Increasing 14N NQR signal by 1H-14N level crossing with small magnetic fields.

Abstract NQR detection of materials, such as TNT, is hindered by the low signal-to-noise ratio at low NQR frequencies. Sweeping small (0–26 mT) magnetic fields to shift the 1 H NMR frequency relative to the 14 N NQR frequencies can provide a significant increase of the 14 N NQR signal-to-noise ratio. Three effects of 1 H– 14 N level crossing are demonstrated in diglycine hydrochloride and TNT. These effects are (1) transferring 1 H polarization to one or more of the 14 N transitions, including the use of an adiabatic flip of the 1 H polarization during the field sweep, (2) shortening the effective 14 N T 1 by the interaction of 1 H with the 14 N transitions, (3) “level transfer” effect where the third 14 N (spin 1) energy level or other 14 N sites with different NQR frequency are used as a reservoir of polarization which is transferred to the measured 14 N transition by the 1 H. The 14 N NQR signal-to-noise ratio can be increased by a factor of 2.5 for one 14 N site in diglycine hydrochloride (and 2.2 in TNT), even though the maximum 1 H frequency used in this work, 1116 kHz, is only 30% larger than the measured 14 N frequencies (834 kHz for diglycine hydrochloride and 843 kHz for TNT).