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Optical properties of 50 CM Browne-Buechner spectrograph Lee, Kai-Chung


A 50-cm Browne-Buechner broad-range spectrograph is being used at TKIOH.F to measure the production cross section of low energy pions when H and other nuclei are bombarded by energetic protons. These pions are detected by a scintillation counter hodoscope located on the focal plane of the spectrograph. This thesis describes the investigation of the optical properties of the spectrograph, such as magnification, dispersion and aberration. In order to measure these properties to the accuracy desired, it was necessary to obtain the complete field maps of the magnet of the spectrograph. In knowing the optical properties, one can then obtain accurate evaluation of the acceptance which can then be directly used to convert event rates into cross sections. The field in the magnet was measured at several levels of excitation, using both a NMR probe and a Hall probe. First, the field was measured by a NMR probe at 2.5, 5 and 10 kG, and second, the field was re-measured using a Hall probe at 5.0 and 10.0 kG. The field maps were used as data for the ray tracing programme to obtain the complete determination of the magnet optics of the spectrograph. The fits showed that for the central ray, the dispersion was 229. 4 cm/(Δp/p) (3%), the magnification was 2.38 (6%), and the total resolution was 0.157 Δp/p (11%). Also for the central ray, the aberration (the size for a point source of monoenergetic pions at the focus) was 1.436 cm (15%). The values in the parenthesis are percentage deviation from the values for a uniform field Browne-Buechner spectrograph of 50 cm effective radius. For the non-central ray optics, the 50 cm uniform Browne-Buechner model also compared as favorably.

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