  The CZTI (Cadmium Zinc Telluride Imager) onboard AstroSat is a high energy
coded mask imager and spectrometer in the energy range of 20 - 100 keV. Above
100 keV, the dominance of Compton scattering cross-section in CZTI results in a
significant number of 2-pixel Compton events and these have been successfully
utilized for polarization analysis of Crab pulsar and nebula (and transients
like Gamma-ray bursts) in 100 - 380 keV. These 2-pixel Compton events can also
be used to extend the spectroscopic energy range of CZTI up to 380 keV for
bright sources. However, unlike the spectroscopy in primary energy range, where
simultaneous background measurement is available from masked pixels, Compton
spectroscopy requires blank sky observation for background measurement.
Background subtraction, in this case, is non-trivial because of the presence of
both short-term and long-term temporal variations in the data, which depend on
multiple factors like earth rotation and the effect of South Atlantic Anomaly
(SAA) regions etc. We have developed a methodology of background selection and
subtraction that takes into account for these effects. Here, we describe these
background selection and subtraction techniques and validate them using
spectroscopy of Crab in the extended energy range of 30 - 380 keV region, and
compare the obtained spectral parameters with the INTEGRAL results. This new
capability allows for the extension of the energy range of AstroSat
spectroscopy and will also enable the simultaneous spectro-polarimetric study
of other bright sources like Cygnus X-1.
