The radiation shield acts to reflect infrared radiation emanating from the reaction zone back toward {TAIL ~ the reaction zone} while permitting the passage of {HEAD ~ the alkali metal vapor} to the condensing means 
The radiation shield acts to reflect infrared radiation emanating from the reaction zone back toward { TAIL ~ the reaction zone } while permitting the passage of { HEAD ~ the alkali metal vapor } to the condensing means
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH