Studies of cosmic rays with the anticoincidence system of the PAMELA satellite experiment

Abstract: PAMELA is a satellite-borne experiment designed to study the charged component of the cosmic radiation of galactic, solar and trapped nature. The main scientific objective is the study of the antimatter component of cosmic rays over a wide range of energies (antiprotons: 80 MeV–190 GeV, positrons: 50 MeV–270 GeV). PAMELA is also searching for antinuclei with a precision ~10^−7 in anti-He/He measurements. PAMELA is mounted on the Resurs DK1 satellite that was launched on June 15th 2006 from the Baikonur cosmodrome and is now on a semipolar (69.9°) elliptical (350 × 600 km) orbit. The experiment has been acquiring data since July 11th 2006 and has a foreseen lifetime of at least 3 years. The PAMELA apparatus consists of a permanent magnet silicon spectrometer, an electromagnetic imaging calorimeter, a time of flight system, a scintillator-based anticoincidence (AC) system, a tail catcher scintillator and a neutron detector. The AC system can be used to reject particles not cleanly entering the PAMELA acceptance.Tests of the PAMELA instrument in its final flight configuration involved long duration acquisition runs with cosmic particles (mainly muons) on ground. A study of the functionality of the AC system during these runs is presented here with a set of selected muons. Studies of activity in the AC detectors as function of the rigidity of the muons and in correlation with the activity in the spectrometer and in the calorimeter are presented.A study of the AC system functionality during in-flight operations provides a map of the particle flux in orbit, and shows the anisotropy in the arrival direction of trapped particles in the Van Allen radiation belts. The singles rates indicate that the AC system saturates in the South Atlantic anomaly (SAA). Information from the AC system in the SAA is therefore not reliable for physics analysis. The timing and multiplicity of AC activity correlated to particle triggers has been studied. A dependence on orbital position was observed.An LED (Light Emitting Diode) based monitoring system was designed to determine the in-orbit behaviour of the AC system independently of the radiation environment and to compare it to the pre-launch behaviour. The LED system shows that the properties of the AC system are stable during flight and that no significant changes in performance occurred as a result of the launch.