Experimental study of time and frequency properties of collective nuclear excitations in a single crystal (gamma-ray resonance)

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Abstract
The angular dependences of the time and frequency parameters of a single-crystal collective nuclear resonance were studied in a unified series of measurements and compared with the resonance parameters of an isolated nucleus. It was shown that, when approaching the Bragg angle ( theta B), the coherent character of gamma radiation interaction with the nuclear system in a crystal causes drastic changes of collective resonance parameters. A sharp reduction of the nuclear system response time-from 230+or-10 ns for an isolated nucleus to 40+or-10 ns at the Bragg peak-takes place. The nuclear resonance width increases from 3.5 Gamma 0 ( Gamma 0 is the natural width) to 20+or-0.5 Gamma 0. The resonance energy changes non-monotonically with the angle of incidence of radiation on the crystal and deviates from the resonance energy of an isolated nucleus by (2.0+or-0.5) Gamma 0 as a maximum. The excitation spectra of the collective nuclear gamma resonances have an asymmetrical shape. The sign of the asymmetry changes when crossing the Bragg angle position. The properties of the collective nuclear excitations were analysed with the theory of Kagan, Afanas'ev and Perstnev, as well as with that of Kagan, Afanas'ev and Kohn.