We report on a new class of coded aperture arrays which has all the desirable imaging characteristics of the Uniformly Redundant Array (URA), yet is constructible in dimensions which are forbidden to that design. In addition, the new arrays (called PNP arrays, for Pseudo-Noise Product) are of self-supporting geometry, simplifying fabrication and making them ideal candidates for situations where active collimators are employed, as for example in high-energy ?-ray imaging. A unique and important feature of all PNP arrays is the ability to produce reconstructed images whose noise is uniform regardless of the original source structure. A comparison of the predicted performance of the PNP, URA, GEOMETRIC, and PINHOLE designs is presented.

PNP - A New Class of Coded Aperture Arrays

The trimodal imager (TMI) images gamma-ray sources from a mobile platform using both coded aperture (CA) and Compton imaging (CI) modalities. In this paper we will discuss development and performance of image reconstruction algorithms for the TMI. In order to develop algorithms in parallel with detector hardware we are using a GEANT4 [J. Allison, K. Amako, J. Apostolakis, H. Araujo, P.A. Dubois, M. Asai, G. Barrand, R. Capra, S. Chauvie, R. Chytracek, G. Cirrone, G. Cooperman, G. Cosmo, G. Cuttone, G. Daquino, et al., IEEE Trans. Nucl. Sci. NS-53 (1) (2006) 270] based simulation package to produce realistic data sets for code development. The simulation code incorporates detailed detector modeling, contributions from natural background radiation, and validation of simulation results against measured data. Maximum likelihood algorithms for both imaging methods are discussed, as well as a hybrid imaging algorithm wherein CA and CI information is fused to generate a higher fidelity reconstruction.

Hybrid coded aperture and Compton imaging using an active mask

A new method for the synthesis of binary arrays which have perfect cross-correlation properties is presented. The arrays, which are discussed with an emphasis on coded aperture imaging, may also be of use in a number of other applications, including signal processing, telecommunications and radio antenna design. The new coded aperture configurations which are made available offer a wider choice of possible aperture parameters, and conveniently fill a gap in the system parameters that are currently available. The increased freedom of system choice, which can be made for both one- and two-dimensional systems, includes the size of the aperture array and its throughput value. The new two-dimensional apertures have the further advantage of being fully self-supporting.

Synthesis of binary arrays with perfect correlation properties — coded aperture imaging

Astrophysical studies with high energy γ-ray telescopes will benefit significantly from improved capability of such telescopes to determine source locations. Instruments under construction use either direct measurement of γ-ray arrival directions (the EGRET instrument on the Gamma Ray Observatory) or coded aperture techniques (the Soviet-French GAMNA-I detector). A comparison of these two approaches shows that either can produce source locations of 5 to 10 arcmin, significantly better than previous results. The conventional telescope achieves this result by using the relatively small number of high energy, high resolution γ-rays, while the coded aperture technique requires high statistics but can use γ-rays of all energies. The coded aperture technique, however, is limited to only the brightest sources. Suggested future direct and coded aperture telescopes can yield better source locations. Conventional high energy γ-ray telescopes offer advantages over coded aperture systems for three reasons: the low statistics inherent in realistic astrophysical observations at these energies limit the operation of the coded aperture detectors; the conventional telescopes gain from the improvement of single photon angular resolution at higher energies; and the cosmic-ray-induced background is a potentially serious problem for coded aperture systems.

Source location capability of present and future high energy gamma ray telescopes

A class of flexible coded apertures, called index class apertures, is presented. The configurations are shown to possess similar properties to the geometric apertures of Gourlay and Stephen [Appl. Opt.22, 4042 (1983)10.1364/AO.22.004042APOPAI0003-6935], and it is demonstrated that the modified uniformly redundant arrays (MURAs) are a special case of the index class apertures. The apertures are shown to offer both a larger range of throughput values and better imaging capability than is available to the geometric apertures, while at the same time possessing more rigidity of structure than other designs, such as the MURAs and the uniformly redundant arrays.

Index class apertures—a class of flexible coded aperture

The coded aperture, a refinement of the scatter-hole camera, offers a method for the improved measurement of γ-ray direction in γ-ray astronomy. Two prototype coded apertures have been built and tested. The more recent of these has 128 active elements of the heavy scintillator BGO. Results of tests for γ-rays in the range 50 - 500 MeV are reported and future application in space discussed.

P. Gorodetsky

Papers

The use of an active coded aperture for improved directional measurements in high energy γ-ray astronomy