Design Details

Introduction to the CZTI

The he Cadmium Zinc Telluride Imager (CZTI) is one among the four X-ray instruments on ASTROSAT. It is a hard X-ray imaging instrument covering the energy band from 10 to 100 keV, has a detector area of 976 cm2 constructed using CZT modules and uses a Coded Aperture Mask (CAM) for imaging.

The CZTI carries a Cesium Iodide (Tl) based scintillator detector for Veto measurements. This is located just under the CZT detector modules. Further, there is a gap of about 8 cm between the base of the collimator slats and the detector plane, in order to accommodate a radioactive calibration source module in each quadrant. This source shines alpha-tagged 60 keV photons on the CZT detector in order to help calibrate the energy response.

The salient features of CZTI

Area 976 cm2
Pixel 16384 (64 modules of 256 pixels each
Pixel Size 2.46 mm X 2.46 mm (5 mm thick); edge row pixels are 2.31 mm wide instead of 2.46 mm
Read-out ASIC based (2 ASICs per module)
Imaging Mathod Coded Aperture Mask (CAM)2
Field of View
(10-100 keV)
4.6º X 4.6º FWHM (primary FOV)
11.8º X 11.8º FWZM (incl. illumination leakage)
Angular Resolution ∼ 8 arcmin (18 arcmin geometric)
Energy Resolution ∼ 8% @ 100 keV
Energy Range 10-100 keV
Up to 1 MeV (Photometric); no imaging above 100 keV
Sensitivity 0.5 mCrab (5 sigma; 104 s)
Memory 50 Mbytes per orbit

The Major Components of CZTI Quadrant

Name Components Mechanical Location
CZT Detector Assembly Detector Modules
Detector Board
Internal Radiator
Detector Board Assembly
HV-CZT Pico DC/DC Detector Board Assembly
Veto Detector CsI(Tl) + 2 PMTs CsI Assembly
HV-Veto HV DC/DC (800V) CsI Assembly
Alpha Tag 241Am Alpha-Box
FPGA Card FPGA, processor,
Processing Electronics Box

Veto Detector

A detector (CsI) covers the large area of 256 cm2 and the light collection is done using two photomultipliers (PMT), viewed from sides. On registering an event, a signal from the detector is sent to a pre-amplifier. This signal is processed and sent to the FEB for further analysis. After amplification of the signal from pre-amplifier, the signal is sent to a comparator via stretcher along with LLD level signal. If LLD is triggered the pulse is digitized to an 8-bit output by ADC through control circuit. This output is used to differentiate Compton scattered events and hence the background in main detector can be reduced.

Alpha - Tag

A calibration source of Am (241), is kept to shine over the detector. A energy of 60 keV is released from the source along with an alpha particle simultaneously. This alpha particle gets detected in the CsI (Tl) detector volume which generates the signal. The photodiode which is coupled to this CsI (Tl) detector generates an electrical pulse, which is amplified in pre-amplifier and post-amplifiers. Postamplifier signal is fed to an analog comparator along with lower-level discriminator level. A relevant digitized signal generated with help of Monoshot is passed on to FPGA as an alphatag event.