part of cancer management increases every year. Thus, the shielding for megavoltage radiotherapy rooms is of great importance. Materials and Methods: In the present study, 14 types of developed high-density concrete with densities ranging from 2.45 to 5.11 were simulated by using Monte Carlo method. The linear attenuation coefficient and the tenth value layer were also calculated. These dosimetric parameters were investigated for megavoltage photon beam spectra for various energies (4, 6, 10, 15, and 18 MeV) of the Varian linac and 60Co gamma rays. The results of simulation were compared with the available published results. Results: The results showed that the attenuation of high-energy photons is primarily administered by the atomic number and density of the concrete. Moreover, the variation of attenuation coefficient with density was not completely linear. Conclusion: It was concluded that the attenuation of high-energy photons not only depends on the density of concrete, but also on the atomic number of its composing elements. dependent on density of concrete but also on atomic number of composing elements.

Surface wave attenuation in the shallow subsurface from multichannel–multishot seismic data: a new approach for detecting fractures and lithological discontinuities

Surface wave analysis generally neglects amplitude information, instead using phase information to delineate near-surface S-wave velocity structures. To effectively characterize subsurface heterogeneities from amplitude information, we propose a method of estimating lateral variation of attenuation coefficients of surface waves from multichannel–multishot (multifold) seismic data. We extend the concept of the common midpoint cross-correlation method, used for phase velocity estimation, to the analysis of attenuation coefficients. Our numerical experiments demonstrated that when used together, attenuation coefficients and phase velocities could characterize a lithological boundary as well as fracture zone. We applied the proposed method to multifold seismic reflection data acquired in Shikoku Island, Japan. We clearly observed abrupt changes in lateral variation of estimated attenuation coefficients around fault locations associated with a lithological boundary and with well-developed fractures, whereas phase velocity results could detect only the lithological boundary. Our study demonstrated that simultaneous interpretation of attenuation coefficients and phase velocities has the potential to distinguish localized fractures from lithological boundaries.

A study on photon attenuation coefficients of different wood materials with different densities

A study on the variation of linear attenuation coefficients with the densities of the wood samples is under taken. The soft wood and hard wood samples were collected from the forest area of Pakal in Warangal district. The linear and mass attenuation coefficients are measured using gamma ray spectrometry based on NaI (Tl) scintillation detector with energies of 662 KeV and 59.5 KeV respectively. The mass attenuation coefficient values measured from experiment and are compared with theoretical methods using XCOM program. The plots of density versus linear attenuation coefficient for different wood materials correspond to higher order polynomial are presented. It is observed that variation of linear attenuation coefficient depends on densities of materials. The Chloroxylon swietenia with more density has more linear attenuation coefficient at 59.5 KeV and 662 KeV. The variation in attenuation coefficient attributed to chemical composition of wood used in the experiment.

Variation of diffuse attenuation coefficient of downwelling irradiance in the Arctic Ocean

The diffuse attenuation coefficient (K d) for downwelling irradiance is calculated from solar irradiance data measured in the Arctic Ocean during 3rd and 4th Chinese National Arctic Research Expedition (CHINARE), including 18 stations and nine stations selected for irradiance profiles in sea water respectively. In this study, the variation of attenuation coefficient in the Arctic Ocean was studied, and the following results were obtained. First, the relationship between attenuation coefficient and chlorophyll concentration in the Arctic Ocean has the form of a power function. The best fit is at 443 nm, and its determination coefficient is more than 0.7. With increasing wavelength, the determination coefficient decreases abruptly. At 550 nm, it even reaches a value lower than 0.2. However, the exponent fitted is only half of that adapted in low-latitude ocean because of the lower chlorophyll-specific absorption in the Arctic Ocean. The upshot was that, in the case of the same chlorophyll concentration, the attenuation caused by phytoplankton chlorophyll in the Arctic Ocean is lower than in low-latitude ocean. Second, the spectral model, which exhibits the relationship of attenuation coefficients between 490 nm and other wavelength, was built and provided a new method to estimate the attenuation coefficient at other wavelength, if the attenuation coefficient at 490 nm was known. Third, the impact factors on attenuation coefficient, including sea ice and sea water mass, were discussed. The influence of sea ice on attenuation coefficient is indirect and is determined through the control of entering solar radiation. The linear relationship between averaging sea ice concentration (ASIC, from 158 Julian day to observation day) and the depth of maximum chlorophyll is fitted by a simple linear equation. In addition, the sea water mass, such as the ACW (Alaskan Coastal Water), directly affects the amount of chlorophyll through taking more nutrient, and results in the higher attenuation coefficient in the layer of 30–60 m. Consequently, the spectral model of diffuse attenuation coefficient, the relationship between attenuation coefficient and chlorophyll and the linear relationship between the ASIC and the depth of maximum chlorophyll, together provide probability for simulating the process of diffuse attenuation coefficient during summer in the Arctic Ocean.

Attenuation of seismic waves in the lithosphere of the northern part of the Basin and Range Province

Attenuation of seismic waves in the lithosphere of the northern part of the Basin and Range Province

Implementation of Histogram Based Soft-Tissue Segmentation for Single Spiral Transmission Scanning in Whole Body PET

3D continuous emission and spiral transmission (CEST) scanning provides a high-throughput whole-body PET study by using two dedicated detectors for 3D emission and singles transmission, together with continuous bed movement. To supress the amount of transmission scatter components (TSC) in singles transmission data, the transmission detector was designed to have a short axial extent with a highly collimated <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">137</sup> Cs point source, and to remove the amount of emission contamination (EC) in post-injection scanning, real-time EC correction was implemented. However, transmission images can be still affected by residual EC and TSC, depending on patient size and injected dose. This produces slight variations in attenuation coefficients, depending on the patient's axial and radial positions. In this study, we developed a new soft-tissue segmentation (STS) method based on histogram scaling at each axial position of the spiral transmission. Peaks, corresponding to soft-tissue in a histogram of attenuation coefficients, were found at each axial position and the transmission image was scaled using the ratio of soft-tissue histogram peaks to the theoretical water attenuation coefficient. In scaled transmission images, pixel values near soft-tissue peaks were replaced with their theoretical water attenuation coefficients. Quantitative evaluation of the transmission images obtained was performed under various acquisition conditions, both with and without the proposed STS method. Final imaging performance evaluations included quantification of emission images reconstructed using both STS attenuation correction and hybrid scatter correction. Results showed that the proposed STS method for spiral transmission scanning provided quantitative images that contained activity, independent of object size.

Velocity Dispersion and Amplitude Attenuation of Rayleigh Waves Across the Philippine Sea

The fundamental-mode interstation group and phase velocities and attenuation coefficients of Rayleigh waves along four paths across the Philippine Sea are determined to have periods of between 20 and 110 seconds by the two-station method. The values of group and phase velocities are quite consistent with the results of previous studies using the single-station method. Group and phase velocities vary significantly with different wave-paths at periods of less than 50 seconds, but for longer periods, the phase velocities are quite close. The inversion results show that the thickness of the lithosphere is about 60 km in the Philippine Sea, except in the area of the eastern volcanic islands where a slightly thinner lithosphere (about 50 km) is deduced. The attenuation coefficients observed in the Philippine Sea are higher than the values obtained in the Pacific. This result simply reflects that the Philippine Sea plate is younger than that of the Pacific. Lateral variation in attenuation coefficient is not as clear as it is in velocity because of its large variance. However, the vertical distribution of Qβ-1 is quite similar to that for the shear velocity model.

Process monitoring of moulding compounds by ultrasonic measurements in a compression mould

Current research addresses ultrasonic quality sensor utilisation for inline cure monitoring, to control and analyse compression moulding. In this investigation, an ultrasonic impulse transmission method was applied to directly monitor changes in the mechanical properties of moulding compounds throughout the whole cure cycle. These measurements were performed during the processing using transducers which were designed to operate at elevated temperature and pressure in a compression mould. Measurements of ultrasound velocity and variation of attenuation coefficient of longitudinal waves were performed following the cure cycle. Ultrasonic measurements were used to investigate the influence of different types of moulding compounds, artificial weathering at elevated temperatures, filler volume, moisture content as well as amount of hardener on the curing process.

AAPM tutorial. CT image detail and noise.

Two important characteristics of the computed tomographic (CT) image that affect the ability to visualize anatomic structures and pathologic features are blur and noise. Increased blurring reduces the visibility of small objects (image detail); increased visual noise reduces the visibility of low-contrast objects. Sources of blurring in CT include the size of the sampling aperture (which can be regulated by the focal spot size and the detector size), the size of the voxels, and the reconstruction filter selected. Noise is caused by the variation in attenuation coefficients between voxels. Use of small voxels and edge-enhancing filters helps reduce blurring and improve visibility of fine details. However, small voxels absorb fewer photons and therefore result in increased noise. Noise can be reduced by using large voxels, increasing radiation dose, or using a smoothing filter, but this filter increases blurring. An optimized protocol for a specific clinical study must take these physical principles into account and be adjusted to give proper balance among detail, low noise, and patient exposure.

Relative Spectral Attenuation Coefficients of Water

The relative variation of attenuation coefficient with wavelength has been studied in the laboratory for distilled water and seawater. The study was for the purpose of locating any very fine structure in the attenuation vs wavelength curve. The spectral region from 3750 A to 6850 A was examined with 0.2 A resolution; water paths were 9.7 m and 19.5 m. Studies of the microphotometer traces show no discernible very fine structure in either distilled water or seawater. The limit of detection was set by plate grain and corresponded to a variation of 0.1 x 10(-4) /cm in the attenuation coefficient. Curves of relative coefficients vs wavelength show very shallow unstructured peaks at about 4700 A, 5150 A, and 5500 A, but these are relatively broad ( approximately 50 A).