The radiation shield 7 including {HEAD ~ a plurality} of {TAIL ~ said metal foils} stacked in layers with orifices in adjacent layers being offset from one another such that said radiation shield is substantially opaque to infrared radiation while permitting the passage of said alkali metal vapor 
The radiation shield 7 including { HEAD ~ a plurality } of { TAIL ~ said metal foils } stacked in layers with orifices in adjacent layers being offset from one another such that said radiation shield is substantially opaque to infrared radiation while permitting the passage of said alkali metal vapor
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