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Generation of 5 mA DC bismuth current

  • Fundamentals of Metrology

In the ion accumulation experiment aimed at linking the atomic mass unit up with the kilogram, a new ion source has been implemented to generate a bismuth ion beam and accumu-late the bismuth ions. It had turned out that the ion source for gold ions so far used did not serve to generate a sufficiently high ion current. The oven version of the new CHORDIS plasma ion source now for the first time allowed bismuth ion beams of approx. 5 mA to be generated. As an example, Figure 1 shows the ion current and the oven temperature as a function of time. At present, the source is working with a one-hole extraction system. Later, it shall be possible to generate even higher ion currents with a two-hole or seven-hole system. The oven could be used in continuous operation for 5 to 6 hours. About 10 days are required to accumulate 10 g of bismuth with an ion current of 5 mA. For maintenance work, for exam-ple on the ion source, accumulation may also be interrupted, part of the vacuum apparatus permanently remaining under vacuum. It is planned to reduce in 2005 the relative uncertainty of 1.5 % so far achieved for the determination of the mass of a gold atom [1] down to 0.04 % for the mass of a bismuth atom.

Ion current (blue) and oven temperature (red, broken line) as a function of time

Figure 1: Ion current (blue) and oven temperature (red, broken line) as a function of time. In the first 150 minutes, the ion source was operated only with xenon gas. After that, the oven was heated up and switched off after approx. 230 minutes. The difference between the total ion current (8 mA to 10 mA) and the xenon current (3 mA) indicates the bismuth contribution. The peak at 150 minutes represents the evaporation of adsorbed water. The two drops before and after the 110-minutes mark were provoked by short-term disconnection of the extraction voltage for test purposes.

Moreover, the following additional improvements were made on the apparatus: A segmented electrode for determination of the transversal alignment of the ion beam and a steerer for setting of the alignment were installed. A cooling trap was incorporated to prevent unwanted bismuth precipitates in the beam handling. A new pressure measurement with a dynamic range of 10-9 - 10-3 mbar was put into operation. A non-contact ion current measurement with a DC current transformer was implemented. Within the scope of the optimization of the ion beam divergence, a new water-cooled and movable mobile faraday cup was integrated in addition into the facility. Figure 2 shows a survey of the ion beam apparatus.

View of the ion beam apparatus.

Figure 2: View of the ion beam apparatus. From left to right: ion source, magnetic quadrupole lens, separator magnet (dipole), collector chamber.

[1] D. Ratschko, D. Knolle, E. Finke, M. Gläser: Accumulation of decelerated gold atoms, Nucl. Instr. and Meth. In Phys. Res. B. 190 (2002), 217-221.

Contact person:

M. Gläser, FB 1.2, AG 1.24, email: Michael.glaeser@ptb.de


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