Disk Sources, Beta (large) These source disks are larger than our standard disks.They measure 1.75" in diameter and just under 1/8" thick. ISOTOPEACTIVITYHALF-LIFEENERGIES (KeV)Carbon1410uCi5730 years49.5 (average)ISOTOPEACTIVITYHALF-LIFEENERGIES (KeV)Iron55100uCi2.73 years5.2 (average)