Why Use Thorium Tungsten Wires as Ion Source?

Many analytical and medical devices are using thorium tungsten wires as ion source. Why use thorium tungsten materials? What about tungsten or neodymium tungsten materials? 

Electrons are made available from a cathode in the source, which in most cases will use thermionic emission, i.e., heating the material so that electrons gain some energy and are able to escape the work function of the cathode material (the energy required per electron to leave the material). To lower work function, manufacturers would like to use thorium tungsten filaments. 

Doping a tungsten wire with thorium can effectively improve the thermionic emission. Doping lowers the work function of the system, resulting in a thermionic current greater than the one produced from a pure W filament. Why it happens? What are the mechanisms that lower the work function?

An idea is that thorium is an α-emitter. The 4He nuclei then produce scattering sources for the electrons, enhancing the emission probability. And according to Electronics Engineer's Reference Book, the mechanism is that thorium is added to the tungsten filament, and then the whole thing is heated significantly hotter than its normal working temperature. This causes the thorium to migrate to the surface of the filament, resulting in essentially a thorium-plated tungsten filament. Thorium has a lower work-function than tungsten so the effective work-function of the filament is that of thorium.

Some manufacturers have tried alternative materials, like neodymium tungsten rods for lamp electrodes, it works good for certain lamps but it does not for other lamps. Just like mercury for many types of lamps, thorium tungsten seems to be very mighty. Only the problem is quantities for these lamp manufacturers and device manufacturers are not large as thorium belongs to radioactive material. 

Due to regulation in Europe, transfer of Thorium powder is very limited if not banned, there are a lot of papers to talk about this Thorium Powder ban, because it can be considered as Radioactive if it is concentrated very high, such as thorium powder. however, for thorium wires or rods, they are usually low in concentration, which is harmless to the health. 

Chinatungsten Online is a professional manufacturer of tungsten for more than two decades, if you have got the purchasing plan for thorium tungsten wires or rods, do feel free to contact us.

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