Abstract A 180° focusing pair spectrometer has been developed in which the use of triggered sonic spark chambers in the focal plane allows the measurement of a broad range (approximately 20 MeV/ c) of values of the total electron pair momentum at a given magnetic field setting. Efficiency calculations give the ratio of spectral width (at half maximum efficiency) to momentum at peak efficiency as 0.70. The instrument described has been used to measure spectra of gamma rays resulting from proton capture into the giant resonance region of nuclear excitation. An energy resolution of 3% fwhm has been obtained, combined with an overall calculated efficiency per gamma ray of approximately 0.4%. The background from competing processes of higher probability, characteristic of an energy absorption spectrometer, is totally absent.