Skip to main content
U.S. flag

An official website of the United States government

Dot gov

The .gov means it’s official.
Federal government websites often end in .gov or .mil. Before sharing sensitive information, make sure you’re on a federal government site.

Https

The site is secure.
The https:// ensures that you are connecting to the official website and that any information you provide is encrypted and transmitted securely.

SEOP METHOD

Spin Exchange Optical Pumping (SEOP)

 

picture of SEOP apparatus

In the spin-exchange method (apparatus shown above), two steps are needed to polarize 3He by this method: polarization of the rubidium (Rb)/potassium (K), and spin exchange between the Rb/K and the 3He nucleus. An optically thick vapor (1014 to 1015 cm−3) of Rb/K is generated by heating a cell filled with 3He, typically 70 mbar of N2, and a small amount of Rb/K metal. The electronic spin of the Rb/K is polarized by optical pumping with near infrared laser light. A simplified two-level pumping scheme (which ignores the nuclear spin of the rubidium) is shown in Fig. 2. circularly polarized (σ+) light excites only atoms in the mj = -1/2 state. Atoms that decay to the mj = +1/2 state remain in that state, but atoms that decay mj = -1/2 state are re-excited by the laser. The nitrogen in the cell suppresses emission of photons, which depolarize other atoms. During binary collisions between the Rb/K atoms and the 3He nucleus, the hyperfine interaction between the unpaired Rb/K electron and the 3He nucleus can transfer spin. This is a slow process, resulting in long time constants (typically from several hours to a few tens of hours) for polarizing 3He, but the time constant is independent of the 3He density. The temporal evolution of the polarization is given by 

$$  P_{\mathrm{He}}(t) = P_A \left (\frac{\gamma_{\mathrm{se}}}{\Gamma_{\mathrm{He}}}\right) \left [1 - e^{-\Gamma_{\mathrm{He}}t}\right]  \tag{1}$$

where \(P_{\mathrm{He}}\) is the 3He polarization, \(\gamma_{\mathrm{se}}\) is the spin exchange rate (proportional to Rb vapor density), \(P_{\mathrm{A}}\) is the alkali polarization, and \(\Gamma_{\mathrm{He}}\) is the 3He spin relaxation rate and is given by

$$  \Gamma_{\mathrm{He}} = \frac{1}{T_{\mathrm{1}}} + (1 + X){\gamma_{\mathrm{se}}} \tag{2}$$

here \(T_{\mathrm{1}}\) is the relaxation time of 3He spin at room temperature. \(X\) is a phenomenological parameter that reflects the recent observation that the slope of the 3He spin relaxation rate with alkali density exceeds the spin-exchange rate. This phenomenon, an "excess" relaxation that scales with alkali density, was recently discovered at the Univ. of Wisconsin {Chann02}, and has been recently studied at Wisconsin and NIST {Chann03 , Gentile05 , Babcock}.     

The above nist-equation (1) describes polarization asymptotically, approaching the limiting value \(P_A \left (\frac{\gamma_{\mathrm{se}}}{He}\right)\) with a pump-up time constant of \(\frac{1}{\Gamma_{\mathrm{He}}}\). Typical values of \(\frac{1}{\gamma_{\mathrm{se}}}\) and \(T_{\mathrm{1}}\) are 5 to 20 hours and 50 to 500 hours, respectively. With sufficient laser power and long lifetime cells, it is possible for \(P_{\mathrm{A}}\) to approach unity and for \(\frac{1}{T_{\mathrm{1}}}\) to be a small correction. In this case, \(P_{\mathrm{He}}\) can approach \(\frac{1}{1 + X}\). Until recently it was thought that \(X\) would be near zero, but the relaxation studies at Wisconsin first revealed a value of 0.33, which yields a polarization limit of 75%. More extensive studies at Wisconsin and NIST have now shown that X varies from cell to cell, in a range between 0.15 and 1. For most of the larger spin filter cells that we have employed, \(X\) is 0.25 to 0.3 {Babcock}. Improving the maximum achievable 3He polarization in SEOP requires us to eliminate or reduce the temperature-dependent relaxation that is the origin of the current limit.

 

Diagram of SEOP Station Optical Pumping Background NSF Figure of Merit

 

Created May 9, 2018, Updated November 15, 2019