DETERMINATION OF THE FINE-STRUCTURE CONSTANT BY MEASURING THE QUOTIENT OF THE PLANCK CONSTANT AND THE NEUTRON MASS

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, the product of these two quantities is equal to the quotient h/m(n) (h Planck constant, m(n) neutron mass). The result is h/m(n) = 3 956 033 320(302) x 10(7) m(2) s(-1) (relative uncertainty: 7.7 x 10(-8). From this value, a value of the fine-structure constant alpha was derived, leading to alpha(-1) = 137 036 011 28(5 24) (relative uncertainty: 3.9 x 10(-8). The experiment was carried out at the high-flux reactor of the Institut Laue-Langevin in Grenoble, France.