The noise immunity of gradiometer coils for14N NQR land mine detection: Practical limitations

Magnetic resonance measurements in the field, such as for land mine detection using¹⁴N nuclear quadrupole resonance (NQR), must deal with large levels of environmental radio-frequency interference. One approach to minimize the level of interference which enters the NQR receiver is the use of a coil with no magnetic dipole moment. Such a coil is, ideally, sensitive only to spatial gradients of the magnetic fields and is referred to as a gradiometer. It is straightforward to estimate the amount of reduction one can expect for an ideal gradiometer. Here it is shown that for¹⁴N NQR land mine detection in practice, the ultimate level of interference received can be expected to be significantly greater than what one would expect solely due to these spatial gradients. This is due to the fact that it is quite difficult to construct an ideal gradiometer.