For shielding or absorbing neutrons, boron and cadmium are the lower
cost material most often used. Lithium and boron are good neutron
absorbers but are both chemical poisons and are difficult to handle in
the metallic state.
Cadmium-113 absorbs thermal neutrons well but is a radioactive material
with a very long half-life and when used as an isotope is high cost.
Also, cadmium is toxic to humans. Because of the undesirable features of
cadmium as a neutron absorber, gadolinium and hafnium have sometimes
substituted, but are higher cost than bulk cadmium. Hafnium is available
as a by-product of refining zirconium - which is the valuable metal
needed for the opposite reason (does not absorb).
Gadolinium is a rare-earth metal existing in seven natural isotopes. It
is five times more abundant on earth than tungsten, for example, so it
is not that rare. It is costly because there is little demand.
Gadolinium is also used as a shielding material as is disclosed by U.S.
Pat. No. 5,015,863 and 4,868,400. In nature, gadolinium occurs mixed
with other rare-earth metals, but can be separated by well known
techniques such as ion-exchange and the like. Gadolinium is malleable,
ductile, electroplate-able and available in a number of forms.
The figure of interest is the thermal neutron cross-section. The Oxford
(Emsley) reference gives these numbers (higher is better):
Boron 3837
Lithium 71
Cadmium 2450
Hafnium 103
Gadolinium 49,000
As you can see - using gadolinium in hot fusion, and by extension,
perhaps even "giving it a try" in LENR does make some sense if there are
any neutrons at all showing up.
Jones
