Improved tungsten moderator structures for slow positron beams

Tungsten filaments from commercial incandescent light bulbs have been tested as slow positron moderators. The filaments were annealed by electric current in the neutral gas atmosphere of the bulbs at or above the normal operating temperature of the lamp. The total efficiency of the positron source/moderator assembly was found to be as high as 4×10−4. This value is almost doubled at high extraction voltage (3000 V). Neither the annealing temperature (i.e. the heating current) nor the filament thickness (related to the nominal power of the lamp) affected significantly the moderation efficiency, most probably due to the relatively thick filaments used and the difficulties in controlling the source geometry. It is suggested that the moderation efficiency can be further improved by using structures with thinner tungsten patterns, optimised configuration and extraction electric field.