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The half lives of many radioactive nuclei can be determined with high precision
By current definitions, Avogadro's number is about 6.022140857x1023. It cannot be defined with any more precision
The half lives ..... are all currently known to within 1ppm.
You can't easily average measurements made on 10 successive days, because the decay rate will differ on every day.While the original element may be quite stable, fission decay products are sometimes a bit unpredictable, and some of them may react with oxygen in the air.
So you want to replace a number known to 1 part in 210 (sic) with one known to 1 part in 26 (sic). In what way does this constitute progress?
There is a special place in Hell for Disciples of the Box - beware of guilt by association!
There has been debate for some time about redefining the SI kilogram.One of the proposals was to base the kilogram on a certain number of atoms of isotopically pure silicon. So various teams produced highly-polished silicon spheres, and then attempted to count the atoms, by various methods. This would have defined Avogadro's number in a new way.For some of the saga, see: http://spectrum.ieee.org/consumer-electronics/standards/the-kilogram-reinvented However, the current recommendation is to define the kilogram in terms of Plank's constant. This in turn will in turn produce a more traceable value for Avogadro's number.See: http://en.wikipedia.org/wiki/Proposed_redefinition_of_SI_base_units#Kilogram
Interesting. I do like the idea of using Plank's constant--really anything to get away from a hunk of metal that has no universal meaning.
Nice idea, but the official value of h has changed by about 3 ppm in the last 100 years!
All that metrologists can say is that the master kilogram seems to have lost as much as 50 µg over the course of a century relative to its siblings.
The Planck constant is (with one or two exceptions) the fundamental physical constant which is known to the lowest level of precision, with a 1σ relative uncertainty ur of 1.2×10−8.
1.2×10−8 is 12 ppb. I think Plank is still ahead.