On excitation of isomeric nuclear states in a crystal by synchrotron radiation

The time dependence of delayed radiation is investigated when a pulse of synchrotron radiation undergoes resonance Bragg scattering by the nuclei in a crystal that have an isomeric level with a Mossbauer transition. Instead of the usual exponential law exp(-t/ tau 0 ) characteristic of an isolated nucleus, a time dependence in the form ( tau 0 /t) 2 exp(-t/ tau 0 ) was discovered at small deviations alpha from the Bragg angle. The acceleration of the decay is connected with the collective nature of the excitation of the nuclei in the crystal-the nuclear exciton formation. The exponential law remains at large alpha but the intensity decreases as 1/ alpha 2 . In the case of large divergence of the incident beam the law ( tau 0 /t) exp(-t/ tau 0 ) is obeyed. The frequency distribution of the reflected pulse and the possibility of formation of a resonance structure with sufficient resolution are analysed in detail.