FIGARO: An experiment for pulsar and variable source studies in the MeV range

Abstract

We present a large area, balloon borne, NaI(Tl) detector for low-energy gamma rays with temporal signature : FIGARO. The main detector is a mosaic of 12 NaI(Tl) tiles 22.5 × 15 × 5 cm, for a total geometric area of 4050 cm 2. In the energy band 140 keV - 6 MeV, the expected background counting rate at float altitude is in the range of two to three thousands counts per second. For pulsar analysis the expected 3δ sensitivity for 5 hours exposition time is 2.5 10 −4 ph/cm 2.s.MeV (150–500 keV) 1.5 10 −4 ph/cm 2.s.MeV (1–6 MeV). This performance, together with the large effective area and the relatively short duration of a balloon flight, make FIGARO particularly suitable for the identification of sources by means of temporal analysis. For objectives in the Northern sky, including the Crab pulsar, a transmediterranean flight is planned for the summer of 1982 ; a Southern mission is scheduled in Brazil for the fall of 1983 (Vela, PSR 1822-09).