Nuclear Quadrupole Resonance
The physical principles of Nuclear Quadrupole Resonance (NQR hereafter) are quite similar to those of Nuclear Magnetic Resonance (NMR). Let us start with NMR, which also historically (first experimental demonstration in 1946) precede NQR and remains to the present day much better developed.. The motive force behind NMR detection and spectroscopic methods, is the magnetic moment of the nuclei in a substance. The protons in the nuclei of a substance have a positive electric charge. Because they spin, this gives rise to a magnetic moment, that is, the proton acts like a tiny bar magnet. Quantum mechanics dictates that when such nuclei are subject to an externally applied magnetic field, they must align themselves along it. The alignment allows only two possible orientations: one in the same direction as the applied magnetic field, and another opposite to it. Due to thermal excitation of the nuclei, almost half of the nuclei's magnetic moments will be aligned anti-parallel, and slightly more nuclei will be aligned parallel to the magnetic field. When the magnetic field is switched off, the nuclei wiil return to their standard thermal distribution, releasing an energy whose spectrum is typical of the substance involved. By spectroscopic analysis one can readily verify the nature of the compound. A short video that shows this process, can be found on the quantum magnetics website.
NQR works pretty much the same way, but there are some striking differences. First of all, NQR works by applying a external electrical field, rather than a magnetic field. Electric fields are easier to generate, and a device to produce an electric field of a given strength is considerably more portable than its magnetic equivalent. The drawback, is that NQR only works when the substance contains an appreciable quadrupole moment. The quantum physical basis for the arisal of quadrupole moments in some substances, is due to their nuclei having a spin number greater than 1/2. When this is the case, the charge density is no longer spherically symmetrical. In stead, the charge density is described by the so-called quadrupole tensor Q. Some 74% of NMR active nuclei have a spin greater than 1/2, and as a consequence posses a quadrupole moment. Most studied examples include Deuterium, Nitrogen 14, Chlorine 37, Copper 63, Zinc 67, or Iodine 127. The quadrupole moment interacts with any electric field gradient. In particular, it interacts with the local electric field generated by the molecular structure of the compound containing the quadrupole moment -from which it gets its compound-specific NQR spectrum- and it interacts with any externally applied field. By applying an external RF field, we can align the spins and see them relax when we terminate the field.
NQR methods typically have a better than 99% compound-specific detection rate
and a false positive rate of less than 1% for very small amounts of plastic
explosives. This makes it an ideal
candidate technology for detecting many of the small, non-metallic land mines that cannot be detected using conventional methods. It is especially suited to act in minefield containing plastic mines, or, in combination with a metal detector (alternatively, NQR equipment can be modified to measure metal content) in minefields with a mixture of metallic and non-metallic mines. One of the problems is that the NQR spectrum is highly temperature dependent. For an article that investigates temperature dependence for subtances containing nitrogen, click here.
Usefulness of NQR to our research program
In the orginal FWO project, a proposal was made to combine the measurement results of two bistatic GPR setups. The reason for choosing this type of setup was twofold. On the one hand GPR is a very promising sensor, as it can detect both plastic and metal mines and its penetration depth is good above 500 Mhz for most types of soil. Typical wave lengths inside the soil are of the order of 10 cm, which is sufficient for demining applications. On the other hand, GPR measures reflections of UHF waves (usually between 1 and 10 Ghz) which are mostly governed by changes in the dielectrical constant of the soil. Mines represent a jump in dielectrical constant, and holes inside the mines are even more easy to image. The dielectrical constant, and hence the GPR image, are highly dependent on the water content of the soil. In simple words: a GPR can see many things, but it is -as of yet- not as strong a distinguisher as efficient demining would require. We conjectured that quantum distinguishability criteria would improve the resolvent power of the GPR setup. Moreover, the dual use of two bistatic GPR's would allow for a data fusion in accordance with state reconstruction schemes from quantum mechanics. The second reason for chosing this type of setup, is that the mathematics of radar is very similar to that of quantum physics in phase space. The main reason for the structural identity has been identified as their common group-theoretical structure, which in both cases involves the irreducible representation of the Poincare group. However, GPR signals are goverened by a different group than normal radar signals, as soil is a much more dispersive medium than air. Because of the changed group structure, much work would need to be re-examined. As a result of these considerations, we are looking closer into the NQR phenomenon. Due to the dependence of the NQR signal on the chemical specificity, the NQR spectrum is specimen dependent. Hence optimal excitation signals are best decided for the explosive content of a landmine retrieved "on the spot". However, for a given specimen of explosive, the returned signal essentially determines a one-parameter group, the parameter being a function of the temperature of the sample. A mine detector which integrates IR measurement, or an NTC probe, could reliably set interval in which the parameter of the NQR signal (if it is there to detect) must fall. An extremely important advantage of NQR with respect to any other mine detection method from the perspective of using quantum methodology, is the fact that the noise can be whitened. An NQR detector with an appropriate whitening filter is able to determine its own false alarm rate, quite independent on anything but the mine and the detector. This is an essential treat of quantum detectors too. As such the NQR setup seems especially appropriate for implementation of the project.
Recommended web publications
Introductions and Tutorials
Methods and groups
Review and Research Papers on NQR
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