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
recommendations.
Alessandro Curioni, Elena Aprile, Karl-Ludwig Giboni, Masanori Kobayashi, Uwe Oberlack, Edward Chupp, Philip Dunphy, Tadayoshi Doke, Jun Kikuchi, Sandro Ventura
KEYWORDS: Data acquisition, Monte Carlo methods, Atmospheric particles, Sensors, Particles, Atmospheric modeling, Telescopes, Energy efficiency, Space telescopes, Liquids
LXeGRIT is the first prototype of a novel Compton telescope for MeV γ-ray astrophysics based on a Liquid Xenon Time Projection Chamber (LXeTPC), sensitive in the energy band of 0.15-10 MeV. In this homogeneous, 3D position sensitive detector, γ-rays with at least two interactions in the sensitive volume of 2800 cm3, are imaged as in a standard Compton telescope. γ-rays with a single interaction cannot be imaged and constitute a background which can be easily identified and rejected. Charged particles and localized β-particles background is also easily suppressed based on the TPC localization capability with millimeter resolution. A measurement of the total γ-ray background rate in near space conditions and the background rejection power of the LXeTPC was a primary goal of the LXeGRIT balloon flight program. We present here a preliminary analysis addressing this question, based on balloon flight data acquired during the Oct 4-5, 2000 LXeGRIT balloon flight from Ft. Sumner, NM. In this long duration balloon experiment, the LXeGRIT TPC was not surrounded by any γ-ray or charged particle shield. Single site events and charged particles were mostly rejcted on-line at the first and second trigger level. The remaining count rate of single-site γ-ray events, at an average atmospheric depth of 3.2 g cm-2, is consistent with that expected from atmospheric and diffuse γ-ray background, taking into account the instrument mass model and response.
Elena Aprile, Alessandro Curioni, Karl-Ludwig Giboni, Masanori Kobayashi, Uwe Oberlack, Edward Chupp, Philip Dunphy, Tadayoshi Doke, Jun Kikuchi, Sandro Ventura
LXeGRIT is the first prototype of a novel concept of Compton telescope, based on the complete 3D reconstruction of the sequence of interactions of individual γ-rays in one position sensitive detector. This balloon-borne telescope consists of an unshielded time projection chamber with an active volume of 400 cm2 × 7 cm filled with high purity liquid xenon. Four VUV PMTs detect the fast xenon scintillation light signal, providing the event trigger. 124 wires and 4 anodes detect the ionization signals, providing the event spatial coordinates and total energy. In the period 1999-2001, LXeGRIT has been extensively tested both in the laboratory and at balloon altitude, and its response in the MeV region has been thoroughly characterized. Here we summarize some of the results on pre-flight calibration, even reconstruction techniques, and performance during a 27 hour balloon flight on October 4-5. We further present briefly the on-going efforts directed to improve the performance of this prototype towards the requirements for a base module of a next-generation Compton telescope.
LXeGRIT is a balloon-borne Compton telescope employing a large volume liquid xenon time projection chamber (LXe-TPC) as the central (gamma) -ray detector. It is designed to image (gamma) - rays in the energy range of approximately 200 keV to 20 MeV, with an angular resolution of about 3 degrees (1 sigma) at 2 MeV, within a field-of-view (FOV) of about 1 sr. The detector's energy and three-dimensional spatial resolution as measured during pre-flight calibration experiments, are (Delta) E1xe/E equals 8.8% (root)1MeV/E and < 1 mm RMS, respectively. The detection efficiency for Compton events varies between 1.5% and 4% depending on energy and event selection. We describe the instrument as flown on May 7, 1999 and review its overall performance at balloon altitude. The launch occurred at 13:26:54 UT from Ft. Sumner, New Mexico and the flight was terminated about 9 hours later. The Crab was in the instrument FOV for a few hours. Analysis of these data is in progress.
LXeGRIT is a balloon-borne Compton telescope based on a liquid xenon time projection chamber (LXeTPC) for imaging cosmic (gamma) -rays in the energy band of 0.2 - 20 MeV. The detector, with 400 cm2 area and 7 cm drift gap, is filled with high purity LXe. Both ionization and scintillation light signal are detected to measure the energy deposits and the three spatial coordinates of individual (gamma) -ray interactions within the sensitive volume. The TPC has been characterized with repeated measurements of its spectral and Compton imaging response to (gamma) -rays from radioactive sources such as 22Na, 137Cs, 88Y and Am-Be. The detector shows a linear response to g-rays in the energy range 511 keV - 4.4 MeV, with an energy resolution (FWHM) of (Delta) E/E equals 8.8% (root)1MeV/E. Compton imaging of 88Y (gamma) -ray events with two detected interactions is consistent with an angular resolution of approximately 3 degrees (RMS) at 1.8 MeV.
KEYWORDS: Sensors, Monte Carlo methods, Reconstruction algorithms, Xenon, Telescopes, Compton scattering, Liquids, Gamma ray imaging, Detection and tracking algorithms, Kinematics
The Liquid Xenon Gamma-Ray Imaging Telescope (LXeGRIT) is a balloon born experiment sensitive to (gamma) -rays in the energy band of 0.2 - 20 MeV. The main detector is a time projection chamber filled with high purity liquid xenon (LXeTPC), in which the three-dimensional location and energy deposit of individual (gamma) -ray interactions are accurately measured in one homogeneous volume. To determine the (gamma) - ray initial direction (Compton imaging), as well as to reject background, the correct sequence of interactions has to be determined. Here we report the development and optimization of an algorithm to reconstruct the Compton scattering sequence and show its performance on Monte Carlo events and LXeGRIT data.
Elena Aprile, Valeri Egorov, Karl-Ludwig Giboni, Steven Kahn, Tomotake Kozu, Uwe Oberlack, S. Centro, Sandro Ventura, Tadayoshi Doke, Jun Kikuchi, Edward Chupp, Philip Dunphy, Dieter Hartmann, Mark Leising, H. Bloemen
XENA is a new Compton telescope concept, designed to image about 50% of the gamma-ray sky with a sensitivity that would significantly surpass CGRO/COMPTEL's multi-year sensitivity with a 2 weeks balloon flight from the Southern Hemisphere. The detector, based on liquid xenon time-projection chambers, is optimized for approximately 0.3 - 10 MeV and combines high efficiency within a 3 sr field-of-view with approximately 1 degree(s) angular resolution and excellent background reduction capability. XENA's primary scientific goal is the discovery and mapping of 60Fe radioactivity from the Galaxy, which is pivotal for understanding nucleosynthesis. XENA will detect 60Fe even if current predictions are 7X overestimated. At 1.8 MeV, XENA's sensitivity (6 10-6 cm-2 s-1) will significantly refine the COMPTEL 26Al mapping along the Southern Milky Way. Also, XENA would be the first instrument capable to decide whether the 3 - 7 MeV excess seen in Orion is indeed due to nuclear lines from 12C and 16O, and it could discover the predicted lower-energy lines. The scanned sky area includes many continuum (gamma) -ray sources as well, such as pulsars and numerous (gamma) -ray AGNs. Secondary scientific objectives include also supernova remnants, gamma-ray bursts, and solar flares.
Recently developed CdTe detectors with a Schottky anode contact are well suited for (gamma) -ray imaging applications in astrophysics and nuclear medicine. Since the response of the detectors to (gamma) -rays is homogeneous throughout their volume the detectors can be oriented to let the radiation enter perpendicular to the electric field. Thus, large area arrays with 10 mm thickness can be assembled. The leakage current and capacitance of the detectors is sufficiently low to connect more than a hundred together in a row-column read out reducing the number of electronic channels.
As part of our ongoing research program to develop a liquid xenon gamma-ray imaging telescope (LXe-GRIT) for medium energy astrophysics, we have built a liquid xenon time projection chamber (LXeTPC) with a total volume of 10 liters and a sensitive are of 20 cm by 20 cm. The detector has been successfully tested with gamma-ray sources in the laboratory and is currently being prepared as balloon-borne payload for imaging MeV gamma-ray emission from the Crab Nebula, Cygnus X-1 and the Orion molecular cloud region. The LXe-TPC, sensitive to gamma-rays from 300 keV to 30 MeV, measures the energy and the 3-D location of each gamma-ray interaction with a resolution of 6% FWHM and 1 mm RMS at 1 MeV, within a 1 sr FOV. Its detection efficiency for Compton events is about 4% in the 1 - 3 MeV, an energy band of great astrophysical interest for both continuum and line emission. Its 3 sigma continuum sensitivity of 1.8 multiplied by 10-7 ph cm-2s-1keV-1 for a nominal 10 hr observation time, will allow us to study a variety of sources with an imaging accuracy as good as 1 degree. We plan to pursue a vigorous program of balloon flights with this telescope to achieve the maximum science return while continuing a strong R&D laboratory program on LXe technology. The ultimate goal is an optimized design of a satellite implementation of a liquid xenon gamma-ray imaging instrument that will lead to drastic improvements in sensitivity and angular resolution in the 0.3 - 30 MeV band and beyond.
The experiment LXe-CAT (Liquid Xenon-Coded Aperture Telescope), which we have proposed for (gamma) -ray astrophysics observations in the 300 keV - 10 MeV energy range, uses a Liquid Xenon Time Projection Chamber (LXe-TPC) as a position sensitive (gamma) -ray detector, and a coded aperture mask to provide a telescope with an angular resolution of 30 ft over a field of view (FOV) of 28 degree(s) X 28 degree(s). The point source localization accuracy is about 2 ft for a 10(sigma) source. To demonstrate the spectroscopy and imaging capabilities of the LXe-TPC as a (gamma) -ray detector we have designed, built and are currently testing a 10 liter prototype with a sensitive area of approximately 400 cm2 and an active liquid depth of 5 cm. Studies on stability, spectroscopy and imaging response of this detector to various (gamma) -ray sources will be presented. The unique capability of the LXe-TPC to identify and reject background events based on Compton kinematics reconstruction has also been demonstrated.
The feasibility of a large volume liquid xenon time projection chamber (LXe-TPC) for three- dimensional imaging and spectroscopy of cosmic gamma-ray sources, was tested with a 3.5 liter prototype. The observation of induction signals produced by MeV gamma rays in liquid xenon is reported, with a good signal-to-noise ratio. The results represent the first experimental demonstration with a liquid xenon ionization chamber of a non-destructive read- out of the electron image produced by point-like charges, using a sense wire configuration of the type originally proposed in 1970 by Gatti et al. An energy resolution as good as that previously measured by us with millimeter size chambers, was achieved with the large prototype of 4.4 cm drift gap.
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