Coded Aperture Imaging System Research Articles

The INTEGRAL IBIS/ISGRI System Point Spread Function and Source Location Accuracy

The imager on board INTEGRAL (IBIS) presently provides the most detailed sky images ever obtained at energies above 30 keV. The telescope is based on a coded aperture imaging system which allows to obtain sky images in a large field of view (29 29) with an angular resolution of 12 0 . The System Point Spread Function of the telescope and its detailed charac- teristics are here described along with the specific analysis algorithms used to derive the accurate point-like source locations. The derived location accuracy is studied using the first in-flight calibration data on strong sources for the IBIS /ISGRI system. The dependence of the calibrated location accuracy with the signal to noise ratio of the sources is presented. These preliminary studies demonstrate that the IBIS/ISGRI telescope and the standard scientific analysis software allow source localizations with accuracy at 90% confidence level better than 1 0 for sources with signal to noise ratios >30 over the whole field of view, in agreement with the expected performances of the instrument.

IBIS detector performance during calibration — preliminary analysis

The IBIS telescope is a high angular resolution gamma-ray imager due to be launched on the INTEGRAL satellite on October 17, 2002. The scientific goal of IBIS is to study astrophysical processes from celestial sources and diffuse regions in the hard X-ray and soft gamma-ray domains. IBIS features a coded aperture imaging system and a novel large area (∼3000 cm 2) multilayer pixellated detector which utilises both cadmium telluride (16,384 detectors) and caesium iodide elements (4096 detectors) surrounded by a BGO active veto shield. We present an overview of, and preliminary analysis from, the IBIS calibration campaign. The performance of each pixel has been characterised, and hence the scientific performance of the IBIS detector system as a whole can now be established.

CAFNA/sup (R)/, coded aperture fast neutron analysis for contraband detection: preliminary results

We have developed a near field coded aperture imaging system for use with fast neutron techniques as a tool for the detection of contraband and hidden explosives through nuclear elemental analysis. The technique relies on the prompt gamma rays produced by fast neutron interactions with the object being examined. The position of the nuclear elements is determined by the location of the gamma emitters. For existing fast neutron techniques, in Pulsed Fast Neutron Analysis (PFNA), neutrons are used with very low efficiency; in Fast Neutron Analysis (FNA), the sensitivity for detection of the signature gamma rays is very low. For the Coded Aperture Fast Neutron Analysis (CAFNA(R)) we have developed, the efficiency for both using the probing fast neutrons and detecting the prompt gamma rays is high. Compared to PFNA, the improvement for neutron utilization is n/sup 2/, where the total number of voxels in the object being examined is n/sup 3/. Compared to FNA, the improvement for gamma-ray imaging is proportional to the total open area of the coded aperture plane; a typical value is n/sup 2//2, where n/sup 2/ is the number of total detector resolution elements or the number of pixels in an object layer. We have performed analysis, Monte Carlo simulations, and preliminary experiments using low and high energy gamma-ray sources. The results show that a high sensitivity 3-D contraband imaging and detection system can be realized by using CAFNA.

Performance of Prototype Segmented CdZnTe Arrays

AbstractThe Burst and All Sky Imaging Survey (BASIS) is a proposed mission to provide ∼3 arc second locations of approximately 90 Gamma-Ray Bursts (GRBs) per year. The BASIS coded aperture imaging system requires a segmented detector plane able to detect the interaction position of (10 - 150 keV) photons to less than 100 μm. To develop prototype detector arrays with such fine position resolution we have fabricated many 15 mm × 15 mm × 2 mm 100 μm pitch CdZnTe strip detectors. We have assembled these fine pitch CdZnTe strip detectors into prototype 2 × 2 and 6 × 6 element arrays read out by ASIC electronics. The assembly and electronics readout of the 6 × 6 flight prototype array will be discussed, and preliminary data illustrating the uniformity and efficiency of the array will be presented.

Neutron penumbral imaging of inertial confinement fusion targets at Phébusa)

First 14 MeV neutron images of imploded microballoons have been obtained at the Phébus laser facility at CEL-V in 1992 [Garconnet et al. Laser Part Beams 11, 3 (1994)]. The sizes of the neutron source have been measured by using a coded-aperture imaging system and a scintillator array as a detector. The threshold of the experimental setup was typically 2×1010 neutrons/shot. 600–800 μm source sizes in direct drive experiments have been measured with a 130 μm two-point resolution. In 1993 we improved the sensitivity of the camera by increasing the light collection efficiency. It can now work at a neutron yield as small as a few 108. Thanks to this improvement some images in indirect drive experiments have been recorded in the range 3×108–5×109 with a 56 μm two-point resolution. Wiener filter, homomorphic Wiener filter, and Nugent’s ‘‘comb filter’’ methods have been used and compared to deconvolve the penumbral images. Design of the camera and numerical method performances will be discussed.

Neutron penumbral imaging of inertial confinement fusion targets at Phébus (abstract)a)

First 14 MeV neutron images of imploded microballoons have been obtained on the Phebus laser facility at CEL-V in 1992 [Garconnet et al. Laser Part Beams 11, 3 (1994)]. The sizes of the neutron source have been measured by using a coded-aperture imaging system and a scintillator array as a detector. The threshold of the experimental setup was typically 2×1010 neutrons/shot. 600–800 μm source sizes in direct drive experiments have been measured with a 130 μm two-point resolution. In 1993 we improved the sensitivity of the camera by increasing the light collection efficiency. It can now work at a neutron yield as small as a few 108. Thanks to this improvement some images in indirect drive experiments have been recorded in the range 3⋅108–5⋅109 with a 56 μm two-point resolution. Wiener filter, homomorphic Wiener filter, and Nugent’s ‘‘comb filter’’ methods have been used and compared to deconvolve the penumbral images. Design of the camera and numerical method performances will be discussed.

Neutron penumbral imaging of inertial confinement fusion targets

The first 14-MeV neutron images of imploded microballoons have been obtained on the Phébus laser facility at CEL-V. The sizes of the source have been measured, in direct-drive experiments, by means of a coded-aperture imaging system. The principle is to use a thick aperture with a diameter larger than that of the source to image the microballoon. The deconvolution of the recorded image allows one to reconstruct the image of the neutron source, and this technique allows one to obtain images at a lower neutron yield than with a conventional pinhole camera. The choice of the experimental conditions is a trade-off between the Phébus conditions, the spatial resolution, and the image reconstruction method that is related to the signalto-noise ratio. The sensitivity of the diagnostic is strongly dependent on the number of scintillator photons that are collected by the recording system. The neutron measurement threshold of our experimental setup is typically 2 × 1010 neutrons/shot for neutron source sizes of 800 μm.

An optimised coded aperture imaging system

A new coded aperture system design for use with circular detectors is discussed, whereby square element coded aperture unit patterns are mosaiced onto a circular detector with a view to maximising the size of the unambiguous field of view for any given detector radius. The results for these optimum configurations are presented for all 50% transparency URA and MURA unit patterns of order υ<600. Improvements in the FOV size over standard rectangular mosaicing are presented with the available increase being found to be around 23% for large values of υ.

Neutron imaging of inertial confinement fusion targets at Nova

Images of the neutron-emitting region of high-yield inertial confinement fusion targets have been obtained with a penumbral coded-aperture imaging system. The major components of the imaging system are the penumbral aperture, neutron detector, alignment hardware, and image reconstruction software. We describe these components and present an example of the neutron imaging results.

Coded-aperture imaging system for reconstructing tomograms of human myocardium

To increase the detection efficiency and improve the spatial resolution, a coded-aperture imaging method is applied to nuclear medicine. The aperture consists of nine pinholes arranged in a square grid. Three kinds of coding are sequentially used to record the same number of projections including parallax and overlap. The overlapped images are partially separated, and good tomograms of a ring phantom and a human myocardium are reconstructed using a modified backprojection algorithm with variable damping factor.

Aberrations in gamma-ray imaging systems

The aberrations present in a coded aperture imaging system have a fundamentally different origin to those found in a focussed optical instrument. A series of laboratory tests is described in which masks with carefully controlled defects were employed to generate non-perfect gamma-ray images, so that the magnitude of the aberrations introduced could be quantitatively investigated. The results of these tests are presented and the extent to which they affect the design of a practical gamma-ray imaging system is discussed.

Performance Characteristics of the Annular Core Research Reactor Fuel Motion Detection System

Recent proof tests have shown that the annular core research reactor (ACRR) fuel motion detection system has reached its design goals of providing high temporal and spatial resolution pictures of fuel distributions in the ACRR. The coded aperture imaging system (CAIS) images the fuel by monitoring the fission gamma rays from the fuel that pass through collimators in the reactor core. The gamma-ray beam is modulated by coded apertures before producing a visible light coded image in thin scintillators. Each coded image is then amplified and recorded by an opticalimage-intensifier/fast-framing-camera combination. The proximity to the core and the coded aperture technique provide a high data collection rate and high resolution. Experiments of CAIS at the ACRR conducted under steady-state operation have documented the beneficial effects of changes in the radiation shielding and imaging technique. The spatial resolutions are 1.7 mm perpendicular to the axis of a single liquid-metal fast breeder reactor fuel pin and 9 mm in the axial dimension. Changes in mass of 100 mg in each resolution element can be detected each frame period, which may be from 5 to 100 ms. This diagnostic instrument may help resolve important questions in fuel motion phenomenology.

Advanced coded-aperture imaging system for nuclear medicine

An advanced coded imaging system is described, and some results of phantom experiments are presented. The advanced method uses a pair of coherent codes (+ 1 and -1 codes) and has many advantages compared with conventional ones. One of the greatest advantages is that there are no sidelobes in the focal plane and only a few in other planes. Therefore, when an object can be regarded as two-dimensional, it is perfectly reconstructed with high detecting efficiency, and this is successfully simulated by a thyroid phantom with 99mTc. Moreover, this system has an ability to reconstruct tomograms, which is also shown by using ring phantoms piled on one another with some cold spots in their shells. From these experimental results it may be concluded that the new system is useful for practical applications, for example, to nuclear medicine.

Pseudo real-time coded aperture imaging system with intensified vidicon cameras

A coded image displayed on a TV monitor was used to directly reconstruct a decoded image. Both the coded and the decoded images were viewed with intensified vidicon cameras. The coded aperture was a 15-element nonredundant pinhole array. The coding and decoding were accomplished simultaneously during the scanning of a single 16-msec TV frame.

Improved depth resolution in coded aperture γ-ray imaging systems

We derive a rigorous expression for the three-dimensional point response function associated with the combined encoding-decoding process of coded aperture imaging. From this general expression we derive the two-dimensional point response function appropriate to a coded aperture imaging system which images a two-dimensional slice through a, three-dimensional source distribution. This point response function is then calculated numerically for a number of different aperture coding functions and decoding functions. The graphed results suggest that simple modifications of the decoding function can lead to significant improvements in the depth resolving capability of a coded aperture imaging system. Suggestions for future research are given.