The Scientific Instruments

In fig.  a sketched view of the scientific payload shows how the various instruments are placed.

Figure: The BeppoSAX Scientific Payload

The instruments are grouped into two classes: the Narrow Field Instruments (NFIs) and the Wide Field Cameras (WFCs). All the NFIs are co-aligned and pointing along the positive direction of the Z axis of the BeppoSAX frame of reference; they consist of the following (Table ): a LECS (Low Energy Concentrator Spectrometer, $0.1-10$ keV), three MECS (Medium Energy Concentrator Spectrometers, $1.3-10$ keV), a HPGSPC (High Pressure Gas Scintillation Proportional Counter, $4-120$ keV), and a PDS (Phoswich Detection System, $15-300$ keV). Each imaging GSPC is located at the focal plane of an X-ray concentrator system. The WFCs ($2-28$ keV) consist of two identical coded mask cameras co-aligned with the Y axis: WFC1 points along the negative direction and WFC2 along the positive one.

Eventually, there is another instrument which, on principle, has no imaging capability: the Gamma-Ray Burst Monitor (GRBM), whose four detector units form a square well around the PDS: the normal directions to GRBM1 and GRBM3 point along the same directions as WFC1 (-Y) and WFC2 (+Y), respectively, while +X for GRBM2 and -X for GRBM4 (fig. ).

Table: Main features of the scientific instruments

Instrument	Band	FOV	Ang. Res.$^{(1)}$	Eff. Area	Energy Res.	Time Res.
units	(keV)	(FWHM)	(FWHM)	(cm$^{2}$)	(% FWHM)	($\mu$s)
1 LECS	0.1-10	37$^\prime$	9.7$^\prime$@0.28 keV	22@0.28 keV	$8.84$x$(E/6)^{-0.42}$	$16 \mu$s
			2.1$^\prime$@6 keV	50@6 keV
			105 $^{\prime\prime}$@1.5 keV	31@1.5 keV
3 MECS	1.3-10	28$^\prime$	75 $^{\prime\prime}$@6.4 keV	150@6.4 keV	$8$x$(E/6)^{-0.5}$	$15 \mu$s
			75 $^{\prime\prime}$@8.1 keV	101@8.1 keV
1 HPGSPC	4-120	1.1$^\circ$	collimated	240@30 keV	$4$x$(E/60)^{-0.5}$	$15 \mu$s
1 PDS	15-300	1.3$^\circ$	collimated	600@80 keV	$15$x$(E/60)^{-0.5}$	$16 \mu$s
2 WFC	1.8-28	$20$$^\circ$x $20$$^\circ$	3	140@10 keV	$18$x$(E/6)^{-0.5}$	$0.5$ ms$^{(2)}$
4 GRBM$^{(3)}$	40-700	open	$-$	500@300 keV	$30$x $(E/100)^{-0.4}$	1 s$^{(4)}$

$^{(1)}$

Radius containing the 50% of the power.

$^{(2)}$

Valid for imaging mode (Normal Mode). In the case of fast non-imaging mode (High Time Resolution Mode) it is 0.25 ms.

$^{(3)}$

The effective area and the energy resolution refer to unit 1.

$^{(4)}$

Valid for ratemeters (normal mode). High Time Resolution (HTR) mode reaches 7.8125ms and 0.488 ms, while the GRBM spectra's temporal resolution is fixed to 128 s.

The LECS ([Parmar et al., 1997]) and the MECS ([Boella et al., 1997b]) are a set of four X-ray concentrators, each one consisting of 30 nested, gold coated, confocal mirrors with a double cone approximation to Wolter I geometry ([Citterio et al., 1985,Conti et al., 1994]), with diameters ranging from $16.2$ to $6.8$ cm, thickness ranging from $0.4$ to $0.2$ mm, with a focal length of 185 cm and a geometric area of 124 cm$^2$. The detector unit, placed in the focal plane, is a position sensitive gas scintillation proportional counter and it is filled with Xenon ($1.0$ atm at 25 $^\circ$C). The gas cell has a cylindrical shape; at the top, the LECS is closed by different layers made of polymide, aluminum nitride, Al and carbon, allowing the detection of down to $0.1$ keV photons and shielding it against space plasma; in the case of the MECS, the top layer consists of a 50 $\mu$m beryllium window.

The HPGSPC instrument ([Manzo et al., 1997]) is a High Pressure Gas Scintillation Proportional Counter filled with a gas mixture of Xe (90%) and He (10%) at the pressure of 5 atm. Its gas cell is cylindrical and consists of a titanium body (3 mm thick, diameter of 360 mm and depth of 184.5 mm) closed by a beryllium entrance window, whose foils, $0.5$ and $0.8$ mm thick and with a diameter of 30 cm, are transparent down to 3 keV photons and, at the same time, stiff enough to withstand the gas pressure.

The WFCs ([Jager et al. 1997]) are two identical coded mask cameras working in the $1.8-28$ keV energy band: their FOV is $20$$^\circ$x $20$$^\circ$FWHM ($40$$^\circ$x $40$$^\circ$at zero response). The detector is a Multi Wire Proportional Counter filled with a gas mixture (94% Xe, 5% CO$_2$, 1% He, at 2.2 bar) and closed by a 150 $\mu$m thick entrance window of beryllium, while the coded mask, made of iron, is placed 70 cm in front of it; both are supported by a stainless steel structure and their sizes are $25.5$x$25.5$ cm$^2$ for the detector and $25.6$x$25.6$ cm$^2$ for the mask, respectively. The angular resolving power is 5$^\prime$FWHM, while the source location accuracy is better than 1$^\prime$, taking into account also the satellite pointing stability. The limiting sensitivity, though depending on the X-ray background flux, is a few mCrab in $10^5$ s.

The PDS is described in the following section, that is mainly devoted to the description of the GRBM structure and data.