Experimental investigations on geometrical resolution of optical transition radiation (OTR)

Optical Transition Radiation (OTR) provides an attractive method for diagnostics on electron/positron beams of small dimensions at high energies (GeV). However, some limits on the geometrical resolution at very high energies have been often discussed in the literature and a minimum value given by ‘γλ’ has been invoked. In order to bring an experimental contribution to the problem, systematic measurements of electron beam profiles, in the energy range of 1–2 GeV and at optical wavelength between 400 and 700 nm, have been carried out at the Orsay 2 GeV Linear Accelerator. OTR emitted from an aluminium foil at a 30° incidence angle was collected by a two-lens telescope and recorded by an intensified CCD camera. The OTR beam profiles were compared to the profiles obtained by a SEM Grid having a resolution better than 0.5 mm. After a theoretical introduction presenting the different resolution limits, which can be invoked with diffraction phenomenon, and a presentation of our calibration procedure, the experimental results are presented and compared to these limits. They show that the resolution in OTR measurements is definitely better than the already invoked γλ-limit. For a small size electron beam an r.m.s. beam width about 170 μm has been determined. This value can be compared to γλ=2.5 mm (E=2 GeV and λ=650 nm). Our theoretical analysis also provides a more precise evaluation of the resolution power of OTR, which depends on the sensitivity of the detector; this could be of interest for much higher energies.