This invention involves an apparatus to measure the nuclear quadrupole (NQR)
response of a specimen using three frequencies. Three-frequency NQR involves excitation
of at least two transitions that causes an observed signal at a third transition
frequency. Thus, the transition excited and detected is not irradiated at all.
This reduces undesirable interfering signals due to the excitation, for example
as a result of acoustic ringing and/or tank circuit ring-down, since the excitation
is not applied at the frequency that is detected. This invention will be particularly
useful to detect substances selected from the group consisting of explosives and
narcotics using nitrogen NQR.
