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Search Results: 1 - 10 of 594 matches for " attenuation. "
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Temperature Dependence of Density and Thermal Expansion of Wrought Aluminum Alloys 7041, 7075 and 7095 by Gamma Ray Attenuation Method  [PDF]
Kethireddy Narender, Ammiraju Sowbhagya Madhusudhan Rao, Kalvala Gopal Kishan Rao, Nallacheruvu Gopi Krishna
Journal of Modern Physics (JMP) , 2013, DOI: 10.4236/jmp.2013.43045
Abstract:

The gamma quanta attenuation studies have been carried out to determine mass attenuation coefficients of 7041, 7075 and 7095 wrought aluminum alloys. The temperature dependence of linear attenuation coefficient, density and thermal expansion of these wrought aluminum alloys in the temperature range 300 K - 850 K have been reported. The measurements were done by using a gamma ray densitometer designed and fabricated in our laboratory. The data on variation of density and linear thermal expansion with temperature have been represented by linear equations. Volume thermal expansion coefficients have been reported.

An Investigation of the Required MR Bone Attenuation Correction for Quantitative Whole-Body PET/MR Imaging Using Clinical NaF PET/CT Studies  [PDF]
Hua A. Ai, Osama Mawlawi, R. Jason Stafford, Jim Bankson, Yiping Shao, Michele Guindani, Richard E. Wendt III
International Journal of Medical Physics,Clinical Engineering and Radiation Oncology (IJMPCERO) , 2018, DOI: 10.4236/ijmpcero.2018.73023
Abstract: Tissue-classification-based attenuation correction strategies have been previously proposed to correct for bone attenuation in PET/MR imaging and simulated using computed tomography. However, the complication of voxel averaging uniquely associated with bone has not been considered explicitly in the past. This study investigated the effect of voxel averaging between bone and soft tissue in attenuation images and determined how accurately bone must be detected in MR images in order to perform acceptable attenuation correction of PET data by using CT-simulated attenuation correction. We found out that treating bone as soft tissue caused a mean quantification difference of -9.9% ± 5.5% in all 119 bone lesions. There were no significant differences between lesions in the pelvis and the vertebrae. The nominal difference in lesions in the ribs was significantly lower, likely due to the spatial misregistration between the emission and attenuation images. Interestingly, a non-monotonic relationship between the bone imaging ability and the absolute PET quantification accuracy was observed, with the minimal quantification difference achieved at a BVF around 40% for skull lesions (2.6% ± 2.1%), and 30% for non-skull lesions (1.4% ± 1.1%) and all lesions (1.5% ± 1.3%). This study established that a bone classification sensitivity of approximately 30% BVF is required in order for MR-based attenuation correction methods to achieve optimal quantification in whole-body PET/MR studies. For this purpose, higher bone imaging ability of MR may not be necessary.
The Effects of Power Control on Free-Space Optical Communications during Snowfall and Rainfall  [PDF]
Salem Salamah, Muhammad A. Alsubaie, Mubarak Alhajri, Mahmoud Alnaser, Ahmed Mohamed Abdalla
Int'l J. of Communications, Network and System Sciences (IJCNS) , 2018, DOI: 10.4236/ijcns.2018.1110013
Abstract: Free-space optical (FSO) communication requires a line-of-sight connection between a transmitter and a receiver in which the information signal is modulated by an optical carrier that propagates in free space. The FSO channel is greatly affected by weather conditions such as fog, rain, and snow. In the literature, several adaptive techniques, such as power control (PC), have been suggested to mitigate channel link degradations. In this paper, we investigate the effects of snow and rain attenuation on the bit error rate (BER) of the FSO system using two types of modulations, the on-off keying (OOK) modulation and the pulse-position modulation (16-PPM). The effect of PC on the performance of FSO communications is also examined in this study. We evaluated the system’s performance with two types of snow, wet snow and dry snow, as well as with different rain regions. Results show that PC improves the BER of the FSO system; a high rate of improvement is found for wet snow and rain. PC has almost no effect with dry snow because of the high attenuation and the limitations on transmitted power. The BER for 16-PPM is better than that for OOK modulation.
Mechanisms of geometrical seismic attenuation
Igor B. Morozov
Annals of Geophysics , 2011, DOI: 10.4401/ag-4780
Abstract: In several recent reports, we have explained the frequency dependence of the apparent seismic quality-factor (Q) observed in many studies according to the effects of geometrical attenuation, which was defined as the zero-frequency limit of the temporal attenuation coefficient. In particular, geometrical attenuation was found to be positive for most waves traveling within the lithosphere. Here, we present three theoretical models that illustrate the origin of this geometrical attenuation, and we investigate the causes of its preferential positive values. In addition, we discuss the physical basis and limitations of both the conventional and new attenuation models. For waves in media with slowly varying properties, geometrical attenuation is caused by variations in the wavefront curvature, which can be both positive (for defocusing) and negative (for focusing). In media with velocity/density contrasts, incoherent reflectivity leads to geometrical-attenuation coefficients which are proportional to the mean squared reflectivity and are always positive. For coherent reflectivity, the geometrical attenuation is approximately zero, and the attenuation process can be described according to the concept of scattering Q . However, the true meaning of this parameter is in describing the mean reflectivity within the medium, and not that of the traditional resonator quality factor known in mechanics. The general conclusion from these models is that non-zero and often positive levels of geometrical attenuation are common in realistic, heterogeneous media, both observationally and theoretically. When transformed into the conventional Q-factor form, this positive geometrical attenuation leads to Q values that quickly increase with frequency. These predictions show that the positive frequency-dependent Q observed in many datasets might represent artifacts of the transformations of the attenuation coefficients into Q.
Rain Attenuation Impact on Performance of Satellite Ground Stations for Low Earth Orbiting (LEO) Satellites in Europe  [PDF]
Shkelzen CAKAJ
Int'l J. of Communications, Network and System Sciences (IJCNS) , 2009, DOI: 10.4236/ijcns.2009.26052
Abstract: Low Earth Orbits (LEO) satellites are used for public communication and for scientific purposes. These satellites provide opportunities for investigations for which alternative techniques are either difficult or impossible to apply. Ground stations have to be established in order to communicate with such satellites. Usually these satellites communicate with ground stations at S-band. The communication quality depends on the performance of the satellite ground station, in addition to that of satellite. The performance of the satellite ground stations is expressed through Figure of Merit. The aim of this paper is to analyze the rain attenuation impact on the performance of the respective ground station. Rain attenuation depends on geographical location where the satellite ground station is implemented. In order to compare this effect on satellite ground station performance, some cities of Europe are considered. Finally, the rain attenuation impact on the satellite ground station Figure of Merit for the hypothetical satellite ground station installed in Prishtina is analyzed.
Surface Wave Characteristics at the Interface of Welded Elastic Halfspaces  [PDF]
J. N. Sharma, K. K. Sharma, Ashwani Kumar
Open Journal of Acoustics (OJA) , 2011, DOI: 10.4236/oja.2011.11001
Abstract: The present article concentrates on the propagation of generalized surface acoustic waves in a composite struc- ture consisting of piezoelectric and non-piezoelectric semiconductor media. The mathematical model of the problem is depicted by a set of partial differential equations of motion, Gauss equation in piezoelectric and elec- tron diffusion equation in semiconductor along with boundary conditions to be satisfied at the interface. The secular equation that governs the propagation of surface waves has been derived in compact form after obtaining the formal solution. The analytic expressions for displacements, stresses, piezoelectric potential and electron concentration during the surface wave propagation at the interface have also been obtained. The numerical solu- tion of the secular equation is carried out for the cadmium selenide and silicon composite by employing fixed point functional iteration numerical method along with irreducible Cardano method. The computer simulated results with the help of MATLAB software in respect of dispersion curves, attenuation coefficient, displace- ments, stresses, carrier concentration and piezoelectric potential are presented graphically. This work may be useful in surface acoustic wave (SAW) devices and electronic industry.
Sensitivity of Nanostructured Iron Metal on Ultrasonic Properties  [PDF]
Alok Kumar Gupta, Archana Gupta, Devraj Singh, Sudhanshu Tripathi
Open Journal of Metal (OJMetal) , 2011, DOI: 10.4236/ojmetal.2011.12005
Abstract: The present investigation is focused on the influence of the nanocrystalline structure of pure iron metal on the ultrasonic properties in the temperature range 100 - 300 K. The ultrasonic attenuation due to phonon- phonon interaction and thermoelastic relaxation phenomena has been evaluated for longitudinal and shear waves along <100>, <110> and <111> crystallographic directions. The second-and third-order elastic constants, ultrasonic velocities, thermal relaxation, anisotropy and acoustic coupling constants were also com- puted for the evaluation of ultrasonic attenuation in this temperature scale. The direction <111> is most ap- propriate to study longitudinal sound waves, while <100>, <110> direction are best to propagate shear waves due to lowest values of attenuation in these directions. Other physical properties correlated with obtained results have been discussed.
Attenuation of P, S and Coda Waves in the NW-Himalayas, India  [PDF]
Imtiyaz A. Parvez, Preeti Yadav, K. Nagaraj
International Journal of Geosciences (IJG) , 2012, DOI: 10.4236/ijg.2012.31020
Abstract: The frequency-dependent characteristics of P- and S-wave attenuation in the upper crust of NW Himalayas have been estimated using local earthquakes for a frequency range of 1.5 to 18 Hz. A total of 43 local events of magnitude 2.1 - 4.8, mostly from the vicinity of Main Boundary Thrust (MBT) and Main Central Thrust (MCT) have been used in the analysis. The extended coda normalization methods were applied to estimate the quality factors for P- waves (QP) and S-waves (QS) and the single back-scattering model has been used earlier (Kumar et al. [1]) to determine the quality factor for coda waves (QC). The observed quality factors QP and QS is strongly frequency dependent and the estimated average frequency dependent relation is given by QP = (97 ± 3)f (1.06 ± 0.06) and QS = (127 ± 6)f (0.96±0.06) respectively for P- and S-waves. A comparison of QS estimated in this study and QC previously reported shows that QC > QS for entire frequency range. This indicates the enrichment of coda waves and the importance of scattering attenuation to the attenuation of S-waves in the study region infested with faults and fractures. The ratio QS/QP is found to be greater than unity for the entire frequency range indicating that the body waves from source to station paths crossed a crustal volume with dry and rigid rocks. The frequency dependent relations developed in this study can be very useful to ground motion modeling which in turn is required in the seismic hazard assessment of the region.
Air Trapping: A Cause of Heterogeneous Attenuation  [PDF]
Keiko Morikawa, Fumito Okada, Yumiko Ando, Asami Ono, Hiromu Mori
Open Journal of Radiology (OJRad) , 2012, DOI: 10.4236/ojrad.2012.23017
Abstract: Purpose: To investigate important factors affecting the frequency of air trapping through observation of normal lung attenuation, and propose a cause of heterogeneous attenuation. Materials and Methods: In this ethical committee-approved study, a total of 109 patients (30 males, 79 females; mean age, 58.2 years; range, 27 - 81 years) were included. All patients had undergone inspiratory and expiratory chest thin-section computed tomography (CT) examinations and pulmonary function tests. Air trapping on CT images was graded subjectively. Hounsfield units (HU) lung attenuation value and lung volume were measured on CT images. All variables (age, sex, indices of pulmonary function test results, air trapping score, HU value, and rate of change in lung volume) were compared by diagnoses and air trap- ping findings cohorts. The correlation between lung function test results and expiratory HU attenuation were analyzed. Results: Interstitial pneumonia showed higher and bronchiolitis obliterans showed lower HU attenuation at normal and air trapping regions. The variables affecting air trapping findings were age, a ratio of forced expiratory volume in 1 second to forced vital capacity (FEV1.0/FVC), maximal expiratory flow at 50% of forced vital capacity (MEF50), HU attenuation at normal regions, and rate of change in lung volume. Compared with expiratory HU attenuation, significant positive correlation was shown to FEV1.0/FVC and negative correlation to single-breath diffusion capacity for carbon monoxide, which was confirmed by a simple regression analysis. Conclusion: It can be suggested that lung attenuation can increase when fibrosis is advanced, and this is exaggerated when lungs are compressed.
Shielding Properties of Lead Zinc Borate Glasses  [PDF]
Samir Yousha El-Kameesy, Sahar Abd El-Ghany, Moenis Abd El-Hakam Azooz, Yaser Abd Allah El-Gammam
World Journal of Condensed Matter Physics (WJCMP) , 2013, DOI: 10.4236/wjcmp.2013.34033
Abstract:

In the present work, ZnO·PbO·B2O3 glasses are prepared to be used as g-ray shielding materials. The attenuation properties of these glasses with different concentrations of PbO (20% - 70%) have been investigated at photon energies 662, 1173, 1332 and 2614 keV. The optical absorption spectra of some glass samples have been measured from 200 to 1100 nm before and after g-ray irradiation. The analyses of these spectra have been interpreted and discussed.

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