Alessandro Curioni

XENON: A 1 TONNE LIQUID XENON EXPERIMENT FOR A SENSITIVE DARK MATTER SEARCH

XENON is a novel liquid xenon experiment concept for a sensitive dark matter search using a 1-tonne active target, distributed in an array of ten independent time projection chambers. The design relies on the simultaneous detection of ionization and scintillation signals in liquid xenon, with the goal of extracting as much information as possible on an event-by-event basis, while maintaining most of the target active. XENON is expected to have effective and redundant background identification and discrimination power, higher than 99.5%, and to achieve a very low threshold, on the order of 4 keV visible recoil energy. nBased on this expectation and the 1-tonne mass of active xenon, we project a sensitivity of 0.0001 events/kg/day, after 3 yr operation in an appropriate underground location. The XENON experiment has been recently proposed to the National Science Foundation (NSF) for an initial development phase leading to the development of the 100 kg unit module.

Compton Imaging of MeV Gamma-Rays with the Liquid Xenon Gamma-Ray Imaging Telescope (LXeGRIT)

The Liquid Xenon Gamma-Ray Imaging Telescope (LXeGRIT) is the first realization of a liquid xenon time projection chamber for Compton imaging of MeV γ-ray sources in astrophysics. By measuring the energy deposit and the three spatial coordinates of individual γ-ray scattering points, the location of the source in the sky is inferred with Compton kinematics reconstruction. The angular resolution is determined by the detectors energy and spatial resolutions, as well as by the separation in space between the first and second scattering. The imaging response of LXeGRIT was established with γ-rays from radioactive sources, during calibration and integration at the Columbia Astrophysics Laboratory, prior to the 2000 balloon flight mission. In this paper we describe in detail the various steps involved in imaging sources with LXeGRIT and present experimental results on angular resolution and other parameters which characterize its performance as a Compton telescope.

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.

A new light readout system for the LXeGRIT time projection chamber

LXeGRIT is a liquid xenon time projection chamber (LXeTPC) used as balloon-borne Compton telescope for imaging cosmic sources in the MeV energy band. The three-dimensional position sensitive charge readout is triggered by the xenon scintillation light. In the original chamber design, the light is detected by four UV sensitive, 2 PMTs (EMI 9813), coupled to the liquid xenon vessel by quartz windows. In order to improve the trigger efficiency and uniformity, a new light readout system has been studied. It consists of 12 UV sensitive, compact 2 PMTs (Hamamatsu R6041Q), mounted in Teflon frames which cover the four sides of the LXeTPC active volume. These all-metal PMTs were especially developed to work at liquid xenon temperature and up to 3.5 atm overpressure. Light simulations promise an increase in light collection efficiency by more than a factor of ten for the new readout. We present simulations of the light collection efficiency and initial results from the successful operation of the new PMT fully immersed in liquid xenon.

LXeGRIT Compton telescope prototype: current status and future prospects

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.

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.

Performance of the light trigger system in the liquid xenon /spl gamma/-ray imaging telescope LXeGRIT

LXeGRIT is a balloon-borne Compton telescope for MeV /spl gamma/-ray astrophysics, based on a liquid xenon time projection chamber with charge and light readout. The energy and direction of an incident /spl gamma/-ray are reconstructed from the three-dimensional locations and energy deposits of individual interactions taking place in the homogeneous detector volume. While the charge signals provide energy information and X-Y-positions, the fast xenon scintillation light signal is used to trigger the detector. The drift time measurement, referred to the time of the trigger signal, gives the Z-position with the known drift velocity. The light is detected by four ultraviolet-sensitive photomultiplier tubes (PMTs). The logical OR of the PMT signals triggers the data-acquisition system with an efficiency that depends on the event energy and location, as well as on the discriminator thresholds used on the individual PMTs. Results from experiments with a tagged /sup 22/Na source give the spatial distribution of the light trigger efficiency for 511-keV /spl gamma/-rays. When averaged over the whole sensitive volume and all PMTs, the trigger efficiency is 47% or 40% for two discriminator windows used during the LXeGRIT balloon flight of 1999. These values are strongly affected by the different sensitivity of each PMT. The corresponding average efficiency at 511 keV for the best of the four PMTs is in fact 63% and approaches 100% for interactions taking place in a small volume right above the PMT.

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...

On the background rate in the LXeGRIT Instrument during the 2000 Balloon Flight

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.