Q-factor switching method and apparatus for detecting nuclear quadrupole and nuclear magnetic resonance signals

A probe ( 80 ) for irradiating a sample with RF energy during transmitting periods and detecting an NQR or NMR signal from a substance contained within the sample during receiving periods. The probe ( 80 ) comprises a variable impedance unit ( 20 ) for changing the Q-factor of the probe and a probe coil. The probe ( 80 ) is responsive to powerful RF pulses applied thereto to excite an RF magnetic field in the probe coil during the transmitting periods. The variable impedance unit ( 20 ) is controllable to provide a Q-factor for the probe ( 80 ) at: (i) an optimal level during a prescribed transmitting period of an RF pulse for irradiating the sample with said RF energy; (ii) a minimal level during a prescribed recovery period immediately following said transmitting period to rapidly dampen transient signals from the probe; and (iii) a maximal level during a prescribed receiving period for detecting an NQR or NMR signal from the target substance if present, immediately following the recovery period. A method for detecting an NQR or NMR signal within a sample using the probe ( 80 ) is also described.