X-ray polarimetry offers a unique vantage to investigate particle acceleration from compact objects and relativistic
outflows. The HX-POL concept uses a combination of Si and Cadmium Zinc Telluride (CZT) detectors to measure
the polarization of 50 keV - 500 keV X-rays from cosmic sources through the azimuthal distribution of Compton
scattered events. HX-POL would allow us to measure the polarization degrees of Crab-like sources well below
10% for a one day balloon flight. A longer (15-30 day) flight would improve the polarization degree sensitivity
to a few percent. In this contribution, we discuss the sensitivity of a space-borne HX-POL payload, and present
new results from laboratory tests of the HX-POL Si and CZT detectors.
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
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 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.
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.