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The kilogram is the only one of the seven SI base units (metre, kilogram, second, ampere, kelvin, mole, candela) that is not yet defined by an atomic or fundamental constant of physics, but by a material artefact, the international prototype of the kilogram. It has been found, that mass standards of the type of the international prototype of the kilogram (e.g. national prototypes) change their mass among one another. Mass changes of the international prototype can therefore not be excluded. Only by comparison with an invariable constant of nature could such changes be proved. Worldwide considerations and experimental approaches are therefore going on for re-defining the kilogram by a physical experiment that is reproducible at any time and at any place - similarly to the metre or the second. It is obvious to choose the mass of a particular atom as a reference quantity. Shurely, it is impossible to weigh a single atom (the mass of a 12C atom, for example, is 0,000 000 000 000 000 000 000 000 02 kg - too small by orders of magnitude). With the ion accumulation experiment, PTB has started an experiment, by which not a single atom, but many charged atoms, for example 197Au or 209Bi atoms, collected up to a weighable mass of about m =10 g, will be weighed using a balance. The number of accumulated atoms will be known by measuring the electrical ion current and the time, and thus the mass of an atom ma can be determined.
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Idea of the experiment
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Ions of a particular element, such as gold or bismuth will be accumulated up to a weighable mass m, and the ion current I will be integrated over the accumulation time t. With the known charge e of an ion we have the mass of an atom ma or the atomic mass unit mu - with the relative atomic mass Ar.
To be independent from the elementary charge e, the ion current I may run through a resistance R with a voltage drop U by comparing R with h/(n1 e2), the Quantum Hall resistance, and U with n2 f h /(2 e), f being the frequency the Josephson voltage. The atomic mass ma can thus be traced to the units kilogram and second.
With a gold or bismuth ion beam of 10 mA, for example, a mass of 10 g could be accumulated in about 6 days.
With a balance having a standard deviation of 0.1 µg, this mass can be determined with a relative uncertainty of 10-8.
A current of 10 mA corresponds to a mass flow of about 1.8 g/day for gold, resp. 1.9 g/day for bismuth, and a particle flow of 6.2·1016 particles/s.
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Experimental set-up
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From a gas discharge ion source (CHORDIS) electrically positive gold ions (and noble gas ions) are extracted and perform an ion beam. The gold ions are focused by a magnetic lens (quadrupole triplett) and are separated from other ions by use of a dipole magnet. The collector is suspended from a balance that allows a comparison between the mass of the accumulated ions and that of weights. The ion beam represents also an electrical current that is measured by means of a Quantum Hall resistance and a Josephson voltage standard.
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Accumulated gold
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Low energy gold ions (< 50 eV) have been collected on an electrode.
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Ion accumulation on a quartz crystal balance
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With a first accumulation experiment, gold ions of an ion beam have been decelerated down to less than 400 eV and accumulated on a quartz crystal balance. The quotient: elementary charge multiplied by the mass flow and divided by the ion current (or: elementary charge e multiplied by the accumulated mass m and divided by the accumulated charge Q) has been measured as a function of the ion energy. In the ideal case, this quotient is the mass of a single gold atom. Because - among other effects - the ions sputter atoms from the quartz surface, the value of this quotient arrives the expected value only at low ion energies.
The measurements have shown a reasonable agreement with the expected value of the mass of a gold atom in the unit kilogram for low ion energies:
mAu = 3,29·10-25 kg U/mAu = 1,5%
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