[en] A spectrometer is being installed in a high-intensity antiproton beam (LESBII) at the AGS. The beam momentum entering the target will be known to approximately +- ¹/₂%. The target is surrounded by a stereoscopic cylindrical drift chamber, and thus the total charge of the event will be measured. Since the vertex will be reconstructed with an accuracy of approximately +- 2 mm, studies of annihilations with high resolution down to approximately 150 MeV/c will be possible. On one side of the target a large-aperture (2.3 x .62 m²) magnet covered on both sides with drift chambers and triggering--TOF scintillation counters, will measure particle and γ-ray energies with high resolution, as well as identify particles. The primary objective is to look for peaks in the γ spectra coming from annihilations at rest by converting them in a thin (.05 x₀) radiator before the magnet. Narrow peaks (< 1 MeV) which are produced with frequencies of approximately 10^-3 or better are expected to be seen as 5 sigma effects. The principle and details of the spectrometer are presented