The Advanced Energetic Pair Telescope (AdEPT), a future NASA/GSFC MIDEX mission, is being developed to perform high-sensitivity medium-energy (5–200 MeV) astronomy and revolutionary gamma-ray polarization measurements. The enabling technology for AdEPT is the ThreeDimensional Track Imager (3-DTI), a large volume gaseous time projection chamber with 2- dimentional micro-well detector (MWD) readout. The low density and high spatial resolution of the 3-DTI allows AdEPT to achieve high angular resolution (~0.5° at 67.5 MeV) and, for the first time, exceptional gamma-ray polarization sensitivity. These capabilities enable a wide range of scientific discovery potential for AdEPT. The key science goals of the AdEPT mission include: 1) Explore fundamental processes of particle acceleration in active astrophysical objects, 2) Reveal the magnetic field configuration of the most energetic accelerators in the Universe, 3) Explore the origins and acceleration of cosmic rays and the Galactic MeV diffuse emission, 4) Search for dark matter in the Galactic center, and 5) Test relativity with polarization measurements. We report on the latest developments of the MWDs for the 3-DTI.
The Advanced Energetic Pair Telescope (AdEPT) will explore the universe in the medium-energy range from about 5 MeV to greater than 200 MeV via gamma-ray pair production. AdEPT will provide high angular resolution observations and for the first time high polarization sensitivity over this essentially unexplored energy range. The NASA/GSFC quasi-monoenergetic 6 MeV Gamma Facility was built to characterize detector’s response of planetary science and astrophysics instrumentation to be done in house. It will provide the ability to study pair production imaging of the AdEPT pair telescope at the difficult low end of its energy range. There is no natural radioactive isotope that provides a gamma ray with energy above the 2.614 MeV line of <sup>228</sup>Th. The quasi-monoenergetic 6 MeV gamma-ray source provides a calibration point at significantly higher energy and is thus necessary to the design and testing of astrophysics and planetary neutron/gamma-ray instruments. This paper presents the mechanical design of the facility and the measured activity of the source.
The Advanced Energetic Pair Telescope (AdEPT) is being developed at GSFC as a future NASA MIDEX mission to
explore the medium-energy (5–200 MeV) gamma-ray range. The enabling technology for AdEPT is the Three-
Dimensional Track Imager (3-DTI), a gaseous time projection chamber. The high spatial resolution 3-D electron
tracking of 3-DTI enables AdEPT to achieve high angular resolution gamma-ray imaging via pair production and triplet
production (pair production on electrons) in the medium-energy range. The low density and high spatial resolution of 3-DTI allows the electron positron track directions to be measured before they are dominated by Coulomb scattering.
Further, the significant reduction of Coulomb scattering allows AdEPT to be the first medium-energy gamma-ray
telescope to have high gamma-ray polarization sensitivity. We review the science goals that can be addressed with a medium-energy pair telescope, how these goals drive the telescope design, and the realization of this design with AdEPT. The AdEPT telescope for a future MIDEX mission is envisioned as a 8 m<sup>3</sup> active volume filled with argon at 2 atm. The design and performance of the 3-DTI detectors for the AdEPT telescope are described as well as the outstanding instrument challenges that need to be met for the AdEPT mission.
Progress in high-energy gamma-ray science has been dramatic since the launch of INTEGRAL, AGILE and FERMI.
These instruments, however, are not optimized for observations in the medium-energy (~0.3< E<sub>γ</sub> < ~200 MeV) regime
where many astrophysical objects exhibit unique, transitory behavior, such as spectral breaks, bursts, and flares. We
outline some of the major science goals of a medium-energy mission. These science goals are best achieved with a
combination of two telescopes, a Compton telescope and a pair telescope, optimized to provide significant improvements
in angular resolution and sensitivity. In this paper we describe the design of the Advanced Energetic Pair Telescope
(AdEPT) based on the Three-Dimensional Track Imager (3-DTI) detector. This technology achieves excellent, mediumenergy
sensitivity, angular resolution near the kinematic limit, and gamma-ray polarization sensitivity, by high resolution
3-D electron tracking. We describe the performance of a 30×30×30 cm<sup>3</sup> prototype of the AdEPT instrument.
We describe a gamma-ray imaging camera (GIC) for active interrogation of explosives being developed by
NASA/GSFC and NSWC/Carderock. The GIC is based on the
Three-dimensional Track Imager (3-DTI) technology
developed at GSFC for gamma-ray astrophysics. The 3-DTI, a large volume time-projection chamber, provides
accurate, ~0.4 mm resolution, 3-D tracking of charged particles. The incident direction of gamma rays, E > 6 MeV, are
reconstructed from the momenta and energies of the electron-positron pair resulting from interactions in the 3-DTI
volume. The optimization of the 3-DTI technology for this specific application and the performance of the GIC from
laboratory tests is presented.
The Neutron Imaging Camera (NIC) is based on the Three-dimensional Track Imager (3_DTI) technology developed at
GSFC for gamma-ray astrophysics applications. The 3-DTI, a large volume time-projection chamber, provides accurate,
~0.4 mm resolution, 3-D tracking of charged particles. The incident direction of fast neutrons, En > 0.5 MeV, are
reconstructed from the momenta and energies of the proton and triton fragments resulting from 3He(n,p)3H interactions
in the 3-DTI volume. The performance of the NIC from laboratory is presented.
We describe our program to develop gas micro-well detectors (MWDs) as three-dimensional charged particle trackers for use in advanced gamma-ray telescope concepts. A micro-well detector consists of an array of individual micro-patterned gas proportional counters opposite a planar drift electrode. The well anodes and cathodes may be connected in X and Y strips, respectively, to provide two-dimensional imaging. When combined with transient digitizer electronics, which record the time signature of the charge collected in the wells of each strip, full three-dimensional reconstruction of charged-particle tracks in large gas volumes is possible. Such detectors hold great promise for advanced Compton telescope (ACT) and advanced pair telescope (APT) concepts due to the very precise measurement of charged particle momenta that is possible (Compton recoil electrons and electron-positron pairs, respectively). We present preliminary lab results, including detector fabrication, prototype electronics, and initial detector testing. We also discuss applications to the ACT and APT mission concepts, based on GEANT3 and GEANT4 simulations.
The Polarized Gamma-ray Observer (PoGO) is a new balloon-borne instrument designed to measure polarization from astrophysical objects in the 30-200 keV range. It is under development for the first flight anticipated in 2008. PoGO is designed to minimize the background by an improved phoswich configuration, which enables a detection of 10 % polarization in a 100 mCrab source in a 6--8 hour observation. To achieve such high sensitivity, low energy response of the detector is important because the source count rate is generally dominated by the lowest energy photons. We have developed new PMT assemblies specifically designed for PoGO to read-out weak scintillation light of one photoelectron (1 p.e.) level. A beam test of a prototype detector array was conducted at the KEK Photon Factory, Tsukuba in Japan. The experimental data confirm that PoGO can detect polarization of 80-85 % polarized beam down to 30 keV with a modulation factor 0.25 ± 0.05.
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.
We present a concept for an imaging gamma-ray polarimeter operating from ~50 MeV to ~1 GeV. Such an instrument would be valuable for the study of high-energy pulsars, active galactic nuclei, supernova
remnants, and gamma-ray bursts. The concept makes use of pixelized gas micro-well detectors, under development at Goddard Space Flight Center, to record the electron-positron tracks from pair-production events in a large gas volume. Pixelized micro-well detectors have
the potential to form large-volume 3-D track imagers with ~100 μm (rms) position resolution at moderate cost. The combination of high spatial resolution and a continuous low-density gas medium permits many thousands of measurements per radiation length, allowing the particle tracks to be imaged accurately before multiple scattering masks their original directions. The polarization of the incoming radiation may then be determined from the azimuthal distribution of the electron-positron pairs. We have performed Geant4 simulations of these processes to estimate the polarization sensitivity of a simple telescope geometry at 100 MeV.
We describe the design of Lobster-ISS, an X-ray imaging all-sky monitor (ASM) to be flown as an attached payload on the International Space Station. Lobster-ISS is the subject of an ESA Phase-A study which will begin in December 2001. With an instantaneous field of view 162 x 22.5 degrees, Lobster-ISS will map almost the complete sky every 90 minute ISS orbit, generating a confusion-limited catalogue of ~250,000 sources every 2 months. Lobster-ISS will use focusing microchannel plate optics and imaging gas proportional micro-well detectors; work is currently underway to improve the MCP optics and to develop proportional counter windows with enhanced transmission and negligible rates of gas leakage, thus improving instrument throughput and reducing mass. Lobster-ISS provides an order of magnitude improvement in the sensitivity of X-ray ASMs, and will, for the first time, provide continuous monitoring of the sky in the soft X-ray region (0.1-3.5 keV). Lobster-ISS provides long term monitoring of all classes of variable X-ray source, and an essential alert facility, with rapid detection of transient X-ray sources such as Gamma-Ray Burst afterglows being relayed to contemporary pointed X-ray observatories. The mission, with a nominal lifetime of 3 years, is scheduled for launch on the Shuttle c.2009.
Gas proportional counter arrays based on the micro-well are an example of a new generation of detectors that exploit narrow anode-cathode gaps, rather than fine anodes, to create gas gain. These are inherently imaging pixel detectors that can be made very large for reasonable costs. Because of their intrinsic gain and room-temperature operation, they can be instrumented at very low power per unit area, making them valuable for a variety of space-flight applications where large-area X-ray imaging or particle tracking is required. We discuss micro-well detectors as focal plane imager for Lobster-ISS, a proposed soft X-ray all-sky monitor, and as electron trackers for the Next Generation High-Energy Gamma Ray mission. We have developed a fabrication technique using a masked UV laser that allows us both to machine micro-wells in polymer substrates and to pattern metal electrodes. We have used this technique to fabricate detectors which image X-rays by simultaneously reading out orthogonal anode and cathode strips. We present imaging results from these detectors, as well as gain and energy resolution measurements that agree well with results from other groups.
Anticoincidence detectors are required for a variety of satellite instruments, including high energy gamma-ray telescopes, in order to differentiate ambient background radiation from signals of interest. Presently, most anticoincidence systems use scintillators coupled to photomultiplier tubes. We have demonstrated that it is now possible to use very high gain solid state avalanche photodiodes (APDs) as photodetectors for this application. A single APD coupled to a 30 cm multiplied by 30 cm multiplied by 0.95 cm plastic scintillator tile demonstrated 100% detection efficiency for minimum ionizing particles, with a low false positive rate. Multiple APDs enhance the signal to noise ratio in addition to providing redundancy. Relative to PMTs, APDs are compact, low power, and mechanically robust devices. Ground test data of APDs for anticoincidence shields is presented.
The development of large area xenon drift chambers as imaging systems for the advanced Gamma-Ray Astronomy Telescope Experiment (AGATE), sensitive in the energy range 20 MeV - 100 GeV, is presented here. AGATE is visualized as the successor to the Energetic Gamma Ray Experiment Telescope (EGRET) on the Compton Gamma-Ray Observatory, and will add to the wide range of important results currently being obtained by EGRET. Experiments were carried out with a laboratory prototype consisting of a stack of sixteen 1/2m X 1/2m active area drift chambers using both xenon and argon gas mixtures. The spatial resolution of the drift chamber stack was measured with a multi-wire readout plane using atmospheric muons traversing the active volume. A spatial resolution of about 0.23 mm was measured with drift chambers using xenon- methane gas mixtures. The experiments with the argon-isobutane gas mixtures yielded a spatial resolution of about 0.14 mm.
In order to continue the achievements in high energy (10 MeV - 100 GeV) gamma-ray astronomy made with the Energetic Gamma-Ray Experiment Telescope (EGRET) instrument on the Compton Gamma Ray Observatory (CGRO), a 'next generation' high energy gamma- ray telescope with a large increase in sensitivity coupled with improved angular resolution will be required. This 'next generation' telescope is envisioned as a 2 m X 2 m active area telescope using drift chambers for the imaging detector. The four major components of the instrument are the anticoincidence shield, the track imaging system, the coincidence/time-of- flight system and the energy measurement system. In this paper we discuss the design goals and challenges for the four subsystems and the techniques we are utilizing to achieve them as well as the design and performance of high speed electronics that we have developed specifically for this application.