E Massa et al 2009 New J. Phys. 11 053013 doi:10.1088/1367-2630/11/5/053013
Measurement of the lattice parameter of a silicon crystal
E Massa1,4, G Mana1, U Kuetgens2 and L Ferroglio1,3
Show affiliationsThe silicon crystal WASO04 is a reference in the adjustment of fundamental physical constants, but its lattice parameter has never been measured in absolute terms. In the framework of an international project meant to base the kilogram definition on the molar volume and the lattice parameter of 28Si, the WASO04 crystal has been used to manufacture an interferometer prototype for the performance testing and the fine-tuning of a new experimental apparatus for lattice parameter measurements by combined x-ray and optical interferometry. The present paper discusses the test results and gives an accurate lattice parameter determination. With respect to previous determinations, the value obtained, d220(WASO04)=192.015 570 2(10) pm, displays a four-fold improvement in accurracy.
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GENERAL SCIENTIFIC SUMMARY
Introduction and background. To allow the kilogram definition to be based on an atomic mass—by counting the number of atoms in a 1 kg silicon crystal-sphere—the uncertainty of the (220) lattice spacing of silicon must be reduced by a factor ten. An accurate value of this quantity is relevant also to establish a consistent set of values of the fundamental physical constant.Main results. To achieve the required 3 × 10−9 relative accuracy, a combined x-ray and optical interferometer is used and an x-ray interferometer with an unusually 5 cm long analyzer-crystal was manufactured. The technologies necessary to accurately measure and control changes in the position and alignment of these crystals at the sub-atomic level and over distances as large as 5 cm were developed as well. Measurements were performed to verify and to test the apparatus performances and to pinpoint the experiment limits. With respect to previous determinations, the value obtained, 192.015 5702(10) pm, for the crystal designated as WASO04, in a vacuum, and at 22.5 °C, displays a four-fold accuracy improvement.
Wider implications. These results represent an improvement of the existing measurement capabilities by more than an order of magnitude and make more numerous systematic effects visible and reproducible.
- PACS
- Subjects
- Dates
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Issue 5 (May 2009)
Received 2 February 2009
Published 27 May 2009
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Measurement of the lattice parameter of a silicon crystal
E Massa et al 2009 New J. Phys. 11 053013
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