Electron-emission densities for LaB6 cathodes were compared with those for other common cathode types. Equations were presented for describing the poisoning mechanism. Poisoning was related to the gas pressure, cathode temperature and heat of adsorption. Posioning was measured using a flowing gas system which maintained a constant pressure in the vicinity of the cathode for various adsorption rates. Poisoning rates were governed by the cathode temperature and gas pressure. An equilibrium poisoned emission level was reached within a few minutes for each cathode temperature and gas pressure. The poisoning gases used were oxygen, carbon dioxide, air, hydrogen, nitrogen and argon. Oxygen was the most active poisoning gas and argon the least active. No poisoning occurred until a critical pressure was reached. The emission then decayed rapidly with increasing pressure. Resistance to poisoning increased with increasing cathode temperature. At a cathode temperature of 1570C, the critical poisoning pressure for oxygen was about 5 x 10-5Torr. At 1400C, the critical poisoning pressures were: 2 x 10-6Torr for oxygen, 2 x 10-5Torr for carbon dioxide, 5 x 10-5Torr for air and greater than 10-2Torr for hydrogen, nitrogen and argon. The critical poisoning pressure for LaB6 cathodes was several orders of magnitude higher than that for oxide and impregnated cathodes.

Poisoning of LaB6 Cathodes. Gallagher, H.E.: Journal of Applied Physics, 1969, 40[1], 44-51