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