Nuclear Quadrupole Resonance (NQR), a type of radio frequency spectroscopy, holds the promise of unambiguous detection of particular explosives; the associated resonant frequencies are virtually unique. This specificity is spoiled by natural and anthropogenic interference that can swamp the NQR signal. Fortunately, the spatial magnetic signature from the explosive differs significantly from that of interference and can be exploited to separate the signals. An array of coils, however, cannot provide truly independent measurements due to the inductive coupling between the coils. Single coil configurations can cancel out constant interference and retain the signal from an NQR coil, but the balance between arms of the coil is compromised by differential coupling to the environment. Atomic magnetometers, an emergent technology predicted to surpass the sensitivity of coil detection, do not suffer from such coupling and have no fundamental limitation to forming an array of independent sensors. We have demonstrated up to 94× interference rejection with a 4-sensor array spanning 25 cm. The array symmetry permits rejection of linearly varying interference. We discuss the prevention of reradiation from DC field coils used to set the magnetometer resonance frequency. Such reradiation leads to non-linear variation. The benefits of larger sensor arrays are also explored.
Robert J. Cooper, Brian L. Mark, David W. Prescott, and Karen L. Sauer, "Improving the design of atomic magnetometer arrays for RF interference mitigation in NQR detection of explosives," Proc. SPIE 10182, Detection and Sensing of Mines, Explosive Objects, and Obscured Targets XXII, 1018208 (Presented at SPIE Defense + Security: April 10, 2017; Published: 4 May 2017); https://doi.org/10.1117/12.2262392.
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