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Krempel, Jochen (2011): A new spectrometer to measure the molar Planck constant. Dissertation, LMU München: Fakultät für Physik



The SI-unit kilogram is scheduled to be re-defined within the next years. In the attempt to link it to a fundamental constant of nature – namely the Planck constant or the Avogadro constant – some discrepancies appeared. A direct determination of the molar Planck constant helps to trace this discrepancy. Such a determination can be done with the gamma-spectrometer Gams, that exists at the ILL. However, the instrument is not stable enough to provide the required accuracy of 2×10^(−8) (relative). To improve stability and accuracy, a complete new instrument was designed and built. For this purpose, the instrument core, an angle interferometer, had to be set up in vacuum. Drift-free fixation-methods and new designs for the optical elements, like corner-cube retro-reflectors, were developed. A new data acquisition system was elaborated, and also the mathematical theory to evaluate the acquired data. The new concepts for the instrument were completed, manufactured and assembled. First performance tests showed a quite convincing result in terms of stability. During the development of the new instrument, the old instrument Gams4 was optimized. The neutron binding energy of chlorine-36 was determined to be (8579797.4±1.8)eV. The relative uncertainty is 2.7 times smaller than the previous value. It is two times smaller than the uncertainty of any other comparable binding energy.