Nuclear Quadrapole Resonance, NQR, is a science tool, technique or instrument of the same family as the more-familiar MRI or Magnetic Resonance Imaging. Both probe the nucleus structure of the atom or molecule using radio waves and coils attached to electronic circuits.
NQR equipment can be built in elementary forms that are simple, undemanding and inexpensive – yet useful. MRI, however, generally requires a larger commitment in engineering, support and financing.
NQR resembles the Fluorescence of minerals under a ‘black-light’ or ultraviolet illumination, which has been used as an exploration tool. This type of ad hoc fluorescence works very well, is easy & cheap … but it does not give the same results for the same materials & substances, in different conditions. Variations of impurities or chemical and environmental factors can lead to different results for different samples of the same material. (This sensitivity can of course also be useful…)
MRI, on the other hand, is not influenced by incidental complications, the way NQR is. This allows certainty that NQR can’t give, and in cases like medicine this can be the difference between acceptable and not.
Quadrapole resonance has received attention in the late 20th & early 21st C, as a practical method of detecting buried landmines. Princess Diana was famous for spearheading the landmine initiative. The high variant-sensitive of NQR, which can impair its formal scientific utility (or disqualify it in medical applications) means that it can detect an arbitrary object in varying ground-materials, and may even detect that specimens of the exact same model of explosive device were made at different (specific) factories, which can then deprive makers, providers and users of plausible deniability.