Effects of neutron irradiation on the thermionic emission properties of LaB 6 -emitters used in neutral gas pressure gauges

After the failure of a LaB 6 -emitter used in a neutral gas pressure gauge in the Large Helical Device during deuterium operation, the effect of neutrons on the thermionic emission properties of LaB 6 is studied in this work. For that purpose, six cylindrical LaB 6 -samples of 8 mm length and 1 mm diameter were irradiated with neutron doses between 1 ⋅ 1 0 16 − 3 ⋅ 1 0 17 n cm 2 at the TRIGA Mark-II reactor. Analysis after neutron irradiation showed accumulation of oxygen on the sample surfaces and the formation of cracks, holes, growth of surface layers and erosion at the sample edges. In order to study the thermionic emission properties of the neutron irradiated LaB 6 -samples, they were used as emitters in a neutral gas pressure gauge under vacuum conditions and in a magnetic field. The tests revealed significantly reduced thermionic emission directly after neutron irradiation that improves when repeating the measurements. Four out of six LaB 6 -emitters eventually reach thermionic emission properties comparable to the reference emitters again, due to either the removal of the lanthanum-oxide surface layer from the emitter surface during operation in the neutral gas pressure gauge or the thermal recovery of neutron induced lattice defects.