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