EXPERIMENTAL INVESTIGATIONS ON GEOMETRICAL RESOLUTION OF OPTICAL-TRANSITION RADIATION (OTR)

Optical Transition Radiation (OTR) provides an attractive method for d iagnostics on electron/positron beams of small dimensions at high ener gies (GeV). However, some limits on the geometrical resolution at very high energies have been often discussed in the literature and a minim um value given by 'gamma lambda' has been involved. In order to bring an experimental contribution to the problem, systematic measurements o f electron beam profiles, in the energy range of 1-2 GeV and at optica l wavelength between 400 and 700 nm, have been carried out at the Orsa y 2 GeV Linear Accelerator, OTR emitted from an aluminium foil at a 30 degrees incidence angle was collected by a two-lens telescope and rec orded by an intensified CCD camera. The OTR beam profiles were compare d to the profiles obtained by a SEM Grid having a resolution better th an 0.5 mm. After a theoretical introduction presenting the different r esolution limits, which can be invoked with diffraction phenomenon. an d a presentation of our calibration procedure. the experimental result s are presented and compared to these limits. They show that the resol ution in OTR measurements is definitely better than the already invoke d gamma lambda-limit. For a small size electron beam an r.m.s. beam wi dth about 170 mu m has been determined. This value can be compared to gamma lambda 2.5 mm (E = 2 GeV and lambda = 650 nm), Our theoretical a nalysis also provides a more precise evaluation of the resolution powe r of OTR, which depends on the sensitivity of the detector; this could be of interest for much higher energies. (C) 1998 Elsevier Science B. V. All rights reserved.