Valeri Egorov

A liquid xenon time projection chamber for γ-ray imaging in astrophysics: present status and future directions

Abstract A liquid xenon time projection chamber (LXeTPC) has been developed to image cosmic γ-rays in the energy band 0.2–20 MeV. Its performance as Gamma Ray Imaging Telescope (LXeGRIT instrument) has been tested during a high altitude balloon flight (Spring ’99, New Mexico). The detector, with 400 cm 2 area and 7 cm drift gap, is filled with 7 l high purity LXe. Both ionization and scintillation light signals are detected to measure the energy deposits and the three spatial coordinates of individual γ-ray interactions within the sensitive volume. During the pre-flight calibration experiments the LXeGRIT instrument was extensively tested with γ-ray sources in the laboratory: a 10% FWHM energy resolution at 1 MeV was determined, scaling with 1/ E . The detector shows a linear response in the energy range 511 keV–4.4 MeV.

Spectroscopy and imaging performance of the Liquid Xenon Gamma-Ray Imaging Telescope (LXeGRIT)

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.

Liquid Xenon Gamma-Ray Imaging Telescope (LXeGRIT) for medium energy astrophysics

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.

Preliminary results from the 1999 balloon flight of the Liquid Xenon Gamma-Ray Imaging Telescope (LXeGRIT)

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 detectors 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: The liquid xenon gamma-ray imaging telescope

The feasibility of a large-volume liquid xenon Compton telescope based on full event imaging in a time projection chamber has been demonstrated with the development of the balloon-borne instrument LXeGRIT. With its 400 cm2 sensitive area and 7 cm drift gap, the liquid xenon detector images γ-rays in the energy range from 200 keV to 25 MeV. The precise 3-dimensional localization of γ-ray interactions within the sensitive volume provides excellent background reduction capabilities. Together with the large efficiency of a homogeneous detector volume, LXeGRIT addresses the primary instrumental limitations encountered in this energy band. Following engineering tests at balloon altitude in 1997, LXeGRIT has been upgraded with a new trigger and data acquisition system, integrated with the existing readout electronics. Enhanced data transfer capability and onboard data storage were also implemented. LXeGRIT was successfully flown from Ft. Sumner, NM on May 7, 1999. The instrument worked as expected at balloon alt...

XENA: a liquid-xenon Compton telescope for gamma-ray astrophysics in the MeV regime

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/COMPTELs 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. XENAs 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, XENAs 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.