Now nearing completion, Prometheus I is a (01.–5) MeV balloon‐borne actively collimated mosaic of 11×11 independent scintillation counters. It improves sensitivity by rejecting background events due to instrument activation. Each counter is an optically isolated 1.2 cm×1.2 cm×5.0 cm (thick) NaI crystal and separate photomultiplier. A beta ray emitted by an activated nucleus in the mosaic causes only one counter to pulse, but a gamma ray causes several counters to pulse simultaneously. Otherwise unvetoed events that involve only one counter are rejected in data analysis. The 30 cm thick active collimator constructed of plastic scintillator vetoes events that are due to charged particles and to gamma radiation from outside its 28° FWHM aperture. Plastic scintillator has a small cross section for activation and its spallation products all emit charged particles simultaneously with the emission of any gamma ray. Surrounding the active collimator in all directions, as well as lining it, is a 2.54 cm thick LiF passive slow‐neutron shield. At midlatitudes, Monte Carlo simulations have shown that the 6‐hour 3‐sigma 1 MeV gamma ray sensitivity is approximately 5×10−5 photons cm−2 s−1, assuming that all the background due to activation and leakage is rejected or accounted for.
With some sensitivity loss, 1° imaging will be feasible when a thick coded mask is placed in the aperture. Monte Carlo simulations have shown that image blurring by Compton scattering is tolerable.
