Four samples of barium carbonate were irradiated with neutrons at three different times and at three different positions in the heavy water pile. A few days after an irradiation, lanthanum separations were made until no further ${\mathrm{La}}^{140}$ activity could be detected. The lanthanum was discarded. At intervals of 4 to 9 days the ${\mathrm{La}}^{140}$ grown from the ${\mathrm{Ba}}^{140}$ was separated and its number of atoms determined with a calibrated Geiger-Müller counter. The fluxes of thermal neutrons were obtained by experiment as far as possible; the maximum used was 2.0×${10}^{13}$ neutrons/${\mathrm{cm}}^2$/sec. The final value for the capture cross section of ${\mathrm{Ba}}^{139}$ is 4.7±1.2 barns.

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