The thermal neutron absorption cross section of boron of different origin has been determined by comparing the decay of the neutron flux from a water solution of the sample with the flux decay from the same volume of distilled water. The ratio of the two decay curves is a pure exponential, independent of slowing down and of the presence of harmonic modes. After application of small corrections the decay constant of the ratio curve gives directly the absorption cross section. The result for the Argonne-Brookhaven standard boron is 764±3 barns as compared to 755±5, 749±4, and 771±5 barns obtained at Argonne, Brookhaven, and Harwell, respectively.

• Received 29 July 1954

DOI:https://doi.org/10.1103/PhysRev.96.1566