The Phoswich Detection System PDS on board the SAX satellite

The Phoswich Detection System (PDS) is one of the four narrow-field instruments on board the SAX satellite. The experiment will be dedicated to temporal and spectral studies of celestial X-ray sources in the 15÷300 keV energy band. The PDS detector is composed of 4 actively shielded NaI(TI)/CsI(Na) phoswich scintillators and has a field of view of 1.4o (FWHM). The total geometric area is 800 cm2. The instrument is designed with the goal to achieve an energy resolution better than 17% at 60 keV and a 3σ flux sensitivity for an observation time of 105 s of about 1×10−6 photons cm−2 s−1 in an energy band 40 keV wide centred at 100 keV. This sensitivity corresponds to about 2 milliCrab flux units in the same energy range. The expected 3σ sensitivity of the PDS to cyclotron emission lines is about 3×10−5 photons cm−2 s−1 at 60 keV in 105 s. The CsI(Na) lateral shields of the PDS will also be used as a gamma-ray burst monitor. In this report we give a detailed description of the experiment design and we discuss the expected in-flight performances of the PDS.