The combinatorics of binary arrays

This paper gives an account of the combinatorics of binary arrays, mainly concerning their randomness properties. In many cases the problem reduces to the investigation on difference sets.postprin

Array Antennas of Size 8×8 Based on Hadamard Difference Sets

The problem of synthesizing the optimized equi-amplitude array antenna (AA) on the 8×8 grid based on a Hadamard difference set is considered. By using newly found sets of this type on the 8×8 grid the 28- and 36-element AAs having a low sidelobe level are obtained. A numerical experiment showed that by a small alteration of the structure of such a set, further reduction of the AA sidelobe radiation is possible.Рассматривается вопрос синтеза антенных решеток (АР) на основе адамаровских разностных множеств. С использованием новых найденных множеств этого типа на решетке 8×8 получены 28- и 36-элементные АР с низким уровнем боковых лепестков. Численный эксперимент показал, что путем небольших изменений структуры таких множеств можно добиться дальнейшего снижения уровня бокового излучения.Розглядається питання щодо синтезу антенних решiток (АР) на базi адамаровських рiзницевих множин. З використанням нових знайдених множин цього типу на решiтцi 8×8 отриманi 28- та 36-елементнi АР з низьким рiвнем бокових пелюсткiв. Числовий експеримент показав, що шляхом незначних змiн у структурi таких множин можна домогтися подальшого зниження рiвня бокового випромiнювання

Time-Encoded Thermal Neutron Imaging using Large-Volume Pixelated CdZnTe Detectors

CdZnTe detectors are commonly used for room-temperature gamma-ray spectroscopy and imaging in a variety of applications including nuclear security, nuclear medicine, and space science. The material's long-established sensitivity to thermal neutrons, however, is less utilized. Generally speaking, the performance of neutron detectors based on the Cd capture reaction is limited by the physical nature of the reaction itself. Multiple gamma rays are emitted promptly following each capture event, which consists of one realization of many possible combinations of gamma-ray lines. Although the gamma-ray cascade can reduce photopeak efficiency in conventional devices, this work demonstrates that pixelated CdZnTe can recover losses by reading out each gamma-ray interaction separately. Including coincident events, the measured 558-keV photopeak efficiency for a 3 by 3 array of 2 cm by 2 cm by 1.5 cm pixelated CdZnTe detectors was about 10%, i.e., ten 558 keV photopeak events per 100 incident thermal neutrons. This was in good agreement with its calculated value. Initial measurements also show that neutron-gamma discrimination beyond simple energy windowing is possible when incorporating the 3-D interaction locations of gamma rays provided by the pixelated readout. In this work, we developed and successfully demonstrated a proof-of-principle time-encoding system for thermal neutron imaging using pixelated CdZnTe. Time encoding was chosen because it is not limited by the detector's position resolution or spatial extent. These issues are exacerbated by Cd capture due to the dispersal of cascade gamma rays throughout the device. The system was first tested using a MURA-based, W-metal mask with both Co-57 and U-metal gamma-ray sources. About 0.3-degree angular resolution within a 22-degree field of view was achieved for gamma rays, and good image uniformity was observed for objects of moderate spatial extent. A MURA-based thermal neutron mask was then constructed using 1-mm-thick BN tiles, which attained roughly 4-degree angular resolution within a 50-degree field of view when measuring HDPE-moderated Cf-252. Two different thermal neutron imaging measurements were taken, with one and two moderators within the field of view. Reconstructed images corresponded well with the 3-D locations and sizes of moderators, and had predictable signal-to-noise ratio. We believe the experimental imaging results provided here warrant further studies on the use of CdZnTe for other thermal neutron imaging scenarios.PHDNuclear Engineering & Radiological SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/137131/1/stbrow_1.pd

Design of a near-field coded aperture cameras for high-resolution medical and industrial gamma-ray imaging

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 2001.Includes bibliographical references (p. [251]-255).Coded Aperture Imaging is a technique originally developed for X-ray astronomy, where typical imaging problems are characterized by far-field geometry and an object made of point sources distributed over a mainly dark background. These conditions provide, respectively, the basis of artifact-free and high Signal-to-Noise Ratio (SNR) imaging. When the coded apertures successful in far-field problems are used in near-field geometry, images are affected by extensive artifacts. The classic remedy is to move away from the object until a far-field geometry is restored, but this is at the expense of counting efficiency and, thus, of the SNR of the images. It is shown in this thesis that the application to near-field of a technique originally developed to mitigate the effects of non-uniform background in far-field applications results in a considerable reduction of near-field artifacts. This result opens the way to the exploitation in near-field problems of the favorable SNR characteristics of coded apertures: images comparable to those provided by state-of-the-art imagers can be obtained in a shorter time or while administering a lower dose to patients. Further developments follow when the SNR increase is traded for better resolution at constant time and dose.(cont.) The main focus of this work is on a coded aperture camera specifically designed for high-resolution single-photon planar imaging with a pre-existing gamma (Anger) camera. Original theoretical findings and the results of computer simulations led to an optimal coded aperture that was tested experimentally in phantom as well as in-vivo studies. Results include, but are not limited to, 1.66-mm-resolution images of 99mTc-labeled blood and bone agents in a mouse. The theoretical bases for extension to sub-millimeter resolution and higher-energy isotopes are also laid and a candidate aperture capable of 0.96-mm resolution proposed. Potential applications are in small-animal imaging, pediatric nuclear medicine and breast imaging, where increased resolution can result in earlier diagnosis of disease. The last Chapter of the thesis extends the ideas developed to the design of a coded aperture suitable for CAFNA (Coded Aperture Fast Neutron Analysis), a contraband detection technique that has been under development at MIT for a number of years.by Roberto Accorsi.Ph.D

Coded aperture breast tumour imaging using a full-size clinical gamma camera

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