Gamma-Ray Imaging Spectrometer Research Articles

Radiation Monitoring for Volatilized Zinc Contamination Using Gamma-Ray Imaging and Spectroscopy

Abstract Gamma-ray imaging is a tool that has grown in importance in the applications of non-destructive assay (NDA) for radioactive survey and analysis of nuclear facilities. Imaging techniques have shown great promise in providing valuable information involving radioactive waste management and contamination prevention. For the application studied in this work, 65Zn has been identified as a radioactive contaminant during tritium extraction. Due to the volatile nature of 65Zn under the pressure and temperature changes during extraction operations, 65Zn can easily travel through components of the extraction system as vapor, making it difficult to trap. Previous research involving the development of a filtration system showed that the 65Zn can be trapped, mitigating product contamination. However, during the extraction process, direct analysis of the equipment to confirm that zinc contamination is trapped in the filter and has not spread to other components is impractical. In this situation, the need to assay the location of the contamination with little-to-no interference with operations is vital. In this work, we demonstrate the use of a commercialized 3D position-sensitive CdZnTe (CZT) gamma-ray imaging spectrometer to provide analysis of the 65Zn contamination. Onsite measurements during an extraction process are studied to assess the location and migration of the 65Zn. The results obtained from real-time glovebox monitoring demonstrate the feasibility of gamma-ray imaging for localizing the contamination and providing a preliminary qualitative assessment that is intended to be used in future work quantifying the contamination build-up and activity over time.

Qualitative measurement of spatial shielding isotopics via Compton imaging neutron-induced gamma rays using 3-D CdZnTe detectors

Abstract Characterization of shielding around special nuclear materials (SNMs) in unknown objects is important for homeland security, international safeguards and potential nuclear disarmament verification. Shielding modulates source photons and neutrons, complicating radiation-based SNM characterization, and indicates object purpose and guides response. Previous gamma-ray-based shielding detection algorithms only estimate effective shielding atomic number and areal thickness, complicating the characterization of compound shields comprised of multiple elements. Neutron-induced gamma rays, from inelastic scatter or capture, provide a complementary, isotope-specific shielding signal which can be localized using Compton imaging. Qualitative, angularly-resolved isotopic distributions in a compound shield were measured using polyethylene and polyvinyl chloride targets, excited via a single PuBe neutron source, and a 3-D, position-sensitive CdZnTe imaging spectrometer. Hydrogen capture gamma rays, which predominately originate from the polyethylene target, were well separated in angular space from chlorine capture gamma rays, originating in the polyvinyl chloride target, using Compton imaging. The described qualitative probe of spatial shielding isotopics can be immediately applied on any commercial gamma-ray imaging spectrometer without modification.

SU-C-201-03: Coded Aperture Gamma-Ray Imaging Using Pixelated Semiconductor Detectors

Purpose: Improved localization of gamma-ray emissions from radiotracers is essential to the progress of nuclear medicine. Polaris is a portable, room-temperature operated gamma-ray imaging spectrometer composed of two 3×3 arrays of thick CdZnTe (CZT) detectors, which detect gammas between 30keV and 3MeV with energy resolution of <1% FWHM at 662keV. Compton imaging is used to map out source distributions in 4-pi space; however, is only effective above 300keV where Compton scatter is dominant. This work extends imaging to photoelectric energies (<300keV) using coded aperture imaging (CAI), which is essential for localization of Tc-99m (140keV). Methods: CAI, similar to the pinhole camera, relies on an attenuating mask, with open/closed elements, placed between the source and position-sensitive detectors. Partial attenuation of the source results in a “shadow” or count distribution that closely matches a portion of the mask pattern. Ideally, each source direction corresponds to a unique count distribution. Using backprojection reconstruction, the source direction is determined within the field of view. The knowledge of 3D position of interaction results in improved image quality. Results: Using a single array of detectors, a coded aperture mask, and multiple Co-57 (122keV) point sources, image reconstruction is performed in real-time, on an event-by-event basis, resulting in images with an angular resolution of ∼6 degrees. Although material nonuniformities contribute to image degradation, the superposition of images from individual detectors results in improved SNR. CAI was integrated with Compton imaging for a seamless transition between energy regimes. Conclusion: For the first time, CAI has been applied to thick, 3D position sensitive CZT detectors. Real-time, combined CAI and Compton imaging is performed using two 3×3 detector arrays, resulting in a source distribution in space. This system has been commercialized by H3D, Inc. and is being acquired for various applications worldwide, including proton therapy imaging R&D.

The Polaris-H imaging spectrometer

Recently, H3D has designed and introduced a gamma-ray imaging spectrometer system named Polaris-H. Polaris-H was designed to perform gamma spectroscopy and imaging throughout nuclear power plants. It integrates a 3D-position-sensitive pixelated CZT detector (20mm×20mm×15mm), associated readout electronics, an embedded computer, a 5-h battery, and an optical camera in a portable water-proof enclosure. The total mass is about 4kg, and the system startup time is 2min. Additionally, it has a connection for a tablet, which displays a gamma-ray spectrum and isotope-specific images of the gamma-ray distribution in all directions in real time. List-mode data is saved to an external USB memory stick. Based on pixelated depth-sensing technology, spectroscopy is routinely better than 1.1% FWHM at 662keV, and imaging efficiency at 662keV varies less than a factor of two for all directions, except through the battery. Measurements have been performed in contaminated environments, in high radiation fields, and in cramped quarters.

Gamma-ray energy-imaging integrated spectral deconvolution

In conventional Compton camera systems, the image reconstruction is performed only in two-dimensional or three-dimensional spatial coordinates for a specific gamma-ray energy. By doing so, a priori knowledge of the incident gamma-ray energy is required, and usually an energy window is applied to select full energy deposition events. In some other applications, spectral-deconvolution algorithms were developed to estimate the incident gamma-ray spectrum by deconvolving the observed energy-loss spectrum. However, usually the spectral system response function of a non-spherical detector depends on the incident gamma-ray's direction, which cannot be modeled by those spectral-deconvolution algorithms. In this paper, we propose a new energy-imaging integrated spectral-deconvolution method, which utilizes both the Compton imaging and the spectral-deconvolution techniques. In the new method, the deconvolution takes place in a integrated spatial and energy space. This technique eliminates the requirement of knowing the gamma-ray energy in the imaging part, and removes the directional dependence in the spectral-deconvolution part. The deconvolved result provides the image at any specific energy, as well as the spectrum at any specific direction. The deconvolution method is based on the maximum likelihood expectation maximization (MLEM) algorithm, which is popular in reconstructing photon-emission images. Since the ML solution estimates the true incident gamma-ray intensity, the deconvolved energy spectrum at the source location is free of Compton continuum. To truthfully reconstruct the source distribution from the observation data, the accuracy of the system response function t ij , i.e. the probability for a photon from source pixel j to be observed as event i , is the most crucial information. Because of the large number of pixels in the energy-imaging integrated space, and the very large number of possible measurement events, it is impossible to pre-calculate the system response function t ij by simulations. In this paper, an analytical approach is introduced so that the system response function can be calculated during the reconstruction process. In order to perform Compton imaging, gamma-ray detectors are required to have position-sensing capability. The energy-imaging integrated deconvolution algorithm is applied to a three-dimensional position-sensitive CdZnTe gamma-ray imaging spectrometer, which can provide not only the energy-deposition information, but also the position information of individual gamma-ray interactions. The results demonstrate that the technique is capable of deconvolving the energy spectrum and of reconstructing the image simultaneously.

The Reuven Ramaty High Energy Solar Spectroscopic Imager Observation of the 1809 keV Line from Galactic 26 Al

Observations of the central radian of the Galaxy by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) have yielded a high-resolution measurement of the 1809 keV line from 26Al, detected at 11 σ significance in 9 months of data. The RHESSI result for the width of the cosmic line is 2.03 keV FWHM. The best-fit line width of 5.4 keV FWHM reported by Naya et al. using the Gamma-Ray Imaging Spectrometer balloon instrument is rejected with high confidence.

Gamma-Ray Limits on Galactic [TSUP]60[/TSUP]F[CLC]e[/CLC] Nucleosynthesis and Implications on the Origin of the [TSUP]26[/TSUP]A[CLC]l[/CLC] Emission

The Gamma-Ray Imaging Spectrometer recently observed the gamma-ray emission from the Galactic center region. We have detected the 1809 keV Galactic 26Al emission at a significance level of 6.8 σ but have found no evidence for emission at 1173 and 1332 keV, which is expected from the decay chain of the nucleosynthetic 60Fe. The isotopic abundances and fluxes are derived for different source distribution models. The resulting abundances are between 2.6±0.4 and 4.5±0.7 M for 26Al, and a 2 σ upper limit for 60Fe is between 1.7 and 3.1 M. The measured 26Al emission flux is significantly higher than that derived from the Compton Gamma Ray Observatory COMPTEL 1.8 MeV sky map. This suggests that a fraction of the 26Al emission may come from extended sources with low surface brightness that are invisible to COMPTEL. We obtain an60Fe/26Al flux ratio 2 σ upper limit of 0.14, which is slightly lower than the 0.16 predicted from current nucleosynthesis models assuming that Type II supernovae are the major contributors to the Galactic 26Al. Since the uncertainties in the predicted fluxes are large (up to a factor of 2), our measurement is still compatible with the theoretical expectations.

A Maximum Entropy Map of the 511 [CLC]ke[/CLC]V Positron Annihilation Line Emission Distribution Near the Galactic Center

We have applied the maximum entropy method to a large data set based on Compton Gamma Ray Observatory (CGRO)/OSSE, Solar Maximum Mission, Transient Gamma-Ray Spectrometer, Gamma-Ray Imaging Spectrometer, HEXAGONE, and FIGARO observational results of the Galactic center 511 keV radiation in order to produce a map of the 511 keV line emission. This map suggests two components: a central bulge with a flux of 5.7 ± 0.29 × 10-4 photons cm-2 s-1 and a Galactic plane (GP) component with a flux of 2.2 ± 0.4 × 10-4 photons cm-2 s-1. The central bulge is located at l = -047 ± 024, b = 01 ± 018, and FWHM = 528 ± 048. We note that the position of the GP component coincides with a strong hot spot in the COMPTEL map of 1.8 MeV26Al line emission. A comparison between CGRO/OSSE and other instruments with larger field of view suggests an extended diffuse 511 keV emission. An interesting hot spot at l = -4°, b = 7° with a flux of ~2 × 10-4 photons cm-2 s-1 is shown on our map. A bootstrap test indicates that the significance level of this feature is 3.5 σ.

GRABIT: a large-area, broad-bandwidth, gamma-ray imaging spectrometer for astrophysics

GRABIT is the gamma-ray bar imaging telescope, a low-energy gamma-ray (60-600 keV) imaging spectrometer that can be extended to a large area at low cost. GRABIT has a gamma-ray position-resolution of 2 mm with an energy resolution typical of alkali-halide detectors (18% at 60 keV). The complete coded-aperture telescope will have arc-minute angular resolution over a large field of view. In addition, the good energy and angular resolutions are obtained over the full bandwidth of the instrument.

GRIS background reduction results using isotopically enriched GE

The Gamma Ray Imaging Spectrometer (GRIS) was flown twice from Alice Springs, Australia, in the spring of 1992 for a total of 32 hr at float altitude. One of the seven Ge detectors was isotopically enriched (greater than 97% Ge-70). This was the first time an enriched-Ge detector was used for astrophysical observations. Because of its thick anticoincidence shield, the GRIS instrument background is dominated by internal beta-decay in the energy range of 200-1000 keV. Half of the contribution in this beta-decay 'hump' is due to neutron-activated Ge-74. In this energy range, GRIS observed a factor of 2 reduction in the background in the enriched detector, as predicted. In future instruments (e.g., INTEGRAL), with thicker anticoincidence shields and smaller apertures, the background reduction will be even larger. Three strong instrumental background lines (54, 67, and 139 keV) are also eliminated. The elimination of the first two is particularly important for cylotron line observations.

GRIS detections of the 511 keV line from the Galactic center region in 1992

The Gamma Ray Imaging Spectrometer (GRIS) was flown on balloons over Alice Springs, Australia on 1992 April 26 and May 7. A full Galactic center transit (about 12 hr) was achieved on both flights with the instrument working normally. The electron/positron annihilation line was detected on both flights. The line fluxes and line widths were found to be (7.7 +/- 1.2) x 10 exp -4 and (8.9 +/- 1.1) x 10 exp -4 photons/sq cm per sec and 1.3 +/- 0.7 and 3.6 +/- 1.0 keV, respectively. These results are compared to each other and earlier (1988) GRIS results to produce suggestive evidence for source variability. Near-contemporaneous OSSE/CGRO Galactic center observations indicate that the GRIS results cannot be due solely to a single point source like IE 1740.7-2942 within a few degrees of the Galactic center. The GRIS 1992 results represent the first time that successive high-resolution balloon measurements have been achieved on a time scale of days.

GRIS observations of Al-26 gamma-ray line emission from two points in the Galactic plane

Both of the Gamma-Ray Imaging Spectrometer (GRIS) experiment's two observations of the Galactic center region, at l = zero and 335 deg respectively, detected Al-26 gamma-ray line emission. While these observations are consistent with the assumed high-energy gamma-ray distribution, they are consistent with other distributions as well. The data suggest that the Al-26 emission is distributed over Galactic longitude rather than being confined to a point source. The GRIS data also indicate that the 1809 keV line is broadened.

GRIS observations of positron annihilation radiation from the Galactic center

In the present Gamma-Ray Imaging Spectrometer (GRIS) observations, the 511 keV line has been spectrally resolved and the Galactic plane and center components have been independently measured. These data, in combination with those from other narrow-field observations in the 1980s, support the two-component model of the positron annihilation source. A strong hard-X-ray continuum was detected in the Galactic plane observation; this 'diffuse' continuum component solves the mystery as to why wide-field instruments have detected such high continuum emission from the Galactic center, in virtue of its source contributions.

Observations of gamma-ray line profiles from SN 1987A

Fully resolved gamma-ray line observations from the decay of Co-56 in SN 1987A are presented. Data are from the first two balloon flights of the Gamma-Ray Imaging Spectrometer. On day 433 (after the initial optical sighting) the 847 and 1238 keV lines from Co-56 were observed at 2.3 and 4.3 sigma significance. On day 613, the lines at 847, 1238, and 2599 keV were observed at 4.6, 3.4, and 1.9 sigma, respectively. The combined significance for the three-line complex in both flights is 7.8 sigma. Gaussian profiles yield acceptable least-squares fits to the lines. The line profiles are centered on the red side of the rest energy with typical velocity dispersions of about 3500 km/s FWHM, consistent with an optically thin source, but the line intensities are less than about 30 percent of those produced by the 0.075 solar mass of Co-56 determined from the bolometric light curve.

Resolution of the 1,238-keV γ-ray line from supernova 1987A

WE report observations of supernova 1987A from the maiden flight of the Gamma-Ray Imaging Spectrometer (GRIS). SN1987A was observed for a period of 11.1 hours on 1 May 1988. Line emission at 1,238 keV and continuum emission (to be reported elsewhere) from 60 to 800 keV were detected1. A gaussian line profile gives an acceptable fit to the 1,238-keV line. The best-fit parameters are: flux = 8.5 (+2.3,−2.2) x 10−4 photons cm−2 s−1 ; peak energy = 1,235.4 (+2.2, −2.4) keV; full width at half maximum = 16.3 (+6.1,−5.7)keV. As the 1,238-keV line is redshifted by 1.1 keV because of the motion of the Large Magellanic Cloud (LMC), we find no evidence for a supernova-produced red- or blueshift in the 1,238-keV line. This is to be compared with model-predicted blueshifts (in the frame of the LMC) of typically 3–4 keV (refs 2–4). Furthermore, our measured linewidfh is a factor of about two greater than the model predictions, although the discrepancy represents only two standard deviations. Our line profiles are characteristic of optically thin regions, whereas the intensity implies a mean optical depth of about 2. Fragmentation or non-spherical geometry of the supernova shell are possible explanations of the data.

Gamma-ray burst and spectroscopy instrumentation development at the goddard space flight center

This paper summarizes the activities within the Low-Energy Gamma-Ray Group of the Laboratory for High-Energy Astrophysics at the Goddard Space Flight Center that are specifically related to the development of instrumentation for gamma-ray astronomy. Three programs are described: 1) the Gamma-Ray Imaging Spectrometer (GRIS), a balloon-borne array of seven germanium detectors for high-resolution spectrographic studies of persistent gamma-ray sources, 2) the Transient Gamma-Ray Spectrometer (TGRS), a single radiatively-cooled germanium detector for the spectrographic study of gamma-ray bursts, and 3) the Rapidly Moving Telescope (RMT), a ground-based optical telescope for the detection and study of short-lived optical transients, particularly those that occur in coincidence with gamma-ray bursts.