## Abstract

Measurements have been made of the neutron density as a function of distance from a polonium-beryllium source in effectively infinite tanks of water and aqueous boric acid. The ratio of the thermal-neutron capture cross-sections of boron and hydrogen was found to be 2317 $\pm $ 27. The cross-section of hydrogen calculated from this result, using the known cross-section of Harwell standard boron, 769 $\pm $ 4 barns (at a neutron velocity of 2200 m/s.), and the known cross-section of the boron used relative to Harwell standard boron, 0$\cdot $985 $\pm $ 0$\cdot $002, was 0$\cdot $327 $\pm $ 0$\cdot $004 barn (at 2200 m/s.) which is consistent with other recent accurate values. In preliminary experiments, it was shown that flux measurements reproducible to better than 3% were possible with the disks of Ilford C2 nuclear research photographic emulsion used as detectors. The main source of error in the result was the statistical error for a total count of 200 000 tracks. The only corrections necessary were 1$\cdot $6% for recoil proton background, and 0$\cdot $7% for the disturbance of the neutron density by the detectors.