Where a metal foil construction is utilized, a plurality of such metal foils may be stacked in layers with orifices in adjacent layers being offset from one another such that the radiation shield is substantially opaque to infrared radiation while permitting {HEAD ~ the passage} of {TAIL ~ the alkali metal vapor} 
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Where a metal foil construction is utilized , a plurality of such metal foils may be stacked in layers with orifices in adjacent layers being offset from one another such that the radiation shield is substantially opaque to infrared radiation while permitting { HEAD ~ the passage } of { TAIL ~ the alkali metal vapor }
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