The transition frequencies between the hyperfine structure sublevels of atomic nitrogen lie in the decametre waveband. As Shklovsky1 predicted, these transitions can be observed in the form of narrow spectral lines. The 14N ground state (4S3/2, nucleus spin I = 1) is split into three sublevels F = 5/2, 3/2 and 1/2 with frequencies and transition probabilities ν = 26.127 MHz, A = 4.32 × 10−20 s−1 for F = 5/2 → 3/2 and ν = 15.676 MHz, A = 1.29 × 10−20 s−1 for F = 3/2 → 1/2 The Einstein coefficients given here have been calculated from the improved values of line intensities2. We describe here an attempt made between 1978 and 1979 at the Institute of Radiophysics and Electronics, Academy of Science of the Ukrainian SSR, to detect the 14N absorption line for some point and extended cosmic objects. This effort has been successful for Cassiopeia A.
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