LXeGRIT (Liquid Xenon Gama-Ray Imaging Telescope)

The Liquid Xenon Gamma-Ray Imaging Telescope or LXeGRIT was a balloon-borne Compton telescope designed to capture and image gamma rays in the energy range of 200 keV to 20 MeV. Its core detector was a large-volume liquid xenon time projection chamber (LXeTPC), which operated within a cryogenics system. The instrument featured an angular resolution of approximately 3 degrees at 2 MeV and a field-of-view spanning 1 steradian. LXeGRIT was developed at the Astrophysics Laboratory of the Columbia University, and counted with an extensive collaboration from institutions like the University of New Hampshire, NASA's Marshall Space Flight Center, The Waseda University from Japan and the Istituto Nazionale di Fisica Nucleare from Italy.

At left we can see an scheme of the instrument (click to enalrge). The LXeTPC was the central gamma-ray detector, housed in a 10-liter cylindrical vessel with active dimensions of 20 cm x 20 cm x 7 cm, corresponding to an absorber thickness of 21 g/cm². Electrons ionized within the xenon volume were drifted in a uniform electric field, their positions recorded in three dimensions using a system of orthogonal wire planes and anodes. UV-sensitive photomultiplier tubes detected scintillation light through quartz windows. Xenon gas, purified to high levels, was liquified and maintained at -95°C and 1.5 atm pressure using a controlled flow of liquid nitrogen.

Surrounding the LXeTPC, a shield-veto system reduced background noise from cosmic rays and gamma-ray interactions. This included active side and back shields constructed of NaI(Tl) scintillators and a plastic scintillator counter atop the TPC. The shields worked in tandem with the detector's trigger system to minimize noise and enhance the signal-to-background ratio, allowing precise offline event discrimination.

The instrument's data acquisition system utilized advanced electronics for signal digitization and processing, with the capability to record detailed information about each detected gamma-ray event. A Motorola-based processor managed data acquisition, transmitted portions of the data to the ground via telemetry, and stored the rest onboard. Telemetry and onboard data handling ensured efficient monitoring of the instrument's performance during its flights.

Since LXeGRIT was a Compton telescope that did not require active pointing, the gondola that housed the instrument was engineered to remain passively aligned. Its zenith direction was determined through pitch and roll measurements using precision inclinometers, while azimuth orientation was obtained from dual magnetometers calibrated before the flight. These orientation measurements, combined with geographical positioning data, were essential for analyzing imaging data. The gondola structure incorporated a suspension system that connected it to the balloon, enabling it to withstand stresses during launch, ascent, flight, and recovery. The instrument's sensitive components, particularly the liquid xenon time projection chamber (LXeTPC), were mounted within a robust pressure enclosure and insulated cryostat. These were supported by the gondola framework to maintain alignment and protect against mechanical shocks, such as those experienced during landing.

The LXeTPC and its associated electronics were enclosed in a vacuum-insulated cryostat to reduce heat transfer and maintain the liquid xenon at its operating temperature of -95°C. The gondola carried a liquid nitrogen (LN2) dewar, modified to include a molecular sieve trap for maintaining vacuum conditions. This dewar supplied LN2 to cool the xenon, with solenoid valves controlled by the Instrumentation and Control System (ICS) ensuring precise temperature regulation. The gondola's thermal design also mitigated risks of overheating in reduced-pressure environments at high altitudes.

Power for the gondola and its systems was supplied by a stack of lithium batteries, each rated at 30 Ah, providing sufficient energy for flights lasting up to 40 hours. These batteries powered the cryogenics, instrumentation, and telemetry systems, ensuring uninterrupted operation throughout the mission.

Balloon launched on: 10/4/2000 at 16:39 utc
Launch site: Scientific Flight Balloon Facility, Fort Sumner, (NM), US  
Balloon launched by: National Scientific Balloon Facility (NSBF)
Balloon manufacturer/size/composition: Zero Pressure Balloon 800.000 m3 - SF3-29.47-.8/.8/.8NHR
Balloon serial number: W29.47-2X-35
Flight identification number: 491N
End of flight (L for landing time, W for last contact, otherwise termination time): 10/5/2000 at 22:06 utc
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): 26 h 30 m
Landing site: 14 miles SW of Buckeye, Arizona, US

After difficult low-level wind conditions had postponed the flight several times, the NSBF team managed to find a slim launch window, during which the payload was smoothly put afloat. The balloon was released at 19:39 utc on October 4th, 2000, which was an unusual early-afternoon launch in NASA balloon base of Ft. Sumner. The balloon went straight up, heading East initially, but soon changing to a south-western and eventually western course. Due to the western course of the balloon, which is very unusual for a flight in early October, the science team was flown from Ft. Sumner to the downlink station in Holbrook, AZ, in the morning of October 5. The gondola landed softly about one hour west of Phoenix, Arizona, near a small town called Buckeye. The payload just tipped over without being dragged, thus producing only minor dammage to the gondola.

The long flight time, the perfect operation of all systems, and the wide field-of-view of the Compton telescope LXeGRIT, which was flown this time without any shields, allowed the science team to obtain data on all MeV gamma-ray sources within ~45 deg zenith angle. The main focus was imaging of the Crab nebula, but other strong sources such as 3C273 or Cygnus X-1 have also been in the field-of-view. During the flight were collected a total of almost 40 GB of event data, of which about 80% have been stored onboard, and the rest downlinked through two 500 kbps telemetry channels.

• A Study of the LXeGRIT Detection Efficiency for MeV Gamma-Rays during the 2000 Balloon Flight Campaign submitted to NIMA
• Compton Imaging of MeV Gamma-Rays with the Liquid Xenon Gamma-Ray Imaging Telescope (LXeGRIT) submitted to NIM A
• LXeGRIT - A Liquid Xenon Gamma-Ray Imaging Telescope Columbia University website