Observation of Electric Synchrotron Radiation in a Crystal

Enhanced radiation of up to 0.4 of the incident beam energy, measured for 4-, 15-, and 17.5-GeV positrons directed along the $〈100〉$ axis of an 80-μm-thick diamond crystal, is reported for the first time. The results are in excellent agreement with the recently proposed theory of crystal-assisted radiation based on the synchrotron formalism.