The LAXPC Instrument:
For X-ray timing and low-resolution spectral studies over a broad energy band (3-80 keV) ASTROSAT will use a cluster of 3 co-aligned identical Large Area X-ray Proportional Counters (LAXPCs), each with a multi-wire-multi-layer configuration and a Field of View of 1ox1o. These detectors are designed to achieve (I) wide energy band of 3-80 keV, (II) high detection efficiency over the entire energy band, (III) narrow field of view to minimize source confusion, (IV) moderate energy resolution, (V) small internal background and (VI) long life time in space.
Xenon based gas mixture at a pressure of two atmospheres will be filled in multilayer 15-cm deep detectors to achieve an average detection efficiency of close to 100% below 15 keV and about 50 % up to 80 keV. A thin (thickness 25/50 micron) aluminized Mylar window for X-ray entrance will ensure a low energy threshold of about 2-3 keV. The Mylar film will be supported by a honeycomb shaped window support collimator with a 5ox5o field of view. A field of view of 1ox1o will be provided by using mechanical collimators made of a sandwich of tin,copper and aluminium coaligned with the window support collimator and sitting above it.
A LAXPC unit for test in a balloon flight experiment. This is similar to the unit to be flown on ASTROSAT except that it has a 3o x 3o field of view collimator.
The LAXPC detector housings are milled out of single blocks of high tensile strength Aluminium alloy to ensure light weight and structural integrity required to withstand large pressure difference in the large volume detectors. Each detector will be shielded with a tin and copper sheet to prevent leakage of X-rays into the detection volume from the detector walls. Mutual and veto layer anticoincidence logic will be employed between the large number of anode wires to ensure rejection of unwanted charged particle background signals and Compton scattered high energy photons. The X-ray detection volume of each detector will consist of 60 anode cells, each with a cross section of 3cm x 3cm, arranged in 5 anode layers.
The LAXPC X-ray detection anode assembly with veto layer on 3 sides mounted on the back plate
The total effective area of the 3 LAXPCs will be about 6000 cm2 at 5keV. Due to its large depth and high gas pressure the LAXPC will have high detection efficiency right up to about 80keV, as shown in the figure below.
Effective area of ASTROSAT LAXPC compared with RXTE PCA and BeppoSAX PDS. The effect of transmision through the Mylar window is included.
To achieve good energy resolution of the detectors, it is necessary to have a uniform gain over the entire area and the gas needs to be free from impurities like oxygen and water vapour. The former will be achieved by precision placement of the anode wires at the center of the cells and by use of anode wire of uniform diameter. An onboard purifier that will be used to purify the gas from time to time will prevent degradation of energy resolution due to slow outgassing from detector walls. Oxisorb purifiers with onboard compressor for gas flow will be used for this purpose.
The high sensitivity of the LAXPC instrument will allow the detection of a 0.1 mCrab source at 5-sigma level in an exposure of about 104 sec. This will enable the LAXPC to address a wide variety of science.