Hybrid K-Rb Spin Exchange Optical Pumping Cells for the Polarization of 3 He UNC/TUNL A.Couture, T.Daniels, C.Arnold, T.Clegg

Outline I. Motivations for Using Hybrid Cells II. Production of Hybrid Cells III. White Light Spectroscopy VI. Polarization Results

Motivation for Using Hybrid Cells The spin exchange efficiency for Rb- 3 He is about 2% under optimum conditions 50 photons to polarize a 3 He nucleus The spin exchange efficiency for K- 3 He is about 23% [Babcock, et.al, Phys. Rev Letter, Vol 91, Num. 12] Thus ~4 photons to polarize a 3 He nucleus

So Why Not Just Use Potassium? Pumping on the D2 line is bad Rb D1 and D2 are separated by ~15nm and absorption on the D2 has been observed Potassium’s D1 and D2 are located at nm and nm, respectively Directly pumping the D1 line of potassium without D2 absorption is currently not feasible.

Hybrid Cells Circumvent This Problem Optically pump Rb at nm D1 resonance K-Rb spin exchange cross section is extremely large ~200 A 2 At densities of cm -3, the spin-exchange rate can exceed 10 5 /s, compared to Rb- 3 He of ~10 -6 /s Thus any Rb polarization is nearly instantaneously transferred to K where the greater 3 He spin exchange efficiency may be realized [Babcock, et.al]

Production of Hybrid Cells Here is the final product. 3 inch diameter Pneumatic aluminum body ¼ in swagelok valve K-Rb with 8 amagat 3 He

The Initial Manifold Two cells are produced at once. Y-shaped retorts for separate introduction of K and Rb into manifold Place alkali ampules in manifold with nitrogen flowing then seal with a torch

The Baking Process

The day before baking is completed the alkali must be chased into the smaller retort. The larger retort is then removed with a torch. This distills the metal, removing impurities. Distillation

The alkali metals are then chased into the cells and the cells are removed. The optimum ratio of K:Rb is 30:1 in liquid form in the cells. This leads to vapor ratios of 10:1 at 250 C. We have as of yet not perfected this, only obtaining at best 2:1 in the vapor phase. Cell Filling

White Light Spectroscopy Standard 60 Watt white halogen bulb Oven at C Ocean Optics Spectrometer

White Light Spectroscopy We then may examine the D1 and D2 absorption cross sections for Rb and K The following formula relates the absorption cross section to the alkali density: ResonanceWavelength ω Alk K-D K-D Rb-D Rb-D

White Light Spectroscopy Thus we may compare either the D1 or the D2 cross sections for the two species to obtain:

White Light Spectroscopy Results for K-Rb pyrex cell Test performed at 200 C Ratio of K-Rb is 0.8±0.1

White Light Spectroscopy GE-180 K-Rb cell Test performed at 230 C Ratio of K:Rb is 2.2±0.3

Polarization Results Saturation polarization 35-40% Spinup time 2.3 hrs

Polarization Results Spindown time 14 hrs

Polarization Results Saturation polarization 35-40% Spinup time 9.2 hrs

Polarization Results Saturation Response Strength (polarization uncalibrated) 737 mV Spinup time 4 hrs

Polarization Results Saturation Response Strength (Polarization uncalibrated) 1350 mV Spinup time 9.2 hrs

References E. Babcock, I. Nelson, B. Driehuys, L. Anderson, F. Herman, and T. Walker, Phys. Rev Letters, 91, 12, Bastiaan Driehuys in the flesh.