IT has been recently reported1 that neutrons are liberated from beryllium by -rays of radium and that these are able to induce radioactivity in iodine. Following up this work, we have attempted to liberate neutrons from beryllium by means of hard X-rays, produced by high-voltage electron tubes. An electron tube, which could conveniently be operated by a high-voltage impulse generator at several million volts2, is at present in use in the High Tension Laboratory of the A.E.G. in Berlin, and has served in the present experiment for the production of X-rays. In September 1934, Leo Szilard and T. H. Chalmers let gamma rays fall onto a beryllium target, noting that emissions from the target induced radioactivity in iodine. "We conclude," they wrote, "that neutrons are liberated from beryllium by gamma rays." Two months later, A. Brasch and colleagues, including Szilard and Chalmers, reported a similar effect using X-rays rather than gamma rays. More ominously, the existence of neutron-induced radioactivity also suggested the possibility of neutron chain-reactions — using the neutrons emitted by radioactive elements to induce radioactivity, and liberate further neutrons, from other nuclei. The first demonstration came four years later, following the discovery of nuclear fission in uranium [see Nature 143, 239-240 (1939)].
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