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Nuclear spectral lines are spectral lines resulting not from changes in an atom's electron excitation that release or absorb photons, but from somewhat-analogous changes within a nucleus, i.e., nuclear processes adding, subtracting, or rearranging nucleons. The lines are evident when some particular transition in large quantity occurs within a transparent region from which photons of a particular photon energy can escape as emission lines. The photons are generally high-energy, e.g., gamma rays and X-rays, and gamma-ray astronomy includes their observation and analysis. Such a situation occurs in the solar atmosphere, e.g., the corona, and the term is used in solar science. The transitions producing such lines are radioactive decay, or when a particle (e.g., proton, neutron, or alpha particle) is absorbed in a nucleus, or when a less-than stable nuclear isomer (typically the result of above-such transitions) later relaxes to a less-energetic state. Nuclear line spectrography refers to spectrography aiming to detect and measure such lines. Such lines offer analysis of some of the same broad mechanisms, e.g., Doppler broadening.
Note that the words line, along with nucleus or nuclear occur in discussions of spectral lines and active galactic nuclei (AGNs): a line generated through electron orbit transitions within atoms is occasionally referred to as a nuclear line to indicate that it is generated within some galaxy's AGN rather than somewhere in the rest of the galaxy. The term LINER is an example. This can also occur for other types of astronomical objects for which the term nucleus is used, such as comets.
Also, certain spectral lines occur in nuclear resonance imaging, and phrases including nuclear and lines are commonly used to indicate them.