Stand-off Detection of Xenon Spin Polarization by Radar REMPI

The development of stand-off methods for the detection of threats is of central importance to the safety of military personnel and the protection of national assets. One well-proven method for detection is through perturbations to the magnetic field of the earth due to hidden threats. Perturbations of the magnetic field are common occurrences and are largely due to metallic objects. The development of a laser-based stand-off method for detecting the local magnetic field will greatly enhance the capabilities of the Navy for the detection of submarines, mines and other objects not otherwise observable. The possibility of achieving that goal involves the use of spin polarized naturally occurring species in the air. Early studies established that the most promising species is xenon 129, which has a nuclear spin of ½. It is desirable because, as an inert atomic species, the spin cannot be disoriented by rotation changing collisions which occur for all diatomic and polyatomic molecular species. As a consequence, the lifetime of the Xe spin polarization is long enough to allow variations in the nutation rate due to magnetic anomalies to be detected. The Larmor nutation frequency of 129Xe is 1.18 kHz/gauss, and the magnetic field of the earth is between 0.25 and 0.65 Gauss, so spin lifetimes of seconds are desirable to see magnetic field variations. The measured 129Xe spin lifetime in air is ~30 seconds, providing sufficient time for high fidelity measurements. Radar REMPI has been shown to be capable of stand-off measurements of species to parts per billion levels. The proposed work will follow from previous work examining the potential for Radar REMPI to selectively detection spin polarized 129Xe using ultrashort pulse lasers. This approach has the advantage of being blind to solar background, well localized by the focal zone of the laser and time synchronized to the laser pulse, so background interference is minimized. Three-photon Radar REMPI is an attractive optical standoff approach to the detection of xenon, which will be demonstrated in a vapor cell ideally achieving parts per million or parts per billion levels at relevant conditions.