A high-resolution pair spectrometer for medium energy photons

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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.

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    ∗∗

    Supported in part by the “Swiss National Science Foundation”.

    Present address: Gymnase Belvédère, Lausanne, Switzerland.

    Present address: Physics Department, Case Western Reserve University, Cleveland, U.S.A.

    ††

    Supported in part by the German “Bundesministerium fü Forschung und Technologie”.

    +

    Present address: Fakultät für Physik der Universität Freiburg i. Br., Freiburg, Germany.

    §

    Present address: Elektrowatt Engineering Services Limited, Zürich, Switzerland.

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