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Linking the kilogram to an atomic mass

In the gold ion experiment at PTB, the atomic mass of gold was determined in the SI base unit "kilogram". In future, this experiment could lead to a new definition of the kilogram not linked to a prototype anymore, but traced back to an atomic mass and thus linked to a physical constant.

Top view on the experimental set-up. In the foreground: ion source and mass separator (blue).

Comparisons between the international and the national kilogram prototypes as well as with the reference standards of the Bureau International des Poids et Mesures (BIPM) have shown mass changes in the range of 50 µg in 100 years. A mass change of the international kilogram prototype itself cannot be ruled out. Proof of such changes must be based on a sufficiently exact comparison of the kilogram with a physical constant. The PTB ion accumulation experiment therefore aims at linking up the atomic mass unit with the kilogram with a relative uncertainty of approximately 10-8. The goal is to open up a new way to redefine the kilogram.

In the experiment, ions from an ion beam are captured in a collector. The current of the ion beam and the accumulation time are measured, and the mass of the accumulated ions is determined. From this, the unit of atomic mass can be quantified, provided the relative atomic mass and the elementary charge are known. With an ion current of approximately 10 mA a mass of 10 g of a heavy element will be accumulated. This procedure takes approximately six days.

In a first accumulation experiment, gold ions were collected on a gold-coated quartz microbalance. The atomic mass of gold was determined in the base unit "kilogram" with a deviation of 0.6 % from the expected value and an uncertainty of 1.5 %. The accumulated mass amounted to approx. 0.5 mg at an ion current of 0.01 mA during an accumulation time of eight hours.

In the meanwhile, total ion beams (xenon and gold) with a current of more than 60 mA have been produced with a new set-up designed to increase the ion current. However, the gold ions only accounted for a current of 0.8 mA. As this current is still insufficient, a new ion source will be used in future. The new source will generate a bismuth ion beam with the aid of an oven. This will probably provide the required current of at least 10 mA. The mass of the accumulated ions will be determined in vacuum with a symmetrical equal-armed balance. This balance has been developed at PTB and is currently undergoing tests. The present standard deviation of the balance is 3 · 10-9 kg. It will be improved to less than 1 · 10-10 kg to allow an accumulated mass of approx. 10 g of bismuth to be determined with the sufficient accuracy.

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