A second level trigger for the PAMELA satellite experiment

The PAMELA space experiment will be launched on-board of a Russian Resurs DK1 satellite towards the end of 2005. The main scientific goal is the study of the antimatter component of the cosmic radiation. Three years of data taking will provide unprecedented statistics for antiprotons (80 MeV-190 GeV) and positrons (50 MeV-270 GeV) and will set the upper limit for the ratio He¯/He below 10−8. PAMELA is built around a permanent magnet silicon spectrometer, surrounded by a plastic scintillator anticoincidence shield. An electromagnetic calorimeter is used for particle identification and energy measurements. If PAMELA data exceed the storage allowance on the satellite or the daily downlink quota (20 GB), a second level trigger may be activated by uplink from ground. Information from the anticoincidence system and from the calorimeter will be included in the second level trigger condition, providing a selective reduction of data. The second level trigger condition provides a reduction of data of ∼60%, with a maximum systematic uncertainty in the proton (electron) spectra of 10% (3%). This uncertainty will be assessed during flight measuring one event every 10 without the second level trigger condition.