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The 21 cm line of hydrogen is a magnetic dipole transition between the two hyperfine levels of the hydrogen ground term. There has been some interest in the possibility of finding other lines of this type in diffuse interstellar regions. Turner searched without success for 23Na for which the transition within the ground term has a wavelength of 17 cm. We have developed a general formula for the magnetic dipole line strength in LS coupling of a hyperfine transition within a single atomic energy level. We have applied our formula to the calculation of the transition probabilities of lines in isotopes of neutral atoms and some atomic ions up to iron.
R. H. Garstang (Sat,) studied this question.