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Nuclear resonance fluorescence (NRF) is a nuclear process in which a nucleus absorbs and emits high-energy photons called gamma rays. NRF interactions typically take place above 1 MeV, and most NRF experiments target heavy nuclei such as uranium and thorium[1]
Mode of interaction

NRF reactions are the result of nuclear absorption and subsequent emission of high-energy photons (gamma rays). As a gamma ray strikes the nucleus, the nucleus becomes excited (that is, the nuclear system as a quantum mechanical ensemble is put into a state with a higher energy). Much like electronic excitation, the nucleus will decay toward its ground state, releasing a high-energy photon at a number of possible, discrete energies. Thus, NRF can be quantified using spectroscopy. Nuclei can be identified by the distinct pattern of NRF emission peaks, although NRF analysis is much less straightforward than typical electronic emissions.[2]

As the energy of incident photons increases, the average spacing between nuclear energy levels decreases. For sufficiently energetic nuclei (i.e. incident photons of over ~1 MeV), the mean spacing between energy levels may be lower than the mean width of each NRF resonance. At this point, determinations of peak spacing cannot be analytical, and must rely on specialized applications of the statistical methods of signal processing.

There is a related phenomenon at the level of electronic orbitals. A photon, generally in a lower energy range, can be absorbed by displacing an orbital electron, and then a new photon having the same energy is emitted in a random direction when the electron drops back down. See resonance fluorescence.
References

http://www.tunl.duke.edu/groups/nnsa/nrf.html
P. G. Hansen, B. Jonson, and A. Richter, Nucl. Phys. A 518, 13 (1990)

Physics Encyclopedia

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