Experimental Observation of "Shadowing" in Optical Transition Radiation.

We report the observation of shadowing between two optical transition radiation (OTR) sources from a 205 MeV electron beam. The total optical intensity is measured as a function of the distance d between the sources, covering the range 0<d<4L_{v}, where L_{v} is the formation length of the particles. Data show that the total optical intensity starts decreasing due to shadowing when d approaches L_{v} until it becomes undetectable for very short distances d/L_{v}→0. A model based solely on interference between the two OTR sources is in good agreement with experimental data. To the knowledge of the authors this is the first systematic experimental observation of shadowing in OTR.