Radiation from 56-MeV positrons ($\gamma =111\mathrm{}$) channeled between the major planes or along the $〈110〉$ axis of an 18-μm-thick silicon crystal has been observed. The energies of the measured planar-channeling spectral peaks agree well with theoretical predictions.

• Received 7 February 1979

DOI:https://doi.org/10.1103/PhysRevLett.42.1148