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# A balloon-borne coded aperture telescope for low-energy gamma-ray astronomy

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## Abstract

A telescope for imaging cosmic γ-ray emission over the energy range 160 keV to 9.3 MeV has been developed and successfully flown on a high altitude balloon over Palestine, Texas on 1 October, 1984. This instrument consists of a coded mask based on a 5 × 7 uniformly redundant array (URA) and a scintillator array consisting of 35 bismuth germanate (BGO) detectors. The telescope can image sources with an intrinsic resolution of 3.8° within a 15.2° × 22.8° field of view. The properties of the instrument are described and its imaging capability is demonstrated with results from an observation of the region of the Crab Nebula. The imaging response to the Crab was found to be well represented by a bivariate Gaussian function of full width at half-maximum (FWHM) 4.8°. The centroid of the response was determined to a precision of ± 12 arc min.

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... To avoid this problem, the Uniformly Redundant Array (URA) mask was proposed by Fenimore and Cannon, which yielded sharp autocorrelation functions with flat sidelobes [19]. Later, modified URA (MURA) with a double-exposure method was developed where the antimask was obtained by rotating the mask 180 degrees, aiding easy implementation of the two-shot-imaging method [20,21]. Later, CAI was adapted for spectral-imaging [22] and spatial-imaging applications [23]. ...
Conference Paper
Computational imaging techniques are indirect ones consisting of two steps: optical recording and computational reconstruction. In this study, deterministic optical fields such as Bessel, Airy, Gaussian and Laguerre-Gaussian were studied in this indirect imaging framework.
... However, the development of CA patterns based on" Cyclic Difference Sets" such as Uniformly Redundant Arrays (URAs) [22][23][24], Modified Uniformly Redundant Arrays (MURAs) [25] and arrays based on MURAs such as No-Two-Hole-Touching (NTHT) [26] have been demonstrated to be the most promising of all the CA patterns. These optimum arrays, as originally developed and extended by Fenimore and Cannon [23,24,27] have become widely used in the detection of X-ray and gamma ray sources in astronomy for imaging stars and more recently in nuclear medicine for small animal imaging [28,29]. These arrays combine the high transmission (have up to 50% open area for the MURA patterns) characteristic with flat (zero) side-lobes in their response function ( Figure 1) The high transmission provides a potential capability to image low-contrast sources and may dramatically enhances the detection efficiency compared to collimators system. ...
Article
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This is a continuous investigation of a possible application of Coded Aperture (CA) for breast tumor imaging. This image formation system allows a high photon transmission, image magnification and to a lesser extent provide (limited-angle) tomographic capability. This paper describes two non-Monte Carlo methods, the first one based on a simple attractive approach called Binary Mask Shift (BMS) representing the action of a distributed source in the projective CA imaging geometry. It allows to investigate all the possible built in artefacts without the effect of solid angle. Thus, it provides understanding of the application of CA patterns in breast tumor imaging through the evaluation of their potential and their performance under a variety of imaging conditions. The second method based on Pseudo-Ray Tracing (PRT) that obtained by purely calculating the angle of incidence of each point in the object that successfully strikes an open aperture element and then hits the detector element. These methods particularly used for CA imaging investigations. Interestingly, these methods yield similar results of a similar CA pattern but takes less computing power, than using a full Monte Carlo Simulation (MCS) approach. The main results demonstrate that Modified Uniformly Redundant Arrays (MURAs) patterns of squares arrays are very promising as it produces excellent decoded images. This is attractive particularly when used for breast tumor imaging i.e. ideal for imaging small isolated sources and thus provides a good match to the imaging of Scintimammography (SM).
... To avoid this problem, the Uniformly Redundant Array (URA) mask was proposed by Fenimore and Cannon, which yielded sharp autocorrelation functions with flat sidelobes [19]. Later, modified URA (MURA) with a double-exposure method was developed where the antimask was obtained by rotating the mask 180 degrees, aiding easy implementation of the two-shot-imaging method [20,21]. Later, CAI was adapted for spectral-imaging [22] and spatial-imaging applications [23]. ...
Article
Full-text available
Indirect-imaging methods involve at least two steps, namely optical recording and computational reconstruction. The optical-recording process uses an optical modulator that transforms the light from the object into a typical intensity distribution. This distribution is numerically processed to reconstruct the object’s image corresponding to different spatial and spectral dimensions. There have been numerous optical-modulation functions and reconstruction methods developed in the past few years for different applications. In most cases, a compatible pair of the optical-modulation function and reconstruction method gives optimal performance. A new reconstruction method, termed nonlinear reconstruction (NLR), was developed in 2017 to reconstruct the object image in the case of optical-scattering modulators. Over the years, it has been revealed that the NLR can reconstruct an object’s image modulated by an axicons, bifocal lenses and even exotic spiral diffractive elements, which generate deterministic optical fields. Apparently, NLR seems to be a universal reconstruction method for indirect imaging. In this review, the performance of NLR isinvestigated for many deterministic and stochastic optical fields. Simulation and experimental results for different cases are presented and discussed.
... A large area photon detection plane simultaneously measures both the position of the photon interaction (in three dimensions to avoid parallax effects) and the energy lost by the photon as a result of that interaction. The most notable satellite applications of this technique are GRANAT/SIGMA [7], BeppoSAX [8], INTEGRAL [9], [10] and the soon-to-be-launched SWIFT [11], [12] all of which follow in the pioneering footsteps of balloonborne instruments such as DGT [13] and GRIP [14], [15]. ...
Conference Paper
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The primary scientific mission of the Black Hole Finder Probe (BHFP), part of the NASA Beyond Einstein program, is to survey the local Universe for black holes over a wide range of mass and accretion rate. One approach to such a survey is a hard X-ray coded-aperture imaging mission operating in the 10-600 keV energy hand, a spectral range that is considered to be especially useful in the detection of black hole sources. The development of new inorganic scintillator materials provides improved performance (for example, with regards to energy resolution and timing) that is well suited to the BHFP science requirements. Detection planes formed with these materials coupled with a new generation of readout devices represent a major advancement in the performance capabilities of scintillator-based gamma cameras. Here, we discuss the Coded Aperture Survey Telescope for Energetic Radiation (CASTER), a concept that represents a BHFP based on the use of the latest scintillator technology.
... Метод «маска -антимаска» [4] применяется для устранения влияния неоднородности фона детектора на результаты измерений и восстановления картины пространственного распределения. Суть метода со-стоит в том, что область интереса наблюдается прибором с кодированной апертурой в течение некоторого времени Т через апертуру, в которой расположена кодирующая маска. ...
Article
Full-text available
In studying the properties of tomographic gamma - ray imaging systems with coded apertures, (gamma emission - tomography) raised the question of the influence of the interaction of gamma - radiation with matter on the quality of the reconstructed image. In this paper describes principles of the tomographical information reconstruction in systems with coded apertures, the results of the software - simulation of decoding the location of point sources of ionizing radiation in the field of view of the complete coding.
... We have experience in effectively using a similar approach with a coded aperture γray telescope. [59][60][61] It is generally believed, however, that the best approach to dealing with the systematics would be to spin the polarimeter during the observation so as to average out the influence of the various detector asymmetries. The use of both approaches may provide an effective combination. ...
Article
Full-text available
We have developed a modular design for a hard X-ray and soft gamma-ray polrimeter that we call GRAPE (Gamma RAy Polarimeter Experiment). Optimized for the energy range of 50-300 keV, the GRAPE design is a Compton polarimeter based on the use of an array of plastic scintillator scattering elements in conjunction with a centrally positioned high-Z calorimeter detector. Here we shall review the results from a laboratory model of the baseline GRAPE design. The baseline design uses a 5-inch diameter position sensitive PMT (PSPMT) for readout of the plastic scintillator array and a small array of CsI detectors for measurement of the scattered photon. An improved design, based on the use of large area multi-anode PMTs (MAPMTs), is also discussed along with plans for laboratory testing of a prototype. An array of GRAPE modules could be used as the basis for a dedicated science mission, either on a long duration balloon or on an orbital mission. With a large effective FoV, a non-imaging GRAPE mission would be ideal for studying polarization in transient sources (gamma ray bursts and solar flares). It may also prove useful for studying periodically varying sources, such as pulsars. An imaging system would improve the sensitivity of the polarization measurements for transient and periodic sources and may also permit the measurement of polarization in steady-state sources.
... However, the development of CA patterns based on "Cyclic Difference Sets" such as URAs MURAs and arrays based on MURAs such as NTHT [2,3,17,22] have been demonstrated to be the most promising of all the CA patterns. These optimum arrays, as originally developed and extended by Fenimore and Cannon [2,3,25,26] have become widely used in the detection of X-ray and gamma-ray sources in astronomy for imaging stars and more recently in nuclear medicine for small animal imaging [18]. is of special interest in artefact reduction and has been used in the past for reducing systemic non-uniform background [27]. Certainly, these advantages and properties have motivated the authors to select this family of CA patterns as appropriate for use in SM. ...
Article
Full-text available
The pinhole camera is a well-established direct-imaging system that produce good quality images, in terms of spatial resolution, but has an associated limited field of view. When used in the context of nuclear medicine imaging, the limited angular acceptance and poor geometric efficiency means the imaging process suffers from lengthy acquisition times as it suffers from low efficiency due to its single small pinhole size. The collimator image formation such as the parallel-hole is limited by the design and also provides a low detection efficiency. The forthcoming review focuses on methods of removing these limitations by investigation using a Coded Aperture (CA) instead of a conventional collimator, coupled to a standard clinical gamma camera for breast tumor imaging applications. This paper introduces the concept of CA imaging and provides an historical background of the different types of CA, families with their mathematical imaging properties. Then, it describes their shape and correlation properties. These include Fresnel zone plate, random array, non-redundant arrays, L shape geometric array, X shape geometric, uniformly redundant, modified uniformly redundant, mosaic uniformly redundant and no-Two-Hole-Touching patterns. Finally, this paper illustrating the principle of pinhole camera to appreciate the theory and the formation of coded image. Then describe the potential of CA imaging in breast tumor imaging. It a general overview of the entire imaging problems associated with the collimator-based SM system closes the paper.
... Under near-field imaging conditions, it is difficult for a coded-hole collimator to provide an ideal point spread function. The different incident angles will change the projection intensity and then generate an intensity modulation to create artifacts [18][19][20][21] . Complementary coded-aperture imaging was used to reduce noise and artifacts [22] . ...
Preprint
The nuclides inhaled during nuclear accidents usually cause internal contamination of the lungs with low activity. Although a parallel-hole imaging system, which is widely used in medical gamma cameras, has a high resolution and good image quality, owing to its extremely low detection efficiency, it remains difficult to obtain images of inhaled lung contamination. In this study, the Monte Carlo method was used to study the internal lung contamination imaging using the MPA-MURA coded-aperture collimator. The imaging system consisted of an adult male lung model, with a mosaicked, pattern-centered, and anti-symmetric MURA coded-aperture collimator model and a CsI(Tl) detector model. The MLEM decoding algorithm was used to reconstruct the internal contamination image, and the complementary imaging method was used to reduce the number of artifacts. The full width at half maximum of the I-131 point source image reconstructed by the mosaicked, pattern-centered, and anti-symmetric Modified uniformly redundant array (MPA-MURA) coded-aperture imaging reached 2.51 mm, and the signal-to-noise ratio of the simplified respiratory tract source (I-131) image reconstructed through MPA-MURA coded-aperture imaging was 3.98 dB. Although the spatial resolution of MPA-MURA coded aperture imaging is not as good as that of parallel-hole imaging, the detection efficiency of PMA-MURA coded-aperture imaging is two orders of magnitude higher than that of parallel hole collimator imaging. Considering the low activity level of internal lung contamination caused by nuclear accidents, PMA-MURA coded-aperture imaging has significant potential for the development of lung contamination imaging.
... To avoid this problem, the Uniformly Redundant Array (URA) mask was proposed by Fenimore and Cannon, which yielded sharp autocorrelation functions with flat sidelobes [19]. Later modified URA (MURA) with a double exposure method was developed where the antimask was obtained by rotating the mask 180 degrees, aiding easy implementation of the two-shots imaging method [20,21]. Later, CAI was adapted for spectral imaging [22] and spatial imaging applications [23]. ...
Preprint
Indirect imaging methods involve at least two steps, namely optical recording, and computational reconstruction. The optical recording process uses an optical modulator that transforms the light from the object into a typical intensity distribution. This distribution is numerically processed to reconstruct the object's image corresponding to different spatial and spectral dimensions. There have been numerous optical modulation functions and reconstruction methods developed in the past years for different applications. In most cases, a compatible pair of optical modulation function and reconstruction method gives optimal performance. A new reconstruction method termed non-linear reconstruction (NLR) was developed in 2017 to reconstruct the object image in the case of optical scattering modulators. During the years, it was revealed that the NLR could reconstruct an object's image modulated by an axicons, bifocal lenses and even exotic spiral diffractive elements, which generate deterministic optical fields. Apparently, NLR seems to be a universal reconstruction method for indirect imaging. In this review, the performance of NLR has been investigated for many deterministic and stochastic optical fields. Simulation and experimental results for different cases are presented and discussed. Preprints (www.preprints.org) | NOT PEER-REVIEWED |
Article
A high resolution telescope for imaging cosmic γ-ray sources in the MeV region, with an angular resolution better than 0.5° is being developed as ballon-borne payload. The instrument consists of a 3-D liquid xenon TPC as γ-ray detector, coupled with a coded aperture at a distance of 1 meter. A study of the actual source distribution of the 1.809 MeV line from the decay of 26Al and the 511 keV positron-electron annihilation line is among the scientific objectives, along with a search for new γ-ray sources. The telescope design parameters and expected minimum flux sensitivity to line and continuum radiation are presented. The unique capability of the LXe-TPC as a Compton Polarimeter is also discussed.
Article
Extensive Monte Carlo modeling of a coded aperture γ-ray telescope based on a high resolution liquid xenon TPC has been performed. Results on efficiency, background reduction capability and source flux sensitivity are presented. We discuss in particular the development of a reconstruction algorithm for events with multiple interaction points. From the energy and spatial information, the kinematics of Compton scattering is used to identify and reduce background events, as well as to improve the detector response in the few MeV region. Assuming a spatial resolution of 1 mm RMS and an energy resolution of 4.5% FWHM at 1 MeV, the algorithm is capable of reducing the background rate expected at balloon altitude by an order of magnitude, thus significantly improving the telescope sensitivity.
Article
The limiting sensitivities of coded-aperture imaging telescopes employing fixed masks are derived for continuum and line emission from cosmic point sources. The sensitivities are calculated for a single-source observation and do not take into consideration the many advantages offered by a multiplex system; for instance, low susceptibility to secular background changes and the ability to observe more than one source during an observation period. For the nuclear transition energy region, it is shown that the utilization of a coded-aperture mask by a particular detection system does not significantly degrade its performance relative to conventional, sequential scanning instruments. It is further shown that for short source observation times (e.g., typical of those obtained from stratospheric balloons), the coded-aperture imaging technique can be particularly advantageous. The effects of a non-uniform instrumental background on the imaging process are discussed and a correction procedure suggested. It is found that by careful planning of the observing program coupled with a stable instrument design, image degradation due to background non-uniformities can be made arbitrarily small and the resulting performance made to approach that predicted for an equivalent mask-antimask system.
Article
Uniform redundant-array masks have been reported as good lenses to form the image of gamma sources, with the correlation between the mask-aperture matrix and the shadows projected on a static position-sensitive detector. We present a modified uniform redundant-array configuration suitable for portable and small-size gamma cameras; its ability to reconstruct the image of several sources is analyzed. We have carried out a Montecarlo simulation of the gamma interactions in the mask, defining the expected response of the correlation process and comparing it with that achieved with the usual uniform redundant-array configurations.
Article
The gamma-ray imaging using coded aperture masks as focusing elements is an extended technique for static position sensitive detectors. Several transfer functions have been proposed to implement mathematically the set of holes in the mask, the uniformly redundant array collimator being the most popular design. A considerable amount of work has been done to improve the digital methods to deconvolve the gamma-ray image, formed at the detector plane, with this transfer function. Here we present a study of the behavior of these techniques when applied to the geometric shadows produced by a set of point emitters. Comparison of the shape of the object reconstructed from these shadows with that resulting from the analytical reconstruction is performed, defining the validity ranges of the usual algorithmic approximations reported in the literature. Finally, several improvements are discussed.
Article
Full-text available
We report the detection of a flaring gamma-ray source by the University of New Hampshire (UNH) balloon-borne coded aperture gamma-ray telescope (DGT) on 1984 October 2. The source was detected at the significance level of 7.2 sigma over the energy range 160-2000 keV. The intensity in the range (160-200) keV was 1.1 Crab. The best-fit position of the source is given by R.A. = 3h 25.8m and Decl. = 67 deg 653 min and is located in the constellation of Camelopardia. The source was visible within the Field of View (FOV) of the telescope for approximately = 2 hr and exhibited signs of flaring. The derived photon spectrum can be equally fitted by an optically thin bremsstrahlung distribution of kT approximately = 52 keV or a power law of the form, dN(E)/dE = 3.7 x 10(exp -6) (E/400)(exp -4.5) photons/sq cm/keV. We compare its spectral characteristics ad energy output to various types of fast X-ray transients. No measurable flux could be detected from CG 135+1, the COS B source which was in the FOV and therefore, we present 2 sigma upper flux limits on its spectral emission over the energy range 160 keV to 9.3 MeV.
Article
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The properties and the expected performance of a liquid xenon (LXe) gamma-ray imaging telescope, optimized for the MeV energy region, are presented. The unique potential of this telescope as a Compton polarimeter is particularly emphasized. Based on Monte Carlo simulations, we show that the modulation factor is as high as 40% at 1 MeV with a detection efficiency close to 20%. These figures of merit, combined with the excellent background suppression capability of the three-dimensional position sensitive LXe detector, yield sensitivity at the 3 sigma level to polarization fractions as small as a few percent for strong sources, even in a balloon flight.
Article
Full-text available
A balloon-borne coded-aperture telescope, measuring gamma-ray photons in the 160 keV to 9.3 MeV range, was used to observe the Cygnus region of the sky on October 1 and 2, 1984. In the 2-9.3-MeV band, evidence is found for a hard spectral component with a mean flux level at the top of the atmosphere of 7.4 + or - 2.5 x 10 to the -7th photons/sq cm per s per keV, inconsistent with the predictions of the inverse Compton models normally used to describe the X-ray emission. Both Cyg X-1 and Cyg X-3 could be observed simultaneously with the telescope. The results are used to establish 1-sigma upper flux limits on the spectral emission from Cyg X-3.
Article
Uniformly redundant array coded apertures have proven to be useful in the design of collimators for x-ray astronomy. They were initially expected to be equally successful in single-photon emission computed tomography (SPECT). Unfortunately, the SPECT images produced by this collimator contain artifacts, which mask the true picture and can lead to false diagnosis. Monte Carlo simulation has shown that the formation of a composite image will significantly reduce these artifacts. A simulation of a tumor in a compressed breast phantom has produced a composite image, which clearly indicates the presence of a 5 mm x 5 mm x 5 mm tumor with a 6:1 intensity ratio relative to the background tissue.
Article
Full-text available
Coded apertures for imaging problems are typically based on arrays having perfect cross-correlation properties. These arrays, however, guarantee a perfect point-spread function in far-field applications only. When these arrays are used in the near-field, artifacts arise. We present a mathematical derivation capable of predicting the shape of such artifacts. The theory shows that methods used in the past to compensate for the effects of background nonuniformities in far-field problems are also effective in reducing near-field artifacts. The case study of a nuclear medicine problem is presented to show good agreement of simulation and experimental results with mathematical predictions.
Article
Reconstruction of multiplex images produced by coded‐aperture telescopes for extended sources of radiation which cannot be reflected or refracted (moderate‐ to high‐energy x rays and low‐energy γ rays) can be accomplished by least‐squares solution of an overdetermined set of simultaneous equations linking the object, the aperture, and the image. A necessary condition is that the object field (an area of the sky) have a smaller number of pixels than the multiplex image recorded by the position‐sensitive detector, which requires that the projection of the object field onto the detector plane be smaller than the detector. This can be achieved by the use of an auxiliary completely open aperture of carefully chosen size and position (‘‘limiter’’) ahead of the coded aperature. The latter consists of regularly spaced elements which are made open or closed either randomly or according to some pattern, subject to solubility of the resulting set of equations. It may be larger or smaller than the detector, the undersize configuration having several advantages. A companion article [T. P. Kohman, Rev. Sci. Instrum. 60, 3410 (1989)] and supporting materials discuss various aspects of the implementation of the design for deployment on balloons or Earth satellites, describe a computer program developed for simulation of the operation of this telescope, and present the results of many simulations, which have demonstrated the feasibility of this approach to coded‐aperture imaging.
Article
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We report the detection of a gamma-ray burst by a balloon-borne gamma-ray telescope at 23:00:22 UT on 1988 May 11. The signal was detected over the energy range 42-205 keV and lasted for ∼80 seconds. The flux in this interval was detected at the 8.2 σ level and is equivalent to ∼1 Crab. Assuming a uniformly distributed source model, a 30 keV energy threshold and a 3 σ detection threshold, we estimate the burst rate to be (3.9 -3.2+9.0) × 104 yr-1 above a size of 7.7 × 10-9 ergs cm-2. The emission has been determined to originate from the direction of Ursa Major, and specifically, from within a 58° square box centered on celestial coordinates 166°.1 in right ascension and +38° in declination. A Fourier power spectrum of the burst data reveals the presence of a marginal (3.2 σ) periodicity of fundamental period (2.17 ± 0.12) s.
Article
LXeGRIT is a balloon-borne Compton telescope employing a large volume liquid xenon time projection chamber (LXeTPC) as the central gamma-ray detector. It is designed to image gamma rays in the energy range of 200 keV to 20 MeV, with an angular resolution of about 3 degrees (1 sigma) at 2 MeV, within a field-of-view (FOV) of about 1 sr. The detector's energy and three-dimensional spatial resolution as measured during pre-flight calibration experiments, are $\Delta E_\mathrm{lxe}/E=8.8% : \sqrt{1\MeV /E}$ and < 1mm RMS, respectively. The detection efficiency for Compton events varies between 1.5% and 4% depending on energy and event selection. We describe the instrument as flown on May 7, 1999 and review its overall performance at balloon altitude. The launch occurred at 13:26:54 UT from Ft. Sumner, New Mexico and the flight was terminated about 9 hours later. The Crab was in the instrument FOV for a few hours. Analysis of these data is in progress.
Conference Paper
A 7.6 cm x 7.6 cm BGO detector was flown in the middeck of the Space Shuttle Columbia August 8-13, 1989 as part of the Shuttle Activation Monitor (SAM) experiment. One of the goals of this experiment was to compare the performance of the BGO to that of NaI in the same environment. Twenty-four hours of data in five-minute time bins were recorded with each detector in each of two locations in a high-inclination orbit (57-degrees, 300 km). Pre- and post-flight low background counting was performed for identification of induced activities produced due to the exposure to the space radiation environment.
Article
A 7.6-cm×7.6-cm BGO (bismuth germanate) detector was flown in the middeck of the Space Shuttle Columbia August 8-13, 1989, as part of the Shuttle Activation Monitor (SAM) experiment. One of the goals of this experiment was to compare the performance of the BGO to that of NaI in the same environment. Twenty-four hours of data in 5-min time bins were recorded with each detector in each of two locations in a high-inclination orbit (57°, 300 km). Pre- aid post-flight low-background counting was preformed for identification of activities induced by the space radiation environment. A number of isotopes attributed to induced activity from exposure to the space radiation environment have been tentatively identified
Article
Most theoretical work on coded aperture masks in X-ray and low-energy gamma-ray astronomy has concentrated on masks with large numbers of elements. For gamma-ray spectrometers in the MeV range, the detector plane usually has only a few discrete elements, so that masks with small numbers of elements are called for. For this case it is feasible to analyze by computer all the possible mask patterns of given dimension to find the ones that best satisfy the desired performance criteria. A particular set of performance criteria for comparing the flux sensitivities, source positioning accuracies and transparencies of different mask patterns is developed. The results of such a computer analysis for masks up to dimension 5 x 5 unit cell are presented and it is concluded that there is a great deal of flexibility in the choice of mask pattern for each dimension.
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The Low Energy Gamma Ray Imager (LEGRI) on board the first MINISAT mission, is a telescope with imaging capabilities working in the 20-100 KeV spectral region. The main goal of LEGRI is to perform a galactic survey in order to observe compact objects like black hole candidates, neutron stars, pulsars and high mass X-ray binaries. By considering LEGRI sensitivity, integration times from 10 5 up to 10 6 seconds will allow to observe a total amount of ≈ 40 galactic objects. Furthermore, a few extragalactic objects (high luminosity X-ray binaries on the Magellanic Clouds and AGN) are expected to be observed. The pointing of the instrument to the objects of interest is one of the responsibilities of the LEGRI Science Operation Centre (SOC). The LEGRI SOC is a structure which contains typical elements of a scientific operation centre as well as those of a scientific data centre. In this paper a brief description of the LEGRI observational goals is presented together with the implementation of the different SOC tasks.
Article
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A new imaging balloon-borne telescope for hard X-rays in the energy range from 30 to 100 keV is described. The imaging capability is provided by the use of an extended URA-based coded-mask. With only one motor and suitable stop pins, we can rotate a carbon-fiber wheel with most of the mask elements attached to it by 180, and a bar, which is also part of the mask pattern and is allowed to rotate freely over the wheel, by 90; this combined rotation creates an antimask of the original mask, except for the central element. This is a novel and elegant manner of providing an antimask without additional weight and complex mechanical manipulations. We show that the use of antimasks is a very effective method of eliminating systematic variations in the background map over the position-sensitive detector area. The expected sensitivity of the instrument for the 30–100 keV range is of the order of 7 10-5 photons cm-2 s-1 keV-1, for an integration time of 104 seconds at a residual atmosphere of 3.5 g cm-2. This telescope will provide imaging observations of bright galactic hard X-ray sources with an angular resolution of 2 in a 10 by 10 FOV, which is defined by a collimator placed in front of the detector system. We are particularly interested in the galactic center region, where recent imaging results in X-rays have shown the presence of an interesting source field. Results of computer simulations of the imaging system are reported.
Article
The Low Energy Gamma Ray Imager (LEGRI) on board the Spanish MINISAT-01 mission is a position-sensitive γ-ray instrument with a 10 × 10 HgI2 elements array on the detector plane. This telescope is dedicated to imaging of astrophysical sources in the 20–200 keV spectral range. In the framework of LEGRI sensitivity evaluations, one of the focal points is the determination of the background noise induced by the cosmic and trapped proton flux. To achieve an accurate estimation of this background noise component, small HgI2 crystals were irradiated with high energy protons at SATURNE (Saclay, France). After the identification of the unstable isotopes generated by the proton flux, their production rates were calculated. The experimental data were compared with the results obtained by a Monte Carlo simulation code. Since a good agreement between both experimental and simulation results was observed, these last ones have been used to perform full calculations of the proton-induced background in LEGRI. The information provided by these calculations has been used to estimate an upper limit of LEGRI sensitivity.
Article
The nuclides inhaled during nuclear accidents usually cause internal contamination of the lungs with low activity. Although a parallel-hole imaging system, which is widely used in medical gamma cameras, has a high resolution and good image quality, owing to its extremely low detection efficiency, it remains difficult to obtain images of inhaled lung contamination. In this study, the Monte Carlo method was used to study the internal lung contamination imaging using the MPA-MURA coded-aperture collimator. The imaging system consisted of an adult male lung model, with a mosaicked, pattern-centered, and anti-symmetric MURA coded-aperture collimator model and a CsI (Tl) detector model. The MLEM decoding algorithm was used to reconstruct the internal contamination image, and the complementary imaging method was used to reduce the number of artifacts. The full width at half maximum of the I-131 point source image reconstructed by the MPA-MURA coded-aperture imaging reached 2.51 mm, and the signal-to-noise ratio of the simplified respiratory tract source (I-131) image reconstructed through MPA-MURA coded-aperture imaging was 3.98 dB. Although the spatial resolution of MPA-MURA coded-aperture imaging is not as good as that of parallel-hole imaging, the detection efficiency of PMA-MURA coded-aperture imaging is two orders of magnitude higher than that of parallel-hole collimator imaging. Considering the low activity level of internal lung contamination caused by nuclear accidents, PMA-MURA coded-aperture imaging has significant potential for the development of lung contamination imaging.
Article
Background: Due to the effect of near-field artifacts, the clearity of reconstructed image obtained by coded aperture imaging was decreased. Purpose: In order to explore a way to eliminate the effect and to enhance the image quality, a Geant4 simulation was performed in this work. Methods: On the basis of the principles of near-field artifact generation and elimination, coded aperture imaging of a 300 keV gamma source was simulated by Geant4 at near-field conditions with a mosaicked MURA(19×19) mask by single-and double-mode measurements, respectively. Results: The near-field artifacts were apparent in the images obtained by single-mode measurement, while they were almost eliminated by double-mode measurement. Similar results showed up in the imaging experiments with a commercial gamma camera. Conclusions: The near-field artifacts would be inhibited or even eliminated by the double-mode measurement otherwise from the conventional single-mode measurement, resulting with an enhanced signal-to-noise ratio (SNR) of reconstructed images.
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Article
A novel design of coded‐aperture x‐ or γ‐ray telescope is described in a companion article [Kohman, Rev. Sci. Instrum. 60, 3396 (1989)] A fortran computer program xgrtel developed for simulation of its operation is described briefly and illustrated with a representative simulation of the imaging of a synthetic object field with a small system; details are presented in supporting material. The simulation is displayed graphically, and significant results from numerous other simulations are presented. These explore the effects of various instrument parameters, object characteristics, etc., and indicate the overall imaging performance. Under appropriate circumstances excellent imaging, limited only by pixel‐size resolution and statistics of counting, is achieved. The point‐spread function contains no side‐lobes, ghost images, or excess noise when the least‐squares reconstruction method is used with random‐coded apertures, which yield just as good results as uniformly redundant‐array apertures. Disadvantages of the design are increased size and mass, possible detector‐background enhancement by the limiter structure, and relatively large amount of computer time required for large systems.
Article
Article
Cosmic gamma-rays are observed to be emitted on a wide range of angular scale and structure from a diverse population of astronomical objects. For the case of Galactic emission a mixture of discrete spectral lines and continuum radiation is derived from point sources, localised regions, as well as a diffuse band along the entire Galactic plane. In the extragalactic context sources range from the “point-like” active galactic nuclei, for which fine angular resolution studies would be a notable advantage, to the all pervading cosmic diffuse gamma-ray background. Nearby galaxies such as the Large Magellanic Cloud and nearby clusters of galaxies provide targets for which a combination of point source and extended emission studies are required. Solar flares require fine angular gamma-ray imaging within a small field of view whereas the enigmatic gamma-ray bursters pose the challenge of the precise positional location of “random” short lived events which arrive isotropically. In this article we have evaluated the imaging requirements of the next generation of gamma-ray telescopes for the known classes of gamma-ray emitting objects and compared them to the performance characteristics achievable with current instrument technology.
Article
High background counting rates are the main limitation on sensitivity of satellite borne gamma-ray instrumentations. The observed background comes from different sources: cosmic diffuse, charged particles and high energetic photons. Among the different background components, that due to the activation of the telescope and satellite passive materials by cosmic protons is the most difficult component to evaluate.In the framework of background studies and sensitivity estimations for the INTEGRAL and MINISAT-01 projects, a wide range of materials has been irradiated with proton beams at different energies to identify the induced unstable isotopes and their production cross-sections.In this paper we present experimental results obtained from the analysis of such irradiation experiments. These values are compared to those obtained by means of the two frequently used prediction methods: GEANT/GCALOR Monte-Carlo code and Silberberg and Tsao semiempirical expressions.
Article
During a Nuclear Medicine project that called for the optimal design of a coded aperture we found that low-throughput masks do not always provide a Signal-to-Noise Ratio (SNR) advantage. In this paper, we present the simulations of the performance of some coded aperture patterns chosen from different families and compare the results with theoretical predictions. A general expression for the SNR and its particular form for different patterns are provided. The choice of the optimal pattern family is discussed with reference to the characteristics of the object to be imaged and in light of the effect of near-field artifacts. No-Two-Holes-Touching (NTHT) arrays based on Modified Uniformly Redundant Arrays (MURAs) proved to offer the best compromise between SNR performance and practical fabrication constraints. r 2001 Elsevier Science B.V. All rights reserved.
Article
Protons generate one of the most important contributions to the background noise in space gamma-ray telescope observations. This contribution comes from their direct interactions with the detector as well as surrounding materials. Cosmic and geomagnetically trapped protons are the main sources of this proton flux. An accurate estimation of this background component before the construction of a telescope is a key point in order to ensure the foreseen instrument performances. In this paper we present a comparison between experimental spectra acquired by CsI(Tl) detector units irradiated with 0.2 and 1.0 GeV protons and the results provided by a complete modelling of these units using different data of unstable isotope production cross-sections (experimental, semiempirical expressions of Silberberg and Tsao and the package GEANT/GCALOR based on Monte Carlo techniques). In all cases a better reproduction of the experimental spectra has been obtained by using the experimental production cross-sections. Data obtained from Silberberg and Tsao expressions generate a more accurate modelling than the GEANT/GCALOR approximations.
Article
High levels of background radiation are one of the major problems associated with satellite-borne telescopes operating in the gamma-ray domain. Although there are many contributions to this background, cosmic-proton induced radioactivity is perhaps the most difficult to remove because of the delayed gamma-emission.As a part of the INTEGRAL project a series of studies have been performed on a range of materials to determine the production cross sections for unstable isotopes produced by energetic protons. In this paper we present results for the irradiation of small (5×5×2 mm) cadmium telluride (CdTe) crystal with 1.7 GeV protons. Data are given for experimentally determined production cross sections which are compared to simulation predictions by the GEANT/GCALOR code.
Article
Mercuric iodide (HgI2) semiconductor crystals are generating a lot of interest as room temperature solid state detectors for hard X-ray astronomy observations. For these applications one of the most important background sources is the cosmic proton induced radioactivity in the detector material. In order to study this background noise contribution a 1×1×1 cm HgI2 crystal was irradiated with high energy protons. The resulting long-lived unstable isotopes and their production rates have been identified and compared with Monte Carlo simulations.
Article
Two major missions of Surveillance systems are imaging and ground moving target indication (GMTI). Recent advances in coded aperture electro optical systems have enabled persistent surveillance systems with extremely large fields of regard. The areas of interest for these surveillance systems are typically urban, with spatial topologies having a very definite structure. We incorporate aspects of a priori information on this structure in our aperture code designs to enable optimized dealiasing operations for undersampled focal plane arrays. Our framework enables us to design aperture codes to minimize mean square error for image reconstruction or to maximize signal to clutter ratio for GMTI detection. In this paper we present a technical overview of our code design methodology and show the results of our designed codes on simulated DIRSIG mega-scene data.
Article
The previously described design of a cosmic x- or γ-ray telescope with least-squares image reconstruction has been enhanced to provide for mask-antimask operation. This cancels and eliminates uncertainties in the detector background. The simulations of its operation have been made more realistic by incorporating instrumental blurring of sources. A second-stage least-squares procedure determines the precise positions and total fluxes of point sources responsible for clusters of above-background pixels in the field resulting from the first-stage reconstruction. Another program converts source positions in the image plane to celestial coordinates and vice versa, the image being a gnomic projection of a region of the sky. A complete sky-to-sky simulation of the imaging of a source field is presented. It is demonstrated that the point-spread function of the overall imaging process is essentially perfect. © 1997 American Institute of Physics.
Article
An instrument is described which will provide a direct image of γ-ray line or continuum sources in the energy range 300 keV-10 MeV. We illustrate the use of this instrument to study the celestial distribution of the 26Al isotope by observing the 1.809 MeV de-excitation γ-ray line. The source location accuracy is 2' or better. The imaging telescope is a liquid xenon time projection chamber coupled with a coded aperture mask (LXeCAT). This instrument will confirm and extend the COMPTEL observations from the Compton Gamma Ray Observatory (CGRO) with an improved capability for identifying the actual Galactic source or sources of 26Al, which are currently not known with certainty. Sources currently under consideration include red giants on the asymptotic giant branch, novae, Type Ib or Type II supernovae, Wolf-Rayet stars, and cosmic rays interacting in molecular clouds. The instrument could also identify a local source of the celestial 1.809 MeV γ-ray line, such as a recent nearby supernova.
Article
The past several years have seen unprecedented growth in the field of gamma-ray astronomy. Highly successful missions such as the Compton Gamma-Ray Observatory (CGRO) have led to both a great increase in the number of detected gamma-ray sources and a more fundamental understanding of the basic physical processes involved for those sources. New ground-based observatories, the Rossi X-Ray Timing Explorer (RXTE), and the SIGMA instrument aboard the GRANAT spacecraft have all contributed to this explosion. Detailed observations of active galaxies, pulsars, accreting binaries, and diffuse emission have had a tremendous impact on our view of the universe. Given that new experiments that will provide a similar increase in source numbers are several years away, it is a good time to take inventory of the state of gamma-ray astronomy. To this end, we have developed a general gamma-ray point-source catalog containing 309 objects that summarize the field. Gamma-ray astronomy, as we define it, includes photon energies from 50 keV to about 1 TeV. While many catalogs concentrate on a single type of astronomical object and/or a very restricted energy range, the nature of this catalog is somewhat different. The large variety of objects and the many orders of magnitude in energy space covered by gamma-ray astronomy presents an organizational challenge. We focus on two main types of information: a general listing of the basic characteristics of each source, and detailed tables of a representative sample of high-energy observations. We also summarize the gamma-ray instruments whose observations are included in the catalog.
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The primary scientific mission of the Black Hole Finder Probe (BHFP), part of the NASA Beyond Einstein program, is to survey the local Universe for black holes over a wide range of mass and accretion rate. One approach to such a survey is a hard X-ray coded-aperture imaging mission operating in the 10--600 keV energy band, a spectral range that is considered to be especially useful in the detection of black hole sources. The development of new inorganic scintillator materials provides improved performance (for example, with regards to energy resolution and timing) that is well suited to the BHFP science requirements. Detection planes formed with these materials coupled with a new generation of readout devices represent a major advancement in the performance capabilities of scintillator-based gamma cameras. Here, we discuss the Coded Aperture Survey Telescope for Energetic Radiation (CASTER), a concept that represents a BHFP based on the use of the latest scintillator technology. Comment: 12 pages; conference paper presented at the SPIE conference "UV, X-Ray, and Gamma-Ray Space Instrumentation for Astronomy XIV." To be published in SPIE Conference Proceedings, vol. 5898
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Coded-mask telescopes which are considered as one of the best solutions for producing images of the sky in hard X-ray and gamma-ray astronomy, because of their optimum signal-to-noise (SNR) ratio, are discussed. It is shown that for a given field of the sky, the mask-pattern design and the decoding technique used, influence the values of the SNR in the individual pixels of the reconstructed image. In different conditions of background and source fluxes, the expressions for the SNR of different replicated masks are derived for the inversion and correlation methods. It is shown that for the inversion method, the SNR is inversely proportional to the square root of the trace of the matrix associated with the mask, whereas for the correlation method, the SNR is limited by an additional systematic term which is due to the cross-talk between sources in the field of view and whose effect is avoided by choosing a uniformly redundant array pattern.
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In view of the very small number of extragalactic objects that have been detected so far as gamma-ray energies, the considered discussion centers on the mechanisms for the production of gamma-rays in the nuclei of active galaxies. Seyfert galaxies and the significance of gamma-ray emission for studying them are considered, taking into account the general properties of Seyferts, a list of X-ray Seyferts, the correlation between the X-ray flux and emission at other wavelengths, the galaxy NGC 4151, and X- and gamma-ray observations of NGC 4151. Other X- and gamma-ray active galaxies are also examined, giving attention to quasars, BL Lacertae objects, and NGC 5128. A description is presented of sources of power in X- and gamma-active galactic nuclei, taking into consideration Compton models, and massive black hole models. All of the observations of extragalactic objects at gamma-ray wavelengths indicate that a break must occur in the spectrum at low gamma-ray energies.
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Available experimental spallation data have been used to construct a five-parameter formula describing the cross-section distribution for a wide range of experimental conditions with a fair degree of accuracy. The application of the cross-section formula to different fields of research is indicated.
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Coded aperture imaging in high energy astronomy represents an important technical advance in instrumentation over the full energy range from X- to -rays and is playing a unique role in those spectral ranges where other techniques become ineffective or impracticable due to limitations connected to the physics of interactions of photons with matter. The theory underlying this method of indirect imaging is of strong relevance both in design optimization of new instruments and in the data analysis process. The coded aperture imaging method is herein reviewed with emphasis on topics of mainly practical interest along with a description of already developed and forthcoming implementations.
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The region of the Galactic anticenter, including the Crab Nebula, was observed during a balloon flight of the University of New Hampshire Directional Gamma-Ray Telescope employing the coded-aperture imaging technique to image celestial gamma-radiation between 160 keV and 9.3 MeV. The background systematics are treated with a simple and relatively straightforward correction procedure. The results demonstrate that the coded-aperture procedure is a viable approach for imaging not only point sources of radiation, but also extended sources of emission. The results for the Crab's photon spectrum are consistent with a power-law spectrum. Upper limits on the flux levels of line emission at 405 keV and 1050 keV and on the flux from the X-ray binary source A0535 + 26 and diffuse Galactic emission from the anticenter region are derived.
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Evaluation of calculations and experimental evidence from 600-MeV proton irradiation indicating that cosmic-ray-induced radioactivity in detectors used to measure the diffuse gamma-ray background produces a significant counting rate in the energy region around 1 MeV. It is concluded that these counts may be responsible for the observed flattening of the diffuse photon spectrum at this energy.
Article
A problem with telescopes utilizing replicated structure coded masks is the possible occurrence of virtual images. A numerical method to discriminate virtual from real images, and to determine their correct location in the field of view, is described. The performance and reliability of this method, based on eight “ad hoc” modifications of the coding array, have been investigated by Monte Carlo simulations.
Article
Measurements of the spectral and angular distributions of atmospheric gamma rays in the energy range 0.3-10 MeV over Palestine, Texas, at residual depths of 2.5 and 70 g/cm 2 are reported here. In confirmation of the general features of a model prediction (Ling, 1975) the measurements show at 2.5 g/cm 2 upward moving fluxes greater than the downward moving fluxes, the effect increasing with energy, and approximate isotropy at 70 g/cm . Numerous characteristic gamma ray lines were observed, most prominently at 0.511, 1.6, 2.3, 4.4, and 6.1 MeV. Their intensities were also compared with model predictions. Observations were made with an actively shielded scintillator counter with two detectors, one of aperture 50 ø FWHM and the other of120 ø FWHM. Above 1 MeV, contributions to the counting rate from photons penetrating the shield annulus and from neutron interactions were large; they were studied by means of a Monte Carlo code and are extensively discussed here.
Article
The propagation of the primary cosmic radiation through the earth's atmosphere is a phenomenon of considerable intrinsic complexity stemming from the large number and variety of parameters involved and their dependence on energy, latitude, and atmospheric depth. Although in the past the study of the hadronic component in the atmosphere has received appreciable attention, not much interest has been focused on the electron-photon component until very recent years. As a consequence, the observational data now available on the latter are meager and fragmentary; such information as is available is reviewed here, and the existing gaps in our knowledge are highlighted. Because of the very complexity of the problem, it has been recognized that extensive and refined calculations are needed to understand the observations and to provide new motivation to experimenters. In the present article the limited theoretical calculations so far attempted are first reviewed; a summary of detailed calculations recently carried out by the present authors follows. Finally, the results from the calculations are compared with those available from experiments; wherever it is necessary, attempts are made to identify the areas in which further work is needed.
Spectral distribution and yields of scintillation pulses from the prompt interaction of neutrons with 7.6 cm diameter × 7.6 cm long bismuth germanate (BGO) and NaI(Tl) detectors have been measured. Neutrons at energies between 0.4 and 10 MeV were produced by the 7Li(p, n) and the 197Au(p, n) reactions, respectively, and identified by their time-of-flight relative to the pulsed proton beam. For both scintillators the neutron response is dominated by (n, n′γ) reactions, and the efficiencies are in qualitative agreement with simple estimates that depend on the non-elastic and elastic cross sections, and on two geometrical factors. Compared to NaI(Tl), BGO exhibits a much superior gamma-ray-to-neutron detection ratio.
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.
Article
The background which arises from the presence of a coded aperture. The major contributions which have been considered here are the interactions with the mask of the isotropic gamma-ray background, a parallel gamma-ray beam, neutrons and the effect of the mask element profile. It is shown that none of these factors contribute to a significant excess or modulation in the background counting rate over the detection plane. In this way the use of a passive rather than an active coded apertuer mask is seen to be suitable for use in a low energy gamma-ray telescope.
Article
The proton component has been extensively studied at high altitudes on a series of 6 Skyhook balloon flights at various latitudes using the Cerenkov-scintillation counter technique. The intensity of primary protons has been measured at 4°, 41°, 53°, and 55° geomagnetic latitude. The proton differential energy spectrum has been measured directly in the region 250-750 Mev. It is observed that primary alphas and protons have the same form of rigidity spectrum from 1 Bv to 17 Bv. A series of cutoff rigidities are measured in the vicinity of lambda=53° and 55°. The intensity and composition of fast splash albedo is determined at lambda=4°, 53°, and 55°. Significant time variations are observed between the three high-latitude flights. Differences observed in the intensity and energy spectra of these three flights are discussed and strong restraints are placed on possible modulating mechanisms.
Article
The diffuse cosmic and the vertical atmospheric component of 1-20 MeV gamma rays are determined from the gamma-ray data obtained during a balloon flight of the large-area Compton telescope of the Max-Planck-Institut. It is shown that the basic improvement of this new measurement above previous measurements with a first version of the Compton telescope is based on the fact that for each registered event various additional parameters are determined. Further, by means of the pulse shape measurement of neutron-induced events it is possible to determine most of the undesired background events from the data themselves. Finally, consideration is given to the origin of the diffuse cosmic gamma-ray component, and the results of the atmospheric gamma-ray component are compared with different model calculations.
Article
It is found that polar solvents provide an effective means for removing surface contaminants and discoloration from NaI(Tl) crystals and can offer a simple alternative to traditional grinding or cleaving.
Article
A prototype imaging telescope of a kind that could be used in a future γ-ray astronomy mission has been tested using a quasi-point source of 20 MeV γ radiation. The results have demonstrated the ability to reconstruct images of the source at various positions within the field of view of the telescope. The point-spread function of the telescope was improved by nearly two orders of magnitude over that of a track chamber alone through the use of a coded aperture mask. This work has demonstrated for the first time the possibility of achieving arcminute source location with future γ-ray space telescopes. This would enable point-like γ-ray sources to be located with that precision while extended objects could be mapped with aresolution of about 10 arc min.
Article
The response of a cylinder to neutrons in the pulse height region between 2.5 and 7 MeV gamma energy equivalent was studied as a function of incident neutron energy for a range of neutron energies between room scattered neutrons and 1.162 MeV. The Li7(p, n) Be7 reaction was used as a source of neutrons since it is relatively free from high energy gamma ray background. Monoergic neutron response curves were obtained for neutron energies of 166 keV, 341 keV, and 469 keV. All other distributions were for proton energies giving rise to two neutron groups. The pulse height distributions were measured for a fixed number of counts of a shielded long counter at each neutron energy. The long counter was calibrated against Ilford C2 emulsions for the case of 1.162 and 0.698 MeV neutrons in order to obtain the neutron flux from the long counter counts. The pulse height distributions are nearly linear between 4 and 6.6 MeV. The extrapolations of these distributions intersect the gamma equivalent axis at approximately the binding energy of the last neutron in I128.
Article
The general behaviour of the atmospheric -ray flux as a function of the residual atmosphere, zenith angle, and photon energy is approximated with simple analytical expressions. The free parameters are determined by comparison with the most recent models and the existing experimental measurements. The resulting formulae have an accuracy comparable to the mutual consistency of the available pieces of information, and at the same time are very convenient to use in numerical simulations.
Article
Laboratory experiments have been performed to demonstrate the capabilities of a Â¿-ray imaging system employing a NaI Anger camera and a rotating coded aperture mask. The mask incorporates in its design a new type of hexagonal uniformly redundant array (HURA) which is essentially antisymmetric under 60Â° rotation. The image formation techniques are described and results are presented that demonstrate the imaging capability of the system for individual and multiple point sources of Â¿-ray emission. The results are cornpared to analytical predictions for the imaging and point source localization capabilities of coded aperture systems using continuous detectors.
Article
On June 10, 1974, a new high-resolution gamma ray spectrometer operating in the energy range of 0.05 to 10 MeV was succesfully balloon-flight tested. This system uses four 40 cm3 Ge(Li) crystals in a single cryostat as prime sensors. A 6.35 cm thick CsI (Na) shield provides background rejection and collimation, defining a solid angle of 0.095 sr. Cooling is provided by a liquid-nitrogen dewar. The system was highly stable with spectral resolution of better than 2.5 keV at 1 MeV for the flight duration. The background total count rate was about 20 counts/s. Numerous lines were observed, most of which originated from secondary neutron interactions in the spectrometer. The sensitivity of the system to line emissions originating in cosmic point sources is 10−4-10−3 photons/cm2 s over the energy range.
Article
Coded aperture imaging with uniformly redundant arrays (URAs) is the standard technique for imaging above the limit of grazing incident x-ray telescopes. It is an ideal technique for high energy astrophysics because it has high throughput, excellent performance on point sources, and the ability to measure simultaneously signal and background. However, many sources of interest in high energy astrophysics are time variable or require detailed energy spectra. Until now, to obtain a single time (or energy) sample, the photons from the particular time (or energy) interval must be formed into an encoded pattern, then processed to obtain an image for that sample. Therefore, massive computations are required to cover the entire time and energy parameter space. We present a new method of coded aperture analysis called URA tagging, which provides time and/or energy resolved histories of sources with known positions without using a correlation operation. It can easily reduce the computation time by orders of magnitude ompared to the next fastest method, the fast delta Hadamard transform. URA tagging can also correct for improperly encoded images or motion blurred images. Whereas previous methods for quantifying performance have not taken into account the finite resolution or the quantized sampling, URA tagging provides a SNR equation that includes all such effects. URA tagging analysis explains why delta decoding has a somewhat poorer SNR than balanced correlation; naively, one expects the better angular resolution to yield a better SNR. In addition, we show that complementary URAs (exchanged opaque and transparent elements) have different properties, and those with an even number of transparent elements should be preferred.
Article
Coded aperture imaging uses many pinholes to increase the SNR for intrinsically weak sources when the radiation can be neither reflected nor refracted. Effectively, the signal is multiplexed onto an image and then decoded, often by computer, to form a reconstructed image. We derive the modulation transfer function (MTF) of such a system employing uniformly redundant arrays (URA). We show that the MTF of a URA system is virtually the same as the MTF of an individual pinhole regardless of the shape or size of the pinhole. Thus, only the location of the pinholes is important for optimum multiplexing and decoding. The shape and size of the pinholes can then be selected based on other criteria. For example, one can generate self-supporting patterns, useful for energies typically encountered in the imaging of laser-driven compressions or in soft x-ray astronomy. Such patterns contain holes that are all the same size, easing the etching or plating fabrication efforts for the apertures. A new reconstruction method is introduced called delta decoding. It improves the resolution capabilities of a coded aperture system by mitigating a blur often introduced during the reconstruction step.
Article
Uniformly redundant arrays (URA) have autocorrelation functions with perfectly flat sidelobes. The URA combines the high-transmission characteristics of the random array with the flat sidelobe advantage of the nonredundant pinhole arrays. A general expression for the signal-to-noise ratio (SNR) has been developed for the URA as a function of the type of object being imaged and the design parameters of the aperture. The SNR expression is used to obtain an expression for the optimum aperture transmission. Currently, the only 2-D URAs known have a transmission of (1/2). This, however, is not a severe limitation because the use of the nonoptimum transmission of (1/2) never causes a reduction in the SNR of more than 30%. The predicted performance of the URA system is compared to the image obtainable from a single pinhole camera. Because the reconstructed image of the URA contains virtually uniform noise regardless of the original object's structure, the improvement over the single pinhole camera is much larger for the bright points than it is for the low intensity points. For a detector with high background noise, the URA will always give a much better image than the single pinhole camera regardless of the structure of the object. In the case of a detector with low background noise, the improvement of the URA relative to the single pinhole camera will have a lower limit of ~(2f)(-(1/2)), where f is the fraction of the field of view that is uniformly filled by the object.
Article
Uniformly redundant arrays (URA) have autocorrelation functions with perfectly flat sidelobes. The URA combines the high-transmission characteristics of the random array with the flat sidelobe advantage of the nonredundant pinhole arrays. This gives the URA the capability to image low-intensity, low-contrast sources. Furthermore, whereas the inherent noise in random array imaging puts a limit on the obtainable SNR, the URA has no such limit. Computer simulations show that the URA with significant shot and background noise is vastly superior to random array techniques without noise. Implementation permits a detector which is smaller than its random array counterpart.
Article
The photofraction of a 5.1 cmn Ã 2.0 cm bismuth germanate (BGO) scintillator was measured over a Â¿-ray energy range of 0.2 to 6.1 MeV. Several methods, used to minimize the effect of "room scattering" on the measurement, are discussed. These include a Â¿-Â¿ coincidence technique, a Ã-Â¿ Coincidence technique, and the use of sources calibrated with a standard 7.6 cm Ã 7.6 cm sodium iodide (NaI) scintillator.
Article
A gamma ray telescope is being developed to operate in the energy range 100 keV to 5 MeV utilizing coded aperture imaging. The design incorporates a mask pattern based on a Uniformly Redundant Array (URA), which has been shown to have ideal imaging characteristics. A mask-anti-mask procedure is used to eliminate the effects of any possible systematic variations in detector background rates. The detector array is composed of 35 elements of the high-Z material Bismuth Germanate (BGO). Results of laboratory testing of the imaging properties will be presented. A southern hemisphere balloon flight is planned for 1982 with the goal of observing the 0.511 MeV radiation from the Galactic Center. Computer calculations show that a point source of this radiation can be located to within Â±1Â°.
Article
A nuclear radiation detector gain stabilizing scheme has been developed which offers the unique advantage of having only one stable operating gain position by utilizing a gated radioactive source and broad digital windows. This scheme can be used on space missions or other remote locations where turn-on adjustments would be difficult or impossible, as well as on large detector arrays where such adjustments would be very time consuming. The technqiue has been incorporated into the Gamma Ray Spectrometer on NASA's Solar Maximum Mission, where an overall pulse height resolution of ~ 7% at 0.662 MeV, with a seven-detector array of 7.6 cm Ã 7.6 cm NaI scintillators, is realized. This work was supported by NASA under Grant NGL 30-002-021 and contract NAS 5-23761.
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