In {HEAD ~ operation}, tube 12 is evacuated to a pressure lower than {TAIL ~ about 0.1 torr}, preferably lower than about 0.001 torr, and then sealed 
In { HEAD ~ operation } , tube 12 is evacuated to a pressure lower than { TAIL ~ about 0.1 torr } , preferably lower than about 0.001 torr , and then sealed
OTH HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH