A magnetic pair spectrometer to detect photon splitting
Sigurdson, James R.
The subject of this thesis is the calibration, energy resolution and efficiency of a magnetic gamma ray pair spectrometer (PS), investigated using the photon tagger at the Saskatchewan Accelerator Laboratory (SAL). This instrument employs the well known process of pair production, with subsequent momentum analysis of the electron-positron pair to reconstruct the photon's energy. Tagged photons impinged on different thicknesses and geometries of copper converters. The resulting charged pairs were recorded by six scintillation detectors (three per focal plane) that were positioned on the focal planes of the pair spectrometer dipole magnet. The total energy of the pair was compared to the photon energy according to the tagger. An adjustable iris and annular scintillation detector were placed at the outrun of the pair spectrometer to study beam halo. The PS design specifications were dictated by the need to perform the first reliable observations on a process known as photon splitting. This phenomenon is characterized by the dissociation (in the electric field of a nucleus) of an incoming photon having energy Ï‰1 into two real photons whose energy sum(Ï‰2+Ï‰3) is equal to that of the incident photon. This process is predicted by the theory of quantum electrodynamics and is a consequence of vacuum polarization. A secondary goal was to use this PS in the related experiment of DelbrÃ¼ck scattering. The PS can also be used as a beam monitor for diagnostic purposes and for photon tagger energy calibrations in the energy regime between 20 - 160 MeV. The energy resolution is less than 2.0 MeV (FWHM) and the energy calibration of the pair spectrometer is within 1 MeV of the SAL tagger. The efficiency of the device is strongly dependent on small angle multiple scattering in the converter and the number of focal plane detectors (energy acceptance) but is sufficiently high to warrant use in an experiment on photon splitting. Simulations are presented that agree with experimental results. Recent results (1994) are given using the final configuration of the PS in energy calibration tests on the SAL photon tagger.