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