Science data modes

The characteristics of the four data modes are given in table 1.

• MPC1 mode: mainly used for spectral studies of faint sources. Events are accumulated in sixteen separate spectra of 48 channels each. The sixteen spectra comprise the top- and mid-layers from each of the 8 detectors. The separation of top- and mid-layers improves the signal to noise ratio for weak sources and helps the background estimation.
• MPC2 mode: provides compression of data by a factor of eight enabling better time resolution. Combines top- and mid-layers from four detectors into one and gives two separate spectra. The combination of layers decreases the signal to noise ratio and background estimation is less precise.
• MPC3 mode: carries the process further by combining the 48 energy channels into twelve and grouping all eight detectors together, giving a further factor of eight compression.
• PC mode: used for timing studies; it by-passes the ADC to avoid dead time effects. Signals are divided into two energy bands by three discriminators (lower, middle and upper) and no other energy information is retained. In this way the dead time is reduced to 16.5µs/event and time resolution down to 976.6µs (=1/1024) is obtained from two energy bands per detector group. The lower discriminator is the same as that used for the pulse height spectra, while the other two descriminators have two commandable levels.