Measurement set-up and strategy

SAX was set onto a rotary table, in the Z axis vertical direction, and the tooling bar was located at about 570 cm from the PDS center (Fig. 2.3). By means of a coordinated shift of the movable horizontal arm of the tooling bar and a movement of the source support onto the extended arm it was possible to reach positions lying onto a virtual spherical surface centered on PDS and having 500 cm radius. During the calibrations, the energy thresholds were fixed at the following levels: LLT at minimum value (nominal 20 keV), ULT at maximum value (nominal 700 keV) and ACT at minimum value (nominal 100 keV). A first set of measurements was carried out with the SAX satellite that rotated around Z axis on the rotary table at a fixed angular speed of $\sim$9 arcmin/s. With this angular speed, a complete rotation of SAX lasted 2304 s, which means 18 time periods of 128 s (accumulation time of GRBM spectra). The sources were positioned at various elevation angles, ranging from $-45^\circ$ to $+60^\circ$with respect to the horizontal (X,Y) plane. The angular response of each GRBM shield was thus measured from GRBM and AC ratemeters.

 

Energy -45 -30 -15 +15 +30 +45 +60

²⁴¹Am Y Y Y Y Y Y Y Y

⁵⁷Co Y Y Y

¹³⁹Ce Y Y Y Y Y

²⁰³Hg, Y

¹¹³Sn Y Y Y Y Y

⁸⁵Sr Y

¹³⁷Cs Y

Also, average energy response over 128 s, corresponding to a rotation of 20 degrees, was determined.

Hereafter we will express source direction with respect to each LS in the altazimuthal coordinates $\theta$, i.e. the angle between source direction and the satellite XY plane, and $\phi$, i.e. the angle between the projection of source direction onto the XY plane and the detector axis.

In Tab. 2.2 we report the energy and $\theta$ values 'covered' by these kind of measurements. The number of runs was limited by time constraints. Several background measurements ware made, with no source in the tooling bar arm end without altering the calibration set-up.

  

Figure 2.2: ESTEC calibrations: radioactive sources set-up

  

Figure 2.3: ESTEC calibrations: global set-up

A second set of measurements was carried out with the SAX satellite fixed in some particular positions, in order to better understand some shielding effects. This was done, in particular, for LS2 and LS4, whose angular responses were expected to be more complex because of the presence in front of them of the HPGSPC and WFCs respectively.

During these calibrations, the PDS was operated in standard mode and all the other scientific instruments were off.