Ideal Point Source Research Articles

A simple acoustic model to characterize the internal sound field in centrifugal pumps originated by blade‐tongue interaction

Conventional centrifugal pumps with volute casing generate fluid‐dynamic noise particularly at the so‐called blade‐passing frequency, due to the interaction of the flow exiting the pump impeller with the volute tongue. The amplitude of the sound generated is very dependent on the pump operating point. Following previous studies by the authors, a methodology has been applied to quantify the generation of tonal noise for a given centrifugal pump, previously tested in laboratory. The procedure is based on a simple acoustic model for the pump, in which one or several ideal point sources are located at some arbitrary position in the volute. These ideal sources are assumed to radiate plane sound waves along the volute, which was considered to be composed by a succession of slices, each of them equivalent to a linear three‐port acoustic system with sound transmission and reflexion coefficients according to the corresponding port areas. A series of tests was conducted to check the assumptions of the acoustic model, by applying external acoustic loads onto the pump outlet duct and measuring the noise reflected. The resulting reflection coefficient was in good agreement with the predictions of the acoustic model.

Quantitative evaluation of discrete failure events in composites using infrared imaging and acoustic emission

We describe the successful synchronization of thermal imaging with acoustic emission to observe and characterize for the first time the thermal responses of discrete fiber breaks and matrix cracks in composite panels under load in real time. These events can be accurately described by ideal buried point and line thermal sources.

Characterization of neutron reference fields at US department of energy calibration fields

The Health Physics Measurements Group at the Los Alamos National Laboratory (LANL) has initiated a study of neutron reference fields at selected US Department of Energy (DOE) calibration facilities. To date, field characterisation has been completed at five facilities. These fields are traceable to the National Institute for Standards and Technology (NIST) through either a primary calibration of the source emission rate or through the use of a secondary standard. However, neutron spectral variation is caused by factors such as room return, scatter from positioning tables and fixtures, source anisotropy and spectral degradation due to source rabbits and guide tubes. Perturbations from the ideal isotropic point source field may impact the accuracy of instrument calibrations. In particular, the thermal neutron component of the spectrum, while contributing only a small fraction of the conventionally true dose, can contribute a significant fraction of a dosemeter's response with the result that the calibration becomes facility-specific. A protocol has been developed to characterise neutron fields that relies primarily on spectral measurements with the Bubble Technology Industries (BTI) rotating neutron spectrometer (ROSPEC) and the LANL Bonner sphere spectrometer. The ROSPEC measurements were supplemented at several sites by the BTI Simple Scintillation Spectrometer probe, which is designed to extend the ROSPEC upper energy range from 5 to 15 MeV. In addition, measurements were performed with several rem meters and neutron dosemeters. Detailed simulations were performed using the LANL MCNPX Monte Carlo code to calculate the magnitude of source anisotropy and scatter factors.

Negative refraction imaging in a hybrid photonic crystal device at near-infrared frequencies

We present the experimental demonstration of imaging of a point source by negative refraction at near-infrared frequencies using a hybrid photonic crystal device. The photonic crystal device, fabricated by patterning holes in 260nm silicon-on-insulator, integrates a triangular-lattice photonic crystal with a large photonic bandgap and square-lattice photonic crystal with negative refraction. Experimental results show that the output of a line-defect photonic bandgap waveguide provides a nearly ideal point source and then is imaged through the photonic crystal by negative refraction.

Scale invariant forces in one-dimensional shuffled lattices

We present a detailed and exact study of the probability density function P(F) of the total force F acting on a point particle belonging to a perturbed lattice of identical point sources of a power-law pair interaction. The main results concern the large-F tail of P(F) for which two cases are mainly distinguished: (i) Gaussian-like fast decreasing P(F) for lattice with perturbations forbidding any pair of particles to be found arbitrarily close to one each other and (ii) Lévy-like power-law decreasing P(F) when this possibility is instead permitted. It is important to note that in the second case the exponent of the power-law tail of P(F) is the same for all perturbations (apart from very singular cases) and is in a one-to-one correspondence with the exponent characterizing the behavior of the pair interaction with the distance between the two particles.

Analysis of μ-Contact Printed Protein Patterns by SPR Imaging with a LED Light Source

We demonstrate the characterization of mu-contact printed protein patterns and analysis of protein-protein interactions by two-dimensional (2-D) surface plasmon resonance imaging (SPRi). Advancements in SPRi image quality from employing a light emitting diode (LED) as the light source are described. We show that a LED offers an ideal point source that can eliminate interference artifacts and speckles found when using a laser source. The attainable thickness resolution in fixed-angle imaging is comparable to that of a monochromatic source, providing a solid foundation for quantitative analysis with the system. The SPR imaging technique reported here affords sub-nanometer thickness sensitivity and micrometer lateral resolution, allowing for convenient studies of biomolecular interactions and surface morphologies of ultrathin films. Spatially well-defined protein patterns of bacterial toxins were obtained by microcontact printing using a polydimethylsiloxane (PDMS) stamp on a functionalized self-assembled monolayer on Au. The influence of protein concentration in the inking solution on transfer efficiency was investigated, and a nonlinear correlation was observed between the solution concentration and the amount of protein immobilized on the surface. Quantitative analysis of protein interaction was performed with toxin-specific antibody, showing a concentration-dependent relationship that verifies the retention of biological activity of the protein after printing. The study demonstrates the feasibility and effectiveness of using LEDs as light sources in SPR imaging, opening doors for developing compact SPR instruments for direct, sensitive, and label-free detection of biohazardous molecules.

Theory of near-field optical imaging with a single molecule as light source

A fluorescing structure (made of one or several molecules) can be considered as an ideal point source of light for use in a near-field optical microscope [Michaelis et al., Nature 405, 325 (2000)]. This paper discusses how the image–object relationship is made easier to understand by introducing the concept of photon local density of states (LDOS). Specifically, we show that the LDOS determined by Green’s dyadic calculations provide the relevant interpretation of the images recorded with a single fluorescing molecule as source of light. Recent experimental results are analyzed on this basis.

Diffraction tomography for multi-monostatic ground penetrating radar imaging

A generalized diffraction tomographic (DT) algorithm is derived for subsurface imaging from multifrequency multi-monostatic ground penetrating radar (GPR) data. The algorithm is based on the Born approximation for vector electromagnetic scattering and incorporates realistic nearfield models for the receiving and transmitting antennas. The forward scattering model is inverted analytically using the regularized pseudoinverse operator to yield an algorithm for imaging the underground region based on scattered field measurements at a set of receiving antennas. Whereas the usual inversion algorithms of DT require a lossless background medium and ideal point sources and receivers, the algorithm described here allows an attenuating background and arbitrary transmitting and receiving antennas. The algorithm places no restrictions on the radar frequency, and can thus include shallow imaging applications where the wavelengths are on the same order as the depth of buried objects of interest. Versions of the algorithm are given for both the three dimensional and the 2.5-dimensional cases. Results are given of computer simulations designed to test the algorithm.

Design and applications of a scanning SQUID microscope

The scanning SQUID (Superconducting Quantum Interference Device) microscope is an extremely sensitive instrument for imaging local magnetic fields. We describe one such instrument which combines a novel pivoting lever mechanism for coarse-scale imaging with a piezoelectric tube scanner for fine-scale scans. The magnetic field sensor is an integrated miniature SQUID magnetometer. This instrument has a demonstrated magnetic field sensitivity of <10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−6</sup> gauss/√Hz at a spatial resolution of ∼10 µm. The design and operation of this scanning SQUID microscope are described, and several illustrations of the capabilities of this technique are presented. The absolute calibration of this instrument with an ideal point source, a single vortex trapped in a superconducting film, is shown. The use of this instrument for the first observation of half-integer flux quanta, in tricrystal thin-film rings of YBa <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> Cu <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7−δ</inf> , is described. The half-integer flux quantum effect is a general test of the symmetry of the superconducting order parameter. One such test rules out symmetry-independent mechanisms for the half-integer flux quantum effect, and proves that the order parameter in YBa <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> Cu <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7−δ</inf> has lobes and nodes consistent with d-wave symmetry.

High-resolution scanning SQUID microscope

We have combined a novel low temperature positioning mechanism with a single-chip miniature superconducting quantum interference device (SQUID) magnetometer to form an extremely sensitive new magnetic microscope, with a demonstrated spatial resolution of ∼10 μm. The design and operation of this scanning SQUID microscope will be described. The absolute calibration of this instrument with an ideal point source, a single vortex trapped in a superconducting film, will be presented, and a representative application will be discussed.

Diameter measurement of single-mode fiber by using interferometric and imaging techniques

Two methods to measure the cladding diameter of single-mode fibers are presented. The first method is based on an interference fringe measurement technique. Interference fringe spacing at two different planes is measured to determine the cladding diameter of the fiber. The theory of the fringe formation in the interferometeric arrangement using single-mode fibers is discussed. It is theoretically shown that the far-field overlapping Gaussian field distribution from the fibers shifts the position of the fringe maxima and minima. As a special case of unit fringe visibility the minima positions do not shift, whereasthe maxima positions are shifted. In the case of a Lloyd mirror arrangement it is shown that fringes can be obtained from a rough surface as well. A lens is used in the second method to image the two identical point sources that cause the interference. Through the use of the magnification and spacing of the images, the cladding diameter is estimated. It is shown that the accuracy of the fiber cladding-diameter measurement can be enhanced by generating multiple point sources. Consistent results of the fiber cladding diameter have been obtained with the proposed methods. Fiber cladding-diameter measurements with a standard error of less than 0.15 µm can be achieved.

Thermoluminescent dosimetry for 103Pd seeds (model 200) in solid water phantom

Dose measurements using LiF thermoluminescent dosimeters (TLD) have been performed for single 103Pd seeds (model 200) at the center of a solid water phantom. TLD cubes 1 mm on an edge were used for measurements from 1 mm to 1 cm at 1-mm intervals. The cubes were centered along transverse and longitudinal axes and along radial lines from seed center at 10 degrees increments. TLD chips of dimension 3.1 X 3.1 X 0.89 mm were used at distances of 2, 2.5, 3, and 4 cm at 15 degrees angular intervals. Data are presented as the product of distance squared and dose rate per unit source strength, plotted versus distance and angle. At 1 cm from seed center along the transverse axis this product was found to be 0.88 cGy cm2 mCi-1h-1. A dose-rate table in polar coordinates has been formulated for use with multiseed dose distribution calculations. Comparison with data of Meigooni et al. [Endocuriether./Hyperthermia Oncol. 6, 107-117 (1990)] shows general agreement for distances of 2 cm or greater. A comparison of our transverse axis data with Russell's calculated values (Theragenics Internal Report, 4 November 1984) for an ideal point source of 103Pd shows very good agreement except at distances less than 0.5 cm, where differences are attributable to the extended source effect in the actual seed.

Optical Tracking Using Charge-Coupled Devices

Techniques and instruments developed for extracting precise positional information from CCD images of point-source and extended optical targets are described. With the use of thinned, backside-illuminated devices, performance levels close to those expected from straightforward geometric considerations are obtained. For ideal point sources (ie., stars outside of the earth's atmosphere), centerfinding accuracy of 1/100 pixel and measurement jitter of less than 1/250 pixel have been measured. Detailed tests showed that the main factor limiting tracker accuracy is small variation in the optical image shape rather than CCD noise or response nonuniformity. Methods of searching the entire field for the desired targets are described, along with windowing techniques for tracking. Also discussed are three specific examples of flight instruments previously developed or now being developed at the Jet Propulsion Laboratory.

Monte Carlo simulation of the energy dissipation profiles of 30, 50 and 100 keV incident beams in a layered structure in electron beam lithography

This paper presents a Monte Carlo simulation of the energy dissipation profiles of 30, 50 and 100 keV incident beams in thin film (0.4μm) of electron resist, polymethyl methacrylate (PMMA), on thick silicon substrate in electron beam lithography. The radial scattering and the energy loss of incident electrons (including backscattered electrons from the substrate) are simulated under the illumination of ideal point source and Gaussian round beam spot source, and the histories of 30000–50000 electrons are computed.

Acoustic intensity measurement error for complex sources

There has been much discussion in recent literature of the accuracy of the two-microphone method of acoustic intensity measurement. The errors introduced by the finite difference formulation for intensity have been a frequent topic of investigation. However, these treatments have been limited to ideal plane wave and point sources. This paper describes a study of finite difference approximation errors for more realistic, finite sized sources. Error expressions are developed for a pulsating sphere, an oscillating sphere, and a piston in an infinite baffle. For the spherical sources, the error expressions derived are identical to those of their ideal equivalents, the point monopole and dipole. In the case of the piston source, large errors are predicted for particular conditions. For nearfield measurements along the piston axis, intensity level measurement errors can approach infinity.

A contribution towards a model for an artificial voice

The acoustic pressure amplitudes radiated from the mouth of a storeroom manikin were measured in a 6 cm × 6 cm vertical plane immediately forward of the lips. The sound was produced by an acoustic driver in the manikin's head. The lips were immoveable, forming an approximately trapezoidal aperture with an area of 6.5 cm2. Pressure measurements were made at the 13‐Octave band center frequencies from 1.0 to 6.3 kHz. The measured results at 1.25 kHz agree well with the speech measurements at 1.3 kHz reported by Leman [4th Int. Conf. Acoust., abstr. G36 (1962)]. As pointed out by Leman, the spreading of the acoustic wave, on axis, can be described by an ideal point source displaced 1 cm horizontally from the tip of the lips. Below the lips, near the chin surface, the acoustic pressure does not spread spherically. Also, the farfield azimuthal directivity of speech measured by Moreno and Pfretzschner [Acoust. Lett. 1, 78–84 (1979)] is shown to be predicted reasonably well by a point source on a sphere, except in an approximate ±20° sector centered at the shadowed pole. However, the median plane vertical directivity differs from that of a point source on a sphere due to body diffraction. [This work was done while the authors were with the Electrical Engineering Department, University of Wyoming, Laramie, WY 82071.]

Bismuth and 208Pb microdistributions in enstatite chondrites

Polished sections of 5 enstatite chondrites have been irradiated with 30 MeV 4He ions to produce the alpha-radioactive nuclei 211At and 210Po from 209Bi and 208Pb, respectively. The distribution of alpha activity can be mapped, using cellulose nitrate as an alpha track detector, to give the corresponding Bi or Pb distributions in the meteorite. No strong localization of Bi or 208Pb was found; relatively uniform track distributions were observed. In particular, metal or sulfide grains are not enriched in Bi or Pb (relative to bulk), which is in agreement with the predictions of nebular condensation calculations. While the track distributions appear uniform, the results of detailed, track-by-track mappings of the Bi detectors indicate that the Bi is not totally randomly distributed; the statistical fluctuations in the observed track density are different for the cases where the Bi is totally randomly distributed and where the Bi is localized in point sources. Assuming that the Bi in a given sample is localized in identical point sources which are uniformly distributed throughout the sample, the observed relative population densities of clusters (‘stars’) of small numbers of tracks (2–5) corresponds to Bi being localized, with ~90% in grains with about 10 −16g-Bi (~3 × 10 5 Bi atoms), and with ~10% in 4 × 10 −14 g-Bi sources. If these are elemental Bi, as predicted theoretically, they are ~ 10 2 Å and 10 3 Å in size, respectively.

Non-Gaussian random fields of independent sources in a discrete lattice

A large class of problems in solid-state physics reduces to the investigation of essentially non-Gaussian random fieldsσ(r) generated by identical point sources placed independently at the sites of a three-dimensional lattice. Finite expressions are obtained for all one- and two-point moments of such a field in terms of the concentration of sources and functionals of the Green's function of the source. In the same form, a description is given of the field of an arbitrary polynomial R(σp) of two fieldsσ1 andσ2 generated by the same sources by means of different Green's functions. The expression obtained for the number of level intersections by the field R(σ) is sufficient to solve a number of problems of statistical geometry in non-Gaussian fields of this class.

Coupling of Finite Sized Sources to a Modal Reverberant Sound Field

Much effort has been devoted to the development of statistical methods for the measurement of acoustic power levels and transmission loss in reverberant rooms. However, history is replete with unexplained interlaboratory variations in these measurements, particularly at low frequencies and when large specimens are involved. This paper examines some of the deterministic factors that govern the excitation of reverberant sound fields by finite sources. Such sound fields are investigated and compared with those produced by ideal point sources. Both analytical and experimental results show that the sound fields produced depend not only on the source position but on the source dimension. The relative amount of acoustic source power that is actually transferred to the modes is determined and the sound levels produced are compared with those that would exist under the assumption of diffuse conditions.

Radiation pattern of mantle Rayleigh waves and the source mechanism of the Hindu Kush earthquake of July 6, 1962

abstract The source mechanism of the Hindu Kush earthquake of July 6, 1962 (magnitude 634-7, focal depth 218 km) was studied by comparing the observed amplitude and phase radiation patterns of mantle Rayleigh waves of 150 sec and 200 sec period with theoretical radiation patterns of Rayleigh waves from single- and double-couple point sources, and by considering evidence from Love waves and the shape of P and S pulses. The solution for the source mechanism, which is consistent with all the body wave and surface wave data available for this earthquake, is a double couple acting as a step function in time, with nodal planes oriented as determined from P wave data. Since for waves with periods greater than about 5 sec, the source appears to be an ideal point source, the radius of the equivalent source volume is estimated to be less than 10 km. For Rayleigh waves of 150 sec period, the agreement between observed and theoretical phases (for the above source model) is greatly improved by assuming regional phase velocities instead of a uniform phase velocity for all areas. It is concluded that with the accuracy currently attainable, a study of Rayleigh waves alone cannot determine the source mechanism of an earthquake uniquely.