Karen L. Sauer

High‐volume production of laser‐polarized 129Xe

A method is described for producing several liters of nuclear spin polarized 129Xe gas via spin exchange with an optically pumped Rb vapor. We use a 140 W AlGaAs laser diode array whose broad spectral output is efficiently absorbed by employing ∼10 atm of 4He to pressure broaden the Rb D1 absorption profile. 129Xe is polarized in a continuous gas flow and is then cryogenically accumulated and stored. Extensions of this technique should enable the production of tens of liters of 129Xe with a nuclear spin polarization of order 50%. Production of laser‐polarized 129Xe in liter quantities is important for the continued development of magnetic resonance imaging using spin‐polarized 129Xe.

Subfemtotesla radio-frequency atomic magnetometer for detection of nuclear quadrupole resonance

A radio-frequency tunable atomic magnetometer is developed for detection of nuclear quadrupole resonance (NQR) from room temperature solids. It has a field sensitivity 0.24fT∕Hz1∕2 at the 423kHz N14 NQR frequency of ammonium nitrate. A potential application of the magnetometer is detection of nitrogen-containing explosives which is difficult with conventional tuned copper coils due to a poor signal-to-noise ratio (SNR) below a few megahertz. The NQR signal from 22g of powdered ammonium nitrate located 2cm away from the sensor is detected with a SNR of 9 in a 4.4-s-long multiple echo sequence, which represents an estimated order-of-magnitude improvement in sensitivity over the pickup coil detection.

CROSS-RELAXATION IN LASER-POLARIZED LIQUID XENON

Abstract By dissolving compounds in hyperpolarized liquid Xe, we have successfully transferred the 129 Xe polarization to other spins in solution. We have observed signal enhancements of over 45 for 1 H, and over 70 for 13 C, compared to their thermal polarization signals at 200 K and 1.4 T. The lifetimes of the enhancements are on the order of 20 min. In addition, in dissolved toluene we have seen preferential enhancement of the ring protons over the methyl protons.

Three-frequency nuclear quadrupole resonance of spin-1 nuclei

Abstract We introduce a new nuclear quadrupole resonance (NQR) method for the detection of spin-1 nuclei, where the transition excited and directly detected is not irradiated at all. It is demonstrated, theoretically and experimentally, that the irradiation of a powder sample containing spin-1 nuclei by two of the three characteristic NQR frequencies can result in free induction decay (FID) and echo signals at the third NQR frequency. We present the optimal conditions for such three-frequency NQR experiments and compare theory with experiment using 14 N ( I =1) in a powder sample of sodium nitrite.

LASER-POLARIZED LIQUID XENON

Abstract We present the first liquid-phase studies of hyperpolarized Xe, Xe in which the polarization of 129 Xe is much greater than thermal. Using grams of Xe of natural isotopic composition with 6% polarization of the 129 Xe, we examined the lifetime of the 129 Xe polarization in the liquid as a function of field and temperature. Our measurements showed that surface effects make a significant contribution to the relaxation rate. Lifetimes in excess of 30 min were observed. The long lifetimes and high polarization present several new applications for liquid Xe.

14N magnetic resonance for materials detection in the field.

Nitrogen is prevalent in many materials, both naturally occurring and man-made. In particular, it is found in many explosives and other contraband materials. One technique for the detection of such materials in the field is the use of the magnetic resonance signal from the nearly 100% abundant, spin-1, 14N nuclei. Some of the difficulties with such measurements in the field include spurious signals from acoustic resonances, radio-frequency interference, and generally low signal-to-noise ratios. A summary of recent work by the authors to help mitigate these difficulties is presented.

Secondary echoes in three-frequency nuclear quadrupole resonance of spin-1 nuclei

We demonstrate, theoretically and experimentally, that the irradiation of a powder sample containing spin-1 nuclei by two of the three characteristic nuclear quadrupole resonance (NQR) frequencies can result in several echo signals at the third NQR frequency. One of these echoes, the principal echo, has the same shape and time of occurrence as an echo produced after a pair of single-frequency excitations. The other echoes are not equivalent to any single-frequency echo. These secondary echoes are the focus of this paper. The time of occurrence and shape of the secondary echoes are determined by the correlation of the distribution in one transition frequency to the distribution in a second transition frequency. This correlation is in turn determined by the correlation between the distributions of the electric field gradient components, which is itself determined by the types and concentrations of crystalline defects present. We present the optimal conditions to observe such secondary echoes and compare the...

Measurement of carrier escape rates, exciton saturation intensity, and saturation density in electrically biased multiple-quantum-well modulators

In this paper, we detail the results of exciton saturation intensity measurements on strained InAsP/InP and InGaAs/GaAs multiple quantum well modulators designed for 1 /spl mu/m operation and under electrical bias as is required for device operation. Carrier escape times from the quantum well were also measured for both electrons and holes. These measurements allow the first experimental determination of the saturation density of the material under electrical bias. This density can also be calculated using a theoretical model proposed by Schmitt-Rink, et al. The experimentally measured density is in good agreement with this theoretical model. >

Magnetic resonance imaging of hyperpolarized 129Xe produced by spin exchange with diode-laser pumped Cs

We report the results of experiments leading to the production of an image of a polarized 129Xe sample prepared by spin exchange with Cs, optically pumped with a spectrally narrowed 894.3 nm diode laser. Representative images of the average electron spin polarization are shown. Appreciable cesium electron polarization values were achieved, and a nuclear polarization of about 2.5% was measured for 129Xe. The absolute nuclear polarization was measured by water-calibrated free induction decay of the nuclear magnetic resonance signal, and the polarized xenon imaged using a 2 T magnetic resonance imaging system.

Low-field approach to double resonance in nuclear quadrupole resonance of spin-1 nuclei

Using double-resonance conditions, in which the Larmor frequency of a spin-1/2 nucleus is matched to one of the nuclear quadrupole resonance frequencies of a spin-1 nucleus, the authors demonstrate increased cross relaxation between the two nuclear spin species. They calculate the cross-relaxation rate using the motionally averaged heterogeneous dipole Hamiltonian as a perturbation to the combined quadrupole and Zeeman Hamiltonians. Using this cross-relaxation rate, in addition to hydrogen and nitrogen autorelaxation rates, expressions governing spin-1/2 and spin-1 spin-lattice relaxation are determined. With ammonium nitrate, containing nitrogen (spin-1) and hydrogen (spin-1/2), increased nitrogen signal and spin-lattice relaxation are demonstrated, using fields less than 120 G. The cross-relaxation rate is also measured and an overall signal/noise improvement by a factor of 2.3+/-0.1 is attained.