The BeppoSAX satellite is a joint program of the Italian Space Agency (ASI) and
the Netherlands Agency for Aerospace Programs (NIVR) devoted to X-ray
astronomical observations in a broad energy band (0.1-300 keV). BeppoSAX is
scheduled to be launched in April 1996 into a circular orbit at 600 km
altitude with an inclination of about 
. The above inclination was
chosen to get a high geomagnetic cut-off with a very low modulation and to
minimize the induced radiation on the payload due to the South Atlantic Anomaly
(SAA). Overviews of the BeppoSAX mission can be found elsewhere (Butler & Scarsi
1990; Scarsi 1993; Piro et al. 1995). The satellite is three-axis-stabilized
with post-facto attitude reconstruction with 1 arcmin accuracy. Its expected
lifetime is from 2 years up to 4 years. The payload includes Narrow Field
Instruments (NFI) and Wide Field Cameras (WFC). The NFIs are four
Concentrators Spectrometers (C/S) with 3 units (MECS) operating in the 
keV energy band and 1 unit (LECS) operating in 
keV, a High Pressure
Gas Scintillation Proportional Counter (HPGSPC) operating in the 
keV
energy band and a Phoswich Detection System (PDS) with four detection units
operating in the 
keV energy band. Orthogonally with respect to the
NFIs there are two WFCs (field of view of 
FWHM)
which operate in the 2-30 keV energy band with imaging capabilities (angular
resolution of 5 arcmin). As a part of the PDS, there is a Gamma Ray Burst
Monitor (GRBM) with nominal energy band of 60 to 600 keV.
Figure 1: A sketch of the major components of the PDS experiment
As can be seen, BeppoSAX covers an unprecedented wide energy
band from 0.1 to 300
keV. The hard X-ray energy band (
keV) covered by the PDS experiment is
now assuming key importance for high energy astrophysics thanks to many
exciting and unexpected results obtained with 
-ray astronomy
experiments like BATSE and OSSE aboard CGRO, SIGMA aboard the GRANAT satellite
and balloon experiments. In addition to well known classes of strong hard
X-ray emitters (galactic X-ray pulsars, black hole candidates, Am Her-type
sources, AGN), other classes of X-ray sources, like X-ray bursters, resulted
to be strong hard X-ray emitters during source states when the low energy
emission is relatively weak (Barret & Vedrenne 1994; Zhang et al. 1996). These
sources were not expected to have hard emission spectra. Soft gamma ray
repeaters have been discovered. Some of them exhibit parossistic phases of
activity, during which up to 
bursts per day are detected
(Kouveliotou et al. 1996; Lewin et al. 1996). Also for extragalactic sources
the hard X-ray sky was full of unexpected results, e.g. for Seyfert 2
(Bassani et al. 1996).
With its high sensitivity, especially in the 20-100 keV band, PDS will offer the opportunity to perform observations of key importance in the hard X-ray band. Many open questions in high energy astrophysics are expected to be clarified and solved with observations in this band. Coordinated observations with GRO-BATSE and XTE will give the opportunity to fully exploit the joint capabilities of the three observatories that cover this important energy band. Also coordinated observations with on-ground and space observatories are extremely important for identifying counterpart of new hard X-ray sources and/or study the complex physics occurring in them.
In this paper we will give a description of the PDS and of its performances as derived from functional tests and pre-flight calibrations. Descriptions of the PDS design have already been reported (Frontera et al. 1991, 1992). Also reported are preliminary results obtained from instrument functional tests (Frontera et al. 1997). The GRBM expected performance has also been investigated and results were reported (Pamini et al. 1990; Alberghini et al. 1994).