The multi-institution Single-Volume Scatter Camera (SVSC) collaboration led by Sandia National Laboratories (SNL) is developing a compact, high-efficiency double-scatter neutron imaging system. Kinematic emission imaging of fission-energy neutrons can be used to detect, locate, and spatially characterize special nuclear material. Neutron-scatter cameras, analogous to Compton imagers for gamma ray detection, have a wide field of view, good event-by-event angular resolution, and spectral sensitivity. Existing systems, however, suffer from large size and/or poor efficiency. We are developing high-efficiency scatter cameras with small form factors by detecting both neutron scatters in a compact active volume. This effort requires development and characterization of individual system components, namely fast organic scintillators, photodetectors, electronics, and reconstruction algorithms. In this presentation, we will focus on characterization measurements of several SVSC candidate scintillators. The SVSC collaboration is investigating two system concepts: the monolithic design in which isotropically emitted photons are detected on the sides of the volume, and the optically segmented design in which scintillation light is channeled along scintillator bars to segmented photodetector readout. For each of these approaches, we will describe the construction and performance of prototype systems. We will conclude by summarizing lessons learned, comparing and contrasting the two system designs, and outlining plans for the next iteration of prototype design and construction.
Coded apertures were originally developed by the high-energy astrophysical community for use in imaging high-energy
photons (x- and γ-rays) for which focusing optics are ineffective. We are now taking what was developed as a tool for
use in the extreme far field at high energies to encode spatial information at optical wavelengths in the extreme near field
to enhance the performance of position-sensitive x- and gamma-ray scintillator detectors. Spatial resolution for events
within bulk scintillators is limited by the size of the light “spot” available at the sides of the scintillator, where
phototransducers convert the light to an electrical signal. The ability to localize an event is determined by how well one
can determine the centroid and the size of the spot. Generally, performance is limited to many millimeters in all three
spatial dimensions, and one cannot resolve simultaneous events that are closer together than the width of the light spot
(frequently of order 10 mm). For this reason, many applications requiring the finest spatial resolution subdivide the
scintillator into tiny elements and use a digital approach to determine event location. However, that technique
significantly complicates the overall instrument and sacrifices energy resolution because the light collection efficiency
varies with event location within the subdivided scintillator. We are building a device that overcomes these shortcomings
by using an optical coded-aperture shadow mask between a bulk crystal and a position-sensitive phototransducer.
Simulations indicate that we can achieve millimeter-scale localization in all three spatial dimensions while resolving
simultaneous energy depositions. The technique and progress toward its realization will be presented.
The use of radiation sensors as portal monitors is increasing due to heightened concerns over the smuggling of fissile
material. Transportable systems that can detect significant quantities of fissile material that might be present in vehicular
traffic are of particular interest, especially if they can be rapidly deployed to different locations. To serve this
application, we have constructed a rapid-deployment portal monitor that uses visible-light and gamma-ray imaging to
allow simultaneous monitoring of multiple lanes of traffic from the side of a roadway. The system operation uses
machine vision methods on the visible-light images to detect vehicles as they enter and exit the field of view and to
measure their position in each frame. The visible-light and gamma-ray cameras are synchronized which allows the
gamma-ray imager to harvest gamma-ray data specific to each vehicle, integrating its radiation signature for the entire
time that it is in the field of view. Thus our system creates vehicle-specific radiation signatures and avoids source
confusion problems that plague non-imaging approaches to the same problem. Our current prototype instrument was
designed for measurement of upto five lanes of freeway traffic with a pair of instruments, one on either side of the
roadway. Stereoscopic cameras are used with a third "alignment" camera for motion compensation and are mounted on
a 50' deployable mast. In this paper we discuss the design considerations for the machine-vision system, the algorithms
used for vehicle detection and position estimates, and the overall architecture of the system. We also discuss system
calibration for rapid deployment. We conclude with notes on preliminary performance and deployment.
The use of radiation sensors as portal monitors is increasing due to heightened concerns over the smuggling of fissile
material. Portable systems that can detect significant quantities of fissile material that might be present in vehicular
traffic are of particular interest. We have constructed a prototype, rapid-deployment portal gamma-ray imaging portal
monitor that uses machine vision and gamma-ray imaging to monitor multiple lanes of traffic. Vehicles are detected
and tracked by using point detection and optical flow methods as implemented in the OpenCV software library. Points
are clustered together but imperfections in the detected points and tracks cause errors in the accuracy of the vehicle
position estimates. The resulting errors cause a "blurring" effect in the gamma image of the vehicle. To minimize these
errors, we have compared a variety of motion estimation techniques including an estimate using the median of the
clustered points, a "best-track" filtering algorithm, and a constant velocity motion estimation model. The accuracy of
these methods are contrasted and compared to a manually verified ground-truth measurement by quantifying the rootmean-
square differences in the times the vehicles cross the gamma-ray image pixel boundaries compared with a groundtruth
Coded aperture imagers provide the optimum means to generate an all-sky survey at gamma-ray energies from 10's of keV to a few MeV. Unfortunately, such imagers are plagued by systematic noise that limits their dynamic range. In par-ticular, spatial gradients in the background radiation across the detector and imperfectly coded signals from strong point sources in the field of view add artifacts to the images. Although theoretical signal-to-noise ratios of order 104 are possi-ble in perfectly coded images, real-world effects have limited performance of past imagers to significantly less than that. One technique to help remove these aberrations is the use of sequential exposures with a mask and it's inverse. However, for large instruments this is an impractical solution. In addition, it does not apply for scenes with rapidly varying sources. In a scanning instrument, one solution to this problem is to interleave the mask and anti-mask patterns in a sin-gle aperture in the direction of scan. A true source will oscillate between positive and negative images (i.e. scintillate) while spatially varying backgrounds are significantly suppressed.
The High Energy Focusing Telescope (HEFT) is a balloon-borne, hard x-ray/gamma ray (20-70 keV) astronomical experiment. HEFT's 10 arcminute field of view and 1 arcminute angular resolution place challenging demands on its attitude control system (ACS). A microprocessor-based ACS has been developed to manage target acquisition and sidereal tracking. The ACS consists of a variety of sensors and actuators, with provisions for 2-way ground communication, all controlled by an on-board computer. Ground based pointing performance measurements indicate 1σ jitter of 7" and gyro drift rates of <1" s-1. Jitter is expected to worsen in the flight environment, but star tracker data are expected to reduce drift rates significantly, enabling a predicted 1σ absolute attitude determination of ≥4.7". HEFT is scheduled for flight in Spring 2004.
We are developing a 2-detector high resolution Compton telescope utilizing 3D imaging germanium detectors (GeDs) to be flown as a balloon payload in Spring 2004. This instrument is a prototype for the larger Nuclear Compton Telescope (NCT), which utilizes 12-GeDs. NCT is a balloon-borne soft γ-ray (0.2-15 MeV) telescope designed to study, through spectroscopy, imaging, and timing, astrophysical sources of nuclear line emission and γ-ray polarization. The NCT program is designed to develop and test the technologies and analysis techniques crucial for the Advanced Compton Telescope, while studying γ-ray radiation with very high spectral resolution, moderate angular resolution, and high sensitivity. NCT has a novel, ultra-compact design optimized for studying nuclear line emission in the critical 0.5-2 MeV range, and polarization in the 0.2-0.5 MeV range. The prototype flight will critically test the novel instrument technologies, analysis techniques, and background rejection procedures we have developed for high resolution Compton telescopes. In this paper we present the design and preliminary results of laboratory performance tests of the NCT flight electronics.
Significant effort currently is being devoted to the development of noninvasive imaging systems that allow in vivo assessment of biological and biomolecular interactions in mice and other small animals. While physiological function in small animals can be localized and imaged using conventional radionuclide imaging techniques such as single-photon emission tomography (SPECT) and positron emission tomography (PET), these techniques inherently are limited to spatial resolutions of 1-2 mm. For this reason, we are developing a small animal radionuclide imaging system (SARIS) using grazing incidence optics to focus gamma-rays emitted by 125I and other radiopharmaceuticals. We have developed a prototype optic with sufficient accuracy and precision to focus the 27.5 keV photons from 125I onto a high-resolution imaging detector. Experimental measurements from the prototype have demonstrated that the optic can focus X-rays from a microfocus X-ray tube to a spot having physical dimensions (approximately 1500 microns half-power diameter) consistent with those predicted by theory. Our theoretical and numerical analysis also indicate that an optic can be designed and build that ultimately can achieve 100 μm spatial resolution with sufficient efficiency to perform it in vivo single photon emission imaging studies in small animal.
Our collaboration is developing a 2-detector prototype high resolution Compton telescope utilizing 3D imaging germanium detectors (GeDs) for a test balloon flight in Spring 2003. This instrument is a prototype for a full 12-GeD instrument, the Nuclear Compton Telescope. NCT is a balloon-borne soft gamma-ray (0.2-15 MeV) telescope designed to study astrophysical sources of nuclear
line emission and polarization. The NCT program is designed to develop and test the technologies and analysis techniques crucial for the Advanced Compton Telescope, while studying gamma-ray radiation with very high spectral resolution, moderate angular resolution, and high sensitivity. NCT has a novel, ultra-compact design optimized for studying nuclear line emission in the critical 0.5-2 MeV range, and polarization in the 0.2-0.5 MeV range. This prototype flight will critically test the novel instrument technologies, analysis techniques, and background rejection procedures we have developed for high resolution Compton telescopes. We present the design and expected performance of this prototype NCT instrument.
A germanium-detector based, gamma-ray imaging system has been designed, fabricated, and tested. The detector, cryostat, electronics, readout, and imaging software are discussed. An 11 millimeter thick, 2 millimeter pitch 19x19 orthogonal strip planar germanium detector is used in front of a coaxial detector to provide broad energy coverage. The planar detector was fabricated using amorphous germanium contacts. Each channel is read out with a compact, low noise external FET preamplifier specially designed for this detector. A bank of shaping amplifiers, fast amplifiers, and fast leading edge discriminators were designed and fabricated to process the signals from preamplifiers. The readout system coordinates time coincident x-y strip addresses with an x-strip spectroscopy signal and a spectroscopy signal from the coaxial detector. This information is sent to a computer where an image is formed. Preliminary shadow and pinhole images demonstrate the viability of a germanium based imaging system. The excellent energy resolution of the germanium detector system provides isotopic imaging.
We have investigated the performance of a position- sensitive, gamma-ray detector based on a CsI(Na) scintillator coupled to a Hamamatsu R3292 Position-Sensitive Photomultiplier Tube. The R3292 has an active area 10.0 cm in diameter. Utilization of the full active area of the photocathode is a goal that has been previously unrealized due to edge effects. Initial measurements with a 0.75 cm thick CsI(Na) crystal indicate that the performance starts to degrade as one reaches a radius of only 3.5 cm, reducing the active area by 60 percent. Measuring the anode wires we have found that this fall off is not solely due to crystal edge effects, but rather is inherent to the tube crystal system. In this paper we describe the results of our measurements and how good performance can be maintained across a full 10cm of the tube face through the use of a few additional electronics channels.
The stellar x-ray polarimeter (SXRP) will be more than an order of magnitude more sensitive than any previous x-ray polarimeter in the 2 - 15 keV energy band. The SXRP is a focal plane detector for a Danish-Russian SODART telescope, which will be launched on the Russian spectrum-x-gamma (SXG) mission. The SXRP exploits the polarization dependence of Bragg reflection from a graphite crystal, and of Thomson scattering from a target of metallic lithium. The SXRP flight model (FM) was calibrated at a facility at Lawrence Livermore National Laboratory (LLNL) equipped with polarized and unpolarized x-ray sources producing x-rays in the band pass for the graphite and lithium scatterers. By adjusting the orientation of the SXRP with respect to the incident x-ray beam, it was possible to simulate the converging beam from a SODART telescope and to measure the SXRP response to telescope pointing errors. In this paper, we describe the SXRP-FM calibration and present results for the graphite polarimeter.
The performance of the engineering prototype Stellar X-Ray Polarimeter (SXRP) has been evaluated. One hundred percent polarized monochromatic x rays at 2.6 keV and 9.7 keV were used to measure the response of the instrument in the energy bands of the graphite and lithium polarizing elements, respectively. On-line analysis showed that the respective depths of modulation are 96% ad 70% as expected. Irradiating SXRP with broadband unpolarized x rays in the energy band 2 - 17 keV demonstrated that the level of spurious modulation inherent in the instrument is less than 3%. Up-to-date results are presented and compared to the predictions of Monte Carlo simulations.
A large area thin graphite crystal and a lithium scattering block are used as the polarization sensitive elements of the Stellar X-Ray Polarimeter. We discuss the construction, selection and characterization of these two polarizing elements. In addition, we describe the plans for calibration of the completed instrument and the facility where it will be conducted.
The Stellar X-ray Polarimeter (SXRP) will be the third orbiting stellar x-ray polarimeter, and should provide an order of magnitude increase in polarization sensitivity over its predecessors. The SXRP exploits the polarization dependence of reflection from a graphite Bragg crystal and scattering from a lithium Thomson scattering target to measure the linear polarization of x- rays from astrophysical sources. In this paper, we review the status of the SXRP instrument.
The Stellar X-Ray Polarimeter (SXRP) uses the polarization sensitivity of a graphite Bragg crystal and a lithium Thomsom scattering target to measure the polarization of X-rays from astrophysical sources. The SXRP is a focal plane detector for the Soviet-Danish SODART telescopes which will be launched on the Soviet Spectrum-X-Gamma mission. The SXRP will be the third orbiting stellar X-ray polarimeter, and should provide an order of magnitude increase in polarization sensitivity over its predecessors.
We are designing a Bragg crystal polarimeter for the focal plane of the SODART telescope on the Spectrum-XGamma
mission. A mosaic graphite crystal will be oriented at 45 0 to the optic axis of the telescope, thereby
preferentially reflecting those x-rays which satisfy the Bragg condition and have electric vectors that are perpendicular
to the plane defmed by the incident and reflected photons. The reflected x-rays will be detected by an imaging
proportional counter with the image providing direct x-ray aspect information. The crystal will be 50 jtm thick to
allow x-rays with energies □ 4 keV to be transmitted to a lithium block mounted below the graphite. The lithium is
used to measure the polarization of these high energy x-rays by exploiting the polarization dependence of Thomson
scattering. The development of thin mosaic graphite crystals is discussed and recent reflectivity, transmission, and
uniformity measurements are presented.
This paper discusses the optimization of the performance of imaging scintillation detectors used in the hard X-ray/soft gamma-ray (20-300) keV region of the spectrum. In these devices, absorption of an incident gamma-ray within an alkali halide crystal induces a scintillation light distribution which is centroided by an imaging photomultiplier tube mounted to the crystal. The ultimate imaging resolution is strongly affected by the detailed propagation of the scintillation light within the crystal and at the interface between the crystal and the phototube face plate. A number of refined techniques for preparing the scintillation crystals so as to optimize the imaging resolution have been investigated. The results indicate very good agreement with relatively simple models of the light propagation. It is shown that it is possible to achieve resolution consistent with the most optimistic models.