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Evaporative Coating of Rb Maser Cells

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IntroductionCoatingCell manifoldsPreparing the filamentCleaning the cell and attaching the Rb ampouleInductive heaterCoating the cellChasing Rb and sealing the cellSealing the cellPost pull-off rubidium chasingRemelting the coatingOptical T1 measurementsTesting the probe strengthOld filamentsPrevious cell coating techniquesCell CleaningBakingDischarge CleaningEvaporative Coating of Rb Maser CellsD. F. Phillips, A. Boca and R. L. WalsworthJune 3, 19991 IntroductionThe double-bulb rubidium maser (DBRM) relies on a vacuumcell for its operation. This differs from the spin-exchange no-ble gas cells which contain Xe, He and N2as well as rubidium.These gases (in addition to playing active roles in the noble gasexperiments) act as a buffer, keeping the Rb away from the wallsof the cell. In the DBRM the lack of buffer gases allows rubid-ium atoms to effuse between the pumping and masing chambersof the device. Without a buffer gas, however, the device is verysensitive to rubidium-wall interactions which can shift the ru-bidium hyperfine frequency and even depolarize the atoms.Long polarization times may be obtained by coating the in-ner surface of the bulb containing the rubidium atoms witha benign substance. Previous work [2, 3]hasshownthatRbbounces hundreds of times from a surface coated with tetracon-tane, C40H82, – a component of standard paraffin – (table 1),without losing its polarization (Fig. 1). Note in the figure thatoctadecyltrichlorosilane (OTS) [4], the coating used for noblegas cells is much less effective with rubidium. (Similar resultshave been found by other groups [5] after careful studies.) Thetetracontane coating process is, however, somewhat complicatedas the tetracontane must be evaporatively coated under vacuum.In the following sections of this document, we will describethe evaporative coating scheme as well as some of the detailsof Rb handling that may differ from the noble gas cells. Wewill also discuss the initial techniques used to characterize thecoating quality as well as some earlier techniques used in coatingDBRM cells.2 CoatingThe aim of the evaporative coating process, as described in detailbelow, is to apply a thin uniform layer of tetracontane to the cell.A metal filament covered with tetracontane is placed inside thecell. The filament is then heated so that the tetracontane meltsand evaporates into the evacuated cell. When the tetracontanehits the room temperature walls of the cell it sticks, formingthe coating. Contaminants inside the tetracontane such as anyJune 3, 1999 cell coating notes 2 of 19121086420Lock-in Signal (arb. units)210200190180 RF frequency (kHz)uncoated cell OTS-coated celltetracontane-coated cell Two Sidebandsfrom AM at 3 kHzFigure 1: Zeeman resonances at B=0.28 gauss from bare, OTScoated and tetracontane coated cells. The linewidth is, in part,determined by the lifetime of the polarized atoms in the cell.If the atoms maintain their polarization for a very short timebecause of depolarizing wall collisions, then their linewidth willbe very broad. The linewidths of uncoated and OTS coatedcells correspond to roughly the time between wall collisions (i.e.one bounce depolarizes them). The tetracontane linewidth cor-responds to several hundred bounces and is limited by magneticinhomogeneities and power broadening in the measurement pro-cess rather than by polarization lifetime. (From [1])solvent, water and oxygen will typically have different vaporpressures than the tetracontane itself. Therefore, they will eitherevaporate before the tetracontane and be pumped away or stickto the glass before the tetracontane or never evaporate at all.This allows us to produce a cleaner coating than simpler wettechniques which apply the tetracontane solution directly to thewalls of the cell in air (see appendix B).2.1 Cell manifoldsThe rubidium bulb is initially made by our glass blower [6]orthequartz workers [7] and attached to a manifold. This manifoldallows the cell to be evacuated by a turbo pump at the cellfilling station and holds the filament containing the coating aswell as the Rb which will be placed in the cell. Typical cellmanifolds are shown in figures throughout the text. A glass,single bulb cell which was used for testing coatings is shown infigure 2. The filament enters the cell through the upper capillaryJune 3, 1999 cell coating notes 3 of 19Table 1: Properties of tetracontaneformula CH3(CH2)38CH3formula weight 536.1 amumelting point 81.3◦Cboiling point 150◦Cand the rubidium is brought in from the lower. Figure 5 showsthe quartz double bulb used for initial maser tests. Figure 6 isa double bulb glass cell for studying light scattering problems.(Except for some additional labels added for this text, this wasthe drawing given to the glass blower.) Note in the figures thatin addition to the cell itself the manifold contains a port whereit is attached to the pumping system, a storage region for thefilament after the coating is completed and a tube where theRb ampoule is attached. (The single bulb cell (Fig. 2)isshownwith the Rb ampoule attached while the double bulb cell (Fig. 6)is shown before the ampoule is attached.) During the pull-off(sec. 3.1), tubes leading from the cell are sealed, leaving onlythe single or double bulb cells with one or two tips.2.2 Preparing the filamentMake filaments from steelbrazing rod, magnet wireand coax inner conductorpressed together.The coating filament is constructed from three pieces of metalto absorb and transport the heat and to hold the tetracontane.First, a 1.200steel brazing rod, 0.100in diameter rests in the cap-illary tube outside the cell. This large diameter piece will beheated via induction heating. As it is magnetic, it can also beused to manipulate the filament with a permanent magnet fromoutside the manifold. Two strands of 200copper magnet wiretransport the heat to the center of the bulb. Finally, a 1/200piece of multistrand inner–conductor from a coax cable holdsthe tetracontane. The multistrand wire wicks the tetracontaneeffectively, stopping it from dropping off the filament onto thebulb when it melts. These three pieces of wire are crimped to-gether as the temperature necessary to vaporize the tetracontaneand coat the cell will melt soft solder. (The pieces could also bespot-welded together. For details on earlier filaments, and theirdifficulties see appendix A.)Aldrich Chemical Co. [8] sells tetracontane (table 1), in theform of flakes. It is melted onto the multistrand wire by


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