Hybrid K-Rb Spin Exchange Optical

Pumping Cells for the Polarization of 3He

UNC/TUNLA.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-3He is about 2% under optimum conditions

• 50 photons to polarize a 3He nucleus• The spin exchange efficiency for K-

3He is about 23% [Babcock, et.al, Phys. Rev Letter, Vol 91, Num. 12]

• Thus ~4 photons to polarize a 3He 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 769.9 nm and 766.4 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 794.7 nm D1 resonance• K-Rb spin exchange cross section is extremely

large ~200 A2

• At densities of 1014 cm-3, the spin-exchange rate can exceed 105/s, compared to Rb- 3He of ~10-6/s

• Thus any Rb polarization is nearly instantaneously transferred to K where the greater 3He 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 3He

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 200-250 C• Ocean Optics Spectrometer

White Light Spectroscopy

AlkAlkAlk Cd

• 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:

Resonance

Wavelength

ωAlk

K-D1 769.9 .339

K-D2 766.4 .682

Rb-D1 780.0 .322

Rb-D2 794.7 .675

White Light Spectroscopy

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

d

d

Rb

K

K

Rb

Rb

K

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, 2003.

• Bastiaan Driehuys in the flesh.