Polarization enhanced Nuclear Quadrupole Resonance with an atomic magnetometer

Abstract

Nuclear Quadrupole Resonance (NQR) has been demonstrated for the detection of 14-N in explosive compounds. Application of a material specific radio-frequency (RF) pulse excites a response typically detected with a wirewound antenna. NQR is non-contact and material specific, however fields produced by NQR are typically very weak, making demonstration of practical utility challenging. For certain materials, the NQR signal can be increased by transferring polarization from hydrogen nuclei to nitrogen nuclei using external magnetic fields. This polarization enhancement (PE) can enhance the NQR signal by an order of magnitude or more. Atomic magnetometers (AM) have been shown to improve detection sensitivity beyond a conventional antenna by a similar amount. AM sensors are immune to piezo-electric effects that hamper conventional NQR, and can be combined to form a gradiometer for effective RF noise cancellation. In principle, combining polarization enhancement with atomic magnetometer detection should yield improvement in signal-to-noise ratio that is the product of the two methods, 100-fold or more over conventional NQR. However both methods are even more exotic than traditional NQR, and have never been combined due to challenges in operating a large magnetic field and ultra-sensitive magnetic field sensor in proximity. Here we present NQR with and without PE with an atomic magnetometer, demonstrating signal enhancement greater than 20-fold for ammonium nitrate. We also demonstrate PE for PETN using a traditional coil for detection with an enhancement factor of 10. Experimental methods and future applications are discussed.

Nuclear quadrupole resonance spectroscopy with a femtotesla diamond magnetometer

“…NQR spectroscopy is used to identify powder substances in ambient conditions for security (29)(30)(31) and pharmaceutical (32)(33)(34) applications and to study the temperature-dependent properties of single-crystal materials (35)(36)(37)(38)(39). These applications typically require the ability to detect kilohertz to megahertz frequencies, at low-bias magnetic fields ≲1 mT, with femtotesla sensitivity (40). Previously, NV centers were used to detect NQR signals arising from nanoscale statistical polarization in single-crystal boron nitride layers in direct contact with the diamond (41,42).…”