title: 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, 111 6 kHz, is only 30% larger than the measured (14)N frequencies (834 kHz for diglycine hydrochloride and 843 kHz for TNT).
authors.affiliation: US Naval Research Laboratory, Washington, DC 20375, USA. thruberk@niddk.nih.gov
authors.forename: Kent R
authors.initials: KR
authors.lastname: Thurber
authors.forename: Karen L
authors.initials: KL
authors.lastname: Sauer
authors.forename: Michael L
authors.initials: ML
authors.lastname: Buess
authors.forename: Christopher A
authors.initials: CA
authors.lastname: Klug
authors.forename: Joel B
authors.initials: JB
authors.lastname: Miller