  Phoswich detectors (RT-2/S & RT-2/G) are major scientific payloads of the
RT-2 Experiment onboard the CORONAS-PHOTON mission, which was launched into a
polar Low Earth Orbit of around 550 km on 2009 January 30. These RT-2
instruments are designed and developed to observe solar flares in hard X-rays
and to understand the energy transport processes associated with these flares.
Apart from this, these instruments are capable of observing Gamma Ray Bursts
(GRBs) and Cosmic diffuse X-ray background (CDXRB). Both detectors consist of
identical NaI(Tl) and CsI(Na) scintillation crystals in a Phoswich combination,
having the same diameter (116 mm) but different thicknesses. The normal working
energy range is from 15 keV to 150 keV, but may be extendable up to ~ 1 MeV. In
this paper, we present the RT-2/S and RT-2/G instruments and discuss their
testing and calibration results. We used different radio-active sources to
calibrate both detectors. The radio-active source Co^57 (122 keV) is used for
onboard calibration of both instruments. During its lifetime (~ 3-5 years),
RT-2 is expected to cover the peak of the 24th solar cycle.

Instrumentation and Methods for Astrophysics