Prometheus I is a very low background 0.03–6.5 MeV gamma‐ray balloon ‐ borne imager. Its position sensitive photon detector eliminates the activation component of instrumental background through use of: (a) A gamma ray detector that is segmented in two dimensions; (b) A 30 cm thick active collimator fabricated from low ‐Z matter (plastic scintillator), which has a 10 keV threshold; (c) An active collimator that vetoes prompt events due to fast neutrons; (d) slow neutron radiationless absorber. Because of its low background, the 3 σ spectral ‐line sensitivity at 1 MeV is expected to be 5×10−5 photons cm−2 sec−1, given six hours observing time at midlatitude balloon altitudes. It will be used produce a 0.5 ‐ degree map of the entire sky, on two Long Duration Balloon flights that are planned for 1994 and 1995. Prometheus has an in‐flight selectable field of view (FOV) and 30 arc‐minute angular resolution. Its selectable FOV will be used to independently measure the ‘‘MeV bump’’ in the spectrum of the diffuse extragalactic background.
Construction of Prometheus I has now been completed. The system is undergoing laboratory calibrations and testing at Rice, prior to undertaking its first, concept ‐verification balloon flight, at NASA’s National Scientific Balloon Facility.
