In thermoelasticity both the second harmonic of the temperature fluctuations and the thermoelastic constant can be used to measure, without contact, residual stress on mechanical components. In this ...paper the combined effect of temperature mean level and constant stress field are analyzed, to better explain and develop a new analytical formulation of the relevant physical phenomena in this complex field of solid mechanics. Second harmonic measurements are then presented for low-yield stress materials: results show that thermoelastic stress analysis can be applied for constant stress measurement on low-yield stress aluminum alloy specimens, whereas for tested plastic materials, the measurement technique cannot be applied.
Detailed and accurate information on the neutron background is relevant for many applications that involve radiation detection, both for non-coincidence and coincidence countings. In particular, for ...the purpose of developing advanced neutron-detection techniques for nuclear non-proliferation and nuclear safeguards, the energy-dependent, ground-level, neutron-background information is needed. There are only a few previous studies available about the neutron background below 10 MeV, which is a typical neutron energy range of interest for nuclear non-proliferation and nuclear-safeguards applications. Thus, there is a potential for further investigation in this energy range. In this paper, neutron-background measurement results using organic-liquid scintillation detectors are described and discussed, with a direct application in optimization simulations of a fast-neutron imager based on liquid scintillators. The measurement was performed in summer 2011 in Ann Arbor, Michigan, USA, and the measurement setup consisted of several EJ-309 liquid scintillators and a fast waveform digitizer. The average neutron flux below 6 MeV was measured to be approximately 4e-4 counts/cm2/s. In addition, the relationship between the neutron-background count rate and various environmental quantities, such as humidity, at Earth's ground level was investigated and the results did not reveal any straightforward dependences. The measured pulse height distribution (PHD) was unfolded to determine the energy spectrum of the background neutrons. The unfolded neutron-background spectrum was implemented to a previously-created MCNPX-PoliMi model of the neutron-scatter camera and simple-backprojection images of the background neutrons were acquired. Furthermore, a simulated PHD was obtained with the MCNPX-PoliMi code using the "Cosmic-Ray Shower Library" (CRY) source sub-routine which returns various types of radiation, including neutrons and photons at a surface, and accounts for solar cycle, altitude, and geomagnetic deflection. Comparison between the measured and simulated PHDs shows very good agreement.
Purpose:
To develop an accurate and fast Monte Carlo (MC) method of simulating CT that is capable of correlating dose with image quality using voxelized phantoms.
Methods:
A realistic voxelized ...phantom based on patient CT data, XCAT, was used with a GPU accelerated MC code for helical MDCT. Simulations were done with both uniform density organs and with textured organs. The organ doses were validated using previous experimentally validated simulations of the same phantom under the same conditions. Images acquired by tracking photons through the phantom with MC require lengthy computation times due to the large number of photon histories necessary for accurate representation of noise. A substantial speed up of the process was attained by using a low number of photon histories with kernel denoising of the projections from the scattered photons. These FBP reconstructed images were validated against those that were acquired in simulations using many photon histories by ensuring a minimal normalized root mean square error.
Results:
Organ doses simulated in the XCAT phantom are within 10% of the reference values. Corresponding images attained using projection kernel smoothing were attained with 3 orders of magnitude less computation time compared to a reference simulation using many photon histories.
Conclusion:
Combining GPU acceleration with kernel denoising of scattered photon projections in MC simulations allows organ dose and corresponding image quality to be attained with reasonable accuracy and substantially reduced computation time than is possible with standard simulation approaches.
The cosmic-ray-induced neutron background at ground level has been measured and simulated in conjunction with EJ-309 organic liquid scintillators with an approximate deposited energy range of ...0.5–6MeV. Specifically, the pulse height distributions, net neutron count rates, and angular dependences were obtained. The simulations were carried out using the Monte Carlo transport code MCNPX-PoliMi combined with the (Cosmic-Ray Shower Generator) CRY source subroutine that returns secondary particles produced by cosmic rays. A scaling formula from literature was also implemented in the simulation. The angular dependence of the neutron count rate was measured by collimating the liquid scintillator with polyethylene to attain 18° angular resolution from 0° downwards to 72° horizontally. The neutron count rate was measured to be 23.10±1.69h−1sr−1 at 0°, and 7.20±0.78h−1sr−1 at 72°. The simulations and measurements compare well and show similar cosine anisotropy for the angular distribution. The study thus shows that the neutron background response in detector systems can be efficiently and accurately simulated using the procedures described.
This work deals with the experimental analysis of wake losses fluctuations at full-scale wind turbines. The test case is a wind farm sited on a moderately complex terrain: 4 turbines are installed, ...having 2 MW of rated power each. The sources of information are the time-resolved data, as collected from the OPC server, and the 10-minutes averaged SCADA data. The objective is to compare the statistical distributions of wake losses for far and middle wakes, as can be observed through the "fast" lens of time-resolved data, for certain selected test-case time series, and through the "slow" lens of SCADA data, on a much longer time basis that allow to set the standards of the mean wake losses along the wind farm. Further, time-resolved data are used for an insight into the spectral properties of wake fluctuations, highlighting the role of the wind turbine as low-pass filter. Summarizing, the wind rose, the layout of the site and the structure of the data sets at disposal allow to study middle and far wake behavior, with a "slow" and "fast" perspective.
This work is part of a research project funded by the Italian Ministry of the University and Research (MIUR), under the call for "National Interest Research Projects 2015 (PRIN 2015)", titled "Smart ...Optimized Fault Tolerant WIND turbines (SOFTWIND)".
Within this project, the research unit of the University of Perugia (UniPG) aims to develop dynamic modeling and simulation methodologies and fatigue behavior evaluation ones for wind turbine as a whole. The development of these methodologies will be aimed at predicting the life of generic wind turbines, also providing important and fundamental parameters for optimizing their control, aimed at reducing the failures of these machines.
In the present paper, a small turbine, developed at the Department of Engineering of the University of Perugia, will be analyzed. The multibody modeling technique adopted and the experimental activity conducted in the wind tunnel of UniPG, needed for the tuning of the model, will be described.
The analysis of both model behavior and experimental data has allowed for the definition of a robust multibody modeling technique that adopts a freeware code (NREL - FAST), universally considered to be a reference in this field.
The goodness of the model guarantees the capabilities of the simulation environment to analyze the real load scenario and the fatigue behavior of this kind of device.
Purpose:
To substantiate the interdependency of contrast dose, radiation dose, and image quality in CT towards the patient‐ specific optimization of the imaging protocols
Methods:
The study deployed ...two phantom platforms. A variable sized (12, 18, 23, 30, 37 cm) phantom (Mercury‐3.0) containing an iodinated insert (8.5 mgI/ml) was imaged on a representative CT scanner at multiple CTDI values (0.7–22.6 mGy). The contrast and noise were measured from the reconstructed images for each phantom diameter. Linearly related to iodine‐concentration, contrast‐to‐noise ratio (CNR), were calculated for 16 iodine‐concentration levels (0–8.5 mgI/ml). The analysis was extended to a recently developed suit of 58 virtual human models (5D XCAT) with added contrast dynamics. Emulating a contrast‐enhanced abdominal image procedure and targeting a peak‐enhancement in aorta, each XCAT phantom was “imaged” using a simulation platform (CatSim, GE). 3D surfaces for each patient/size established the relationship between iodine‐concentration, dose, and CNR. The ratios of change in iodine‐concentration versus dose (IDR) to yield a constant change in CNR were calculated for each patient size.
Results:
Mercury phantom results show the image‐quality size‐ dependence on CTDI and IC levels. For desired image‐quality values, the iso‐contour‐lines reflect the trade off between contrast‐material and radiation doses. For a fixed iodine‐concentration (4 mgI/mL), the IDR values for low (1.4 mGy) and high (11.5 mGy) dose levels were 1.02, 1.07, 1.19, 1.65, 1.54, and 3.14, 3.12, 3.52, 3.76, 4.06, respectively across five sizes. The simulation data from XCAT models confirmed the empirical results from Mercury phantom.
Conclusion:
The iodine‐concentration, image quality, and radiation dose are interdependent. The understanding of the relationships between iodine‐concentration, image quality, and radiation dose will allow for a more comprehensive optimization of CT imaging devices and techniques, providing the methodology to balance iodine‐concentration and dose based on patient's attributes.
Measurements and simulations of the cosmic-ray-induced neutron background Becchetti, M. F.; Flaska, M.; Clarke, S. D. ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
11/2014, Letnik:
777, Številka:
C
Journal Article
Recenzirano
Odprti dostop
The cosmic-ray-induced neutron background at ground level has been measured and simulated in conjunction with EJ-309 organic liquid scintillators with an approximate deposited energy range of 0.5–6 ...MeV. Specifically, the pulse height distributions, net neutron count rates, and angular dependences were obtained. The simulations were carried out using the Monte Carlo transport code MCNPX-PoliMi combined with the (Cosmic-Ray Shower Generator) CRY source subroutine that returns secondary particles produced by cosmic rays. A scaling formula from literature was also implemented in the simulation. The angular dependence of the neutron count rate was measured by collimating the liquid scintillator with polyethylene to attain 18° angular resolution from 0° downwards to 72° horizontally. The neutron count rate was measured to be 23.10±1.69 h-1 sr-1 at 0°, and 7.20±0.78 h-1 sr-1 at 72°. The simulations and measurements compare well and show similar cosine anisotropy for the angular distribution. Finally, the study thus shows that the neutron background response in detector systems can be efficiently and accurately simulated using the procedures described.
Nanocrystalline semiconductor detectors comprised of either cadmium or lead chalcogenide nanoparticles have resulted in excellent energy resolution. For instance, a blended assembly of 4 nm PbSe ...star-shaped particles, has yielded an energy resolution of 0.42 % (1.5 keV) at 356 keV, compared with a resolution of 0.96 % (3.4 keV) for CZT and 0.39 % (1.4 keV) for HPGe. The spectra are characterized however, by x-ray escape features due to the high probability of photoelectric absorption in the lead-based assembly, as well as the high x-ray energies associated with high Z materials. In this paper, we simulate the spectral response of a composite detector composed of PbSe and para-MEH-PPV in order to aid in the identification of the various features.
PDF
