Aperture Assembly Research Articles

Signal to Noise Ratio optimization for extended sources with a new kind of MURA masks

Using coded aperture, for localization of radioactive hot-spots, results in enhanced efficiency and under certain configurations wide Field of View (FOV). We present a coded aperture assembly technique which can be restructured easily, as well as the reduction of the intrinsic noise of coded apertures constructed with this technique, when they localize spatially extended γ-emitters. Specifically, Modified-Uniformly-Redundant-Array (MURA) coded apertures are structured by embedding lead spheres in a matrix of positions machined inside a transparent medium such as acrylic glass, resulting in an advantageous transparent to opaque area ratio and thus an improved detection efficiency. This configuration also induces a systematic, element-wise, noise on the Point-Spread-Function (PSF) of the correlation matrix. When imaging with these apertures extended hot-spots, a penumbra phenomenon occurrs and reduces this intrinsic noise, in the way a kernel filter does. Fast-Fourier-Transform (FFT) is used to analyze the effect of this phenomenon on the correlation matrix and to explain the maximization of its Signal-to-Noise Ratio (SNR) for certain extent of the hot-spots. Simulations have been used for the detailed study of the SNR dependence on the dimensions of the hot-spot, while experiments with two 99mTc cylindrical sources with 11 mm and 24 mm diameter, respectively and 1.5 MBq activity each, confirm the reduction of the intrinsic noise. The results define the way of optimization of the imaging setup for the detection of extended hot-spots. Such an optimization could be useful for example in the case of lymph nodes or thyroid remnant imaging in nuclear medicine. Finally, we propose a kernel filter, derived by the Auto-Correlation-Function (ACF), to be applied on PSFs with high intrinsic noise, in order to eliminate it.

Design, implementation, and performance of the Astro-H soft x-ray spectrometer aperture assembly and blocking filters

The calorimeter array of the JAXA Astro-H (renamed Hitomi) soft x-ray spectrometer (SXS) was designed to provide unprecedented spectral resolution of spatially extended cosmic x-ray sources and of all cosmic x-ray sources in the Fe-K band around 6 keV. The properties that made the SXS array a powerful x-ray spectrometer also made it sensitive to photons from the entire electromagnetic band as well as particles. If characterized as a bolometer, it would have had a noise equivalent power of <4 × 10 ? 18 W / (Hz)0.5. Thus, it was imperative to shield the detector from thermal radiation from the instrument and optical and UV photons from the sky. In addition, it was necessary to shield the coldest stages of the instrument from the thermal radiation emanating from the warmer stages. These needs were addressed by a series of five thin-film radiation-blocking filters, anchored to the nested temperature stages, that blocked long-wavelength radiation while minimizing x-ray attenuation. The aperture assembly was a system of barriers, baffles, filter carriers, and filter mounts that supported the filters and inhibited their potential contamination. The three outer filters also had been equipped with thermometers and heaters for decontamination. We present the requirements, design, implementation, and performance of the SXS aperture assembly and blocking filters.

Noise analysis for lensless compressive imaging

We analyze the signal to noise ratio (SNR) in a recently proposed lensless compressive imaging architecture. The architecture consists of a sensor of a single detector element and an aperture assembly of an array of aperture elements, each of which has a programmable transmittance. This lensless compressive imaging architecture can be used in conjunction with compressive sensing to capture images in a compressed form of compressive measurements. In this paper, we perform noise analysis of this lensless compressive imaging architecture and compare it with pinhole aperture imaging and lens aperture imaging. We will show that the SNR in the lensless compressive imaging is independent of the image resolution, while that in either pinhole aperture imaging or lens aperture imaging decreases as the image resolution increases. Consequently, the SNR in the lensless compressive imaging can be much higher if the image resolution is large enough.

Fixture layout optimization in multi-station assembly processes using augmented ant colony algorithm

In recent years, ant colony algorithms (ACAs) are used to solve the fixture layout optimization problem for a single workpiece machined in a single manufacturing stage. Assembly processes, however, are normally multi-station manufacturing processes, whose fixture layout optimization problem is much more complex. The purpose of this research is to develop an augmented ACA based on continuous optimization methods to optimize fixture layouts for 2D rigid parts in multi-station assembly processes. The algorithm is augmented by changing the mutation step size in the global search, the limiting step size in the local search, the new pheromone value's expression of the ant, etc. The augmented ACA is used to properly select the coordinates of two locating pins to minimize the sensitivity index. A case about three-station automotive side aperture assembly processes is studied to verify the effectiveness of the augmented ACA. The results show that the augmented ACA can generate more accurate results with a faster rate of convergence and a better stability than the basic ACA. This work could also be applied to fixture layouts optimization problems for 3D rigid parts in multi-station manufacturing processes.

AUTOMATIC ANTI-GLARE SYSTEM FOR NIGHT TIME DRIVING USING LIQUID CRYSTAL SCREENS

This paper proposes a prototype model for an automatic anti-glare system to reduce the risk of accidents during night time driving. The architecture consists of an aperture assembly and sensors. The aperture assembly is a two dimensional array of aperture elements, and their transmittances can be independently controllable. According to the transmittance, each aperture element will act either as opaque or transparent. The intensity of light ray is modulated by the transmittance of aperture elements, which causes the reduction of intensity of light passing through it. The prototype is implemented using liquid crystal display (LCD) and photovoltaic sensors. This system provides an automated mechanism to control the illumination according to the direction of incoming light. The fundamental idea is to measure the incoming light and using measured values to generate patterns on the LCD to reduce the illumination. The proposed prototype can be implemented in two ways: i) as spectacles with liquid crystal sheets placed in front of the glasses, which can be used while driving, ii) by placing large liquid crystal sheets in front of the windshields of vehicles. This method provides a flexible low cost implementation to build simple, reliable device to reduce the intensity of the light on-the-fly.

Improvements to the internal and external antenna H− ion sources at the Spallation Neutron Source

The Spallation Neutron Source (SNS), a large scale neutron production facility, routinely operates with 30-40 mA peak current in the linac. Recent measurements have shown that our RF-driven internal antenna, Cs-enhanced, multi-cusp ion sources injects ∼55 mA of H(-) beam current (∼1 ms, 60 Hz) at 65-kV into a Radio Frequency Quadrupole (RFQ) accelerator through a closely coupled electrostatic Low-Energy Beam Transport system. Over the last several years a decrease in RFQ transmission and issues with internal antennas has stimulated source development at the SNS both for the internal and external antenna ion sources. This report discusses progress in improving internal antenna reliability, H(-) yield improvements which resulted from modifications to the outlet aperture assembly (applicable to both internal and external antenna sources) and studies made of the long standing problem of beam persistence with the external antenna source. The current status of the external antenna ion source will also be presented.

Calibration of High Heat Flux Sensors at NIST.

An ongoing program at the National Institute of Standards and Technology (NIST) is aimed at improving and standardizing heat-flux sensor calibration methods. The current calibration needs of U.S. science and industry exceed the current NIST capability of 40 kW/m2 irradiance. In achieving this goal, as well as meeting lower-level non-radiative heat flux calibration needs of science and industry, three different types of calibration facilities currently are under development at NIST: convection, conduction, and radiation. This paper describes the research activities associated with the NIST Radiation Calibration Facility. Two different techniques, transfer and absolute, are presented. The transfer calibration technique employs a transfer standard calibrated with reference to a radiometric standard for calibrating the sensors using a graphite tube blackbody. Plans for an absolute calibration facility include the use of a spherical blackbody and a cooled aperture and sensor-housing assembly to calibrate the sensors in a low convective environment.

Three Dimensional materials characterisation at ultrahigh resolution using a position-sensitive atom probe

The conventional atom probe field ion microscope permits very high resolution chemical information to be determined with a lateral spatial resolution of typically 2 nm. This spatial resolution is determined by the need to define the analysis area using an aperture. A recent development, the position sensitive atom probe (POSAP), has largely removed this limitation. In a conventional atom probe the ions passing through the aperture, which have come from a circular area of the order of 2 nm in diameter, travel along a long flight path where the mass to charge ratios are determined with high precision. In the position sensitive atom probe the aperture assembly, long flight tube and ion detector (a channel plate) are replaced with a position sensitive detector held at a known distance from the specimen surface. This detector consists of two parts, a channel plate component which permits the flight times (and hence mass to charge ratios) to be determined, and a wedge and strip anode which permits the position of the incoming ion to be calculated. This arrival position corresponds directly to the position on the specimen from which the ion came. The total field of view of the POSAP is a disc approximately 20 nm in diameter. With a conventional atom probe the data acquired during the evaporation sequence can be considered as a core extracted from the specimen, where the average composition as a function of depth is known. The position sensitive atom probe permits us to record data from a much wider core (20 nm rather than 2 nm in diameter), and also to retain the spatial information within the core. As the evaporation proceeds the two dimensional information yielded by the position sensitive detector builds up into a three dimensional block of data. We have, therefore, both chemical and spatial information in three dimensions at very high resolution from the sampled volume of material.

The Design of a Portable Polychromator to Monitor Contaminants in Tungsten Arc—Inert Gas Shields

A small portable spectrometer was developed to monitor oxygen and hydrogen as the free or dissociation products in the tungsten arc–inert gas welding process. The hydrogen (6562.8 Å) and oxygen (7771.9 Å) lines were monitored and the argon (8521.4 Å) line was used for the internal standard. The instrument, a fixed Eagle mount, used a 76×50 mm, 60 cm focal length, concave grating with 610 grooves/mm blazed in the second order. The reciprocal dispersion at 6562 Å was 12 Å/mm. The oxygen triplet was resolved. Optical design requirements included the use of very short arc lengths in the order of 0.5 mm and less, an optical plane of 65° to 75deg; with a large flat plate instead of the conventional 90° with respect to the electrodes, and an arc that does not remain in a fixed optical position in space. The focusing lens-limiting aperture assembly was mounted on the torch head with a fiber optic bundle transmitting the light to the spectrometer. The instrument recorded only the signal above background, including the very high background produced by introducing the electrode tip into the optical path. This was accomplished by the use of an oscillating optical block in the entrance beam which yields an ac signal above the dc signal from background. The direct ratio readout system used a Devar ratio computer in each analytical channel and was essentially instantaneous.

Externally adjusted Rayleigh aperture for the Spinco model E ultracentrifuge

The use of the Rayleigh interference optical system in analytical ultracentrifugation has found an ever-increasing popularity since it was first demonstrated by Richards and Schachman (1) that it can provide an extremely precise record of refractive index differences as a function of radius within the centrifuge cell. As theoretical and experimental developments in this field have imposed increased demands for accurate optical alignment, provisions have been made available as standard equipment on the Spinco model E ultracentrifuge for precise and flexible adjustability of the Rayleigh optical components to allow fulfillment of alignment criteria by incremental photographic determination of optimal settings (2). The currently available Rayleigh aperture assembly has provisions for transverse and rotational adjustment. However, it is not accessible during centrifugation required to provide gradients in the cell used in many procedures of rotational alignment, so that incremental adjustment for photographic examination of fringe patterns as a function of angle becomes a time-consuming operation. In this communication, we describe a procedure for minimal modification of existing Spinco components which removes this restriction and ensures rapid and reproducible rotational alignment of the Rayleigh aperture.

The RCA Recording System and its Adaptation to Various Types of Sound-Track&#x002A;

The photographic recording of sound is accomplished by modulating a narrow beam of light and projecting it upon a strip of moving film. There are three ways in which the amount of exposing light may be varied. A light-beam of fixed dimensions may have its intensity varied; a beam of constant intensity and length may have its width varied; or a beam of constant intensity and width may have its length varied. The first two types of modulation produce variable-density soundtracks, while the third type produces variable-width tracks. — The recording optical system can be made to modulate either the intensity or the length of the light-beam. The unit consists essentially of an incandescent lamp to produce the light, a system of lenses to direct the light, an aperture and slit to limit the light, and a reflecting mirror galvanometer to modulate the light. A magnetic shutter for ground-noise reduction is also part of the standard variable-width recording unit. A system of mirrors and lenses intercepts a small portion of the recording light and projects it upon an external card. This system magnifies the deflection of the galvanometer and shutter to such extent that the degree of modulation and the zero settings can be observed easily by the unaided eye. — Many different types of sound-track can be made with the recording optical system without sacrificing any of its advantages as a light modulator. By the use of the appropriate condenser and aperture assembly, the system will record standard bilateral variable-width track, standard variable-density track, push-pull class B variable-width track, push-pull class A variable-density track, and push-pull class A variable-width track. The manner in which each of these systems functions is shown and described in detail.