Nitrogen-14 quadrupole resonance detection of RDX and HMX based explosives
Concerns the use of 14N signals in RDX and HMX. The principle is to subject the quadrupole nuclei to bursts of RF radiation and monitor the signals generated in the quiescent periods between pulses. These signals may be divided into two types: free induction decays (FID) and echoes. The first is the decaying signal observed immediately following the pulse; it is generated by the interaction of the oscillating magnetic field B of the applied RF with the magnetic moment of the quadrupolar nucleus. Often, this B field is generated by surrounding the sample with a small induction coil; the pulse may be imagined to tip the nuclear quadrupole moments away from their equilibrium orientation in the electric field gradient of their surroundings by a so-called "flip" angle α; they then precess at one of their allowed frequencies inducing a voltage in the same RF coil used to excite the transition. This voltage is observed to decay with a time constant, the FID or spin phase memory decay time. In many solids there are variations in the quadrupole frequencies due, for example, to defects or impurities in the material which alter the electric field gradient in their vicinity. The result is to produce a de-phasing of the NQR precession frequencies and a decay of the RF signal. Practical equipment is developed.