Excitation Energy of the First Excited State ofC12, and Observation of a Coherent Doppler Effect

The excitation energy of the first $2^{+}$ state in ${\mathrm{C}}^{12}$ has been determined to be 4440.0±0.5 keV by means of the ($p, p^{\prime }\gamma$) reaction at 23 MeV incident proton energy. $\gamma$ rays were detected by a Ge(Li) detector in coincidence with inelastically scattered protons to reduce Doppler broadening. An anomaly in which a symmetrical doublet appeared in the $\gamma$-ray singles spectrum observed at 90° to the beam direction is explained on the basis of a coherent Doppler effect involving selective population of the magnetic sublevels of the $2^{+}$ state in the inelastic scattering process.