Precise Atomic Masses for Determining Fundamental Constants.
Abstract
The atomic masses of several isotopes ( ^1H, ^2H, ^{13}C, ^{14 }N, ^{15}N, ^{16}O, ^ {20}Ne, ^{28} Si, and ^{40}Ar) are determined by Penning trap mass spectrometry with an accuracy as high as 7 times 10 ^{-11}, which is an improvement of a factor of 10-1000 compared to results from conventional mass spectrometry. Relative ion mass ratios are measured by comparing the cyclotron frequencies of single trapped ions. The precision is limited by short-term fluctuations in the magnetic field. The mass ratios are converted to atomic masses (relative to ^{12} C) after accounting for the chemical energies and performing a global least squares fit. The ratios constitute an overdetermined set, allowing checks on random and systematic errors which are consistent within the reported uncertainties. Precision mass spectrometry offers several contributions to the field of fundamental constants. The energy released in the neutron capture reactions ^{14 }N(n, gamma) and ^{12}C(n, gamma) could be determined by measuring the wavelengths of the gamma-rays emitted and also by measuring the mass differences ^{14}N + ^2H-^{15}N-^1H and ^{12}C+ ^2H -^{13}C-^1H, respectively. These mass differences are reported here with an accuracy of {~}10^{-7}. Combining the wavelength and mass measurements determines the fine structure constant alpha and the molar Planck constant N_{A}h to the same level of precision. Compared with values of alpha from other experiments, the consistency of the underlying physical laws could be verified. Another contribution is the improvement of the atomic mass of ^{28}Si, which is needed for replacing the present artifact kilogram standard with a new atomic mass standard. Ways to improve the experiment in the future also are discussed. Magnetic field fluctuations could be reduced by installing a self-shielding coil around the trap. Two ions could be simultaneously measured in order to cancel the field noise. External ion loading offers many benefits to the experiment, in particular to increase the variety of ions which could be measured. Classical amplitude squeezing techniques are proposed for reducing the effects of thermal noise and may have general applicability to other experiments. Experimentally, thermal noise for a trapped ion is characterized, and a reduction in amplitude uncertainty is demonstrated. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617-253-5668; Fax 617-253 -1690.).
- Publication:
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Ph.D. Thesis
- Pub Date:
- 1994
- Bibcode:
- 1994PhDT........47D
- Keywords:
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- Physics: Atomic; Physics: Nuclear; Physics: General