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