DETERMINATION OF THE FINE-STRUCTURE CONSTANT BY A PRECISE MEASUREMENT OF H/M(N)

The quotient h/m(n) (h Planck constant, m(n) neutron mass) has been measured with a relative uncertainty of 7,7 x 10(-8). The result is h/m(n) = 3,956033320(302) x 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 alpha can be derived. The measured value of h/m(n) leads to alpha(-1) = 137,03601082(524) (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.