S. L. Popov

Coherent re-emission of gamma -quanta in the forward direction after a stepwise change of the energy of nuclear excitation

The re-emission of gamma -quanta in the forward direction by an ensemble of excited nuclei after abruptly changing the energy of the nuclear excitation is studied. A sublevel of the 14.4 keV excited state with a definite spin projection of 57Fe nuclei in a magnetic 57FeBO3 crystal is selectively populated. The abrupt change of the energy of the excited nuclear state is achieved by fast (<or=5 ns) reversal of the hyperfine (HF) magnetic field direction at the nuclei. The nuclear target was a black absorber prior to HF field reversal. A short (about 10 ns) intense flash followed by beats of gamma -radiation intensity is detected with some delay after reversal. The delay of the flash and the beat period depend on the HF nuclear transition excited. The duration of the flash depends on the effective thickness of the nuclear target. The theory developed interprets the time evolution of the re-emission as a result of the interference of two main spectral components of gamma -radiation. The first one has the original frequency, which represents the primary radiation. The second one has a shifted frequency, which represents radiation coherently re-emitted in the forward direction from the nuclear sublevel with energy changed by the HF field reversal. The observations reveal the feasibility of inelastic coherent gamma -resonance scattering and demonstrate the enhancement of the radiative channel in the coherent re-emission of gamma -radiation in the forward direction.

Enhancement of nuclear reaction radiative channel in vibrating absorber

The intensity of sidebands in the spectrum of radiation resonantly scattered in forward direction by a thick vibrating absorber was measured. It was 37.8% of the incident beam intensity. This is an evidence of the enhancement of radiative channel in the nuclear reaction.

Inversion of Target Sequence in Nuclear Forward Scattering of Synchrotron Radiation

The radiation fields reemitted in nuclear forward scattering of synchrotron radiation by a system of two single-line targets of highly different linewidths mounted downstream behind each other were analyzed by applying a stepwise energy shift to one of the targets. The results reveal a pronounced dependence of the fields on target sequence, which demonstrates the important role of radiative coupling.