The block is usually filled with a lasing gas such as {HEAD ~ a mixture} of helium and {TAIL ~ neon} 
of
The block is usually filled with a lasing gas such as { HEAD ~ a mixture } of helium and { TAIL ~ neon }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL OTH OTH TAIL TAIL TAIL TAIL TAIL