Cadmium-Zinc-Telluride Imager (CZTI)
Cadmium - Zinc - Telluride Imager (CZTI) is truly a hard X-ray imaging instrument in the energy range 10-100 keV with a collecting area of 976 cm2. This is a solid state detector and the entire detector assembly is divided into four identical and independent quadrants. In each quadrant, 16 CZT modules each of area 15.25 cm2 are used. CZT modules are pixelated with pixel size 2.46 mm x 2.46 mm and 5 mm thickness. Individual pixels are connected with an electronic assembly to detect the incident X-ray photons as output voltage. Very high energy particles can simply pass through the CZT detector with a partial energy deposition and is a source of background noise. A Cesium - Iodide - Thallium [CsI(Tl)] crystal is used just under the CZT detector panel for background rejection. An X-ray photon in the energy range 10-100 keV deposits the full energy only in the CZT whereas a high energy charged particle deposits energy both in the CZT and the CsI detectors. This can be used to detect which events are due to X-ray photons and which are due to charged particles. The detector has a detection efficiency of 95% in 10-100 keV range.
A collimator, made of 0.07 mm thick Tantalum sheet sandwiched between 0.2 mm thick Aluminum with a field of view 4.6ox4.6o, is placed above the CZT detector assembly allowing nearly parallel incidence of photons onto the detectors. A Coded Aperture Mask (CAM) made of 0.5 mm tantalum is placed above the collimator. The CAM consists of predetermined pattern of rectangle/square holes matched with the size of the CZT pixel straight down to it and the CAM casts a shadow onto the detector with 50% transparency (roughly equal number of close and open cells). The exact position of the source above the detector can be determined from the pattern of the shadow that it casts. CZT modules perform best in the temperature range 0o-15oC and hence the heat generated by the detector assembly is drained out continuously by a radiator panel assembly.
The geometric area and mass of CZTI are 976 cm2 and 56 kg.
This payload is developed by Tata Institute of Fundamental Research (TIFR), Mumbai and Vikram Sarabhai Space Centre (VSSC), Trivandrum and IUCAA.