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AIP Advances

Efficient longitudinal relaxation time measurement of 129Xe via bias-approach

Xiang Zhan, Jiang Qiyuan, Wang Zhiguo, Hui Luo, Hongchang Zhao

Published Sep 1, 2018• 5 citations
Publisher

Abstract

We proposed a new bias-approach method for measuring longitudinal relaxation time of 129Xe. Compared with the delayed pulse method, the measuring time of the proposed single-scan bias-approach is significantly decreased by more than 20 times, and the differences are 2% and 0.7% of two sample cell A and B respectively. By giving a small static magnetic field in the transverse domain, a π-pulse-induced 87Rb magnetometer signal bias with respect to the longitudinal magnetization of xenon was obtained. We then use a π/2 pulse to find the half-life time of longitudinal relaxation. Since the bias-approach has a low requirement of the systematic robust, it naturally becomes a more efficient method to measure the longitudinal relaxation time with respect to the noble gas in the vapor cell.

DOI
10.1063/1.5049178

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We proposed a new bias-approach method for measuring longitudinal relaxation time of 129Xe. Compared with the delayed pulse method, the measuring time of the proposed single-scan bias-approach is significantly decreased by more than 20 times, and the differences are 2% and 0.7% of two sample cell A and B respectively. By giving a small static magnetic field in the transverse domain, a π-pulse-induced 87Rb magnetometer signal bias with respect to the longitudinal magnetization of xenon was obtained. We then use a π/2 pulse to find the half-life time of longitudinal relaxation. Since the bias-approach has a low requirement of the systematic robust, it naturally becomes a more efficient method to measure the longitudinal relaxation time with respect to the noble gas in the vapor cell.
We propose a fast and accurate method for 129 X e − R b spin-exchange rate determination based on an atomic magnetometer at low magnetic field and low pressure. The 129 X e − R b spin-exchange rate can be estimated via the product of the maximum transverse spin polarization of 129 X e nuclei and the corresponding Rabi frequency. The velocity-averaged binary spin-exchange cross section and the rate constant for spin-exchange in 129 X e − R b van der Waals complexes are measured to be ⟨ σ ν ⟩ S E = ( 1.26 ± 0.13 ) × 10 − 16 c m 3 / s and κ 4 H e = 1326 ± ( 47 ) s − 1 in 4 H e buffer gas, respectively. With respect to the method based on the longitudinal relaxation rate, the method presented in this work can greatly facilitate the measurement process, determining the spin-exchange rate in several minutes. Moreover, this method is more suitable for vapor cells with long longitudinal relaxation time.
In the atomic cell of the spin exchange optical pumping system, the optical pumping inevitably introduces a magnetic field gradient, which affects the measurement result of the nuclear spin relaxation time a lot. To address this issue, our method achieves the measurement while excluding the influence of this magnetic field gradient. In this method, the pump laser is turned off after the spins of 87Rb and 129Xe have been effectively polarized, which results in the spin polarization of the 87Rb atoms being provided only by the spin exchange with 129Xe. Ultimately, the transverse and longitudinal components of the polarization of 129Xe are detected by the in situ87Rb atom magnetometer to obtain the T1 and T2 of 129Xe under different conditions. Using our method, since the influence of inhomogeneity caused by the pump laser is eliminated, the relaxation characteristic of atomic cells can be measured more quickly, essentially, and conveniently, which helps compare the performance of different atomic cells and find the best cell fabrication process further.
We demonstrate the measurement of the transverse spin relaxation rate of Xe without the influence of the Rb polarization-induced magnetic field gradient. The optical pumping beam and probe beam turn on and off repeatedly during the measurement to reduce the relaxation originating from the Rb polarization-induced magnetic field gradient. During the absence of the optical pumping beam, the nuclear spin of the noble gas atom does not experience the alkali-polarization-induced magnetic field gradient so that the transverse spin relaxation rate ( 1 / T 2 ) decreases. When the duty cycle reaches zero, the measured transverse relaxation time T 2 approaches the longitudinal spin relaxation time T 1 . Our method helps in roughly estimating the longitudinal spin relaxation time with a single measurement of the free induction decay.
We analyze the transverse nuclear magnetizations of 129Xe and 131Xe in a vapor cell containing natural Xe, 87Rb, and buffer gases. The Xe atoms are polarized through spin-exchange optical pumping (SEOP) with Rb atoms under low-magnetic-field conditions. From the free-induction-decay (FID) signal, we measure the nuclear magnetization of the Xe atoms in the Xe-Rb vapor cell. Furthermore, we measure the dependence of the gyromagnetic ratio on the magnetization of 129Xe and 131Xe by examining the amplitude of the FID signal of each isotope, and we evaluate the relationship between the magnetic field gradient and transverse relaxation rate for both of the 129Xe and 131Xe isotopes.
The transverse relaxation time (T2) is an important indicator to determine the fundamental sensitivity limit of alkali-metal atomic magnetometers. We propose a method based on the principle of longitudinal field modulation that obtains T2 by scanning the transverse static magnetic field. The previous technique of extracting T2 from the linewidth of the modulation frequency and the traditional magnetic-resonance-broadening-fitting method are also described. The T2 measurements of Cesium (Cs) atoms are carried out through these three methods, whose operating environments are applicable to different atomic magnetometers, respectively. The method that we propose can be used for obtaining the T2 of Cs atoms as well as detecting the transverse static magnetic field and is customized for the study of the Cs–Xenon ensemble for the construction of nuclear magnetic resonance gyroscopes. Moreover, the relationship between the limit sensitivities and cell temperatures is further studied in the experiment.
The accuracy of atomic sensor is determined by the transverse nuclear spin relaxation time (T 2 ). As the longitudinal nuclear spin relaxation time (T 1 ) is the upper limit of T 2 , it becomes an important metric for evaluating the merits of alkali vapor cells. However, the conventional method for measuring T 1 of pulse delay takes dozens of minutes, which will increase the effect of random errors during the testing process. In order to solve this problem, a method to quickly estimate the value of T 1 is proposed by suppressing the influence of spin exchange relaxation and magnetic field gradient relaxation terms on T 2 . Experimental results indicate that the accuracy is maintained at above 95%, and the testing time is reduced to less than 5%.