Ask Sawal

Discussion Forum
Notification Icon1
Write Answer Icon
Add Question Icon

what is nqr all about?

1 Answer(s) Available
Answer # 1 #

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 of electron density (e.g. from bonding electrons) and/or surrounding ions. As in the case of NMR, irradiation of the nucleus with a burst of RF electromagnetic radiation may result in absorption of some energy by the nucleus which can be viewed as a perturbation of the quadrupole energy level. Unlike the NMR case, NQR absorption takes place in the absence of an external magnetic field. Application of an external static field to a quadrupolar nucleus splits the quadrupole levels by the energy predicted from the Zeeman interaction. The technique is very sensitive to the nature and symmetry of the bonding around the nucleus. It can characterize phase transitions in solids when performed at varying temperature. Due to symmetry, the shifts become averaged to zero in the liquid phase, so NQR spectra can only be measured for solids.

In the case of 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 Larmor frequency:

where γ {\displaystyle \gamma } is the gyromagnetic ratio and B {\displaystyle B} is the (normally applied) magnetic field external to the nucleus.

In the case of NQR, nuclei with spin ≥ 1, such as 14N, 17O, 35Cl and 63Cu, also have an electric quadrupole moment. The nuclear quadrupole moment is associated with non-spherical nuclear charge distributions. As such it is a measure of the degree to which the nuclear charge distribution deviates from that of a sphere; that is, the prolate or oblate shape of the nucleus. NQR is a direct observation of the interaction of the quadrupole moment with the local electric field gradient (EFG) created by the electronic structure of its environment. The NQR transition frequencies are proportional to the product of the electric quadrupole moment of the nucleus and a measure of the strength of the local EFG:

where q is related to the largest principal component of the EFG tensor at the nucleus. C q {\displaystyle C_{q}} is referred to as the quadrupole coupling constant.

In principle, the NQR experimenter could apply a specified EFG in order to influence ω Q {\displaystyle \omega _{Q}} just as the NMR experimenter is free to choose the Larmor frequency by adjusting the magnetic field. However, in solids, the strength of the EFG is many kV/m^2, making the application of EFG's for NQR in the manner that external magnetic fields are chosen for NMR impractical. Consequently, the NQR spectrum of a substance is specific to the substance - and NQR spectrum is a so called "chemical fingerprint." Because NQR frequencies are not chosen by the experimenter, they can be difficult to find making NQR a technically difficult technique to carry out. Since 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.

Consider a nucleus with a non-zero quadrupole moment Q {\textstyle {\textbf {Q}}} and charge density ρ ( r ) {\textstyle \rho ({\textbf {r}})} , which is surrounded by a potential V ( r ) {\textstyle V({\textbf {r}})} . This potential may be produced by the electrons as stated above, whose probability distribution might be non-isotropic in general. The potential energy in this system equals to the integral over the charge distribution ρ ( r ) {\textstyle \rho ({\textbf {r}})} and the potential V ( r ) {\textstyle V({\textbf {r}})} within a domain D {\textstyle {\mathcal {D}}} :

The first term involving V ( 0 ) {\textstyle V(0)} will not be relevant and can therefore be omitted. Since nuclei do not have an electric dipole moment p {\textstyle {\textbf {p}}} , which would interact with the electric field E = − g r a d V ( r ) {\textstyle {\textbf {E}}=-\mathrm {grad} V({\textbf {r}})} , the first derivatives can also be neglected. One is therefore left with all nine combinations of second derivatives. However if one deals with a homogeneous oblate or prolate nucleus the matrix Q i j {\textstyle Q_{ij}} will be diagonal and elements with i ≠ j {\textstyle i\neq j} vanish. This leads to a simplification because the equation for the potential energy now contains only the second derivatives in respect to the same variable:

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.

A fake device known as the ADE 651 claimed to exploit NQR to detect explosives but in fact could do no such thing. Nonetheless, the device was successfully sold for millions to dozens of countries, including the government of Iraq.

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.

Jordi Merkel