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available by PHP) ALPHA RAYS EMITTING IMPURITIES IN ULTRA PURE
ALUMINIUM : EVOLUTION THROUGH THE SUCCESSIVE REFINING STEPS
Michel LEROY, R&D Manager, ALUMINIUM
PECHINEY / PECHINEY HIGH PURITY, B.P. 7, 38340 VOREPPE, France
Abstract. Alpha rays are a major source of
soft-errors in integrated circuits, especially in DRAM memories. A major aim of refining
techniques used for production of Ultra Pure aluminium alloys is then to eliminate
radioactive impurities. This elimination was studied all along the refining steps from the
ore (bauxite) to Ultra Pure aluminium. A major observation is that Radium isotopes
accumulate in the caustic soda liquor, used for production of alumina from bauxite and in
the cryolithic melt used for production of primary aluminium from alumina. Accumulated
isotope Radium 228, which has a rather short half-life, disintegrates by b-emission to
Thorium 228, which then transfers immediately to cathodic aluminium metal This induces a
strong desequilibrium in the decay chain of Thorium 232, and, as a result, more than 80 %
of the total alpha-emission of Ultra Pure aluminium comes from alpha-decay of Thorium 228
and its daughters, which are not in equilibrium with Thorium 232. As a consequense,
the content of Aluminium in U 238 and Th 232 (the only isotopes measured by GDMS or
ICP/MS) can just give a rough order of magnitude of Alpha-emission, Th 232 family being by
far the most deleterious as regards soft-errors.
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