The Strontium Iodide Radiation Instrumentation (SIRI) is designed to space-qualify new gamma-ray detector technology for space-based astrophysical and defense applications. This new technology offers improved energy resolution, lower power consumption and reduced size compared to similar systems. The SIRI instrument consists of a single europiumdoped strontium iodide (SrI<sub>2</sub>:Eu) scintillation detector. The crystal has an energy resolution of 3% at 662 keV compared to the 6.5% of traditional sodium iodide and was developed for terrestrial-based weapons of mass destruction (WMD) detection. SIRI’s objective is to study the internal activation of the SrI<sub>2</sub>:Eu material and measure the performance of the silicon photomultiplier (SiPM) readouts over a 1-year mission. The combined detector and readout measure the gammaray spectrum over the energy range of 0.04 - 4 MeV. The SIRI mission payoff is a space-qualified compact, highsensitivity gamma-ray spectrometer with improved energy resolution relative to previous sensors. Scientific applications in solar physics and astrophysics include solar flares, Gamma Ray Bursts, novae, supernovae, and the synthesis of the elements. Department of Defense (DoD) and security applications are also possible. Construction of the SIRI instrument has been completed, and it is currently awaiting integration onto the spacecraft. The expected launch date is May 2018 onboard STPSat-5. This work discusses the objectives, design details and the STPSat-5 mission concept of operations of the SIRI spectrometer.
The work reports on the development of a Strontium Iodide Coded Aperture (SICA) instrument for use in space-based astrophysics, solar physics, and high-energy atmospheric physics. The Naval Research Laboratory is developing a prototype coded aperture imager that will consist of an 8 x 8 array of SrI<sub>2</sub>:Eu detectors, each read out by a silicon photomultiplier. The array would be used to demonstrate SrI<sub>2</sub>:Eu detector performance for space-based missions. Europium-doped strontium iodide (SrI<sub>2</sub>:Eu) detectors have recently become available, and the material is a strong candidate to replace existing detector technology currently used for space-based gamma-ray astrophysics research. The detectors have a typical energy resolution of 3.2% at 662 keV, a significant improvement over the 6.5% energy resolution of thallium-doped sodium iodide. With a density of 4.59 g/cm and a Z<sub>eff</sub> of 49, SrI<sub>2</sub>:Eu has a high efficiency for MeV gamma-ray detection. Coupling this with recent improvements in silicon photomultiplier technology (i.e., no bulky photomultiplier tubes) creates high-density, large-area, low-power detector arrays with good energy resolution. Also, the energy resolution of SrI2:Eu makes it ideal for use as the back plane of a Compton telescope.
Modern nano-metrology instruments require calibration references with nanometer accuracy in the x, y, and z directions.
A common problem is the accurate calibration in the z direction (height). For example, it is generally not difficult to
obtain accurate x- and y- calibration references for a Scanning Probe Microscope (SPM). It is, however, much more
difficult to obtain accurate z-axis results. It is difficult to control z-axis piezo dynamics because during scanning in the
xy-plane the x-and y-axes move at a constant rate whiles the z axis does not. Furthermore due to the high cost of
producing calibration standards, the microscope is often calibrated at only one height. However, if the relationship
between the measured z height and the actual z height is not linear, then the height measurements will not be correct. In
this paper, we will present a method for the fabrication of calibration references with: (i) sub-10 nm features and (ii)
multiple step heights on one reference, allowing for better calibration of the non-linearity in the z direction.
Pulse shape discrimination (PSD) is a common method to distinguish between pulses produced by gamma rays and neutrons in scintillator detectors. This technique takes advantage of the property of many scintillators that excitations by recoil protons and electrons produce pulses with different characteristic shapes. Unfortunately, many scintillating materials with good PSD properties have other, undesirable properties such as flammability, toxicity, low availability, high cost, and/or limited size. In contrast, plastic scintillator detectors are relatively low-cost, and easily handled and mass-produced. Recent studies have demonstrated efficient PSD in plastic scintillators using a high concentration of fluorescent dyes. To further investigate the PSD properties of such systems, mixed plastic scintillator samples were produced and tested. The addition of up to 30 wt. % diphenyloxazole (DPO) and other chromophores in polyvinyltoluene (PVT) results in efficient detection with commercial detectors. These plastic scintillators are produced in large diameters up to 4 inches by melt blending directly in a container suitable for in-line detector use. This allows recycling and reuse of materials while varying the compositions. This strategy also avoids additional sample handling and polishing steps required when using removable molds. In this presentation, results will be presented for different mixed-plastic compositions and compared with known scintillating materials
The Advanced X-ray Timing Array (AXTAR) is a mission concept for X-ray timing of compact objects that
combines very large collecting area, broadband spectral coverage, high time resolution, highly flexible scheduling,
and an ability to respond promptly to time-critical targets of opportunity. It is optimized for submillisecond
timing of bright Galactic X-ray sources in order to study phenomena at the natural time scales of neutron star
surfaces and black hole event horizons, thus probing the physics of ultradense matter, strongly curved spacetimes,
and intense magnetic fields. AXTAR's main instrument, the Large Area Timing Array (LATA) is a collimated
instrument with 2-50 keV coverage and over 3 square meters effective area. The LATA is made up of an array
of supermodules that house 2-mm thick silicon pixel detectors. AXTAR will provide a significant improvement
in effective area (a factor of 7 at 4 keV and a factor of 36 at 30 keV) over the RXTE PCA. AXTAR will also
carry a sensitive Sky Monitor (SM) that acts as a trigger for pointed observations of X-ray transients in addition
to providing high duty cycle monitoring of the X-ray sky. We review the science goals and technical concept for
AXTAR and present results from a preliminary mission design study.
The Advanced Compton Telescope (ACT), the next major step in gamma-ray astronomy, will probe the fires where
chemical elements are formed by enabling high-resolution spectroscopy of nuclear emission from supernova explosions.
During the past two years, our collaboration has been undertaking a NASA mission concept study for ACT. This study
was designed to (1) transform the key scientific objectives into specific instrument requirements, (2) to identify the most
promising technologies to meet those requirements, and (3) to design a viable mission concept for this instrument. We
present the results of this study, including scientific goals and expected performance, mission design, and technology
The detection of shielded special nuclear materials is of great concern to the homeland security community. It is a challenging task that typically requires large detectors arrays to achieve the required sensitivity to detect shielded enriched uranium. We simulated the performance of three different configurations of scintillation detectors in a realistic gamma ray background. The simulations were performed using the GEANT4 simulation package fine tuned for low energy photon transport. The background spectrum was obtained by modeling high-resolution background spectra obtained by various groups in various locations. The performance of a non-imaging scintillating array was compared to the performance of two imaging arrays: a coded aperture imager and a Compton imager. The sensitivity was modeled at three energies for the emission from a 1 kg sphere of uranium enriched to 95% U-235: the 185 keV emission from U-235, the 1001 keV emission from U-238, and the 2614 keV emission from U-232. The instruments were modeled with and without passive shielding. The most detectable signal is the 2.614 MeV emission from U-232 contamination if present at a level greater than tens of parts per trillion. While the non-imaging array has the highest efficiency, it also has the highest background rate and is therefore not the most sensitive instrument. We present the expected performance for the three different configurations.
Compton imagers offer a method for passive detection of nuclear material over background radiation. A prototype Compton imager has been constructed using 8 layers of silicon detectors. Each layer consists of a 2×2 array of 2 mm thick cross-strip double-sided silicon detectors with active areas of 5.7 × 5.7 cm<sup>2</sup> and 64 strips per side. The detectors are daisy-chained together in the array so that only 256 channels of electronics are needed to read-out each layer of the instrument. This imager is a prototype for a large, high-efficiency Compton imager that will meet operational requirements of Homeland Security for detection of shielded uranium. The instrument can differentiate between different radioisotopes using the reconstructed gamma-ray energy and can also show the location of the emissions with respect to the detector location. Results from the current instrument as well as simulations of the next generation instrument are presented.
The Medium Energy Gamma-ray Astronomy (MEGA) telescope concept will soon be proposed as a MIDEX mission. This mission would enable a sensitive all-sky survey of the medium-energy gamma-ray sky (0.4 - 50 MeV) and bridge the huge sensitivity gap between the COMPTEL and
OSSE experiments on the Compton Gamma Ray Observatory, the SPI and IBIS instruments on INTEGRAL, and the visionary Advanced Compton Telescope (ACT) mission. The scientific goals include, among other things, compiling a much larger catalog of sources in this energy
range, performing far deeper searches for supernovae, better measuring the galactic continuum and line emissions, and identifying the components of the cosmic diffuse gamma-ray emission. MEGA will accomplish these goals using a tracker made of Si strip detector (SSD) planes surrounded by a dense high-Z calorimeter. At lower photon energies (below ~ 30 MeV), the design is sensitive to Compton interactions, with the SSD system serving as a scattering medium that also detects and measures the Compton recoil energy deposit. If the energy of the recoil electron is sufficiently high (> 2 MeV) its momentum vector can also be measured. At higher photon energies (above ~ 10 MeV), the design is sensitive to pair production
events, with the SSD system measuring the tracks of the electron and positron. A prototype instrument has been developed and calibrated in the laboratory and at a gamma-ray beam facility. We present calibration results from the prototype and describe the proposed satellite mission.
The MEGA mission would enable a sensitive all-sky survey of the medium-energy ?-ray sky (0.3-50 MeV). This mission will bridge the huge sensitivity gap between the COMPTEL and OSSE experiments on the Compton Gamma Ray Observatory, the SPI and IBIS instruments on INTEGRAL and the visionary ACT mission. It will, among other things, serve to compile a much larger catalog of sources in this energy range, perform far deeper searches for supernovae, better measure the galactic continuum emission as well as identify the components of the cosmic diffuse emission. The large field of view will allow MEGA to continuously monitor the sky for transient and variable sources. It will accomplish these goals with a stack of Si-strip detector (SSD) planes surrounded by a dense high-Z calorimeter. At lower photon energies (below ~30 MeV), the design is sensitive to Compton interactions, with the SSD system serving as a scattering medium that also detects and measures the Compton recoil energy deposit. If the energy of the recoil electron is sufficiently high (> 2 MeV), the track of the recoil electron can also be defined. At higher photon energies (above ~10 MeV), the design is sensitive to pair production events, with the SSD system measuring the tracks of the electron and positron. We will discuss the various types of event signatures in detail and describe the advantages of this design over previous Compton telescope designs. Effective area, sensitivity and resolving power estimates are also presented along with simulations of expected scientific results and beam calibration results from the prototype instrument.
The Advanced Compton Telescope (ACT) should provide well over an order-of-magnitude improvement in sensitivity compared to other previous or planned instruments in low-energy gamma-ray astronomy. This will be needed in the study of the nuclear line/MeV region of the gamma-ray spectrum. Such an instrument covers a broad range of science objectives, ranging from the study the <sup>56</sup>Ni light curves of supernovae and provide measurements of supernova dynamics, to <sup>26</sup>Al, <sup>22</sup>Na, and <sup>60</sup>Fe maps of the galaxy, and the first gamma-ray polarization observations probing the geometry of the emission regions of a variety of objects such as AGN, pulsars, and gamma ray bursts. These objectives depend critically on the sensitivity that can be achieved. We present a study of the sensitivity that can be achieved by the ACT, considering estimates of backgrounds, position resolution, energy resolution, Doppler broadening, and recoil electron tracking. Efficiency questions are considered that arise from passive materials within the active volume and track reconstruction. A sensitivity estimate for ACT is presented for a reasonable instrument size and configuration.
We present an instrument concept called GIPSI that uses germanium strip detectors in an imaging system to provide narrow line sensitivity less than 8.0 multiplied by 10<SUP>-6</SUP> gamma cm<SUP>-2</SUP>s<SUP>-1</SUP> at 100 keV in a 2 week exposure (3 sigma), and which has a point spread function (spatial resolution of approximately 20 arc minutes rms. The germanium strip detectors also make an excellent polarimeter by capitalizing on the angular dependence of the Compton scattering cross section. Gamma-ray polarimetry in the energy band around 60 - 300 keV is an interesting area of high energy astrophysics where observations have not been possible with the technologies employed in current and past space missions. We have tested a prototype detector with polarized beams and have measured a modulation factor of approximately 0.8 at 100 keV. A sensitive instrument can be realized on a modest space mission or a long duration balloon flight. Linear polarization can be detected in sources such as the Crab Pulsar, Cen A, Cyg X-1, and solar flares down to less than 5% of the source flux. The proposed instrument would have a collecting area of 400 cm<SUP>2</SUP>.
Burst locations with an arc second telescope (BLAST) is a new mission concept being studied for NASA's medium explorer (MIDEX) mission opportunities. The principal scientific objectives of the BLAST mission are (1) to localize gamma- ray burst (GRB) positions to arcsec accuracy; (2) to search for enhancements in the rate of GRBs toward M31; and (3) to conduct the most sensitive sky survey to date of x-ray sources in the 7 - 200 keV regime. These objectives are achieved using a large array of position-sensitive scintillation detectors with a total area of 17,000 cm<SUP>2</SUP>. This array is combined with a large field of view telescope (greater than 1 steradian) comprising two separate imaging systems. A coded aperture telescope provides arcminute source localization. For low energy x-rays (less than 50 keV), the aperture is also defined by phase modulation grids with provide complementary arcsecond information. The grid system consists of two aperture planes with 'checker board' patterns of slightly different pitch. The beating between the two grid pitches casts a broad interference pattern on the detector plane. Determining the phase of this interference pattern in both coordinates gives the location of a point source source in the sky, with aliased positions at approximately 1 arcmin spacing. The arcmin ambiguity is resolved by the coded aperture image. BLAST has a sensitivity to bursts of 0.03 photons cm<SUP>-2</SUP> s<SUP>-1</SUP>, almost ten times more sensitive than BATSE. We expect to position 20 bursts per year to better than 2 arcsec accuracy and 35 bursts per year to better than 5 arcsec. BLAST will provide an all sky survey in hard x-rays with a sensitivity of 0.2 milliCrab at low energies.
Germanium strip detectors combine high quality spectral resolution with two-dimensional positioning of gamma-ray interactions. Readout is accomplished using crossed electrodes on opposite faces of a planar germanium detector. Potential astrophysics applications include focal plane detectors for coded-aperture or grazing incidence x-ray mirror imagers, and as detection elements of a high resolution Compton telescope. We report on test results of two germanium strips detectors, one with 2 mm position resolution, the other with 9 mm. We discuss general device performance in terms of energy and position resolution, crosstalk effects, potential applications, and a demonstration of imaging properties.