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Proceedings Paper

The stonehenge technique: a new method of crystal alignment for coherent bremsstrahlung experiments
Author(s): Kenneth Livingston
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Paper Abstract

In the coherent bremsstrahlung technique a thin diamond crystal oriented correctly in an electron beam can produce photons with a high degree of linear polarization.1 The crystal is mounted on a goniometer to control its orientation and it is necessary to measure the angular offsets a) between the crystal axes and the goniometer axes and b) between the goniometer and the electron beam axis. A method for measuring these offsets and aligning the crystal was developed by Lohman et al, and has been used successfully in Mainz.2 However, recent attempts to investigate new crystals have shown that this approach has limitations which become more serious at higher beam energies where more accurate setting of the crystal angles, which scale with l/Ebeam, is required. (Eg. the recent installation of coherent bremsstrahlung facility at Jlab, with Ebeam = 6 GeV ) This paper describes a new, more general alignment technique, which overcomes these limitations. The technique is based on scans where the horizontal and vertical rotation axes of the goniometer are adjusted in a series of steps to make the normal to the crystal describe a cone of a given angle. For each step in the scan, the photon energy spectrum is measured using a tagging spectrometer, and the offsets between the electron beam and the crystal lattice are inferred from the resulting 2D plot. Using this method, it is possible to align the crystal with the beam quickly, and hence to set any desired orientation of the crystal relative to the beam. This is essential for any experiment requiring linearly polarized photons produced via coherent bremsstrahlung, and is also required for a systematic study of the channeling radiation produced by the electron beam incident on the crystal.

Paper Details

Date Published: 7 December 2006
PDF: 7 pages
Proc. SPIE 5974, International Conference on Charged and Neutral Particles Channeling Phenomena, 59740G (7 December 2006); doi: 10.1117/12.639988
Show Author Affiliations
Kenneth Livingston, Univ. of Glasgow (United Kingdom)

Published in SPIE Proceedings Vol. 5974:
International Conference on Charged and Neutral Particles Channeling Phenomena
Sultan B. Dabagov, Editor(s)

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