Determination of the fine-structure constant by a precise measurement ofh/mn

Abstract

The quotient h/m n ( h Planck constant, m n neutron mass) has been measured with a relative uncertainty of 7,7 × 10 -8 . The result is h/m n = 3,956 033 320 (302) × 10 -7 m 2 s -1 . To obtain it, a beam of neutrons with an identical and well-known wavelength was produced by Bragg reflection in a perfect silicon crystal; the velocity of the neutrons was measured using a very precise time-of-flight method. According to the de Broglie equation, h/m n is equal to the product of these two quantities. From h/m n the value of the fine-structure constant α can be derived. The measured value of h/m n leads to α -1 = 137,03601082 (524) (relative uncertainty 3,9 × 10 -8 ). The experiment was carried out at the high-flux reactor of the Institut Laue-Langevin in Grenoble, France.

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