Vacuum tube. 
This is an Eimac 4-65A/8165 radio tube that contains a thoriated tungsten (i.e. radioactive) electrode.  They are quite common actually, or at least used to be when vacuum tubes in general were common. 
Source: MDB Ventures 
Contributor: Theodore Gray 
Acquired: 7 August, 2004 
Price: $30 
Size: 3" 
Purity: 2%
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