A high-resolution pair spectrometer for medium energy photons

Abstract The construction, performance and associated on- and off-line analysis procedures of a pair spectrometer for medium energy photons (50–150 MeV) is described. In a large aperture window frame magnet electrons and positrons from photons converting in thin gold foils are bent by 180°. The trajectories are observed with large multiwire proportional chambers. The performance of the system was tested with stopped π − and a liquid hydrogen target. The resolution is 840 keV (fwhm) at 129.4 MeV; the overall acceptance which includes solid angle probability, conversion efficiency, e + e − pattern recognition efficiency and trigger detector efficiency, is 2.5 × 10 −5 (or 1.030 MeV and 4.5 × 10 −5 respectively). With special care, the position of a photon line in the vicinity of the H line can be determined to an accuracy of a few keV, while the known absolute energy of the 129.4 MeV H line has been determined to an accuracy of about 0.1 MeV.