Gravitational Acceleration of Free Neutrons

The gravitational acceleration of free neutrons from the Oak Ridge Research Reactor has been measured in the evacuated 180-m flight path at Oak Ridge National Laboratory. The transmission edges associated with the 〈100〉 and 〈002〉 lattice spacings in a polycrystalline beryllium filter have been used to define particular neutron velocities v₇₊₋= (h2m{d₇₊₋}) in the "slow beam. " Boral filters permitted the selection of a "fast beam" which did not fall appreciably. The differences between the vertical positions of the slit image formed by the "fast beam" and of the two transmission edges (12. 7 and 15. 5 cm lower) have been determined with the aid of least-squares fits to theoretical curves. An x-ray examination of the beryllium filter established the values for 2d₇₊₋. We find the acceleration of free neutrons due to gravity to be g=975. 43 cm/sec^2 (100) planes and g=973. 17 cm/sec^2 (002) planes. These may be compared with the local value g=979. 74 cm/sec^2 and McReynolds' early result g=93570 cm/sec^2. A recent suggestion of Spitzer that there might be a difference in g for the two vertical neutron-spin projections does not appear to be present. No splitting greater than a few percent of g is found.