Nuclear quadrupole resonance

Nuclear quadrupole resonance

Nuclear quadrupole resonance spectroscopy or NQR is a chemical analysis technique related to nuclear magnetic resonance (NMR).[1]

Principle

In NMR, nuclei with spin ≥ 1/2 have a magnetic dipole moment so that their energies are split by a magnetic field, allowing resonance absorption of energy related to the difference between the ground state energy and the excited state. In NQR, on the other hand, nuclei with spin ≥ 1 , such as 14N, 35Cl and 63Cu, also have an electric quadrupole moment so that their energies are split by an electric field gradient, created by the electronic bonds in the local environment. Since unlike NMR, NQR is done in an environment without a static (or DC) magnetic field, it is sometimes called "zero field NMR". Many NQR transition frequencies depend strongly upon temperature.

Any nucleus with more than one unpaired nuclear particle (protons or neutrons) will have a charge distribution which results in an electric quadrupole moment. Allowed nuclear energy levels are shifted unequally due to the interaction of the nuclear charge with an electric field gradient supplied by the non-uniform distribution electron density (e.g. from bonding electrons) and/or surrounding ions. The NQR effect results when transitions are induced between these nuclear levels by an externally applied radio frequency (RF) magnetic field. The technique is very sensitive to the nature and symmetry of the bonding around the nucleus. The energy level shifts are much larger than the chemical shifts measured in NMR. Due to symmetry, the shifts become averaged to zero in the liquid phase, so NQR spectra can only be measured for solids.

Applications

There are several research groups around the world currently working on ways to use NQR to detect explosives. Units designed to detect landmines[2] and explosives concealed in luggage have been tested. A detection system consists of a radio frequency (RF) power source, a coil to produce the magnetic excitation field and a detector circuit which monitors for a RF NQR response coming from the explosive component of the object.

Another practical use for NQR is measuring the water/gas/oil coming out of an oil well in realtime. This particular technique allows local or remote monitoring of the extraction process, calculation of the well's remaining capacity and the water/detergents ratio the input pump must send to efficiently extract oil.[citation needed]

The strong temperature dependance of NQR's frequency allows to make a precise temperature sensor with resolution 10-4 °C[3].

References

  1. ^ [1]
  2. ^ Appendix K: Nuclear quadrupole resonance, by Allen N. Garroway, Naval Research Laboratory. In Jacqueline MacDonald, J. R. Lockwood: Alternatives for Landmine Detection. Report MR-1608, Rand Corporation, 2003.
  3. ^ Leigh, James R. (1988). Temperature measurement & control. London: Peter Peregrinus Ltd.. p. 48. ISBN 0 86341 111 8. 



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