LAXPC (Large Area Xenon Proportional Counters)

The LAXPC or Large Area X-ray Proportional Counters balloon instrument was an experiment aimed to assess the performance of X-Ray detectors for ASTROSAT, India's first satellite fully devoted to astronomical observations. The satellite covered a wide spectral band from optical to hard X-rays by a complement of 4 co-aligned instruments and a Scanning Sky X-ray Monitor. One of the instruments was a LAXPC with 3 identical detectors. In the image at left we can see the LAXPC instrument balloon gondola ready for flight and below one of the LAXPC detectors before being sealed and installed in the gondola (click to enlarge). The experiment included two prototype of these LAXPCs very similar to those on the ASTROSAT except that their field of view (FOV) was 3º x 3º versus FOV of 1º x 1º for the three LAXPCs installed on the ASTROSAT.

THE DETECTORS

Each one of the two identical LAXPC detectors was a multi-anode, multi-layer proportional chamber that has 60 anode cells arranged in 5 layers in anode frames which served as the X-ray detection volume. Each anode cell had a cross-section of 3 cm x 3 cm and a length of 100 cm. The anode consisted of a gold coated stainless steel wire of 37 µ diameter, which was held under tension by soldering it at either end to the silver plated brass pin held in Teflon insulators mounted in the anode frames. Each anode cell was electrically isolated from the neighboring cells by a wall of 50 µ diameter beryllium-copper wires spaced 3 mm apart. Thus the X-ray detection volume was 15 cm deep, 36 cm wide and 100 cm long to achieve high detection eficiency for X-rays of up to 80 keV. The X-ray detecting anode cells were surrounded on three sides by veto layers. The veto layer had 46 veto cells, 11 on each long side and 24 below the bottom anode layer. The veto layers consisted of veto cells of 1.5 cm x 1.5 cm cross-section and used 37 µ gold plated stainless steel wire as the anode of the veto cells.

In the first 2 anode layers alternate anode cells were linked together to provide 2 outputs from each layer which were then operated in mutual anticoincidence. The anode cells in each of the other 3 layers were linked together to provide a single output from each of the 3 layers. Thus there were 7 outputs from the X-ray detector which were fed to seven independent Charge Sensitive Pre-amplifiers (CSPA's). All the 7 CSPA outputs were operated in mutual anticoincidence and an event was considered to be due to X-ray only if the pulse was detected from only one of the 7 outputs.

The entire anode cell assembly was then mounted on a milled out Back Plate (BP) whose rear part was milled out to create pockets in which CSPA's, High Voltage generation cards, distribution cards and command control electronics were housed. A sandwich of 25 µ perforated polypropylene film and 50 µ thick Mylar sheet, aluminized on one side was used to seal the detector on the top side. The Mylar sheet served as a gas barrier as well as X-ray entrance window. The detectors then were filled with Xenon (90%) and Methane gas (10%) at a pressure of about 2 atmospheres.

THE GONDOLA

The two LAXPCs detectors were mounted in a sturdy cradle that was driven in elevation by a geared DC motor. The detector cradle was mounted on an oriented platform which was coupled at the top to a servo control system consisting of a reaction wheel driven by a DC torque motor. This in turn coupled the payload with the balloon. The azimuth angle of the payload could be controlled and rotational perturbations due to balloons were corrected by the servo system. A flux gate sensor mounted on a small platform driven by a geared motor, was used as a sensor for the magnetic field of the Earth to derive error signal from the null position for controlling the azimuth orientation. The azimuth angle is measured by a 10 bit shaft encoder.

The movements in azimuth and elevation were controlled by a source tracking program pre-loaded in an onboard memory which also could be overrun by radio command. This permited acquisition, pointing and tracking of any source whose coordinates were stored in the onboard memory. These values were compared during the flight with the actual readings to activate the motors and correct the difference. The system start time was clocked by an alarm time which was set on the ground just before the launch of the payload. This method provided the on-board tracking of a given X-ray source during the flight. In case of any malfunction of the program, the source tracking could also be done by sending manually real time telecommands.

Telemetry and telecommand were sent to the payload trought a radio frequency link operating in S band, allowing control of the LAXPC instrument, and the operation of complementary flight support subsystems.

Balloon launched on: 4/14/2008 at 00:40 ist
Launch site: TIFR National Balloon Facility, Hyderabad, India  
Balloon launched by: National Balloon Facility, Tata Institute of Fundamental Research
Balloon manufacturer/size/composition: Zero Pressure Balloon Antrix (15 micron + 38 micron) - 738.900 m3
Flight identification number: 470
End of flight (L for landing time, W for last contact, otherwise termination time): 4/14/2008 at 9:16 ist
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): 9 h
Payload weight: 954 kgs

The balloon was launched from the National Balloon Facility in Hyderabad, at 12.40 Indian Standard Time on April 14, 2008. After a nominal ascent phase, it reached ceiling altitude of 41 km corresponding to a vertical residual atmosphere of 2.5 g/cm2 at 2.40 hours. The balloon floated at the ceiling for 6 hours and 36 minutes and the mission was terminated by telecommand at 9:16 IST.

The main objective of the flight was to assess the performance of LAXPCs by making measurements of the background as well as pointed mode observations of X-ray binary Cyg X-1, the brightest X-ray source above 20 keV at balloon altitudes. The observation program included pointed mode Cyg X-1 data and measurements of the background by pointing away from the source before and after Cyg X-1 observation. These data was used for assessing the performance of LAXPC detectors and analysing time variability of Cyg X-1. After the balloon reached the ceiling altitude a source-free region in the sky was observed to measure the background count rates. Following this, Cyg X-1 was observed for 3 hours and 19 minutes. The tracking was so programmed that observations of the source free background region and Cyg X-1 were carried out alternately during the flight to correct for any systematic variations in the background count rates. Cyg X-1 data was acquired from both the LAXPC detectors.

• Balloon launch records Tata Institute of Fundamental Research
• Current Developments and Future Plans at NBF@Hyderabad and Prospects for Long Duration Balloon Flights Trans. JSASS Aerospace Tech. Japan Vol. 8, No. Ists27, Pag 55
• Performance of large area x-ray proportional counters in a balloon experiment Exp Astron (2016)
• The LAXPC Instrument Astrosat website