Technology Home Page Index Site Map

Nuclear Quadrupole Resonance (Research & Development Emerging Sensor Technologies)

Nuclear Quadrupole Resonance (NQR), a derivative of NMR, is a bulk inspection technology which can be used to detect certain chemical elements which have a magnetic dipole moment. Amongst these is nitrogen-14 (14N) which is a major constituent of explosives used in landmines, such as RDX and TNT. NQR has been described as "an electromagnetic resonance screening technique with the specificity of chemical spectroscopy" as it not only detects but can be used to identify the exact chemical compound used.

Unlike NMR where a powerful external magnetic field is needed, quadrupole resonance takes advantage of a material´s natural electric field gradient, i.e. the electrical gradients available within certain assymentrical atomic nuclei. These gradients are due to the distribution of the electrical charge and do therefore strongly depend on the chemical structure (they will be different for RDX, for TNT, etc.)

When a low-intensity RF signal of the correct frequency is applied to the explosive, usually in the range 0.5 to 6 MHz, the energy state of some of the 14N nuclei can be altered. After the RF stimulation is removed, the nuclei can return to their original state, releasing anergy and producing a characteristic radio signal. The signal can be detected using a special radio reciever and be measured for analysis of the compounds present. Detecting the presence of explosives becomes similar to tuning a radio to a particular station and detecting the signal, and the uniqueness of a molecule´s electric field allows NQR technology to be highly compound specific. This high selectivity is partly a disadvantage, as it is not straightforward to build a highly specific multichannel system necessary to cover a wide range of target substances, and the precise frequencies drift with temperature.

The impossibility of detecting substances fully screened by metallic enclosures (also foils, depending on their thickness) is an issue for NQR. However, the presence of such objects can be detected by the detuning effect on the NQR probe. It may also still be possible to detect explosives in imperfectly shielded objects, e.g. within metallic containers having holes or slots or other regions where there are poor electrical connections (possibly even some UXO!), but this will result in a correspondingly weaker NQR signal. Practical applicability is therefore likely to be an issue which requires extensive testing.

Detection times are likely to from a few seconds to tens of seconds, depending on type and quantity of the target substance (especially on its T1 relaxation time which limits the rate of repeating mesurements). It also depends on distance for one-sided applications such as buried mine detection where the suspected mine cannot be put inside a coil. In addition, the signal to noise ratio increases with frequency as w3/2, which implies that detection becomes much easier with increasing frequency and hence detection of (low-frequency) TNT is much harder than detection of RDX - for which NQR has already been confirmed as very promising. Signals are in general rather weak, so that some form of signal averaging is usually necessary - as well as shielding and active cancellation of interference, because the detector will have to work, in the case of TNT, within the AM (medium wave) broadcasting band! Spurious signals have also been reported due to "acoustic ringing" effects (due to certain metals and metal coatings), as well as due to piezoelectric responses from silica in the soil (for applications such as landmine detection). All these effects are being tackled using appropriate pulsing sequences and detection software, as well as specific hardware. Care will have also to be taken of the temperature dependency of the spectral lines, selecting for example those NQR transitions which are least affected by temperature changes (e.g. 3.410 MHz line instead of 5.192 MHz for RDX).

TNT also presents further problems due to TNT cast in mines usually being a solid solution of different crystalline forms which can affect the characteristic frequency response.

Blind tests in the USA have demonstrated that NQR is close to readiness for field testing for use as a confirmation detector for shallow-buried plastic-cased anti-tank mines containing kilograms of explosive. Application for buried anti-personnel mines with only 100g, or less, of explosive still appears to be extremely elusive for TNT though research is continuing in several contries. As electronic systems become cheaper and more powerful it may be possible to substantially improve performance in the future. For a general introduction to NQR please see American Scientist magazine.

Nuclear Quadrupole Resonance is a bulk explosive detection technique, allowing the direct detection of a macroscopic mass of explosive material. [Principal source: ExploStudy.]

Involved Organisations:

Involved Projects:

Related Events:

Related Publications: