RF magnetic fields

Properly designed hardware is important to any successful detection scheme. At first glance, the NQR hardware requirements are deceptively simple a device to generate a strong RF magnetic field and another to detect a weak RF magnetic field. Clearly, there are parallels between these requirements and those of pulsed radar, and some early NMR systems were built around surplus radar hardware. In this section, NQR hardware and how it affects detection sensitivity is discussed. 

We cannot discuss the sensitivity of NQR detection without considering the hardware used for detection. As discussed in Section 2.1.2., NQR signals are commonly excited with an RF magnetic fieldpulse. An RF magnetic field is easily generated by flowing... 

Faraday detection as described in Section 3.1., is the traditional method for recording NQR signals. As discussed earlier, Faraday detection sensitivity is limited by the detector s own thermal noise. The desire for improved detection sensitivity has driven the search for non-traditional methods and lower noise Faraday methods of RF magnetic field detection. In this section, several methods that have been applied to NQR signal detection will be discussed. 

Barras and coworkers [160,161] built and demonstrated a prototype NQR vehicle screener to detect ammonium nitrate. Ammonium nitrate-based bombs were the vehicle-borne IED of choice in terrorist attacks in the United Kingdom at the time the research started. In tests on two small vehicles, they determined that cross-axial RF magnetic fields (parallel to the wheel axles) gave the best penetration into the vehicle. Maximum attenuations of 14 and 20 dB were measured for the two vehicles. 

J. B. Miller, A. N. Garroway, and B. H. Suits, Nuclear Quadrupole Resonance (NQR) Method and Probe for Generating RF Magnetic Fields in Different Directions to Distinguish NQR from Acoustic Ringing Induced in the Sample, US Patent No. 6 522 135 (2003). 

The above has adopted a classical picture for describing the interaction between an rf field and the magnetisation. However, just as for the interaction with the main static applied magnetic field there is an analogous quantum mechanical description. The interaction between the rf magnetic field and the nuclear spins is simply another Zeeman interaction. The difference for this field is that it is time-dependent. In practice, the sample is irradiated with a linearly polarised rf-field of strength 2Bj, frequency corf and phase a. 

Bottomley, P. A. and Andrew, E. R. (1978) RF magnetic field penetration, phase shift and power dissipation in biological tissue implication for NMR imaging. Physics in Medicine and Biology, 23, 630-643. 

It is this energy which eventually limits the energy deposition in the material inside the coil. When H denotes the magnitude of a rf magnetic field ffrf in particular, the sample heating depends on this equation, where the permeability p, is frequency dependent. 

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