Nuclear quadrupole resonance

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Nuclear quadrupole resonance or NQR is a technique related to nuclear magnetic resonance (NMR). In NMR nuclei with a magnetic dipole moment have their energies 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 an electric quadrupole moment, such as ¹⁴N, ³⁵Cl and ⁶³Cu, have their energies 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 quadrupolar charge distribution. The NQR effect results from the inteaction of this quadrupole with an electric field gradient supplied by the non-uniform distribution electron density (from bonding electrons). So the technique is very sensitive to the nature of the bonding around the nucleus. Unlike the chemical shift measured in NMR the interaction is much larger, but is averaged to zero in the liquid phase. So NQR spectra can only be measured for solids which is both a strength and a weakness.

There are several research groups around the world currently working on ways to use NQR to detect explosives. Units designed to detect landmines 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.

http://scienceweek.com/2005/sb050128-5.htm